Coronary venular responses to flow and pressure.

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Abstract
In previous studies, we demonstrated that both endothelium-dependent flow-induced vasodilation and endothelium-independent myogenic responses occur in porcine coronary arterioles. However, it was not established whether these responses are present in the coronary venular microcirculation. The aim of this study was to test the hypotheses that 1) coronary venules, like arterioles, exhibit flow-induced dilation and myogenic responsiveness, and 2) venular flow-induced dilation is endothelium-dependent and is mediated by the release of a nitrovasodilator. Experiments were performed in porcine subepicardial coronary venules, 80-120 microns in diameter, by using cannulated isolated vessel techniques to allow intraluminal pressure and flow to be independently controlled. Flow was initiated by simultaneously moving two perfusion reservoirs connected to the cannulating pipettes in equal amounts but in opposite directions. In the absence of flow, i.e., zero pressure gradient (delta P) between the two reservoirs, venules developed spontaneous tone to 75-80% of maximum diameter at 10 cm H2O intraluminal pressure. Venules gradually dilated in response to stepwise increases in flow (i.e., delta P). The threshold for the flow-induced dilation occurred at delta P = 1 cm H2O (flow = 3.5 nl/sec), and the maximal response (dilation to 93 +/- 2% of maximum diameter) occurred when delta P was elevated to > or = 6 cm H2O (flow = 21 nl/sec at delta P = 6 cm H2O). Flow-induced dilation was abolished after the endothelium was damaged by perfusion of an air bolus through the lumen. Vasoconstriction was observed when denuded venules were subjected to relatively high luminal flows (> or = 21 nl/sec).(ABSTRACT TRUNCATED AT 250 WORDS)