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‐membrane Ca2+‐ATPase (PMCA) has been studied in rat and rabbit skeletal muscle cells by indirect (calmodulin gel overlays) and direct (Western blotting 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 dihydropyridine receptor, which has been studied by Western blotting with specific antibodies. Both PMCA and the Na+/Ca2+ exchanger had a dual localization, i.e., they were found in the plasma membrane and in the transverse‐tubule fractions of the two main types of skeletal muscles studied. The pump and the exchanger were not diffusely distributed in the transverse‐tubule‐membrane system, but specifically confined to the membrane domain where the dihydropyridine receptor was also localized, i.e., the junctional membrane. Experiments with isoform‐specific antibodies have shown that the pump isoform expressed in skeletal muscle is PMCA 1.