Effect of prolonged bodily hydration on the renal concentrating operation

¹ Department of Physiology, Yonsei University College of Medicine, Seoul, Korea

The maximal renal concentrating ability was studied in six healthy men while 150 ml water/kg body wt. was forced daily for 9 days. The maximally concentrated urine samples were obtained after intramuscular injection of 5 units of surgical Pituitrin on the 3rd, 6th, and 9th day of the forced hydration period. The renal concentrating ability was already reduced on the 3rd day of the forced hydration, and there was very little further reduction during the subsequent period. All urinary constituents lost concentration in parallel. The rates of excretion of various urinary constituents through maximally concentrated urine did not show any consistent change. Moreover, all impairments were reversible and were not accompanied by any alteration in the glomerular filtration rate. It is thus considered that the observed reduction in renal concentrating ability during the forced hydration period is due to the reduction of tubular function. It is postulated that the collecting duct undergoes a certain mechanical deformation during the forced hydration period and, hence, is not able to perform the concentrating operation as much as it would do otherwise.