ROLE OF SODIUM AND WATER-EXCRETION IN THE ANTIHYPERTENSIVE EFFECT OF VASOPRESSIN IN THE SPONTANEOUSLY HYPERTENSIVE RAT

Mean arterial pressure (mmHg (1 mmHg = 133.322 Pa)), sodium excretion rate (mumol . kg-1 . min-1), and urine flow (muL . kg-1 . min-1) were measured in conscious unrestrained spontaneously hyptertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) before, during, and after a 3-h intravenous infusion of arginine vasopressin (20 ng . kg-1 . min-1), an equipressor dose of phenylephrine, or an infusion of the vehicle. Cessation of the phenylephrine infusion was associated with a return of arterial pressure to preinfusion control values in both SHR and WKY. Cessation of the vasopressin infusion was also associated with a return of arterial pressure to preinfusion values in WKY. In contrast, in the SHR, arterial pressure fell from a preinfusion control level of 164 +/- 6.2 to 137 +/- 4 mmHg within 1 h of stopping the vasopressin infusion. Five hours after stopping the infusion, pressure was 134 +/- 3 mmHg (29 +/- 5 mmHg below preinfusion levels). Similar to the WKY, cessation of a vasopressin infusion was associated with a return of arterial pressure to preinfusion values in Sprague-Dawley rats. Thus, the failure to observe a hypotensive response in normotensive rats was not a peculiarity of the WKY strain. Sodium excretion rates increased during the infusions of vasopressin to a greater extent in SHR than in WKY. However, the natriuresis induced by phenylephrine was not significantly different from that generated by vasopressin in SHR, and in WKY, the natriuresis was greater for phenylephrine than for vasopressin. Urine output increased to a greater extent during the infusions of phenylephrine in both SHR and WKY than during vasopressin infusion. Because the infusions of phenylephrine were associated with either a similar or greater natriuresis and diuresis than the infusions of vasopressin, it is unlikely that the large fall in arterial pressure that occurred following the withdrawal of the vasopressin infusion (the ''withdrawal-induced antihypertensive phenomenon'') was related to the preceding natriuresis and diuresis.