Metabolic Responses of the ToadBufo marinusto Environmental Hypoxia: An Analysis of the Critical Po2

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Abstract
Characteristics of the critical Po₂ have been investigated by analyzing both O₂ uptake rates and modes of energy production during hypoxic exposure in the toad Bufo marinus. After preacclimatization to moderate hypoxia, this oxyregulator exhibits constant rates of O₂ consumption in a wide range of Po₂. Below 30-37 Torr, an increase in O₂ consumption coincides with the onset of anaerobic metabolism. Consequently, a Po₂ in this range is concluded to be equivalent to the critical Po₂. The early decrease in the levels of high-energy phosphates and the accumulation of large amounts of lactate in the kidney and, even more so, the heart indicate that limits in the O₂ supply to aerobic organs determine the hypoxia tolerance of the animal. The appearance of lactate in the plasma and the concomitant increase in O₂ consumption suggest that lactate may mediate this increase, thus eliciting a stress response.