THE IMPORTANCE OF GLUCOSE FOR THE FREEZING TOLERANCE/INTOLERANCE OF THE ANURAN AMPHIBIANS Rana catesbeiana AND Bufo paracnemis

(With 4 figures) Several species of terrestrially hibernating frogs, turtles and insects have developed mechanisms, such as increased plasma glucose, anti-freeze proteins and antioxidant enzymes that resist to freezing, for survival at subzero temperatures. In the present study, we assessed the importance of glucose to cryoresistance of two anuran amphibians: the frog Rana catesbeiana and the toad Bufo paracnemis. Both animals were exposed to –2ºC for measurements of plasma glucose levels, liver and muscle glycogen content, haematocrit and red blood cell volume. Frogs survived cold exposure but toads did not. Blood glucose concentration increased from 40.35 ± 7.25 to 131.87 ± 20.72 mg/dl (P < 0.01) when the frogs were transferred from 20 to –2ºC. Glucose accumulation in response to cold expo-sition in the frogs was accompanied by a decrease (P < 0.05) in liver glycogen content from 3.94 ± 0.42 to 1.33 ± 0.36 mg/100 mg tissue, indicating that liver carbohydrate reserves were probably the primary carbon source of glucose synthesis whereas muscle carbohydrate seems unimportant. In the toads, the cold-induced hyperglycaemia was less (P < 0.05) pronounced (from 27.25 ± 1.14 to 73.72 ± 13.50 mg/dl) and no significant change could be measured in liver or muscle glycogen. Cold expositio