Biphasic Effect of Pitressin on Sodium Tolerance in the Hypertensive Rat

¹ From the Department of Anatomy, The University of British Columbia, Vancouver, Canada

The effect of a large dose of Pitressin (2 iu subcutaneously) on Na and water excretion under load was studied in rats made hypertensive by DCA. This dose is calculated to be within the upper limit of physiological secretion by the neurohypophysis. The excretion pattern was determined during the 4 1/2 hours following gavage with 100 mg of NaCl in 3 cc of water, both for the period immediately following Pitressin administration and for the period 3 hours after administration. These two periods were selected as corresponding to the pressor and depressor phases of Pitressin action, respectively. The procedure was carried out after one, four and eight daily injections of Pitressin. The selection of control and hypertensive animals was based on a preliminary blood pressure determination obtained by direct electromanometry from the femoral artery. The experiments demonstrated that Pitressin at this dose level has a biphasic action on the handling of Na and water as it has on blood pressure. The first phase consists of a limited tendency to accentuate the already accelerated rate of Na and water excretion found in the hypertensive state. This first effect was not obtained after eight injections. The second phase is characterized by a marked decrease in Na and water excretion which appears to be independent both of the antecedent rate of Na and water loss and of the number of injections.