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

Select hepatic changes in the freeze-tolerant hatchling turtle, Chrysemys p icta marginata, were studied in response to freezing at -2.5 degrees C and thawing. Upon freezing, a small, selective increase in the liver weight wit h no increase in body weight was seen suggestive of an hepatic capacitance response. In all turtles studied, lobular differences in the hepatic conten t of glycogen were evident: the smaller lobe contained twice as much glycog en as the larger lobe. The response to freezing and thawing was comparable. Total hepatic glycogen levels of turtles were reduced similar to 60 per ce nt from control levels in the frozen state and recovered to > 80 per cent o f 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 sta te; and increased 4.5-fold in the thawed state. Thus, changes in hepatic gl ycogen metabolism occur without large changes in blood glucose levels. In t urtle liver plasma membranes, the hepatic alpha(1)-adrenergic receptor was barely detectable and did not change. The beta(2)-adrenergic receptor was e xpressed at high levels and, compared to control levels, was: unchanged aft er 12 h in the cool state; reduced 20 per cent after 24 h and 40 per cent a fter 48 h in the frozen state. On thawing, this receptor was 50 per cent of control levels. While catecholamines working through the beta(2)-adrenergi c receptor may effect early hepatic glycogen breakdown in response to freez ing, other factors must be involved to complete the process. The plasma mem brane-bound enzyme gamma-glutamyltranspeptidase displayed a different patte rn 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 w as decreased in the cool state and slightly increased in the frozen and tha wed states emphasizing the tissue-specific nature of the changes in the act ivity of gamma-glutamyltranspeptidase in response to freezing and thawing. The similarities and differences of the hepatic changes in response to free zing and thawing in the freeze-tolerant hatchling turtle to those we have p reviously reported for the freeze-tolerant frog are discussed. Copyright (C ) 2000 John Wiley & Sons, Ltd.