Hypometabolic Homeostasis in Overwintering Aquatic Amphibians

Many amphibians encounter conditions each winter when their body temperature is so low that normal activities are suspended and the animals enter into a state of torpor. In ice-covered ponds or lakes, oxygen levels may also become limiting, thereby forcing animals to endure prolonged periods of severe hypoxia or anoxia. Certain frogs (e.g. Rana temporaria) can dramatically suppress their metabolism in anoxia but are not as tolerant as other facultative vertebrate anaerobes (e.g. turtle, goldfish) of prolonged periods of complete O₂ lack. Many overwintering amphibians do, however, tolerate prolonged bouts of severe hypoxia, relying exclusively on cutaneous gas exchange. Rana temporaria overwintering for 2 months in hypoxic water (⁠ approximately 25 mmHg) at 3 °C progressively reduce their blood to levels characteristic of water-breathing fish. The result is that blood pH rises and presumably facilitates transcutaneous O₂ transfer by increasing Hb O₂-affinity. Even after months of severe hypoxia, there is no substantial build-up of lactate as the animals continue to rely on cutaneous gas exchange to satisfy the requirements of a suppressed aerobic metabolism. Our recent experiments have shown that the skeletal muscle of frogs oxyconforms in vitro to the amount of O₂ available. The cellular basis for the oxyconformation of skeletal muscle is unknown, but the hypothesis driving our continuing experiments theorises that metabolic suppression at a cellular level is synonymous with suppressed ion leak across cellular membranes.