Interaction of vasopressin and the baroreceptor reflex system in the regulation of arterial blood pressure in the dog.

The hemodynamic effects of 1-hour intravenous infusions of vasopressin were evaluated in trained, unanesthetized dogs in the normal state and following sinoaortic baroreceptor denervation. Pressor sensitivity to vasopressin was greatly enhanced following baroreceptor denervation; threshold sensitivity was increased 11-fold and sensitivity at higher dose levels was increased 60-100-fold. Infusion of physiological levels of vasopressin caused an average increase in arterial blood pressure of 33 mm Hg in conscious, baroreceptor-denervated dogs compared with an increase of 5 mm Hg in normal dogs. In contrast, similar intravenous infusions of norepinephrine at physiological levels resulted in a 3-fold increase in pressor sensitivity with no change in threshold dose. Hypophysectomy of baroreceptor-denervated dogs did not significantly alter their pressor sensitivity to vasopressin in the conscious state. The arterial blood pressure response to intravenous vasopressin infusions was greatly depressed when a high background level of circulating vasopressin was present. Decapitated, spinal, anesthetized dogs maintained with a small continuous infusion of norepinephrine exhibited the greatest sensitivity to vasopressin; the threshold dose for a pressor response was similar to that in conscious baroreceptor-denervated dogs, but pressor sensitivity at physiological dose levels was increased nearly 8,000-fold. The elevations in arterial blood pressure resulting from vasopressin infusions of less than 1.0 munits/kg min" were large enough to implicate the direct pressor effect of vasopressin in the normal control of arterial blood pressure.