Atropine Potentiates the Pressor Effect of Arginine-Vasopressin in Conscious Dogs

The hemodynamic effects of injections of arginine-vasopressin (AVP) at different doses were measured before and after administration of atropine (250 micrograms/kg), propranolol (2 mg/kg), or phenoxybenzamine (5 mg/kg) in conscious dogs. Measurements of arterial pressure derived from a chronic indwelling catheter, and aortic flow derived from an aortic electromagnetic flow probe, were digitized and analyzed by computer to assess mean and pulsatile arterial pressure, cardiac output, total peripheral resistance, heart rate, and other hemodynamic variables. AVP alone moderately increased arterial pressure and decreased cardiac output and heart rate. For doses between 20 and 40 ng/kg, mean arterial pressure increased by 17.4 +/- 1.5 mm Hg. After atropine, the same dose of AVP increased pressure by 51.6 +/- 3.2 mm Hg. The bradycardiac response to AVP was blunted by atropine as was the decrease in cardiac output. The effect of AVP on total peripheral resistance was not affected by atropine. Neither alpha- nor beta-adrenergic blockade enhanced the pressor effect of arginine-vasopressin. Our results indicate that the potentiation of the pressor response to AVP previously reported with total autonomic blockade is largely due to preventing a vagallymediated decrease in cardiac output in conscious dogs.