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Regulation of Cellular Calcium in Cardiac Myocytes

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Abstract

The sections in this article are:

1 General Aspects of Cellular Calcium Regulation
1.1 Ca Transport Systems
1.2 Cytosolic Volume Conventions
1.3 Ca Binding Sites and Buffering in Cardiac Myocytes
1.4 Ca Requirements for Contractile Activation
1.5 Simplified Kinetic Considerations During a Ca Transient
2 Sarcolemmal Ca Transport Systems
2.1 Ca Channels
2.2 Na/Ca Exchange
2.3 Sarcolemmal Ca‐ATPase
3 Intracellular Ca Transporters
3.1 Sarcoplasmic Reticulum Ca‐ATPase and Sarcoplasmic Reticulum Ca Content
3.2 Sarcoplasmic Reticulum Ca Release Channels
3.3 Mitochondrial Ca Transport
4 Ca Removal from the Cytoplasm During Relaxation
4.1 Relative Contributions of Ca Transporters
4.2 Species, Developmental, and Temperature Dependence
4.3 Ca Recycling from Mitochondria Back to the SR
5 The Ca Supply that Activates Contraction
5.1 Ca Influx vs SR Ca Release
5.2 Fraction of SR Ca Released During a Twitch
6 Perturbations of Cellular Ca Balance
6.1 Rest‐Dependent Changes in Cellular and SR Ca Content
6.2 Refilling Depleted Internal Ca Stores
6.3 Rest‐Decay and Rest‐Potentiation of Twitches
6.4 Force‐Frequency Relationships
Figure 1. Figure 1.

Simplified cardiac myocyte Ca fluxes. Ca enters during the action potential via ICa and Na/Ca exchange (Na/CaX). Ca entry triggers Ca release from the SR and the combination activates the myofilaments (MF). Elevated [Ca]i stimulates Ca removal from the cytosol by the SR Ca‐ATPase (modulated by phospholamban, PLB), the Na/Ca exchange, the sarcolemmal Ca‐ATPase, and the mitochondrial uniporter. As [Ca]i declines Ca dissociates from the myofilaments allowing relaxation. The sarcolemmal (Na + K)ATPase and Na/H exchange are also indicated, along with other key mitochondrial ion transporters.

Modified from Bers
Figure 2. Figure 2.

Passive Ca buffering in cardiac myocytes. The six curves represent different estimates of how passive Ca buffering changes as [Ca]i increases from 100 to 1500 nM. These values are indicated as increments above the amount of Ca bound at 100 nM [Ca]i (see also Table and discussion in text). Fabiato‐Orig Fast, Berlin‐Fast and Hove‐Madsen‐Equilibrium are taken directly from values in the relevant papers . Fab/Bers‐New Fast updates some binding constants from the original estimates of Fabiato , as described in the text. Berlin‐Fast+MF Ca/Mg adds known myofilament sites at which Ca and Mg compete. Calculated Total‐New refers to values resulting from constants in Table .

Figure 3. Figure 3.

Measurement of fast Ca buffering in a voltage clamped rat ventricular myocyte. Cells were exposed to 10 μM thapsigargin to prevent SR Ca uptake and were in Na‐free conditions (inside and out) to prevent Na/Ca exchange. A. Voltage clamp pulses (200 ms from −40 to 0 mV) activated ICa and produced nearly step‐like increases in [Ca]i. B: The integral of the ICa from the fourth pulse in A (∫ ICa follows the kinetics of the rise in [Ca]i. The sag in the [Ca]i trace may represent slow buffering that is not in phase with Ca entry via ICa. The “ S” indicates where [Ca]i would be predicted to fall to eventually if the slow buffering by myofilament Ca/Mg sites are included with the fast buffering measured (see text). The points marked [A] and [B] in panel A are the predicted final settling points for [Ca]i if the slow buffering is included for the total ∫ICa throughout the train (as for S) and when the equilibrium buffering from Fig and Table is assumed, respectively

Modified from Berlin et al.
Figure 4. Figure 4.

Free and total Ca requirements for myofilament activation in cardiac myocytes. A: Two different sets of parameters are used for the Hill equation (100/(1 + (Km/[Ca])n) describing the steady‐state relationship between force and [Ca]. The solid curve (Km = 600 nM and n = 4) is more consistent with recent estimates in intact ventricular muscle, whereas the lower dotted curve (Km = 1000 nM and n = 2) is closer to traditional data from skinned muscle fibers (see text). B. The same relationships as in A, but the free [Ca] axis has been altered to account for the cytosolic Ca buffering relationship (Calculated‐Total‐New in Figure and ). Thus, this predicts the force as a function of the total amount of Ca added to the cytosol from a resting level of 150 nM free [Ca]. Coincidentally, almost the same curves are obtained if resting [Ca]i is assumed to be 100 nM and the intermediate buffering curve from Figure is used (Berlin‐Fast+MF Ca/Mg).

Figure 5. Figure 5.

Cytosolic Ca buffering and transport during a Ca transient. A. A Ca transient is calculated from the product of a rising and declining exponential which rises from 100 to 744 nM and back with time constants (τ) of 30 ms (rising) and 300 ms (falling). This was used as a driving function to calculate the time‐dependent changes in binding to various cytosolic buffers (B) using the rate constants and concentrations in Table (TnC is troponin C, SL is total sarcolemmal site in Table , CaM is calmodulin, TnC‐Ca/Mg and My‐Ca/Mg are Ca binding to Ca/Mg sites on TnC and myosin). The change in total cytosolic [Ca] (ΔTotal CaCyt in A) is the sum of the curves in B and the free [Ca]i curve. C: Ca fluxes associated with the Ca transient. ICa activation was calculated by a rising (τ = 3 ms) and falling exponential (τ = 40 ms), peaking at 6.8 pA/pF and bringing in 16 μmol/l cytosol. The SR Ca‐pump rate was 210/(1 + (300 nM/[Ca]i)2) in mol/l cytosol. The SR Ca leak was initiated to counterbalance the SR Ca‐pump rate at rest, and changed linearly as a function of free [Ca] in the SR (based on intra‐SR Ca buffering with a Kd = 600 μM and Bmax equivalent to 180 μmol/l cytosol in an SR volume occupying 3.5% of cell volume). The SR Ca release flux was taken as the residual Ca flux required to produce the driving Ca transient.

Figure 6. Figure 6.

Currents via T‐and L‐type Ca channels. A. Barium currents (with 115 mM Ba) induced by depolarizations to various test potentials from holding potentials of 80 or 30 mV. The Em protocol is shown in the top trace, the currents in the middle trace, and the difference between these currents in the bottom panel. Peak IBa from 30 mV is attributed to L‐type Ca channels and the additional transient difference current activated from 80 mV is attributed to T‐type Ca channels. B: ICa in dog Purkinje fiber cell with 2 mM Ca as charge carrier. The more positive Em required in IBa is due to the higher surface potential in 2 mM Ca vs 115 mM Ba. C: ICa from several species (from 80 to 100 mV), where the hump at ∼−40 mV is due to T‐type current and differs among the tissues studied. Dog Purkinje (ref. ) and rabbit ventricle (G. M. Briggs and D. M. Bers, unpublished) are in 2 mM Ca. Dog atrium (ref. ) and guinea‐pig ventricle (from Mitra & Morad, ) are with 5 mM Ca, but are shifted by 10 mV to compensate for surface potential differences.

From Bean , with permission. From Hirano et al. , with permission. From Bers with permission)
Figure 7. Figure 7.

Ca channel inactivation with different charge carriers. Normalized current amplitudes measured at 0 mV (except Ins at −30 mV to match activation state). ICa was recorded under both perforated patch (allowing normal SR Ca release and Ca transients) and ruptured patch with cells dialyzed with 10 mM EGTA (to prevent global Ca transients). IBa was also recorded with ruptured patch (with 10 mM EGTA in the pipette). Extracellular [Ca] and [Ba] were both 2 mM and Ins was measured in divalent‐free conditions (10 mM EDTA inside and out) with [Na]o at 20 mM and [Na]i at 10 mM. Peak currents were 1370, 808, 780, and 5200 pA and were attained at 5, 7, 10, and 14 ms for ICa (perforated), ICa (ruptured), IBa and Ins respectively. Halftimes of current decline were 17, 37, 161, and >500 ms, respectively.

Recordings made by Dr. W. Yuan
Figure 8. Figure 8.

Ca‐dependent inactivation by SR Ca release and recovery of ICa. Long (4 sec.) voltage clamp pulses activated ICa and SR Ca release and Ca transient in Na‐free conditions and 5.4 mM [Ca]o. The pulse labeled a was the first pulse after depletion of the SR and b was after 5–10 pulses when the SR was loaded. The smaller Ca transient produces less ICa inactivation. The larger Ca transient produces marked inactivation, but the current partially recovers as [Ca]i declines. The lower difference traces show the similarity between the ICa inactivation and Cai transient

Modified from Sipido et al. , with permission
Figure 9. Figure 9.

Ca currents measured during square pulse and action potential clamp in rat and rabbit ventricular myocytes. Command Em waveforms were either the traditional 200 ms square depolarization to 0 mV or an action potential (AP), which was recorded from normal rat and rabbit ventricular myocytes under more physiological conditions (i.e. normal Ca Na and K concentrations, ref ). ICa was then recorded under conditions where all other currents were blocked (e.g. Na‐free and Cs‐rich inside and out). With the action potential waveform, the peak ICa is smaller in both species and occurs later (middle panel). The ICa integral (lower panels) for the AP clamp vs the square, pulses are smaller for the rat but larger for the rabbit (see ref. for additional information).

Figure 10. Figure 10.

Simulations of guinea‐pig ventricular action potential using Oxsoft Heart (v 4.5). This comprehensive simulation created by Professor Denis Noble and colleagues (at Oxford, UK) calculates the Em and many of the ionic currents that are known to flow during the action potential. Shown here are the Em (top), ICa, INa/Ca (middle panel), [Ca]i, and force (lower panel) for two different initial [Na]i values (5 and 8 mM).

Figure 11. Figure 11.

Na/Ca exchange current in intact ventricular myocytes recorded under controlled conditions. A. Very slow speed recording of Em (top) and INa/Ca where the spikes are from ramp depolarizations that were used to generate the current voltage relationships in B, C, and D at the times indicated (a–h). The bars in A are when [Na]o was applied (replacing Li) to activate INa/Ca ([Ca]o was 1 mM throughout). At the arrow [Ca]i was increased from nominally Ca free to 430 nM with 42 mM EGTA and 140 CsCl in the dialyzing pipette throughout. Ouabain, Ba, Cs, D600 and tetraethylammonium were used to inhibit other ionic currents. Application of [Na]o stimulated INa/Ca only after [Ca]i was raised. The gradual decline in INa/Ca (d–g) was supposed to be due to depletion of [Ca]i.

From Kimura et al. , with permission
Figure 12. Figure 12.

Current–voltage relationships for INa/Ca recorded in excised giant patches from guinea pig ventricular myocytes. The patches were treated with chymotrypsin to remove INa/Ca inactivation and all other known currents were blocked . A: With constant [Ca]o (2 mM), [Na]o (150 mM) and [Ca]i (1 μM), the [Na]i was varied from 5 to 100 mM. The lower 3–4 curves probably reflect the INa/Ca expected in intact cells when [Ca]i is high (e.g. peak systole). B: The same conditions as A, except [Ca]i is reduced to 100 nM, comparable to diastolic [Ca]i.

From Matsuoka and Hilgemann , with permission
Figure 13. Figure 13.

Na/Ca exchange currents and [Ca]i in a guinea pig ventricular myocyte. The Ca transient (using fura‐2) show that Ca influx via Na/Ca exchange can bring in large quantities of Ca during large sustained depolarizations (A and B). The [Na] in the dialyzing pipette was 7.5 mM. Outward currents during the depolarization were off‐scale. C: The [Ca]i‐dependence of the “ tail” current was observed upon repolarization to −80 mV. Other ionic currents are blocked by Cs, tetraethylammonium, verapamil, and ryanodine.

From Barcenas‐Ruiz et al., , with permission
Figure 14. Figure 14.

Model of the Na/Ca exchanger based on recent work by Nicoll et al. and Iwamoto et al.. The structure is consistent with 9 membrane spanning segments and a glycosylation site (CH2O). The large cytoplasmic loop contains the Na/Ca exchange inhibitory peptide (XIP) domain (also associated closely with the region responsible for Na‐dependent inactivation) and also contains the site for allosteric regulation by [Ca]i*. The two homologous α repeats (α‐1 and α‐2) are also indicated.

Figure 15. Figure 15.

Na/Ca exchange reversal potential (ENa/Ca) during the rabbit ventricular action potential. This schematic diagram shows how ENa/Ca is expected to change during the action potential for two different levels of intracellular Na activity (aNai = 5 and 8 mM), where aNai is roughly 0.78 · [Na]i. When Em is positive to ENa/Ca, Ca influx via the Na/Ca exchange is thermodynamically favored (shaded areas). When Em is negative to ENa/Ca, Ca extrusion is favored. Resting [Ca]i =150 nM, [Ca]o = 2 mM and aNao = 110 mM for both traces and aNai and peak [Ca]i are as indicated. The [Ca]i trace reaches a peak 40 msec after the action potential begins.

After Bers
Figure 16. Figure 16.

Extracellular [Ca] depletion in rat‐rabbit ventricular muscle. Changes in [Ca]o were measured with double‐barreled Ca‐selective microelectrodes during individual contractions in rabbit (A) and rat (B) ventricular muscle (0.5 Hz, 30°C). The traces show [Ca]o (top) and tension (bottom) in the absence and presence of 10 mM citrate (which limits [Ca]o depletion by buffering [Ca]. The bath [Ca]o was 0.5 mM and is indicated by the dotted line.

A is modified from Shattock and Bers and composite from Bers , with permission
Figure 17. Figure 17.

Changes in Na/Ca exchange driving force during action potential in rat and rabbit ventricle. ENa/Ca is expected to change during the action potential and Ca transient in rabbit and rat ventricle (top). The estimated changes in the net electrochemical driving force for Na/Ca exchange (ENa/Ca – Em) are shown in the bottom panel. Na/Ca exchange stoichiometry of 3Na:1Ca was assumed, the aNai values were measured and, for simplicity, the Ca transient accompanying the contraction, has been assumed to be the same for both species. Resting [Ca]i was assumed to be 150 nM, rising to a peak of 1 μM, 40 msec after the upstroke of the action potential. Note the similarity between the lower panels and the [Ca]o traces in Figure. .

Modified from Shattock and Bers and Bers , with permission
Figure 18. Figure 18.

Caffeine‐induced Ca transient and Na/Ca exchange current in a ferret ventricular myocyte. A: After a steady‐state series of voltage clamp pulses, 10 mM caffeine was rapidly and continuously applied to release SR Ca. Experimental conditions blocked most other ionic currents (e.g. Cs inside and out) and indo‐1 was used as the Ca indicator. The inward INa/Ca rises to a peak before the Ca transient. B: Instantaneous INa/Ca from panel A is plotted as a function of the global [Ca]i reported by indo‐1. The hysteresis shows that during the rising phase of the Ca transient, more inward INa/Ca is observed for a given [Ca]i than during relaxation. The relationship between INa/Ca and [Ca]i during relaxation was fit to a linear regression (for [Ca]i between 300 and 450 nM (dashed line; see also Fig. C).

Experiment performed by Dr. Li Li. C. INa/Ca and [Ca]i data are reproduced from panel A and the subsarcolemmal [Ca]i sensed by the Na/Ca exchanger ([Ca]Na/CaX) based on the linear regression in panel B is also shown
Figure 19. Figure 19.

Ca transport by the SR Ca‐ATPase based on literature values (see text). Ca pump rates were calculated based on Eq. using Vmax = 200 μmol/liter cytosol, Km = 600 nM and n= 2. Activation of protein kinase A (PKA) decreases the Km to 250 nM, but does not affect Vmax.

Figure 20. Figure 20.

Measurement of SR Ca uptake rate in digitonin permeabilized myocytes. A. Free [Ca] was measured simultaneously with indo‐1 and Ca electrode. Ca was added at time 0 to a suspension of rabbit ventricular myocytes permeabilized with digitonin and incubated with 25 μM ruthenium red, 12.5 mM creatine phosphate, and 10 mM oxalate. B: The change in free [Ca] (Ca, solid line, from A), protein bound Ca (Ca‐Pn), Ca‐oxalate (Ca‐Ox), Ca‐indo (Ca‐In), and the sum of free and bound Ca (ΣCa) after addition of 14 μM total Ca. After an initial increase in free [Ca], Ca uptake by the SR lowered the free [Ca] back to the level before the Ca addition. The Ca bound to protein, oxalate, and indo‐1 was calculated from the passive Ca binding to permeabilized myocytes and indo‐1 . C. The Ca uptake rate was calculated as the rate of change in the total non‐SR Ca with time (i.e. dΣCa/dr). D: [Ca] dependence of Ca uptake by the SR. The Ca uptake rate determined in C was plotted versus the corresponding free [Ca] measured in A. Data were fit with Eq. to determine the maximal Ca uptake rate, Km, and Hill coefficient. Data are shown for addition of two different amounts of Ca to the myocyte suspension (14 μM total Ca = diamonds and 43 μM total Ca = circles.

From Hove‐Madsen and Bers , with permission
Figure 21. Figure 21.

Unidirectional SR Ca fluxes at rest. At rest when [Ca]SR is changing only very slowly, net SR Ca flux must be close to zero, implying the forward rate (VFor) is equal to the sum of reverse flux and leak from the SR (VRev + VLeak). The forward rate predicted from the Hill curve is 27 μmol/liter cytosol/sec. For a VLeak of 0.3 μmol/l cytosol/sec the reverse flux must be 26.7 μmol/liter cytosol/sec.

Figure 22. Figure 22.

The influence of SR Ca leak on SR Ca content. The effect of different leak values on steady‐state SR Ca content with [Ca]i = 150 nM was calculated using Eq. and with Vmax = 207 μmol/liter cytosol/sec, Kmf = 300 nM, BmaxSR = 3.95 mM and KdSR = 600 μM. For pump inhibition by thapsigargin (TG) Vmax was decreased by 50%. For Ca‐pump stimulation by isoproterenol, Km was reduced by 50%. The SR Ca leak rate (0.3 μmol/liter cytosol/sec) measured by Bassani and Bers is indicated. (See text and ref for further discussion.)

Figure 23. Figure 23.

Ionic fluxes across the mitochondrial membrane. Ca enters via a uniport, down an electrical gradient established by the proton pump at bottom. Ca can be extruded by a Na/Ca antiport and Na is extruded by Na/H exchange thereby completing the cycle. Elevated cytoplasmic [Ca] can lead to elevated mitochondrial [Ca] and increased activity of mitochondrial dehydrogenases and NADH production.

Figure 24. Figure 24.

Mitochondria free [Ca] ([Ca]m) as a function of cytosolic [Ca] ([Ca]c). Increases of [Ca]c in rat ventricular myocytes were induced by reduction of extracellular [Na] (i.e. via Na/Ca exchange). Mean [Ca]c was measured using indo‐1 (loaded as the salt form) and [Ca]m was measured using indo‐1 (loaded as the AM form) with Mn quench of cytosolic indo‐1. Data are taken from Miyata et al., and have been redrawn (without error bars) and including a broken line corresponding to [Ca]m = [Ca]c (slope = 1).

Figure 25. Figure 25.

Relaxation in rabbit ventricular myocyte with selective inhibition of Ca transporters. Normalized cell relaxations are shown and conditions were either (1) with all Ca transporters functional during relaxation of a steady‐state twitch (Tw), (2) preventing net SR Ca uptake during a caffeine‐induced contracture in NT (Caff), (3) additionally inhibiting Na/Ca exchange in 0Na, 0Ca solution (Caff,0Na,0Ca). To further analyze relaxation during Caff, 0Na, 0Ca, this was coupled with either (4) inhibition of mitochondrial Ca uptake with 1 μM FCCP + 1 μM oligomycin so only sarcolemmal Ca‐ATPase was functional (Caff,0Na,0Ca + FCCP) or (5) inhibition of the sarcolemmal Ca‐pump by elevating [Ca]o to 10 mM after pre‐depletion of [Na]i, so that only mitochondrial Ca uptake was functional (Caff,0Na,10Ca). All four Ca removal systems were also blocked by combining the caffeine application with 0Na, 10Ca and FCCP (after pre‐depletion of [Na]i). The traces are based on mean t1/2 values (shown along the traces without standard error values) for relaxation measured in experiments described by Bassani et al. .

Figure 26. Figure 26.

Twitch Ca transients in rabbit and rat ventricular myocytes with SR Ca‐ATPase blocked by thapsigargin. Ca transient were measured using indo‐1 fluorescence during electrically stimulated twitches in rabbit (A) and rat (B) ventricular myocytes before (Control) and after treatment with 2.5 μM thapsigargin (TG). Twitches were evoked 10 sec after switching to control solution (after 5–7 min pre‐perfusion with 0Na,0Ca solution). For the TG twitch the cells were exposed to TG for 2 min during the last part of the pre‐perfusion period. SR Ca load was maintained despite complete block of the SR Ca‐pump. Time constants (τ) of [Ca]i decline are shown

Modified from Bassani et al.,
Figure 27. Figure 27.

Ca transport functions derived from Ca transients in intact rabbit and rat ventricular myocytes. Ca transport rates as functions of [Ca]i for the SR Ca‐ATPase, Na/Ca exchange and combined slow systems (mitochondrial and sarcolemmal Ca‐ATPase) were determined as described in the text and with respect to Eq. and . Independent values obtained for JSR, JNa/CaX and JSlow respectively were: Vmax (in μmol/liter cytosol/sec) = 82, 46 and 3.9 for rabbit and 207, 27 and 4 for rat; Km (in nM) = 264, 316, and 362 in rabbit and 184, 257 and 268 in rat; n = 3.7, 3.7, 3.2 in rabbit and 3.9, 3.4 an 3.5 in rat.

Based on data from Bassani et al.
Figure 28. Figure 28.

Integrated Ca fluxes during twitch relaxation in rabbit and rat ventricular myocytes. Free [Ca]i during relaxation of a normal twitch was used as a driving function so that Ca flux via each system could be integrated over time using the [Ca]i dependence as described in Figure and Eq. and .

Modified from Bassani et al.
Figure 29. Figure 29.

Ca transients ICa during a twitch and INa/Ca used to measure SR Ca content. Ca transients and inward currents (ICa and INa/Ca) in a rat ventricular cardiomyocyte (dialyzed with 50 μM indo‐1) were recorded during the last two (of 8) conditioning voltage clamp pulses to 0 mV and during two rapid applications of 10 mM caffeine (bars, holding potential −70 mV). Amplitude and time scales for ICa (left) and INa/Ca (right) are different.

From Delbridge et al. , with permission
Figure 30. Figure 30.

Relative reliance on Ca influx in different cardiac muscle preparations. The effect of caffeine (10 mM) or ryanodine (100 nM) on steady‐state twitch contraction amplitude (0.5 Hz at 30°C or 23°C for frog) in various cardiac muscle preparations.

Data are from refs ; modified from Bers
Figure 31. Figure 31.

Fractional SR Ca release during the twitch depends on both ICa trigger and SR Ca load in ferret ventricular myocytes. A. Cells were loaded to a constant level (by stimulation at 0.5 Hz). Extracellular [Ca] was changed just prior to the test contraction where the fraction of SR Ca release was measured. Thus increasing trigger (at constant SR Ca load) increases fractional release. Parallel voltage clamp experiments were done to evaluate how ICa changes over this range of [Ca]o. B. Using a constant trigger (with 2 mM [Ca]o) the SR of cells were Ca loaded to different levels by varying frequency (as indicated) and also increasing [Ca]o at the highest load. SR Ca content was determined by translating the peak and resting [Ca]i during caffeine‐induced contractures to total cytosolic [Ca], using values determined by Hove‐Madsen and Bers for cytosolic Ca buffering.

Modified from Bassani et al. , with permission
Figure 32. Figure 32.

Rest decay and rest potentiation of twitches and SR Ca content in rabbit, rat and ferret myocytes. Both post‐rest twitches and caffeine‐induced contractures (Caff) were measured either in a modified normal Tyrode's solution (NT), when Na/Ca exchange was blocked during the rest (0Na,0Ca) or after pre‐depletion of [Na]i and rest in Ca‐free, 140 mM Na solution (0Cao) to stimulate Ca efflux via Na/Ca exchange. Caff data are indicative of SR Ca load. SR Ca content data (Caff) was fit with [Ca]SRC = A · exp(–t/τRD)(1 –exp{–t/τRF}) +B, where t is time, τRD is time constant, of rest decay of SR Ca content and τRF is a rest‐dependent filling time constant used to explain the delay in rest decay of SR Ca content. A and B are constants for scaling and baseline respectively. Twitch data were fit with the expression C[Ca]SRC(1–exp{–t/τFCC}) + D, where τECC: is the time constant for recovery of E‐C coupling and C and D are scaling and baseline constants. A small second exponential factor was also included to explain the slow decline in twitch amplitude in rabbit ventricle after long rests in 0Na, 0Ca.

Data are from Bers et al. and Bassani and Bers and have been combined and reanalyzed
Figure 33. Figure 33.

Refilling of SR Ca stores after depletion in rabbit ventricle. SR Ca content was measured by either caffeine‐induced contracture amplitude (in isolated myocytes at 23°C) or rapid cooling contractures (RCC) in trabeculae from rabbit ventricle (at 30°C). SR Ca was depleted by either a long rest (5 min) in normal solution (muscle) or a 10 sec application of 10 mM caffeine (myocyte). After SR Ca depletion, stimulation was started at 0.5 Hz and the SR Ca content was assessed after the beat number indicated.

Data are from Bers and Bassani et al.
Figure 34. Figure 34.

Changes in contraction force with abrupt changes in frequency. Frequency of stimulation of a rabbit ventricular trabecula (at 30°C) was increased from 0.5 to 1.5 Hz and then returned to 0.5 Hz.

Figure 35. Figure 35.

Steady‐state force‐frequency relationships. The effect of frequency (from 0.5 to 2 Hz) on twitch force in rabbit (circles), rat (squares) and guinea pig (triangles) ventricular muscle. Data for rabbit and rat are at 30°C.

From Bers (33 and unpublished.] Data for guinea pig are at 36.5°C and were taken from Kurihara and Sakai . RCCs were initiated within 5 sec of the last steady‐state stimulated contraction. [Redrawn from Bers


Figure 1.

Simplified cardiac myocyte Ca fluxes. Ca enters during the action potential via ICa and Na/Ca exchange (Na/CaX). Ca entry triggers Ca release from the SR and the combination activates the myofilaments (MF). Elevated [Ca]i stimulates Ca removal from the cytosol by the SR Ca‐ATPase (modulated by phospholamban, PLB), the Na/Ca exchange, the sarcolemmal Ca‐ATPase, and the mitochondrial uniporter. As [Ca]i declines Ca dissociates from the myofilaments allowing relaxation. The sarcolemmal (Na + K)ATPase and Na/H exchange are also indicated, along with other key mitochondrial ion transporters.

Modified from Bers


Figure 2.

Passive Ca buffering in cardiac myocytes. The six curves represent different estimates of how passive Ca buffering changes as [Ca]i increases from 100 to 1500 nM. These values are indicated as increments above the amount of Ca bound at 100 nM [Ca]i (see also Table and discussion in text). Fabiato‐Orig Fast, Berlin‐Fast and Hove‐Madsen‐Equilibrium are taken directly from values in the relevant papers . Fab/Bers‐New Fast updates some binding constants from the original estimates of Fabiato , as described in the text. Berlin‐Fast+MF Ca/Mg adds known myofilament sites at which Ca and Mg compete. Calculated Total‐New refers to values resulting from constants in Table .



Figure 3.

Measurement of fast Ca buffering in a voltage clamped rat ventricular myocyte. Cells were exposed to 10 μM thapsigargin to prevent SR Ca uptake and were in Na‐free conditions (inside and out) to prevent Na/Ca exchange. A. Voltage clamp pulses (200 ms from −40 to 0 mV) activated ICa and produced nearly step‐like increases in [Ca]i. B: The integral of the ICa from the fourth pulse in A (∫ ICa follows the kinetics of the rise in [Ca]i. The sag in the [Ca]i trace may represent slow buffering that is not in phase with Ca entry via ICa. The “ S” indicates where [Ca]i would be predicted to fall to eventually if the slow buffering by myofilament Ca/Mg sites are included with the fast buffering measured (see text). The points marked [A] and [B] in panel A are the predicted final settling points for [Ca]i if the slow buffering is included for the total ∫ICa throughout the train (as for S) and when the equilibrium buffering from Fig and Table is assumed, respectively

Modified from Berlin et al.


Figure 4.

Free and total Ca requirements for myofilament activation in cardiac myocytes. A: Two different sets of parameters are used for the Hill equation (100/(1 + (Km/[Ca])n) describing the steady‐state relationship between force and [Ca]. The solid curve (Km = 600 nM and n = 4) is more consistent with recent estimates in intact ventricular muscle, whereas the lower dotted curve (Km = 1000 nM and n = 2) is closer to traditional data from skinned muscle fibers (see text). B. The same relationships as in A, but the free [Ca] axis has been altered to account for the cytosolic Ca buffering relationship (Calculated‐Total‐New in Figure and ). Thus, this predicts the force as a function of the total amount of Ca added to the cytosol from a resting level of 150 nM free [Ca]. Coincidentally, almost the same curves are obtained if resting [Ca]i is assumed to be 100 nM and the intermediate buffering curve from Figure is used (Berlin‐Fast+MF Ca/Mg).



Figure 5.

Cytosolic Ca buffering and transport during a Ca transient. A. A Ca transient is calculated from the product of a rising and declining exponential which rises from 100 to 744 nM and back with time constants (τ) of 30 ms (rising) and 300 ms (falling). This was used as a driving function to calculate the time‐dependent changes in binding to various cytosolic buffers (B) using the rate constants and concentrations in Table (TnC is troponin C, SL is total sarcolemmal site in Table , CaM is calmodulin, TnC‐Ca/Mg and My‐Ca/Mg are Ca binding to Ca/Mg sites on TnC and myosin). The change in total cytosolic [Ca] (ΔTotal CaCyt in A) is the sum of the curves in B and the free [Ca]i curve. C: Ca fluxes associated with the Ca transient. ICa activation was calculated by a rising (τ = 3 ms) and falling exponential (τ = 40 ms), peaking at 6.8 pA/pF and bringing in 16 μmol/l cytosol. The SR Ca‐pump rate was 210/(1 + (300 nM/[Ca]i)2) in mol/l cytosol. The SR Ca leak was initiated to counterbalance the SR Ca‐pump rate at rest, and changed linearly as a function of free [Ca] in the SR (based on intra‐SR Ca buffering with a Kd = 600 μM and Bmax equivalent to 180 μmol/l cytosol in an SR volume occupying 3.5% of cell volume). The SR Ca release flux was taken as the residual Ca flux required to produce the driving Ca transient.



Figure 6.

Currents via T‐and L‐type Ca channels. A. Barium currents (with 115 mM Ba) induced by depolarizations to various test potentials from holding potentials of 80 or 30 mV. The Em protocol is shown in the top trace, the currents in the middle trace, and the difference between these currents in the bottom panel. Peak IBa from 30 mV is attributed to L‐type Ca channels and the additional transient difference current activated from 80 mV is attributed to T‐type Ca channels. B: ICa in dog Purkinje fiber cell with 2 mM Ca as charge carrier. The more positive Em required in IBa is due to the higher surface potential in 2 mM Ca vs 115 mM Ba. C: ICa from several species (from 80 to 100 mV), where the hump at ∼−40 mV is due to T‐type current and differs among the tissues studied. Dog Purkinje (ref. ) and rabbit ventricle (G. M. Briggs and D. M. Bers, unpublished) are in 2 mM Ca. Dog atrium (ref. ) and guinea‐pig ventricle (from Mitra & Morad, ) are with 5 mM Ca, but are shifted by 10 mV to compensate for surface potential differences.

From Bean , with permission. From Hirano et al. , with permission. From Bers with permission)


Figure 7.

Ca channel inactivation with different charge carriers. Normalized current amplitudes measured at 0 mV (except Ins at −30 mV to match activation state). ICa was recorded under both perforated patch (allowing normal SR Ca release and Ca transients) and ruptured patch with cells dialyzed with 10 mM EGTA (to prevent global Ca transients). IBa was also recorded with ruptured patch (with 10 mM EGTA in the pipette). Extracellular [Ca] and [Ba] were both 2 mM and Ins was measured in divalent‐free conditions (10 mM EDTA inside and out) with [Na]o at 20 mM and [Na]i at 10 mM. Peak currents were 1370, 808, 780, and 5200 pA and were attained at 5, 7, 10, and 14 ms for ICa (perforated), ICa (ruptured), IBa and Ins respectively. Halftimes of current decline were 17, 37, 161, and >500 ms, respectively.

Recordings made by Dr. W. Yuan


Figure 8.

Ca‐dependent inactivation by SR Ca release and recovery of ICa. Long (4 sec.) voltage clamp pulses activated ICa and SR Ca release and Ca transient in Na‐free conditions and 5.4 mM [Ca]o. The pulse labeled a was the first pulse after depletion of the SR and b was after 5–10 pulses when the SR was loaded. The smaller Ca transient produces less ICa inactivation. The larger Ca transient produces marked inactivation, but the current partially recovers as [Ca]i declines. The lower difference traces show the similarity between the ICa inactivation and Cai transient

Modified from Sipido et al. , with permission


Figure 9.

Ca currents measured during square pulse and action potential clamp in rat and rabbit ventricular myocytes. Command Em waveforms were either the traditional 200 ms square depolarization to 0 mV or an action potential (AP), which was recorded from normal rat and rabbit ventricular myocytes under more physiological conditions (i.e. normal Ca Na and K concentrations, ref ). ICa was then recorded under conditions where all other currents were blocked (e.g. Na‐free and Cs‐rich inside and out). With the action potential waveform, the peak ICa is smaller in both species and occurs later (middle panel). The ICa integral (lower panels) for the AP clamp vs the square, pulses are smaller for the rat but larger for the rabbit (see ref. for additional information).



Figure 10.

Simulations of guinea‐pig ventricular action potential using Oxsoft Heart (v 4.5). This comprehensive simulation created by Professor Denis Noble and colleagues (at Oxford, UK) calculates the Em and many of the ionic currents that are known to flow during the action potential. Shown here are the Em (top), ICa, INa/Ca (middle panel), [Ca]i, and force (lower panel) for two different initial [Na]i values (5 and 8 mM).



Figure 11.

Na/Ca exchange current in intact ventricular myocytes recorded under controlled conditions. A. Very slow speed recording of Em (top) and INa/Ca where the spikes are from ramp depolarizations that were used to generate the current voltage relationships in B, C, and D at the times indicated (a–h). The bars in A are when [Na]o was applied (replacing Li) to activate INa/Ca ([Ca]o was 1 mM throughout). At the arrow [Ca]i was increased from nominally Ca free to 430 nM with 42 mM EGTA and 140 CsCl in the dialyzing pipette throughout. Ouabain, Ba, Cs, D600 and tetraethylammonium were used to inhibit other ionic currents. Application of [Na]o stimulated INa/Ca only after [Ca]i was raised. The gradual decline in INa/Ca (d–g) was supposed to be due to depletion of [Ca]i.

From Kimura et al. , with permission


Figure 12.

Current–voltage relationships for INa/Ca recorded in excised giant patches from guinea pig ventricular myocytes. The patches were treated with chymotrypsin to remove INa/Ca inactivation and all other known currents were blocked . A: With constant [Ca]o (2 mM), [Na]o (150 mM) and [Ca]i (1 μM), the [Na]i was varied from 5 to 100 mM. The lower 3–4 curves probably reflect the INa/Ca expected in intact cells when [Ca]i is high (e.g. peak systole). B: The same conditions as A, except [Ca]i is reduced to 100 nM, comparable to diastolic [Ca]i.

From Matsuoka and Hilgemann , with permission


Figure 13.

Na/Ca exchange currents and [Ca]i in a guinea pig ventricular myocyte. The Ca transient (using fura‐2) show that Ca influx via Na/Ca exchange can bring in large quantities of Ca during large sustained depolarizations (A and B). The [Na] in the dialyzing pipette was 7.5 mM. Outward currents during the depolarization were off‐scale. C: The [Ca]i‐dependence of the “ tail” current was observed upon repolarization to −80 mV. Other ionic currents are blocked by Cs, tetraethylammonium, verapamil, and ryanodine.

From Barcenas‐Ruiz et al., , with permission


Figure 14.

Model of the Na/Ca exchanger based on recent work by Nicoll et al. and Iwamoto et al.. The structure is consistent with 9 membrane spanning segments and a glycosylation site (CH2O). The large cytoplasmic loop contains the Na/Ca exchange inhibitory peptide (XIP) domain (also associated closely with the region responsible for Na‐dependent inactivation) and also contains the site for allosteric regulation by [Ca]i*. The two homologous α repeats (α‐1 and α‐2) are also indicated.



Figure 15.

Na/Ca exchange reversal potential (ENa/Ca) during the rabbit ventricular action potential. This schematic diagram shows how ENa/Ca is expected to change during the action potential for two different levels of intracellular Na activity (aNai = 5 and 8 mM), where aNai is roughly 0.78 · [Na]i. When Em is positive to ENa/Ca, Ca influx via the Na/Ca exchange is thermodynamically favored (shaded areas). When Em is negative to ENa/Ca, Ca extrusion is favored. Resting [Ca]i =150 nM, [Ca]o = 2 mM and aNao = 110 mM for both traces and aNai and peak [Ca]i are as indicated. The [Ca]i trace reaches a peak 40 msec after the action potential begins.

After Bers


Figure 16.

Extracellular [Ca] depletion in rat‐rabbit ventricular muscle. Changes in [Ca]o were measured with double‐barreled Ca‐selective microelectrodes during individual contractions in rabbit (A) and rat (B) ventricular muscle (0.5 Hz, 30°C). The traces show [Ca]o (top) and tension (bottom) in the absence and presence of 10 mM citrate (which limits [Ca]o depletion by buffering [Ca]. The bath [Ca]o was 0.5 mM and is indicated by the dotted line.

A is modified from Shattock and Bers and composite from Bers , with permission


Figure 17.

Changes in Na/Ca exchange driving force during action potential in rat and rabbit ventricle. ENa/Ca is expected to change during the action potential and Ca transient in rabbit and rat ventricle (top). The estimated changes in the net electrochemical driving force for Na/Ca exchange (ENa/Ca – Em) are shown in the bottom panel. Na/Ca exchange stoichiometry of 3Na:1Ca was assumed, the aNai values were measured and, for simplicity, the Ca transient accompanying the contraction, has been assumed to be the same for both species. Resting [Ca]i was assumed to be 150 nM, rising to a peak of 1 μM, 40 msec after the upstroke of the action potential. Note the similarity between the lower panels and the [Ca]o traces in Figure. .

Modified from Shattock and Bers and Bers , with permission


Figure 18.

Caffeine‐induced Ca transient and Na/Ca exchange current in a ferret ventricular myocyte. A: After a steady‐state series of voltage clamp pulses, 10 mM caffeine was rapidly and continuously applied to release SR Ca. Experimental conditions blocked most other ionic currents (e.g. Cs inside and out) and indo‐1 was used as the Ca indicator. The inward INa/Ca rises to a peak before the Ca transient. B: Instantaneous INa/Ca from panel A is plotted as a function of the global [Ca]i reported by indo‐1. The hysteresis shows that during the rising phase of the Ca transient, more inward INa/Ca is observed for a given [Ca]i than during relaxation. The relationship between INa/Ca and [Ca]i during relaxation was fit to a linear regression (for [Ca]i between 300 and 450 nM (dashed line; see also Fig. C).

Experiment performed by Dr. Li Li. C. INa/Ca and [Ca]i data are reproduced from panel A and the subsarcolemmal [Ca]i sensed by the Na/Ca exchanger ([Ca]Na/CaX) based on the linear regression in panel B is also shown


Figure 19.

Ca transport by the SR Ca‐ATPase based on literature values (see text). Ca pump rates were calculated based on Eq. using Vmax = 200 μmol/liter cytosol, Km = 600 nM and n= 2. Activation of protein kinase A (PKA) decreases the Km to 250 nM, but does not affect Vmax.



Figure 20.

Measurement of SR Ca uptake rate in digitonin permeabilized myocytes. A. Free [Ca] was measured simultaneously with indo‐1 and Ca electrode. Ca was added at time 0 to a suspension of rabbit ventricular myocytes permeabilized with digitonin and incubated with 25 μM ruthenium red, 12.5 mM creatine phosphate, and 10 mM oxalate. B: The change in free [Ca] (Ca, solid line, from A), protein bound Ca (Ca‐Pn), Ca‐oxalate (Ca‐Ox), Ca‐indo (Ca‐In), and the sum of free and bound Ca (ΣCa) after addition of 14 μM total Ca. After an initial increase in free [Ca], Ca uptake by the SR lowered the free [Ca] back to the level before the Ca addition. The Ca bound to protein, oxalate, and indo‐1 was calculated from the passive Ca binding to permeabilized myocytes and indo‐1 . C. The Ca uptake rate was calculated as the rate of change in the total non‐SR Ca with time (i.e. dΣCa/dr). D: [Ca] dependence of Ca uptake by the SR. The Ca uptake rate determined in C was plotted versus the corresponding free [Ca] measured in A. Data were fit with Eq. to determine the maximal Ca uptake rate, Km, and Hill coefficient. Data are shown for addition of two different amounts of Ca to the myocyte suspension (14 μM total Ca = diamonds and 43 μM total Ca = circles.

From Hove‐Madsen and Bers , with permission


Figure 21.

Unidirectional SR Ca fluxes at rest. At rest when [Ca]SR is changing only very slowly, net SR Ca flux must be close to zero, implying the forward rate (VFor) is equal to the sum of reverse flux and leak from the SR (VRev + VLeak). The forward rate predicted from the Hill curve is 27 μmol/liter cytosol/sec. For a VLeak of 0.3 μmol/l cytosol/sec the reverse flux must be 26.7 μmol/liter cytosol/sec.



Figure 22.

The influence of SR Ca leak on SR Ca content. The effect of different leak values on steady‐state SR Ca content with [Ca]i = 150 nM was calculated using Eq. and with Vmax = 207 μmol/liter cytosol/sec, Kmf = 300 nM, BmaxSR = 3.95 mM and KdSR = 600 μM. For pump inhibition by thapsigargin (TG) Vmax was decreased by 50%. For Ca‐pump stimulation by isoproterenol, Km was reduced by 50%. The SR Ca leak rate (0.3 μmol/liter cytosol/sec) measured by Bassani and Bers is indicated. (See text and ref for further discussion.)



Figure 23.

Ionic fluxes across the mitochondrial membrane. Ca enters via a uniport, down an electrical gradient established by the proton pump at bottom. Ca can be extruded by a Na/Ca antiport and Na is extruded by Na/H exchange thereby completing the cycle. Elevated cytoplasmic [Ca] can lead to elevated mitochondrial [Ca] and increased activity of mitochondrial dehydrogenases and NADH production.



Figure 24.

Mitochondria free [Ca] ([Ca]m) as a function of cytosolic [Ca] ([Ca]c). Increases of [Ca]c in rat ventricular myocytes were induced by reduction of extracellular [Na] (i.e. via Na/Ca exchange). Mean [Ca]c was measured using indo‐1 (loaded as the salt form) and [Ca]m was measured using indo‐1 (loaded as the AM form) with Mn quench of cytosolic indo‐1. Data are taken from Miyata et al., and have been redrawn (without error bars) and including a broken line corresponding to [Ca]m = [Ca]c (slope = 1).



Figure 25.

Relaxation in rabbit ventricular myocyte with selective inhibition of Ca transporters. Normalized cell relaxations are shown and conditions were either (1) with all Ca transporters functional during relaxation of a steady‐state twitch (Tw), (2) preventing net SR Ca uptake during a caffeine‐induced contracture in NT (Caff), (3) additionally inhibiting Na/Ca exchange in 0Na, 0Ca solution (Caff,0Na,0Ca). To further analyze relaxation during Caff, 0Na, 0Ca, this was coupled with either (4) inhibition of mitochondrial Ca uptake with 1 μM FCCP + 1 μM oligomycin so only sarcolemmal Ca‐ATPase was functional (Caff,0Na,0Ca + FCCP) or (5) inhibition of the sarcolemmal Ca‐pump by elevating [Ca]o to 10 mM after pre‐depletion of [Na]i, so that only mitochondrial Ca uptake was functional (Caff,0Na,10Ca). All four Ca removal systems were also blocked by combining the caffeine application with 0Na, 10Ca and FCCP (after pre‐depletion of [Na]i). The traces are based on mean t1/2 values (shown along the traces without standard error values) for relaxation measured in experiments described by Bassani et al. .



Figure 26.

Twitch Ca transients in rabbit and rat ventricular myocytes with SR Ca‐ATPase blocked by thapsigargin. Ca transient were measured using indo‐1 fluorescence during electrically stimulated twitches in rabbit (A) and rat (B) ventricular myocytes before (Control) and after treatment with 2.5 μM thapsigargin (TG). Twitches were evoked 10 sec after switching to control solution (after 5–7 min pre‐perfusion with 0Na,0Ca solution). For the TG twitch the cells were exposed to TG for 2 min during the last part of the pre‐perfusion period. SR Ca load was maintained despite complete block of the SR Ca‐pump. Time constants (τ) of [Ca]i decline are shown

Modified from Bassani et al.,


Figure 27.

Ca transport functions derived from Ca transients in intact rabbit and rat ventricular myocytes. Ca transport rates as functions of [Ca]i for the SR Ca‐ATPase, Na/Ca exchange and combined slow systems (mitochondrial and sarcolemmal Ca‐ATPase) were determined as described in the text and with respect to Eq. and . Independent values obtained for JSR, JNa/CaX and JSlow respectively were: Vmax (in μmol/liter cytosol/sec) = 82, 46 and 3.9 for rabbit and 207, 27 and 4 for rat; Km (in nM) = 264, 316, and 362 in rabbit and 184, 257 and 268 in rat; n = 3.7, 3.7, 3.2 in rabbit and 3.9, 3.4 an 3.5 in rat.

Based on data from Bassani et al.


Figure 28.

Integrated Ca fluxes during twitch relaxation in rabbit and rat ventricular myocytes. Free [Ca]i during relaxation of a normal twitch was used as a driving function so that Ca flux via each system could be integrated over time using the [Ca]i dependence as described in Figure and Eq. and .

Modified from Bassani et al.


Figure 29.

Ca transients ICa during a twitch and INa/Ca used to measure SR Ca content. Ca transients and inward currents (ICa and INa/Ca) in a rat ventricular cardiomyocyte (dialyzed with 50 μM indo‐1) were recorded during the last two (of 8) conditioning voltage clamp pulses to 0 mV and during two rapid applications of 10 mM caffeine (bars, holding potential −70 mV). Amplitude and time scales for ICa (left) and INa/Ca (right) are different.

From Delbridge et al. , with permission


Figure 30.

Relative reliance on Ca influx in different cardiac muscle preparations. The effect of caffeine (10 mM) or ryanodine (100 nM) on steady‐state twitch contraction amplitude (0.5 Hz at 30°C or 23°C for frog) in various cardiac muscle preparations.

Data are from refs ; modified from Bers


Figure 31.

Fractional SR Ca release during the twitch depends on both ICa trigger and SR Ca load in ferret ventricular myocytes. A. Cells were loaded to a constant level (by stimulation at 0.5 Hz). Extracellular [Ca] was changed just prior to the test contraction where the fraction of SR Ca release was measured. Thus increasing trigger (at constant SR Ca load) increases fractional release. Parallel voltage clamp experiments were done to evaluate how ICa changes over this range of [Ca]o. B. Using a constant trigger (with 2 mM [Ca]o) the SR of cells were Ca loaded to different levels by varying frequency (as indicated) and also increasing [Ca]o at the highest load. SR Ca content was determined by translating the peak and resting [Ca]i during caffeine‐induced contractures to total cytosolic [Ca], using values determined by Hove‐Madsen and Bers for cytosolic Ca buffering.

Modified from Bassani et al. , with permission


Figure 32.

Rest decay and rest potentiation of twitches and SR Ca content in rabbit, rat and ferret myocytes. Both post‐rest twitches and caffeine‐induced contractures (Caff) were measured either in a modified normal Tyrode's solution (NT), when Na/Ca exchange was blocked during the rest (0Na,0Ca) or after pre‐depletion of [Na]i and rest in Ca‐free, 140 mM Na solution (0Cao) to stimulate Ca efflux via Na/Ca exchange. Caff data are indicative of SR Ca load. SR Ca content data (Caff) was fit with [Ca]SRC = A · exp(–t/τRD)(1 –exp{–t/τRF}) +B, where t is time, τRD is time constant, of rest decay of SR Ca content and τRF is a rest‐dependent filling time constant used to explain the delay in rest decay of SR Ca content. A and B are constants for scaling and baseline respectively. Twitch data were fit with the expression C[Ca]SRC(1–exp{–t/τFCC}) + D, where τECC: is the time constant for recovery of E‐C coupling and C and D are scaling and baseline constants. A small second exponential factor was also included to explain the slow decline in twitch amplitude in rabbit ventricle after long rests in 0Na, 0Ca.

Data are from Bers et al. and Bassani and Bers and have been combined and reanalyzed


Figure 33.

Refilling of SR Ca stores after depletion in rabbit ventricle. SR Ca content was measured by either caffeine‐induced contracture amplitude (in isolated myocytes at 23°C) or rapid cooling contractures (RCC) in trabeculae from rabbit ventricle (at 30°C). SR Ca was depleted by either a long rest (5 min) in normal solution (muscle) or a 10 sec application of 10 mM caffeine (myocyte). After SR Ca depletion, stimulation was started at 0.5 Hz and the SR Ca content was assessed after the beat number indicated.

Data are from Bers and Bassani et al.


Figure 34.

Changes in contraction force with abrupt changes in frequency. Frequency of stimulation of a rabbit ventricular trabecula (at 30°C) was increased from 0.5 to 1.5 Hz and then returned to 0.5 Hz.



Figure 35.

Steady‐state force‐frequency relationships. The effect of frequency (from 0.5 to 2 Hz) on twitch force in rabbit (circles), rat (squares) and guinea pig (triangles) ventricular muscle. Data for rabbit and rat are at 30°C.

From Bers (33 and unpublished.] Data for guinea pig are at 36.5°C and were taken from Kurihara and Sakai . RCCs were initiated within 5 sec of the last steady‐state stimulated contraction. [Redrawn from Bers
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Donald M. Bers. Regulation of Cellular Calcium in Cardiac Myocytes. Compr Physiol 2011, Supplement 6: Handbook of Physiology, The Cardiovascular System, The Heart: 335-387. First published in print 2002. doi: 10.1002/cphy.cp020109