Renal function during hypothermia and hibernation

The alterations of urine flow and composition during hypothermia were studied in unanesthetized rats and ground squirrels. One ureter or the urethra was cannulated in some animals for continuous urine collection. Within 5–10 minutes of the onset of cooling, the urine flow diminished to an almost immeasurable level and then recovered to a level lower than the precooling one. This diminution may be due to the stimulation of the sympathetic nervous system. As the body temperature was further lowered, urine flow progressively decreased. Both water and solute diuresis, if already under way before the onset of hypothermia, were also reduced during hypothermia without increase in solute concentration. However, any infusion of fluid during hypothermia augmented the urine flow. This augmented urine flow was not inhibited by injected Pituitrin. During hypothermia the renal blood flow was quantitatively greatly reduced, while the changes in the hematocrit ratio or in the refractive index of plasma were not consistent. These facts suggest that decrease in glomerular filtration rate, which follows the reduction in renal blood flow, may be the main cause for this great reduction in urine flow during hypothermia. The solute concentration of the urine formed during hypothermia was more independent of the urine flow, so that the normal volume-concentration relationship is upset, suggesting that water and solute reabsorptions are dissociated. When 0.50 m NaCl solution was infused during hypothermia, a large amount of dilute urine was excreted, probably due to the inability of tubules to reabsorb the water and chloride loads presented. The hypothermic animals also failed to concentrate urea in the urine. Reducing substances were excreted. The excretion of phenolsulfonephthalein (PSP), in terms either of the percentage return in the urine after intravenous administration or of the blood plasma clearance, was also lowered during hypothermia. Urine/plasma concentration ratio of chloride or PSP was lower during hypothermia. These suggest that tubular functions, both reabsorptive and secretory, are depressed during hypothermia. During hibernation the urine flow was similarly slow and the urine was dilute.