Dihydropyridine binding sites on transverse tubules isolated from triads of rabbit skeletal muscle

Receptors for the dihydropyridine class of Ca2+ channel antagonists are present on transverse tubules isolated by mechanical disruption of skeletal muscle triads. These observations account for the previously reported presence of nitrendipine binding sites in heavy sarcoplasmic reticulum. Nitrendipine receptors were not found in the terminal cisternae after disruption of the triad junctions. The number of sites in junctional T-tubules (27 pmol/mg) is only half that reported for transverse tubules isolated as free vesicles (pmol/mg). The presence of nitrendipine receptors in that region of the transverse tubules held in close apposition to the SR cisternae is consistent with the architectural requirements for the Ca2+ induced Ca2+ release phenomenon to be the mechanism of excitation-contraction coupling.