EFFECT OF THE CESSATION OF AN INFUSION OF VASOPRESSIN IN ACUTELY HYPOPHYSECTOMIZED ANAESTHETIZED DOGS

¹ The Department of Cardiovascular Studies, University of Leeds, Leeds LS2 9JT
² Department of Physiology, University of Sheffield, Sheffield

Stimulation of left atrial receptors in dogs anaesthetized with chloralose results in a reflex diuresis and a small decrease in the plasma concentration of vasopressin. It has been suggested that this diuretic response is solely mediated by the antidiuretic hormone, vasopressin (Gauer & Henry, 1976; Ledsome, 1985). The present investigation was designed to determine how these anaesthetized dogs, which are thought to be insensitive to changes in plasma vasopressin, respond to small decreases in plasma vasopressin similar to those observed during stimulation of left atrial receptors. In the event it was shown for the first time that it is possible to obtain a water diuresis in an anaesthetized animal in response to a small fall in the concentration of plasma vasopressin. In order to produce changes in plasma vasopressin similar to those seen during stimulation of left atrial receptors the pituitary gland was removed and an intravenous infusion of arginine vasopressin was given in eleven dogs anaesthetized with chloralose; a small decrease in plasma vasopressin was then produced by replacing the vasopressin with an infusion of saline for 30 min. A decrease in the plasma concentration of 1·29 pg cm^-3 from a control concentration of 1-85 pg cm^-3 resulted in a 96·7% increase in urine flow. The fall in concentration of vasopressin consequent upon stimulation of atrial receptors could, therefore, explain the water diuresis in the anaesthetized dog preparation. However, it is argued that results from the previous papers, in one of which a humorally mediated diuresis was obtained without the involvement of vasopressin, suggest that vasopressin may not be solely responsible for the diuretic response from the left atrial receptors; it may act as a ‘trigger’ which allows another (diuretic) substance to act.