Vasopressin-induced bradycardia in barodenervated rats

Experiments were performed on conscious chronically instrumented rats to determine the contribution of baroreceptor reflex (BRR)-independent mechanisms in the bradycardia associated with intravenous administration of arginine vasopressin (AVP). At least 2 wk prior to experimentation, the aortic and carotid baroreceptors were denervated in six rats. Following recovery, the maximum bradycardic responses to pressor doses of phenylephrine (PE) or AVP were determined on separate days in each animal. Pulse interval (PI) was calculated from heart rate (HR), and the slopes of PI vs. mean arterial blood pressure (MABP) and PI vs. dose were determined for each experiment by linear regression. There was no significant correlation of PI vs. MABP in response to bolus PE in these barodenervated rats. However, there was a significant correlation of PI vs. MABP and PI vs. dose in response to AVP in five of the six barodenervated rats. These data suggest that pressor levels of AVP may elicit bradycardia independent of the BRR. Another group of experiments was performed in the Langendorff-isolated rat heart preparation to assess the direct chronotropic effect of AVP. Hearts were not paced, and the coronary arteries were perfused at a constant flow. HR was determined at various concentrations of AVP (n = 4 at each dose). In addition, because unpaced hearts exhibited low intrinsic rates, the same protocol was performed in hearts receiving isoproterenol to elevate basal HR (n = 4 at each AVP dose). There was no chronotropic effect of AVP in isolated hearts with or without isoproterenol administration.(ABSTRACT TRUNCATED AT 250 WORDS)