Temperature Effects on the Responses to Prolonged Submergence in the Turtle Chrysemys picta bellii. II. Metabolic Rate, Blood Acid-Base and Ionic Changes, and Cardiovascular Function in Aerated and Anoxic Water

Pulmonary and nonpulmonary O2 uptake (V̇o2) were measured on freshwater turtles (Chrysemys picta bellii) at 3, 10, 15, and 20 C, and these data were related to the physiological responses of turtles to submergence in aerated vs. anoxic water. Total V̇o2 increased with temperature with a progressive fall in Q10 from a peak of 8.5 between 3 and 10 C. The contribution of nonpulmonary exchange decreased with temperature from 21% of the total at 3 C to 5% at 20 C. Anaerobic metabolic rate, estimated from the rate of plasma lactate accumulation during anoxic submergence, increased with temperature, and again a high Q10 value (13.3) was found between 3 and 10 C. At each temperature, anoxic submergence caused a sharp fall in metabolic rate that was most pronounced at 3 C (<90%). The combined effects of low temperature and anoxia, therefore, resulted in a profound depression of metabolism at 3 C, to 0.6% of the aerobic rate at 20 C. Turtles submerged in aerated water experienced less severe blood acid base and ionic disturbances at 3 and 10 C than those submerged in anoxic water, but aeration of the water had no beneficial effect at 15 and 20 C. This observation agrees with the comparatively greater importance of nonpulmonary gas exchange at low temperature; however, turtles became acidotic under all submergence conditions employed in this study. Blood pressure varied directly with temperature in normoxic turtles but was unaffected by anoxic submergence, except at 3 C, where a significant decrease was observed. Heart rate decreased significantly during anoxic submergence at each temperature tested (3, 10, and 20 C). We conclude that the greatly prolonged submergence tolerance of turtles at 3 C is principally due to a profound decrease in metabolic rate. Nonpulmonary gas exchange is also important in limiting hypercapnia and supporting a low-level aerobic metabolism when aquatic O2 is available.