Circadian variation of plasma atrial natriuretic peptide, cortisol and fluid balance in the goat

Circadian variation of plasma atrial natriuretic peptide (ANP), cortisol an d fluid balance was studied in ten adult female goats. The concentrations o f plasma antidiuretic hormone (ADH), electrolytes, creatinine and total pro tein, as well as plasma and urine osmolalities and renal electrolyte excret ion and clearances (C-osm, C-H2O. C-crea), were used to evaluate fluid bala nce. At 3-h intervals, urine was collected from five goats and venous blood samples from all ten goats. Urethral catheterization had no effect on the results. Besides the lower plasma creatinine level in the dark than in dayl ight, no other changes were observed in relation to luminousness. Plasma co ncentrations of ANP, ADH, total protein and K, urine flow rare and osmolali ty, urine concentrations of Na, K and creatinine. renal Na and K excretion, C-osm. C-H2O and C-crea, and haematocrit showed no circadian variation. Ci rcadian variation was observed in plasma osmolality (P < 0.05) and the conc entrations of Na (P < 0.05) and creatinine (P < 0.05), with achrophases aro und 16:00 hours and nadirs between 01:00 and 07:00 hours. Changes in osmola lity and Na followed the feeding schedule. There was a small elevation in p lasma cortisol levels in six goats after midnight, which may be the consequ ence of circadian rhythm. In conclusion, the results suggest that in plasma ANP no circadian rhythm exists.