Influence of arterial diameter on vasomotor responses in the porcine coronary vasculature

The aim was to determine whether regional differences in arterial responses to vasoconstrictor and vasorelaxant agonists exist within the minipig coronary vasculature.

Hearts were obtained from miniature pigs (20-40 kg) immediately after death. First and third order arterial branches of the left anterior descending artery were dissected from within the subepicardium and mounted as ring preparations in a small vessel myograph for measurement of isometric tension under standardised conditions. Contractile responses to acetylcholine, noradrenaline, and U46619, and the relaxation responses to noradrenaline, bradykinin, and substance P were measured. Arterial tone was increased with KCl or acetylcholine prior to addition of vasodilator agonists.

First order branches were more sensitive to the constrictor influence of acetylcholine than third order branches [pD2 values 6.42(SEM 0.07), n = 13, and 6.26(0.07), n = 13, for first and third order respectively, p < 0.05]. U46619 did not induce contractile responses in arteries less than 210 microns in diameter. Noradrenaline only induced small contractile responses in the presence of propranolol following removal of the endothelium. In arteries preconstricted with 40 mM KCl, noradrenaline induced relaxation which was inhibited by propranolol and was uninfluenced by arterial calibre. In the presence of propranolol, noradrenaline-mediated relaxations of acetylcholine-preconstricted arteries were endothelium dependent and alpha 2 adrenoceptor mediated, and greater in first order than in third order branches [58(5)%, n = 9, and 26(8)%, n = 9, for first and third order branches respectively, p < 0.05]. Relaxations mediated by bradykinin and substance P were not influenced significantly by arterial calibre but were greater in arteries preconstricted with acetylcholine than with KCl.

In isolated minipig coronary arteries the vasoconstrictor responses to acetylcholine and U46619, and the endothelium dependent, noradrenaline mediated relaxations, differ according to the branching order studied. These data provide further evidence for a regional heterogeneity of vascular responses in the porcine coronary vasculature.