Contribution of plasma vasopressin concentration and blood pressure to norepinephrine-induced diuresis in conscious dogs.

Infusion of norepinephrine (NE) into humans and experimental animals induces diuresis by mechanisms that are not completely understood. Two series of experiments were performed to determine whether changes in plasma levels of vasopressin or changes in mean arterial pressure (MAP) are important-factors in NE-induced diuresis in conscious dogs. First, plasma vasopressin (PAVP) levels were measured in normal and cardiac-denervated conscious dogs during a 30-min intravenous infusion of NE (0.5 micrograms.kg-1.min-1). When NE was administered to normal dogs, urine flow increased from 0.3 +/- 0.1 to 0.9 +/- 0.4 ml/min. PAVP did not decrease, in spite of increases in mean arterial pressure (from 103 +/- 4 to 123 +/- 6 mm Hg) and left atrial pressure (from 5.2 +/- 0.9 to 8.6 +/- 1.4 mm Hg). The same dose of NE infused into cardiac-denervated dogs significantly increased urine flow (from 0.2 +/- 0.1 to 0.8 +/- 0.3 ml/min) and MAP (from 107 +/- 5 to 147 +/- 10 mm Hg), and decreased PAVP (from 1.8 +/- 0.3 pg/ml to 1.2 +/- 0.3 pg/ml). In the second series of experiments, NE was infused into cardiac-denervated dogs for 40 min. During the final 20 min of NE infusion, nitroprusside was infused to offset the pressor effect of NE by returning MAP to the initial control level. Urine flow increased during the first 20 min in which NE alone was given; however, when MAP was returned to the control level by nitroprusside infusion, urine flow also returned to the control level. PAVP increased from a control value of 3.6 +/- 0.6 to 18.9 +/- 3.8 pg/ml 15 min after the NP infusion had begun. We conclude that a decrease in PAVP is not required to elicit diuresis during NE infusion in normal conscious dogs and that the pressor effect of NE appears to play a major role in NE-induced diuresis.