Regions of the skeletal muscle dihydropyridine receptor critical for excitation–contraction coupling

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Abstract
IT is thought that in skeletal muscle excitation–contraction (EC) coupling, the release of Ca2+ from the sarcoplasmic reticulum is controlled by the dihydropyridine (DHP) receptor in the transverse tubular membrane, where it serves as the voltage sensor1–3. We have shown previously4 that injection of an expression plasmid carrying the skeletal muscle DHP receptor complementary DNA3 restores EC coupling and L-type calcium current that are missing in skeletal muscle myotubes from mutant mice with muscular dysgenesis5–9. This restored coupling resembles normal skeletal muscle EC coupling4, which does not require entry of extracellular Ca2+ (refs 10, 11). By contrast, injection into dysgenic myotubes of an expression plasmid carrying the cardiac DHP receptor cDNA12 produces L-type calcium current and cardiac-type EC coupling13, which does require entry of extracellular Ca2+ (refs 14–16). To identify the regions responsible for this important functional difference between the two structurally similar DHP receptors, we have expressed various chimaeric DHP receptor cDNAs in dysgenic myotubes. The results obtained indicate that the putative cytoplasmic region between repeats II and III of the skeletal muscle DHP receptor3 is an important determinant of skeletal-type EC coupling.