Ca2+-ATPase activity and Ca2+ uptake by sarcoplasmic reticulum in fish heart: effects of thermal acclimation.

This study was designed to compare the activities of sarcoplasmic (SR) Ca2+-ATPase and Ca2+ uptake in fish and mammalian hearts and to determine whether thermal acclimation has any effect on the function of the cardiac SR in fish. To this end, we measured thapsigargin-sensitive Ca2+-ATPase activity and thapsigargin-inhibitable Ca2+ uptake velocity in crude cardiac homogenates of newborn and adult rats and of two teleost fish (crucian carp and rainbow trout) acclimated to low (4 degrees C) and high (17 degrees C and 24 degrees C for trout and carp, respectively) ambient temperatures. The TG-sensitive Ca2+-ATPase activity was highest in adult rat, and the corresponding activities of cold-acclimated trout, warm-acclimated trout, warm-acclimated carp, cold-acclimated carp and newborn rat were 76, 58, 43, 28 and 23 %, respectively, of that of the adult rat at 25 degrees C. SR Ca2+ uptake velocity, measured using Fura-2 at room temperature (approximately 22 degrees C), was highest in cold-acclimated trout, and the values for adult rat, warm-acclimated trout, newborn rat, warm-acclimated carp and cold-acclimated carp were 93, 56, 24, 21 and 14 % of the uptake velocity of cold-acclimated trout, respectively. When corrected to the body temperature of the animal, the relative rates of SR Ca2+ uptake were 100, 26, 19, 18, 11 and 2 % for adult rat, newborn rat, cold-acclimated trout, warm-acclimated trout, warm-acclimated carp and cold-acclimated carp, respectively. These findings show that SR Ca2+ uptake is slower in fish than in mammalian hearts and that marked species-specific differences exist among teleost fish in this respect. Furthermore, acclimation to cold increases the Ca2+ uptake rate of trout cardiac SR (complete thermal compensation) but decreases the SR Ca2+ uptake rate of crucian carp heart. This difference in acclimation response probably reflects the different activity patterns of the two species in their natural habitat during the cold season.