Integration Conclusion

It is clear from the preceding discussion that a host of different microcirculatory control mechanisms contribute to exercise hyperemia in skeletal muscle. Although much is known about these mechanisms, the complete blood flow response to exercise is brought about by a coordinated and complex interaction of these factors, about which there is still much to learn. Figure 1 shows potential mechanisms involved at the onset of exercise and during sustained exercise. The hyperemic response to the onset of exercise is likely due to some combination of mechanical factors associated with muscle contraction, such as the muscle pump and/or vessel distortion. Whether those factors cause immediate vasodilation and an increase in vascular conductance or produce increased blood flow without vasodilation is not known. The hyperemic response to sustained exercise likely begins with metabolically induced vasodilation of small arterioles located within the contracting muscle, and vasodilators released by red blood cells in response to a hypoxic environment contribute as well. The dilation spreads upstream via one or more of the following mechanisms: flow-induced dilation, retrograde propagation, and arterial-venous coupling. The overall responses are complicated by the spatial and temporal heterogeneity described earlier.

Figure 2 illustrates another type of spatial heterogeneity—a variation in the relative importance of each vascular control mechanism throughout the vascular tree. That is, although each vascular control mechanism described appears to be present throughout the vascular tree, the relative effectiveness of each varies depending on location in the tree. For example, metabolic dilation and the myogenic mechanism both have a greater impact in smaller than in larger arterioles. Also, the efficiency of sympatholysis is enhanced by the fact that metabolic dilation is greatest in the same sized arterioles as sympathetic constriction. Finally, flow-induced dilation, a mechanism that may be important in the upstream spread of vasodilation, is more important in the larger than in the smaller arterioles. Thus the variations

Conduit Arteries

Microcirculation

Myogenic Mechanism

Flow Dilation

Figure 2 This figure illustrates the relative responsiveness of each section of the arterial tree (at the top) for the myogenic mechanism, flow-induced dilation (representing endothelium-dependent control), metabolic dilation, and sympathetic constriction. Redrawn based on a figure of the coronary circulation presented by Kuo et al. [12].

Metabolic Dilation

Sympathetic Constriction

Figure 2 This figure illustrates the relative responsiveness of each section of the arterial tree (at the top) for the myogenic mechanism, flow-induced dilation (representing endothelium-dependent control), metabolic dilation, and sympathetic constriction. Redrawn based on a figure of the coronary circulation presented by Kuo et al. [12].

and Integration of Multiple Systems (L. B. Rowell and J. T. Shepherd, eds.), pp. 705-769. New York: Oxford University Press. Saltin, B., Radegran, G., Koskolou, M. D., and Roach, R. C. (1998). Skeletal muscle blood flow in humans and its regulation during exercise. Acta Physiol. Scand. 162, 421-436.

Delp, M. D., and Laughlin, M. H. (1998). Regulation of skeletal muscle perfusion during exercise. Acta Physiol. Scand. 162, 411-419. Buckwalter, J. B., and Clifford, P. S. (2001). The paradox of sympathetic vasoconstriction in exercising skeletal muscle. Exer. Sport Sci. Rev. 29(4), 159-163.

Segal, S. S. (2000). Dynamics of microvascular control in skeletal muscle. In Exercise and Circulation in Health and Disease (B. Saltin, R. Boushel, N. Secher, and J. Mitchell, eds.), pp. 141-153. Champaign, IL: Human Kinetics.

Buckwalter, J. B., Ruble, S. B., Mueller, P. J., and Clifford, P. S. (1998). Skeletal muscle vasodilation at the onset of exercise. J. Appl. Physiol. 85, 1649-1654.

Duling, B. R., and Dora, K. (1997). Control of striated muscle blood flow. In The Lung: Scientific Foundations (R. G. Crystal, J. B. West, et al., eds.), 2nd ed., pp. 1935-1943. Philadelphia: Lippencott-Raven. Lash, J. M. (1996). Regulation of skeletal muscle blood flow during contractions. Proc. Soc. Exper. Biol. Med. 211, 218-235. Mohrman, D., and Regal, R. (1988). Relation of blood flow to Vo2, Po2, and CO2 in dog gastrocnemius muscle. Am. J. Physiol. 255, H1004-H1010.

Pawloski, J. R., and Stamler, J. S. (2002). Nitric oxide in RBCs. Transfusion 42, 1603-1609.

Hester, R. L., and Choi, J. (2002). Blood flow during exercise: Role for the venular endothelium? Exer. Sport Sci. Rev. 30(4), 147-151. Kuo, L., Davis, M. J., and Chilian, W. M. (1992). Endothelial modulation of arteriolar tone. News Physiol. Sci. 7, 5-9.

in the importance of each vascular control mechanism throughout the vascular tree contribute to the efficiency of the hyperemic response to exercise.

Glossary

Adenosine: A purine nucleotide consisting of adenine and ribose. It is a degradation product of ATP and thus is produced in increased quantities during increases in cellular metabolism.

Metabolic vasodilation: Refers to the relaxation of vascular smooth muscle caused by many products of metabolism. When present in increased quantities, these products cause vasodilation and help increase blood flow to active regions of the body.

Muscle pump: The cyclical compression of blood vessels that occurs within contracting skeletal muscle. This compression helps increase venous return of blood to the heart and is thought to contribute to increase blood flow through contracting muscle.

Myogenic mechanism: Refers to the property of smooth muscle that causes it to contract when it is stretched. Contributes to autoregulation of blood flow in a vascular bed because increased pressure elicits vascular constriction and increased resistance to blood flow.

Sympatholysis: Refers to the decreased effects of sympathetic nerve activity on blood vessel constriction in contracting skeletal muscle. It is thought to result from metabolic by-products interfering with some aspect of sympathetic neurotransmission or postsynaptic signaling.

References

1. Laughlin, M. H., Korthuis, R. J., Duncker, D. J., and Bache, R. J. (1996). Control of blood flow to cardiac and skeletal muscle during exercise. In Handbook of Physiology Section 12: Exercise: Regulation

Further Reading

Duling, B. R., and Berne, R. M. (1970). Propagated vasodilation in the microcirculation of the hamster cheek pouch. Circ. Res. 26, 163-170.

Classic paper is the first demonstration of retrograde propagation of vasodilatory responses in the microcirculation.

Furchgott, R. F., and Zawadski, J. V. (1980). The obligatory role of endothelial cells in the relaxation of arterial smooth muscle by acetyl-choline. Nature 288(5789), 373-376. Classic demonstration of the role of the endothelium in blood flow control by secretion of an endothelium-derived relaxing factor. Furchgott was one of three winners of the 1998 Nobel Prize in Medicine for this work.

Kingwell, B. A. (2000). Nitric oxide-mediated metabolic regulation during exercise: Effects of training in health and cardiovascular disease. FASEB J. 14(12), 1685-1896. Good review of the role of nitric oxide in skeletal muscle exercise hyperemia.

Lash, J. M. (1996). Regulation of skeletal muscle blood flow during contractions. Proc. Soc. Exp. Biol. Med. 211, 218-235. Very good and complete review of the mechanisms controlling skeletal muscle blood flow during exercise.

Laughlin, M. H. (1987). Skeletal muscle blood flow capacity: Role of muscle pump in exercise hyperemia. Am. J. Physiol. 253, H993-H1004.

Complete (and most widely cited) discussion of various factors involved in the muscle pump and its effect on skeletal muscle blood flow during exercise.

Laughlin, M. H., and Armstrong, R. B. (1982). Muscular blood flow distribution patterns as a function of running speed in rats. Am. J. Physiol. 243, H296-H306. This is the first paper showing the muscle fiber type dependency of blood flow during exercise.

Laughlin, M. H., Korthuis, R. J., Duncker, D. J., and Bache, R. J. (1996). Control of blood flow to cardiac and skeletal muscle during exercise. In Handbook of Physiology Section 12: Exercise: Regulation and Integration of Multiple Systems (L. B. Rowell and J. T. Shepherd, eds.), pp. 705-769. New York: Oxford University Press. This is the most complete and most widely cited recent review of the various mechanisms involved in the regulation of skeletal muscle and coronary blood flow during exercise. This article also examines the effects of exercise training on these mechanisms.

Prior, B. M., Lloyd, P. G., Yang, H. T., and Terjung, R. L. (2003). Exercise-induced vascular remodeling. Exer. Sport Sci Rev. 31(1), 26-33. Good review of structural changes in the vasculature induced by exercise training.

Segal, S. S., and Duling, B. R. (1986). Flow control among microvessels coordinated by intercellular conduction. Science 234(4778), 868-870.

Important paper demonstrating the role of propagated vasodilation in the control of blood flow in the microcirculation.

Welsh, D. G., and Segal, S. S. (1996). Muscle length directs sympathetic nerve activity and vasomotor tone in resistance vessels of hamster retractor. Circ. Res. 79(3), 551-559. Seminal paper showing mechanical effects of muscle shortening on vascular resistance.

Capsule Biography

Dr. Jasperse is a Seaver Fellow in the Department of Sports Medicine at Pepperdine University. His research focuses on control of blood flow to skeletal muscle and how it is altered by changes in chronic activity.

Dr. Laughlin is chair of the Department of Biomedical Sciences at the University of Missouri College of Veterinary Medicine. A Fellow in the American College of Sports Medicine (ACSM) and the Circulation Council of the American Heart Association, his research examines the effects of exercise training on the coronary circulation and skeletal muscle vascular beds. His work is funded by the National Institutes of Health and the American Heart Association, and he was winner of the 2003 ACSM Joseph Wolffe Award and the 1998 ACSM Citation Award for his career contributions to exercise science.

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