Effect of vasoconstriction on coronary artery resistance changes caused bystretching surrounding myocardial tissue

We previously showed that deformation of the cardiac tissue surrounding a d ilated coronary artery changes its hydraulic resistance depending on the di rection of stretch. Stretch parallel, but not perpendicular, to the vessel axis increased the hydraulic resistance. This asymmetric dependence of resi stance on the direction of stretch was found at a low perfusion pressure on ly, presumably because this was the state in which surrounding fibers were sufficiently stretched to be able to exert their effects. When the vessel i s vasoconstricted and its diameter decreases, this might alter the coupling between tissue and vessel. On the other hand, the stiffer vessel wall woul d 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 vasodila ted and vasoconstricted states. We compared how the hydraulic resistance of an in situ segment of a vasodilated and then vasoconstricted epicardial co ronary artery was affected by stretching the surrounding tissue by 10% in a direction parallel and then perpendicular to the vessel axis. Vasoconstric tion increased the unstretched resistance of the vessel, demonstrating that the vessel diameter was decreased. In both vasomotor states the relative r esistance changes to parallel and perpendicular tissue stretches were found to be similar. Thus, the effects of subtle differences in vessel cross-sec tional shape underlying the resistance changes to tissue stretch in the vas odilated state - that should have been altered by vasoconstriction were see mingly counterbalanced by increased vessel wall stiffness that decreased th e manifestation of coupling between the vessel and the surrounding tissue.