Vasopressin and angiotensin II in reflex regulation of heart rate: Effect of water deprivation

This study tested the hypothesis that endogenous angiotensin II (ANG II) and vasopressin enhance baroreflex-mediated increases in heart rate in water- replete dogs and in dogs water deprived to chronically elevate plasma ANG II and vasopressin concentrations. The baroreflex was assessed by examining the heart rate response to infusion of increasing doses of nitroprusside (0.3, 0.6, 1.5, and 3.0 μg · kg^-1 · min^-1). The effect on the baroreflex of pretreating the dogs with the competitive ANG II antagonist saralasin, a V₁- vasopressin antagonist, or a combined V₁/V₂-vasopressin antagonist, alone or in combination, was determined. Nitroprusside infusion produced dose- dependent increases in heart rate, and the heart rate response was greater in water-deprived dogs in association with higher plasma levels of ANG II and vasopressin than in water-replete dogs. ANG II blockade alone depressed reflex increases in heart rate in water-deprived but not water-replete dogs. In both water-replete and water-deprived dogs, blockade of V₁-vasopressin receptors reduced the heart rate response to hypotension, but this effect could be produced only when ANG II receptors were also blocked. In addition, administration of saralasin and the V₁/V₂-vasopressin antagonist led to a further reduction of the reflex tachycardia. These data suggest that endogenous vasopressin, acting at both V₁- and V₂-receptors, can amplify the increase in heart rate produced by hypotension. In addition, the results further support a physiological role for chronic elevations in endogenous ANG II in the maintenance of normal baroreflex function.