Metabolic Responses of the Toad<i>Bufo marinus</i>to Environmental Hypoxia: An Analysis of the Critical P<scp>o</scp>₂

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.