Calcium uptake and ATPase activity of microsomes from heterotopically transplanted rabbit heart.

Heterotopic cardiac transplantation was performed in rabbits. The calcium uptake and ATPase activities of microsomal fraction were studied in three groups (group I, normal controls; group II, early transplants; group III, late transplants) during severe rejection as characterized histologically and electrocardiographically. Group III showed a significant decrease in calcium uptake but no difference in ATPase activity compared with groups I or II. The results demonstrate that in the rejecting transplanted hearts there is an uncoupling of microsomal calcium uptake from this ATP splitting function.