Renal concentrating ability in the uninephrectomized rat.

To investigate the effects of uninephrectomy on renal concentrating ability, studies were performed on unanesthetized rats 5-11 days after uninephrectomy (UN) or a sham operation (SO). Female rats were deprived of water for 27 h prior to the infusion of inulin and para-aminohippurate and urine collection. They were also preconditioned to being handled and to the experimental locale. During a nondiuretic state urine osmolality was the same for all UN and SO groups (mean about 1,700 micro osmol/g H2O), whereas the mean solute excretion rate (micro osmol/min per kg body wt per kidney) was 74 in the UN and 35 in the SO rats. When SO rats were infused with mannitol or isotonic saline to increase their solute excretion rate per kidney to the level of the UN rats, urine osmolality dropped 200-1,000 micro osmol/g H2O; when urea was infused, urine osmolality did not drop. Thus, after uninephrectomy and a consequent doubling of the solute excretion rate per kidney, renal concentrating ability was higher than predicted on the basis of a comparable but acute elevation of the solute excretion rate. The glomerular filtration rate was about 17 ml/min per kg body wt in the SO rats and was 1.2 times greater (on a per kidney basis) in the UN rats. These exceptionally high glomerular filtration rats are attributed to preexperimental conditioning of the rats and the absence of stress during urine collection.