Effects of thermal acclimation on the relaxation system of crucian carp white myotomal muscle

Many cyprinid fish are able to compensate for a decrease in ambient tempera ture by process of physiological adaptation in the function of muscles. In the winter habitat of crucian carp (Carassius carassius L.), low temperatur e is associated with simultaneous oxygen shortage. Because of the oxygen de privation, there is probably little space for compensatory adaptation becau se positive thermal compensation would increase energy demand and accelerat e depletion of glycogen reserves. Thus, we assumed that the crucian carp, u nlike many other cyprinid fish, would not show positive thermal compensatio n but either no compensation or inverse compensation in muscle function. To test this hypothesis in the relaxation system of skeletal muscles, we dete rmined the parvalbumin content and the activity of sarcoplasmic reticular ( SR) Ca-ATPase in white myotomal muscle of winter- and summer-acclimated cru cian carp. In the laboratory, the winter fish were kept at 2 degrees C and the summer fish at 22 degrees C for a minimum of 3 weeks before the experim ents. The specific activity of SR Ca-ATPase at low experimental temperature (2 degrees C) was similar in summer- and winter-acclimated fish (0.26 +/- 0.04 vs. 0.25 +/- 0.04 mM/mg/min; P > 0.05). Because of the bigger Q(10) of cold-acclimated carp, the enzyme activity at 30 degrees C was higher in co ld-acclimated winter fish than in warm-acclimated summer fish (7.42 +/- 0.9 0 vs. 5.18 +/- 0.53 mM/mg/min; P < 0.05). In contrast, the yield of SR prot ein was 70% higher in summer than winter fish (0.315 +/- 0.045 vs. 0.187 +/ - 0.017 mg/g; P < 0.001). Because of these opposing changes, total Ca-ATPas e activity of SR (per gram muscle weight) remained relatively constant. Sim ilarly, the parvalbumin content of the myotomal muscle was not different be tween summer (4.09 +/- 0.95 mg/g) and winter (3.70 +/- 0.60 mg/g) fish. Alt hough there were no seasonal changes in the total relaxing system of the cr ucian carp white myotomal muscle, the same activity of SR Ca-ATPase in wint er fish was obtained with less amount of SR pump protein, owing to the incr eased catalytic activity of the enzyme. The higher catalytic activity of wi nter fish SR Ca-ATPase might be caused by differences in fatty acid composi tion noted in membrane lipids; i.e., fewer saturated fatty acids and more n -6 polyunsaturated fatty acids (PUFAs), at the expense of n-3 PUFAs, were p resent in the SR of cold-acclimated winter fish. Temperature-induced change s in enzyme protein, however, cannot be excluded. Thus, the present results indicate the absence of positive thermal compensation in the relaxing syst em of crucian carp white muscle. It seems, however, that lipid composition of SR membranes and temperature dependence of SR Ca-ATPase are altered by s easonal acclimation. (C) 1999 Wiley-Liss, Inc.