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Regulation of Extramuscular Fuel Sources During Exercise

David H. Wasserman, Alan D. Cherrington

10.1002/cphy.cp120123

Source: Supplement 29: Handbook of Physiology, Exercise: Regulation and Integration of Multiple Systems

Originally published: 1996

Published online: January 2011

Full Article on Wiley Online Library



Abstract

The sections in this article are:

1 Fuel Requirements of the Working Muscle
1.1 Effect of Exercise Duration
1.2 Effect of Exercise Intensity
2 Exercise Responses of Hormones and Nerves Involved in Acute Metabolic Regulation
2.1 Insulin Response
2.2 Glucagon Response
2.3 Catecholamine Responses
3 Extramuscular Fuel Sources
4 NEFA Mobilization from Adipose Tissue
4.1 Basic Mechanisms that Influence NEFA Availability
4.2 Regulatory Factors
5 Hepatic Glucose Production
5.1 Exercise Response
5.2 Regulation by the Endocrine Pancreas
5.3 Role of Epinephrine
5.4 Role of Sympathetic Nerve Activity and Norepinephrine
5.5 Role of Cortisol
5.6 Regulation during High‐Intensity Exercise
6 Hepatic Fat Metabolism: Oxidation and Triglyceride Synthesis
6.1 Regulation of Ketogenesis
6.2 Triglyceride Synthesis
7 Splanchnic Bed Amino Acid Metabolism
7.1 Protein Breakdown
7.2 Amino Acids as a Carbon Source for Gluconeogenesis and Oxidation
7.3 Uptake of Nitrogenous Compounds by the Splanchnic Bed
7.4 Fate of Nitrogenous Compounds in the Liver
8 Gastrointestinal Tract as a Source of Glucose: Effect of Carbohydrate Ingestion
8.1 Importance
8.2 Determinants of the Metabolic Availability of Ingested Carbohydrates
8.3 Effect of Carbohydrate Ingestion on Endogenous Substrates
9 Training‐Induced Adaptations in Extramuscular Fuel Mobilization
9.1 Endocrine Adaptations to Physical Training
9.2 Adaptations of Extramuscular Fuel Sources to Physical Training
10 Summary
Figure 1. Figure 1.

The effect of the absolute exercise intensity (expressed as O2 uptake) on limb glucose uptake (A) and hepatic glycogenolysis and gluconeogenesis (B) in untrained subjects. The increase in limb glucose uptake and hepatic glucose production per increase in work intensity is greater at higher work rates. The increase in hepatic glucose production is due strictly to an increase in hepatic glycogenolysis (see HEPATIC GLUCOSE PRODUCTION). All measurements were made at 40 min of exercise.

Modified from the data of Wahren et al. 292
Figure 2. Figure 2.

Mechanisms that have been proposed to control the secretion of hormones involved in acute metabolic regulation and the activity of sympathetic nerves during exercise. Glucagon and insulin are secreted from the pancreas into the portal vein after which a percentage is extracted by the liver before reaching the systemic circulation. Sympathetic nerve activity is increased to specific target organs where norepinephrine is released into the synaptic cleft. Norepinephrine levels in the blood represent mainly that which escapes reuptake and spills over from the synaptic cleft.
Figure 3. Figure 3.

Pathways involved in the regulation of NEFA mobilization and availability to working muscle. Substrate cycling occurs due to concurrent lipolysis in adipocytes and re‐esterification in liver (see HEPATIC FAT METABOLISM: OXIDATION AND TRIGLYCERIDE SYNTHESIS) and adipocytes. These cycles increase the sensitivity of NEFA fluxes to regulatory factors. TG refers to triglycerides.

Adapted from Wolfe and George 329
Figure 4. Figure 4.

Gluconeogenic regulation during exercise. Gluconeogenesis is increased during exercise as accelerated rates of protein degradation, lipolysis, and glycolysis lead to increased rates of amino acid, glycerol, lactate, and pyruvate production and subsequent delivery to the liver. The hepatic extraction of gluconeogenic precursors is enhanced by exercise, as is the efficiency of intrahepatic conversion of precursors into glucose. The importance of each of these regulatory sites is determined by the intensity and duration of exercise and the absorptive state of the subject.

Modified from Cherrington 47
Figure 5. Figure 5.

Schematic representation of the minimal glycogenolytic and maximal gluconeogenic contributions to total glucose production during rest, exercise, and recovery. These responses are based on studies in the overnight‐fasted dog 300,312.

From Wasserman and Cherrington 298
Figure 6. Figure 6.

Role of the exercise‐induced increase in glucagon in gluconeogenic regulation. Effect of exercise alone (shaded area), exercise with somatostatin + simulated glucagon and insulin (solid line) and somatostatin + basal glucagon and simulated insulin (dashed line) on (A) gluconeogenic conversion from alanine; (B) intrahepatic gluconeogenic efficiency from alanine; and (C) hepatic fractional alanine extraction. The exercise‐induced increment in glucagon increases gluconeogenesis by stimulating the gluconeogenic precursor extraction by the liver and channeling into glucose within the liver. Data are mean ± SE.

Modified from Wasserman et al. 310
Figure 7. Figure 7.

Schematic representation of the rise in glucose production during moderate‐intensity exercise and the impact of the fall in insulin and rise in glucagon and the role of the increase in epinephrine on this response.

Modified from Wasserman and Cherrington 298
Figure 8. Figure 8.

Proposed pathways for amino acid metabolism in the splanchnic bed. Amino acids, primarily alanine and glutamine, are released by working muscle. Glutamine (GLN) is deaminated in the gastrointestinal tract forming glutamate (GLT), which is released or oxidized. Amino acids are released from the gastrointestinal tract as a result of proteolysis. The liver takes up amino acids where they are converted into glucose, oxidized, or incorporated into protein. Only the branched chain amino acids (leucine, isoleucine, valine) are consistently released from the splanchnic bed in a net sense. Nitrogen released during metabolism of amino acids may be converted to urea.
Figure 9. Figure 9.

Rate of appearance of plasma glucose at rest and during exercise before (closed circles) and after 10 days (open circles) and 12 weeks (open squares) of endurance training. Subjects were exercised at 60% of their pretraining maximum oxygen uptakes. Significantly different than before training (P<0.05). ‡Significantly different than before training (P<0.001).

Modified from Mendenhall et al. 204
Figure 10. Figure 10.

Summary of hormones and nerves involved in the regulation of glucose from the liver and NEFA from adipose tissue during moderate‐intensity exercise.


Figure 1.

The effect of the absolute exercise intensity (expressed as O2 uptake) on limb glucose uptake (A) and hepatic glycogenolysis and gluconeogenesis (B) in untrained subjects. The increase in limb glucose uptake and hepatic glucose production per increase in work intensity is greater at higher work rates. The increase in hepatic glucose production is due strictly to an increase in hepatic glycogenolysis (see HEPATIC GLUCOSE PRODUCTION). All measurements were made at 40 min of exercise.

Modified from the data of Wahren et al. 292


Figure 2.

Mechanisms that have been proposed to control the secretion of hormones involved in acute metabolic regulation and the activity of sympathetic nerves during exercise. Glucagon and insulin are secreted from the pancreas into the portal vein after which a percentage is extracted by the liver before reaching the systemic circulation. Sympathetic nerve activity is increased to specific target organs where norepinephrine is released into the synaptic cleft. Norepinephrine levels in the blood represent mainly that which escapes reuptake and spills over from the synaptic cleft.



Figure 3.

Pathways involved in the regulation of NEFA mobilization and availability to working muscle. Substrate cycling occurs due to concurrent lipolysis in adipocytes and re‐esterification in liver (see HEPATIC FAT METABOLISM: OXIDATION AND TRIGLYCERIDE SYNTHESIS) and adipocytes. These cycles increase the sensitivity of NEFA fluxes to regulatory factors. TG refers to triglycerides.

Adapted from Wolfe and George 329


Figure 4.

Gluconeogenic regulation during exercise. Gluconeogenesis is increased during exercise as accelerated rates of protein degradation, lipolysis, and glycolysis lead to increased rates of amino acid, glycerol, lactate, and pyruvate production and subsequent delivery to the liver. The hepatic extraction of gluconeogenic precursors is enhanced by exercise, as is the efficiency of intrahepatic conversion of precursors into glucose. The importance of each of these regulatory sites is determined by the intensity and duration of exercise and the absorptive state of the subject.

Modified from Cherrington 47


Figure 5.

Schematic representation of the minimal glycogenolytic and maximal gluconeogenic contributions to total glucose production during rest, exercise, and recovery. These responses are based on studies in the overnight‐fasted dog 300,312.

From Wasserman and Cherrington 298


Figure 6.

Role of the exercise‐induced increase in glucagon in gluconeogenic regulation. Effect of exercise alone (shaded area), exercise with somatostatin + simulated glucagon and insulin (solid line) and somatostatin + basal glucagon and simulated insulin (dashed line) on (A) gluconeogenic conversion from alanine; (B) intrahepatic gluconeogenic efficiency from alanine; and (C) hepatic fractional alanine extraction. The exercise‐induced increment in glucagon increases gluconeogenesis by stimulating the gluconeogenic precursor extraction by the liver and channeling into glucose within the liver. Data are mean ± SE.

Modified from Wasserman et al. 310


Figure 7.

Schematic representation of the rise in glucose production during moderate‐intensity exercise and the impact of the fall in insulin and rise in glucagon and the role of the increase in epinephrine on this response.

Modified from Wasserman and Cherrington 298


Figure 8.

Proposed pathways for amino acid metabolism in the splanchnic bed. Amino acids, primarily alanine and glutamine, are released by working muscle. Glutamine (GLN) is deaminated in the gastrointestinal tract forming glutamate (GLT), which is released or oxidized. Amino acids are released from the gastrointestinal tract as a result of proteolysis. The liver takes up amino acids where they are converted into glucose, oxidized, or incorporated into protein. Only the branched chain amino acids (leucine, isoleucine, valine) are consistently released from the splanchnic bed in a net sense. Nitrogen released during metabolism of amino acids may be converted to urea.



Figure 9.

Rate of appearance of plasma glucose at rest and during exercise before (closed circles) and after 10 days (open circles) and 12 weeks (open squares) of endurance training. Subjects were exercised at 60% of their pretraining maximum oxygen uptakes. Significantly different than before training (P<0.05). ‡Significantly different than before training (P<0.001).

Modified from Mendenhall et al. 204


Figure 10.

Summary of hormones and nerves involved in the regulation of glucose from the liver and NEFA from adipose tissue during moderate‐intensity exercise.

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Article Title: Regulation of Extramuscular Fuel Sources During Exercise
 

How to Cite

David H. Wasserman, Alan D. Cherrington. Regulation of Extramuscular Fuel Sources During Exercise. Compr Physiol 2011, Supplement 29: Handbook of Physiology, Exercise: Regulation and Integration of Multiple Systems: 1036-1074. First published in print 1996. doi: 10.1002/cphy.cp120123