Pressure-dependent vasoactive effects of histamine in the coronary circulation

Twenty-one isolated, perfused, spontaneously rhythmic guinea pig hearts (Langendorff preparation) were used to investigate the effects of coronary perfusion pressure (CPP) on the coronary vasoactive response to a continuous infusion of histamine. Heart rate (HR), coronary perfusate flow (CPF), left ventricular pressure, dp/dtmax, oxygen extraction, and myocardial oxygen consumption (MVO2) were measured at constant CPP of 40 (n = 9), 53 (n = 6), and 65 cm H2O (n = 6) in the absence and presence of continuous intracoronary infusion of histamine [0.9 +/- 0.2 microgram/(min X g)]. At 40 cm H2O histamine caused significant coronary vasodilation. At 65 cm H2O histamine caused significant coronary vasoconstriction. At an intermediate pressure of 53 cm H2O histamine had no effect on CPF. At all three pressures HR, left ventricular pressure, dp/dtmax, and oxygen extraction increased significantly in response to histamine. MVO2 was unchanged by histamine at 65 cm H2O (flow was reduced but extraction increased. MVO2 increased modestly but significantly at 53 cm H2O (12% increase; flow unchanged but extraction increased), and increased prominently at 40 cm H2O (50% increase; flow and extraction increased). We conclude that the coronary vascular effects of continuously infused histamine are dependent on the preexisting, steady-state level of CPP in the isolated perfused guinea pig heart.