Vascular effects of arginine vasopressin during fluid deprivation in the rat

Abstract

The vascular effects of arginine vasopressin (AVP) were examined in conscious Sprague-Dawley rats. In 6 control rats, synthetic AVP at a dose of 40 ng/kg, injected as an intravenous bolus, resulted in a rise in mean arterial blood pressure (BP) from 127 to 149 mm Hg (P < 0.005). No tachyphylaxis was observed after a second AVP bolus administered 30 min later, as BP increased from 125 to 150 mm Hg, P < 0.005. In a second group of 6 rats, 1-deamino penicillamine, 2-(O-methyl) tyrosine AVP ([dPTyr (Me)]AVP), was administered intravenously at a dose of 10 μg/kg, just before the second AVP bolus. In this group of studies BP rose from 124 to 150 mm Hg (P < 0.01) after the first AVP bolus, but not after the second AVP bolus, which was administered after [dPTyr (Me)]AVP (129 vs. 129 mm Hg, NS). To assess the effect of this AVP pressor antagonist on BP in rats with suppressed endogenous vasopressin, 6 water-diuresing rats (mean urinary osmolality, 99 mosmol/kg H2O) were administered the analogue at the same dose as the first group of rats. The analogue exerted no demonstrable effect on mean BP (128 before vs. 129 mm Hg after [dPTyr (Me)]AVP, NS). In these rats, mean radioimmunoassayable levels of AVP were at or below the detectable limits of our assay (0.5 pg/ml). In contrast, 6 rats in which endogenous AVP was stimulated by fluid deprivation for 24 h (mean urinary osmolality, 2,489 mosmol/kg H2O and mean AVP level of 21.6 pg/ml) had a marked fall in BP when administered the AVP analogue. In these animals [dPTyr (Me)]AVP caused a fall in BP from 124 to 110 mm Hg (P < 0.005). This fall in blood pressure was due to a fall in peripheral vascular resistance (0.35 vs. 0.30 mm Hg/ml per min per kg, P < 0.02) after [dPTyr (Me)]AVP, as cardiac index remained unchanged. To eliminate the possibility that this AVP analogue was antagonistic to endogenous pressor substances other than AVP, additional studies were performed. In homozygous Brattleboro (diabetes insipidus) rats receiving exogenous AVP, the vasopressin analogue lowered BP (133 to 112 mm Hg, P < 0.001), but failed to lower BP (112 vs. 112 mm Hg) in rats not receiving AVP. BP in a group of bilaterally nephrectomized Sprague-Dawley rats, after 24 h of fluid deprivation, fell from 130 to 118 mm Hg (P < 0.02) after the AVP analogue, precluding an effect of the analogue on lowering BP by inhibiting the renin-angiotensin system. Finally, the AVP analogue failed to alter the pressor response to exogenous infusions of either norepinephrine or angiotensin II. These results demonstrate that (a) the AVP analogue [dPTyr (Me)]AVP abolishes the pressor effect of large exogenous doses of AVP; (b) the analogue has no effect on BP in rats with suppressed or absent endogenous AVP; (c) the depressor effect of the analogue does not involve antagonism of the vasoconstrictors, norepinephrine or angiotensin; and (d) most importantly, BP fell significantly after AVP antagonist administration in intact, conscious, fluid-deprived rats with elevated endogenous AVP levels. This effect of the AVP antagonist to block endogenous AVP and lower BP was primarily due to a fall in peripheral vascular resistance.