Changes in the flow and composition of the urine induced by the 18 monoacetate of D-aldosterone, in cats.

1. Isolated hearts release a steroid-like substance into the blood stream when venous return is decreased. This substance very closely resembles the 18 monoacetate of D-aldosterone (18 MA) in physicochemical and biological properties.2. Hence the changes in the flow and composition of the urine caused by infusions of the 18 MA have been examined in cats.3. The threshold effect of both 18 MA and of antidiuretic hormone (ADH) may be antidiuretic, but higher concentrations of both substances usually produce diuresis in cats under chloralose anaesthesia.4. Both ADH and 18 MA are antidiuretic in decerebrate cats.5. In decerebrate cats and in cats under chloralose anaesthesia the renal actions of 18 MA, 0.003-0.02 mug/kg.min are abolished by hypophysectomy: responses to ADH remain unchanged.6. 18 MA, 0.012 mug/kg.min causes the appearance of thioglycollate labile antidiuretic activity in blood taken from the terminal descending portion of the left lateral sinus.7. 18 MA, 0.05-0.15 mug/kg.min causes diuresis followed by antidiuresis in cats under chloralose anaesthesia. Hypophysectomy abolishes the diuretic phase of the response and uncovers the short latency of the antidiuretic. The antidiuresis is accompanied by reduction in the concentration of urinary Na and a fall in the ratio Na/K of the urine.