Renal adaptation during hibernation

Abstract

Hibernators periodically undergo profound physiological changes including dramatic reductions in metabolic, heart, and respiratory rates and core body temperature. This review discusses the effect of hypoperfusion and hypothermia observed during hibernation on glomerular filtration and renal plasma flow, as well as specific adaptations in renal architecture, vasculature, the renin-angiotensin system, and upregulation of possible protective mechanisms during the extreme conditions endured by hibernating mammals. Understanding the mechanisms of protection against organ injury during hibernation may provide insights into potential therapies for organ injury during cold storage and reimplantation during transplantation.hibernation; kidney; torpor; metabolism; electrolytes MAMMALIAN HIBERNATORS EXHIBIT heterothermy during winter months, cycling through periods of low core body temperature (CBT) for days to weeks (22) during torpor, followed by periodic arousals when CBT is elevated to 37°C for ϳ12 h ( Fig. 1) (63). CBT during torpor can fall to as low as Ϫ3°C in arctic ground squirrels (7) or 2-10°C in temperate-zone hibernators (22). During torpor, hibernators undergo profound physiological changes, reducing their heart rate from a summertime level of 200 -300 to 3-5 beats/min (128), their respiratory rate from 100 -200 to 4 -6 breaths/min (33, 46) and their metabolic rate to as low as 1-5% of basal metabolic rate (47). During arousal, organs undergo rapid metabolic reactivation, reperfusion, and rewarming to near normal levels (22). After maintaining euthermia and high metabolic activity for 12-18 h, the hibernator reenters torpor (Figs. 1 and 2). This review will focus on the renal adaptations employed by hibernators that permit their kidneys to withstand extreme fluctuations in CBT and organ perfusion during winter heterothermy. Studies of hibernation have been conducted for over a century and the nomenclature used to describe the various stages of hibernation, and the criteria used to define stages of hibernation, have evolved. Earlier studies often simply compared "hibernating" with "nonhibernating," and both terms were variably used. Hibernating often meant mid-winter torpor but was also used to refer to animals that were aroused, either naturally or more often, artificially. Nonhibernating was used to denote midsummer euthermic animals or winter aroused animals (94). As a further complication in the nomenclatures used, some investigators examine monthly or seasonal changes whereas others examine changes that occur during one torpor-arousal cycle of a few days duration. More recent studies have employed specific definitions of stages, representing both seasonal and torpor-arousal cycles, based on CBT monitoring using radiotelemetry (62,63,70,83) (Fig. 1). In addition, torpor can occur daily (referred to as "daily torpor" in "daily heterotherms") or over several days to weeks typically during winter in hibernators (46,125). For the purposes of this review we will 1) focus on renal function in hibernators that e...

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