Abstract
Summary
Precapillary sphincters are currently thought by most investigators to completely close or open in response to intrinsic and extrinsic stimuli. In vivo experiments, using the rat mesentery and a high magnification (up to X6500) image-splitting television microscope recording system, were designed to test this hypothesis. The results clearly, and quantitatively, demonstrate that precapillary sphincters, at least in rat mesentery, do not respond (i.e., constrict or dilate) to vasoactive substances in an all-or-none manner but in a dose-dependent, graded fashion.
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