Magnitude of beta-adrenoceptor-mediated responses of dog epicardial coronary arteries: inverse relation to diameter.

Regional blood flow patterns vary within myocardium to suggest that intrinsic regional differences occur in vasoregulation. Accordingly, we used standard in vitro methodology (isometric force transducer) to study adrenergic responses in epicardial left anterior descending (LAD) artery and right coronary arteries (RCA) obtained from dogs (n = 9). In the presence of propranolol (10(-6) M) and blockers of uptake 1 and 2, norepinephrine (NE) elicited minimal, if any, constriction. After preincubation with phentolamine (10(-6) M) and preconstriction with prostaglandin (PG)F2 alpha or a thromboxane (Tx)A2 analogue, maximum NE relaxation (as % of induced tone) for the RCA was 61 +/- 3 (SE) %, which was significantly greater than the LAD (46 +/- 5%, P < 0.01). ED50 values were not different. Endothelial removal and forskolin relaxations did not change the sensitivity or maximal response between arteries. Expressing beta-adrenoceptor-mediated relaxation as a function of vessel diameter revealed a common regression for RCA and LAD (r = -0.56, P < 0.001). ED50 and diameter were minimally related. Thus the RCA has a greater beta-adrenoceptor response than the LAD in dogs. The difference appears to be reconciled by a common inverse relationship between vessel size and beta-adrenoceptor response. The difference was independent of alpha-adrenoceptor, endothelium, and second messenger processing, suggesting a mechanism based on beta-adrenoceptor density.