Effects of vasopressin on heart rate in conscious rabbits

The effects of vasopressin on heart rate and on the baroreceptor-heart period reflex were assessed during graded intravenous infusions of arginine vasopressin. Infusions which elevated plasma arginine vasopressin to 200 pg/ml had no effect on blood pressure, but induced a fall in heart rate and cardiac output and an increase in peripheral resistance. These effects were unaltered by vagal blockade with methylscopolamine and cardiac sympathetic blockade with propranolol but were prevented by pretreatment with a specific vascular antagonist to vasopressin, d(CH2)5Tyr(Me)AVP. Baroreflex control of heart rate was studied during vasopressin infusion by monitoring the heart period responses to graded changes in mean arterial blood pressure produced by inflation of balloon occluders around the abdominal aorta and thoracic vena cava. Elevation of plasma arginine vasopressin to 50 pg/ml and 200 pg/ml had no significant effect on the slope or sensitivity of the baroreceptor-heart period reflex but increased the maximum bradycardia elicited in response to large increases in blood pressure. We conclude that at physiological levels, arginine vasopressin has a direct cardiodepressant action that is not dependent on cardiac vagal or sympathetic activity. Our results indicate that arginine vasopressin increases the maximum bradycardia that can be elicited through baroreceptor reflexes but does not alter the slope relating change in heart rate to change in blood pressure.