Characterization of myocardial Na(+)-Ca2+ exchange in rainbow trout

Abstract

This study compared Na(+)-Ca2+ exchange from the hearts of rainbow trout with that from canines. In several respects, trout cardiac Na(+)-Ca2+ exchange is functionally similar to that from dogs and other mammals. Trout cardiac Na(+)-Ca2+ exchange is stimulated approximately 200% after 30-min incubation with 10 micrograms/ml chymotrypsin at 21 degrees C, similar to mammals. On the other hand, both the temperature and pH dependencies are strikingly different between the trout and canine myocardial Na(+)-Ca2+ exchange. While canine heart Na(+)-Ca2+ exchange exhibits a Q10 of greater than 2 (similar to values observed in other mammals), that from trout is relatively insensitive to temperature with a Q10 of approximately 1.2. The absolute rates of Na(+)-Ca2+ exchange in trout heart are four- to sixfold higher than that in mammals when measured at 7 degrees C. Furthermore, the temperature insensitivity of trout myocardial Na(+)-Ca2+ exchange is retained when the exchanger is reconstituted into an asolectin bilayer, suggesting that this property is intrinsic to the protein and not dependent on species differences in lipid bilayer composition. Trout Na(+)-Ca2+ exchange is not markedly stimulated by alkaline pH, in contrast to mammals, and this characteristic is also maintained after reconstitution. Western blots of trout cardiac sarcolemma run on 7.5% sodium dodecyl sulfate-polyacrylamide gel electrophoresis react with antibodies raised against the canine Na(+)-Ca2+ exchanger with a similar pattern of bands (70, 120, and 160 kDa). Furthermore, a cDNA probe from canine Na(+)-Ca2+ exchanger hybridizes on Northern blots of trout heart mRNA to a 7-kb band, similar to that in mammals. Thus, while important functional differences in Na(+)-Ca2+ exchange exist between trout and mammalian hearts, the molecular basis is not yet known.