Differential vasodilator effect of nitroglycerin on systemic capacitance and resistance vessels in anesthetized dogs, and the mechanism of its action. A study by measuring mean circulatory pressure and total peripheral resistance

The aim of our studies was to examine if the baroreceptor reflex induced by intravenous injection of nitroglycerin (NG) nearly nullifies the vasodilator action of NG on the systemic capacitance vessels in dogs anesthetized with pentobarbital. We performed total spinal anesthesia (TSA) in open-chest dogs to eliminate the baroreceptor reflex. Dogs in which mean blood pressure (MBP) was maintained at about 100 mmHg by continuous intravenous infusion of epinephrine formed the TSA 100 group, and those in which MBP was maintained at about 70 mmHg formed the TSA 70 group. Dose-response curves relating 5∼6 different doses (0.8 to 200μg/kg) of intravenous NG, to changes in mean circulatory pressure (%ΔMCP) and to changes in total peripheral resistance (%ΔTPR), were constructed. These data were compared with those from untreated dogs. In addition, we also studied plasma catecholamine (CA) concentrations before and after the intravenous injection of NG. (1) There was no significant difference in the dose-response curves of NG for %ΔTPR between the 3 groups. (2) There was no significant difference in the dose-response curves for %ΔMCP between the 2 TSA groups. However, there was significant difference in the dose-response curves for %ΔMCP between the TSA groups and the untreated dogs. The dose-response curves for %ΔMCP in the TSA groups shifted upwards as compared with that of the untreated dogs. (3) In the TSA 100 group, there was no significant increase in the plasma CA concentrations with 100μg/kg of NG. In the untreated group, intravenous injection of 12.5μg/kg and 100μg/kg of NG caused a dose-related fall in MBP and a dose-related rise in the plasma norepinephrine and epinephrine concentrations. Our results suggest that the vasodilator action of NG on the systemic capacitance vessels in the untreated dogs was counterbalanced partly by the vasoconstriction produced by the baroreceptor reflex, with its direct vasodilator effect masked when the hypotensive doses of NG were given acutely.