RELATIONSHIP OF THE ANTIHYPERTENSIVE EFFECT OF VASOPRESSIN WITHDRAWALTO SODIUM-EXCRETION IN THE DOCA-SALT HYPERTENSIVE RAT

Arterial pressure, sodium excretion, urine output, and plasma atrial n atriuretic peptide (ANP) concentrations were measured before, during, and after a 3-h i.v. infusion of arginine-vasopressin (vasopressin; 20 ng/kg/min) in conscious Doca-salt hypertensive rats. Arterial pressur e was 166 +/- 8 mm Hg before the infusion of vasopressin; in compariso n, pressure was only 130 +/- 4 mm Hg 5 h after stopping the infusion. The fall in pressure after withdrawal of an equipressor dose of phenyl ephrine in hypertensive animals was much less. In sham normotensive ra ts, pressure did not fall below control levels after stopping either t he vasopressin or phenylephrine infusion. Sodium excretion rates were higher during infusions of vasopressin than during phenylephrine infus ions. However, the elevations observed during vasopressin were similar in the hypertensive (25.3 +/- 4.9 mumol/kg/min) and normotensive (22. 9 +/- 2.7 mumol/kg/min) groups. Urinary output increased to a greater extent in the hypertensive rats during the infusions of both vasopress in and phenylephrine, but the increases were similar for the 2 pressor agents. Plasma levels of ANP were elevated during the infusions of va sopressin in the normotensive rats, but not in hypertensive rats. The results indicate that the fall in pressure associated with cessation o f a pressor dose of vasopressin appears specific to the hypertensive s tate, and relatively specific to vasopressin. This withdrawal-induced antihypertensive phenomenon (WAP) does not appear to be due solely to the preceding natriuresis and diuresis during the infusion of vasopres sin. However, because the hypertensive animal may be more sensitive to a given degree of sodium loss, the possibility that the natriuresis c ould play a contributing or permissive role cannot be excluded.