Effect of vasoconstriction on coronary artery resistance changes caused by stretching surrounding myocardial tissue.

We previously showed that deformation of the cardiac tissue surrounding a dilated coronary artery changes its hydraulic resistance depending on the direction of stretch. Stretch parallel, but not perpendicular, to the vessel axis increased the hydraulic resistance. This asymmetric dependence of resistance on the direction of stretch was found at a low perfusion pressure only, presumably because this was the state in which surrounding fibers were sufficiently stretched to be able to exert their effects. When the vessel is vasoconstricted and its diameter decreases, this might alter the coupling between tissue and vessel. On the other hand, the stiffer vessel wall would be more difficult to deform, making the coupling less evident. The aim of this study was to test the hypothesis that, at this low perfusion pressure, the asymmetric resistance response to strain differs between the vasodilated and vasoconstricted states. We compared how the hydraulic resistance of an in situ segment of a vasodilated and then vasoconstricted epicardial coronary artery was affected by stretching the surrounding tissue by 10% in a direction parallel and then perpendicular to the vessel axis. Vasoconstriction increased the unstretched resistance of the vessel, demonstrating that the vessel diameter was decreased. In both vasomotor states the relative resistance changes to parallel and perpendicular tissue stretches were found to be similar. Thus, the effects of subtle differences in vessel cross-sectional shape underlying the resistance changes to tissue stretch in the vasodilated state - that should have been altered by vasoconstriction - were seemingly counterbalanced by increased vessel wall stiffness that decreased the manifestation of coupling between the vessel and the surrounding tissue.