Summary

The temperature dependence of heart rate and isometric contractile properties of cardiac tissue was determined in thermally acclimated crucian carp (Carassius carassius L.). Fish were adapted for more than 3 weeks to 5 and 15°C during both summer and winter. At all but the lowest experimental temperatures, heart rate (fH), maximal isometric force (F_max) and pumping capacity were higher in summer fish acclimated to 15°C than in winter fish acclimated to 5°C. Except for fH, similar acclimation effects were observed when comparing cold- and warm-acclimated crucian carp in both seasons. In contrast, the kinetics of isometric contraction was not modified either by seasonal acclimatization or by thermal acclimation within a given season: the temperature dependence of time to peak force (TPT) and time to half-relaxation (T_1/2R) was independent of the previous thermal history of the fish. The hearts of winter fish were about 10% smaller than those of summer fish. These results show that acclimation to cold does not induce positive compensatory changes in function or size of crucian carp heart, as happens in many other teleost fishes. Instead, an inverse or noncompensatory acclimation was found. Owing to the inverse thermal acclimation, in winter (at 4°C) the contractile performance of the crucian carp heart is less than one-third of that in summer (at 20°C). The low cardiac activity in cold-acclimated crucian carp seems to be associated with its inactive life style, which is necessary during the winter months to minimize energy consumption in a severely hypoxic environment.

Note: To whom reprint requests should be addressed.