Physiology of hibernation in Rana pipiens: metabolic rate, critical oxygen tension, and the effects of hypoxia on several plasma variables.

Rates of O(2) consumption (.VO(2)) were determined for adult northern leopard frogs (Rana pipiens) submerged at 3 degrees C at water PO(2)s (P(w)O(2)) ranging from 0-160 mmHg. The critical O(2) tension (P(c)) was 36.4 mmHg. Hematocrit and blood levels of PO(2), glucose, lactate, pH, [Na(+)], [K(+)], and osmolality were determined for frogs submerged for two days. Above a P(w)O(2) of 50 mmHg, blood PO(2) ranged from 1-7 mmHg, which was sufficient to allow the frogs to function entirely aerobically. Plasma [lactate] increased as P(w)O(2) fell below 50 mmHg, the increase preceding significant changes in any other variable, and apparently preceding a fall in .VO(2). Most other variables showed little or no change from those of air-breathing control animals, even during anoxia. We present an analysis of the importance of a large decrease in P(c) in permitting frogs to successfully overwinter in icebound ponds and of the factors that contribute to that decrease. Copyright 2004 Wiley Liss, Inc.