Day-night variations in urine excretions and hormones in dogs: role of autonomic innervation

Under controlled light-dark cycle (LD 12:12) and two hydratory conditions, mongrel dogs display marked day-night variations in urine flow and in its constituents. Diurnal peaks were found in urine flow and sodium excretion, while nocturnal peaks were observed in osmolality and potassium excretion. Diuretic responses, after fluid intake, were consistently faster and higher during the day than during the night. Nocturnal increases were found in plasma antidiuretic hormone (ADH), in aldosterone and in plasma renin activity (PRA), which could partially explain the present results. In all dogs there was a decrease in ADH level in the early morning hours which could account for the increased urine flow and decreased osmolality observed at this time. Plasma cortisol did not show significant temporal variations throughout the 24-hour period. Kidney denervation did not alter the day-night variations in urinary flow, sodium and osmolality, but affected the pattern of potassium excretion. A neural control of potassium excretion pattern, probably mediated by PRA-aldosterone is tentatively postulated.