Hepatic changes in the freeze-tolerant turtle Chrysemys picta marginata in response to freezing and thawing

Select hepatic changes in the freeze-tolerant hatchling turtle, Chrysemys picta marginata, were studied in response to freezing at -2.5 degrees C and thawing. Upon freezing, a small, selective increase in the liver weight with no increase in body weight was seen suggestive of an hepatic capacitance response. In all turtles studies, lobular differences in the hepatic content of glycogen were evident: the smaller lobe contained twice as much glycogen as the larger lobe. The response to freezing and thawing was comparable. Total hepatic glycogen levels of turtles were reduced approximately 60 per cent from control levels in the frozen state and recovered to >80 per cent of control levels in the thawed state. Compared to the control state, turtle blood glucose levels were: unchanged after 12 h in the cool state; reduced 28 per cent after 24 h and increased two-fold after 48 h in the frozen state; and increased 4.5-fold in the thawed state. Thus, changes in hepatic glycogen metabolism occur without large changes in blood glucose levels. In turtle liver plasma membranes, the hepatic alpha(1)-adrenergic receptor was barely detectable and did not change. The beta(2)-adrenergic receptor was expressed at high levels and, compared to control levels, was: unchanged after 12 h in the cool state; reduced 20 per cent after 24 h and 40 per cent after 48 h in the frozen state. On thawing, this receptor was 50 per cent of control levels. While catecholamines working through the beta(2)-adrenergic receptor may effect early hepatic glycogen breakdown in response to freezing, other factors must be involved to complete the process. The plasma membrane-bound enzyme gamma-glutamyltranspeptidase displayed a different pattern of changes indicative of selective modulation: it was increased 2.7-fold over control levels in the cool state; unchanged in the frozen state; and increased 1.8-fold in the thawed state. The activity of the kidney enzyme was decreased in the cool state and slightly increased in the frozen and thawed states emphasizing the tissue-specific nature of the changes in the activity of gamma-glutamyltranspeptidase in response to freezing and thawing. The similarities and differences of the hepatic changes in response to freezing and thawing in the freeze-tolerant hatchling turtle to those we have previously reported for the freeze-tolerant frog are discussed.