University of Rochester, Rochester, New York
In skeletal muscle, blood flow is very tightly coupled to metabolism. That is, as the metabolic rate of the muscle tissue changes (for example, during exercise) blood flow through the tissue changes proportionately as a result of changes in tone (i.e., the relative dilation) of the arterioles. In resting muscle, arterioles are partially constricted; generally this constriction is measured at 40 to 60 percent in arterioles of resting muscle. Thus there is considerable capacity in most skeletal muscle beds to change the resistance to flow through the tissue, or through some regions with respect to others. Studies such as those of Folkow and Halicka in the late 1960s established that the metabolically related increase in blood flow was greater in oxidative versus glycolytic muscles; this response differential can occur between different regions in a single muscle where the metabolism is different. These kinds of studies, especially when undertaken in isolated muscles, established that there must be direct coupling between arteriolar responses and metabolic rate, and a great deal of effort, over many decades, has identified roles for a variety of metabolically related products in vascular control. These include, for example, changed PO2, purines (especially adenosine), K+, H+, lactate, and many others. Duling's group, in the 1970s, showed, using direct electrically induced contraction of individual skeletal muscle fibers, that arterioles can indeed be directly and locally coupled to a metabolic stimulus: Interestingly, it is only recently that new advances have been made in identifying mechanisms underlying this response (see Section IV). The goal of this chapter is to summarize what is known about the mechanisms underlying principally the local control of skeletal muscle arteriolar tone. Where appropriate, contributions to these local responses by integrating signals (such as those from neural inputs, or from flow) will be discussed, and important gaps in our knowledge will be identified.
Not all arterioles respond similarly to the same stimuli, and an important aspect of understanding the control of these vessels is to identify how different inputs differentially affect different parts of the microcirculation, and how this is integrated into a coordinated vascular response subserving changes in muscle blood flow. Topics that will be covered in this chapter therefore include the organization of skeletal muscle microvasculature with particular reference to the responses of different parts of the arteriolar tree; mechanisms of communication among the cells of the arteriolar wall; local metabolic response mechanisms; and control of arteriolar function via capillaries and venules.
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