COLOCALIZATION OF THE DIHYDROPYRIDINE RECEPTOR, THE PLASMA-MEMBRANE CALCIUM-ATPASE ISOFORM-1 AND THE SODIUM CALCIUM EXCHANGER TO THE JUNCTIONAL-MEMBRANE DOMAIN OF TRANSVERSE TUBULES OF RABBIT SKELETAL-MUSCLE/

The subcellular distribution of the calmodulin-stimulated plasma-membr ane Ca2+-ATPase (PMCA) has been studied in rat and rabbit skeletal mus cle cells by indirect (calmodulin gel overlaps) and direct (Western bl otting with specific antibodies) methods. It has also been studied in situ in immunocytochemistry experiments. The distribution of PMCA has been compared with that of the Na+/Ca2+ exchanger and of the dihydropy ridine receptor, which has been studied by Western blotting with speci fic antibodies. Both PMCA and the Na+/Ca2+ exchanger had a dual locali zation, i.e., they were found in the plasma membrane and in the transv erse-tubule fractions of the two main types of skeletal muscles studie d. The pump and the exchanger were not diffusely distributed in the tr ansverse-tubule-membrane system, but specifically confined to the memb rane domain where the dihydropyridine receptor was also localized, i.e ., the junctional membrane. Experiments with isoform-specific antibodi es have shown that the pump isoform expressed in skeletal muscle is PM CA 1.