CA2-ATPASE IN CARDIAC MYOFIBRILS OF RAINBOW-TROUT, FRESH-WATER TURTLE, AND RAT( ACTIVATED MYOSIN)

The Ca2+-activated myosin-ATPase and its dependence on hypoxia were as sessed in freshwater turtle, rainbow trout, and in some cases rat. At 20 degrees C and pH 7.3, the maximal ATPase activity was (mean +/- SEM ): turtle 0.040 +/- 0.003, trout 0.090 +/- 0.005, and rat 0.12 +/- 0.0 04 mmolmin(-1)*g(-1) myofibrillar dry weight. The turnover number was about three times lower for turtle than for trout. Trout is typically active at lower temperatures than turtle, and its myosin-ATPase activ ity was about three times lower at 10 degrees than at 20 degrees C. Ad dition of 12 mM phosphocreatine showed that the myosin-ATPase activity covered by myofibrillar creatine kinase was 22 +/- 2% for turtle, 14 +/- 2% for trout, and 69 +/- 5% for rat. At pH 6.8 relative to 7.3, th e maximal M-ATPase activity was the same, whereas the Ca2+-sensitivity decreased, and more so for trout than for turtle. This difference dis appeared, when trout myocardium was examined at 10 degrees C. P-i (15 mM) affected neither maximal activity nor Ca2+-sensitivity. ADP, howev er, reduced maximal myosin-ATPase activity, and more so in trout than in turtle. In conclusion, the ''slow''-type myosin, the low sensitivit y of acidification and ADP, and the high creatine kinase/myosin-ATPase ratio in turtle relative to trout accord with the well-known ability of turtle myocardium to work during hypoxia. However, the difference i n living temperature between turtle and trout obscures the situation ( e.g. inclusion of rat data suggests that the creatine kinase/myosin-AT Pase ratio is related to temperature). (C) 1997 Wiley-Liss, Inc.