Participation of the autonomic nervous system in the cardiovascular effects of milrinone

The cardiodynamic activity of intravenously administered milrinone was examined in α-chloralose anesthetized dogs. Two groups of dogs were used, one pretreated with hexamethonium to block autonomic reflexes, and a second group which received no pretreatment. In the untreated group milrinone produced dose-dependent increases in +dP/dt and heart rate while decreasing both systolic and diastolic blood pressure and left ventricular end diastolic pressure (LVEDP). After treatment with hexamethonium basal heart rate was significantly increased, whereas reflex changes in heart rate in response to i.v. norepinephrine or nitroglycerin were ablated. Systolic, but not diastolic blood pressure was also markedly reduced by hexamethonium. In the presence of hexamethonium responses to milrinone were qualitatively similar to milrinone responses in the absence of hexamethonium. However, the dose-response curves for milrinone were shifted dextrally for changes in +dP/dt and LVEDP, whereas the dose- response curve for blood pressure was shifted sinistrally. Thus, it appears that the autonomic nervous system enhances the effect of milrinone on +dP/dt and LVEDP, but attenuates its effect on blood pressure.