Water metabolism and renal function during hibernation and hypothermia.

Total water turnover in normothermic, cold-exposed 13-lined ground squirrels was about half that predicted from allometric relationships of water loss and body weight. During hibernation pulmocutaneous water loss and oxygen consumption were 1/77th and 1/52nd of values during normothermia, respectively, in this species. Renal function during deep torpor with body temperatures of 8 C or lower has also been assessed in ground squirrels and hamsters. The weight of evidence suggests that glomerular filtration and urine formation are absent of undetectable in these animals. We feel that the reduction in arterial blood pressure may be the primary adaptation for eliminating filtration. If filtration (which is not thermally vulnerable, being dependent on hydrostatic pressure) occurred, it is difficult to imagine how reabsorption (which has critical metabolic steps such as ion transport that would be thermally vulnerable) could keep pace. Filtration and hypertonic urine formation occur at body temperatures of 20 to 30 C during the arousal process. Marmots differ from other hibernating rodents in that measurable filtration and hypertonic urine formation occurs during hibernation.