Interrelations between contracting striated muscle and precapillary microvessels.

Arterioles and capillaries in the hamster cremaster muscle were observed during electrical stimulation of striated muscle fibers in order to characterize the microcirculatory basis of functional hyperemia. When contraction was restricted to single muscle fibers, responses were variable and frequently transient. Stimulation of either small bundles of muscle fibers or the entire cremaster muscle resulted in reproducible responses typified by: 1) a latency period, 2) an early, often transient phase of dilation, and 3) a second, slower phase of dilation. The latency varied inversely with contraction frequency, and the magnitude of the dilation varied directly with contraction frequency over the range 1--8/s. With stimulation of single fibers and small groups of fibers, arteriolar vasodilation was highly localized to regions of the arterioles that were in close apposition to the stimulated fibers. The number of capillaries with red blood cell flow increased during contraction, and the increase was graded with contraction frequency. The changes observed suggest that the vascular response during functional hyperemia is a two-part process and that the control processes are influenced by contraction frequency.