Sarcolemmal Alterations in Unloaded Rat Heart After Heterotopic Transplantation

Following heterotopic transplantation, the rat heart undergoes atrophy and exhibits delayed cardiac relaxation without any changes in contraction and systolic Ca transients. Furthermore, the sarcoplasmic reticular Ca uptake and release activities were reduced and Ca influx through L-type Ca channels was increased in the atrophied heart. Since Ca movements at sarcolemma are intimately involved in the regulation of intracellular Ca concentration, the present study was undertaken to test if sarcolemma plays any role to maintain cardiac function in the atrophied heart.The characteristics of sarcolemmal Ca pump and Na -Ca exchange activities were examined in 8 weeks heterotopically isotransplanted rat hearts which did not support hemodynamic load and underwent atrophy. Sarcolemmal ATP (adenosine triphosphate)-dependent Ca uptake and Ca -stimulated ATPase (adenosine triphosphatase) activities were increased without any changes in Na -K ATPase activities in the transplanted hearts. Although no alterations in the Na -dependent Ca uptake were evident, Na -induced Ca release was increased in the transplanted heart sarcolemmal vesicles. The increase in Na -induced Ca release was observed at different times of incubation as well as at 5, 20, and 40 mM Na . The sarcolemma from transplanted hearts also showed higher contents of phosphatidic acid, sphingomyelin, and cholesterol.These results indicate that increases in the sarcolemmal, Ca transport activities in unloaded heart may provide an insight into adaptive mechanism to maintain normal contractile behavior of the atrophic heart.