Hypotension 1.5 min after 10% hemorrhage permits evaluation of rabbit's baroreflex

In seven chronically instrumented rabbits, we studied the influence of the vasopressin system on the evaluation of acute reflex compensation of arterial pressure for a mild hemorrhage. Rabbits were bled 10% of estimated blood volume in 1 min. The hemorrhage was repeated before and after administration of a vasopressin antagonist [d(CH2)5Tyr(Me)arginine vasopressin, 10 micrograms/kg] and in the conscious and anesthetized (pentobarbital sodium 20 mg/kg) state. The efficacy of the vasopressin antagonist was preliminarily tested in each rabbit by confirming its antagonism against the hemodynamic effects of infused vasopressin (50 ng X min-1 X kg-1). Mean arterial pressure (MAP), heart rate (HR), mean aortic flow (MAF), and total peripheral resistance (TPR) were measured before and at 1.5 min after completion of hemorrhage. The hemorrhage caused statistically significant decrease in MAP (84 +/- 6 to 78 +/- 6 mmHg) and MAF (195 +/- 28 to 158 +/- 26 ml X kg-1 X min-1) and increase in TRP (0.44 +/- 0.05 to 0.51 +/- 0.07 mmHg X kg X min X ml-1) and HR (271 +/- 21 to 284 +/- 22 beat/min). However, there was no statistically significant interaction in any variables between vasopressin and hemorrhage, i.e., whether vasopressin was blocked or intact did not affect the fall in MAP by hemorrhage. We conclude that in the rabbit vasopressin's role in the acute compensation of arterial pressure for a quick 10% hemorrhage is small enough to permit accurate estimation of the arterial baroreflex gain from the depressor responses to mild hemorrhage with and without the reflex function.