Expression of excitation-contraction coupling proteins during muscle differentiation

The goal of this study was to characterize three major excitation-contraction (E-C) coupling proteins: ryanodine receptor [RyR, the calcium release channel in the sarcoplasmic reticulum (SR)], dihydropyridine receptor (DHPR, the voltage-gated L-type calcium channel in the transverse tubule) and SR Ca2+-ATPase (SERCA, the calcium pump in the SR) in the differentiating primary cultures of rat skeletal and cardiac muscle cells. The expression levels of these proteins were determined by ligand binding assays and/ or immunoblottings along with an examination of the morphological changes in differentiating muscles by phase-contrast microscopy. In the skeletal cells, the expression levels of the E-C coupling proteins generally increased in the course of differentiation with the peak expression at the 9th day of culture. Simultaneous with the increased expression of the proteins, the myoblasts elongated, followed by alignment and fusion of the cells to form multinucleated myotubes. In the cardiac cells, on the contrary, the peak expression levels of DHPR, SERCA and RyR were reached within 2, 4, and 7 d of culture, respectively. Alignment of the cardiac muscle cells and spontaneous contraction occurred as early as several h after plating. These results suggest that expression patterns of E-C coupling proteins are different between skeletal and cardiac muscles, and that DHPR could play an important role in Ca2+ metabolism during the early stages of differentiation.