Two domains in dihydropyridine receptor activate the skeletal muscle Ca2+ release channel

The II-III cytoplasmic loop of the skeletal muscle dihydropyridine receptor (DHPR) alpha (1)-subunit is essential for skeletal-type excitation-contraction coupling. Single channel and [H-3]ryanodine binding studies with a full-length recombinant peptide (p(666-791)) confirmed that this region specifically activates skeletal muscle Ca2+ release channels (CRCs). However, attempts to identify shorter domains of the II-III loop specific for skeletal CRC activation have yielded contradictory results. We assessed the specificity of the interaction of five truncated II-III loop peptides by comparing their effects on skeletal and cardiac CRCs in lipid bilayer experiments; p(671-680) and p(720-765) specifically activated the submaximally Ca2+-activatecl skeletal CRC in experiments using both mono and divalent ions as current carriers. A third peptide, p(671-690) showed a bimodal activation/inactivation behavior indicating a high-affinity activating and low-affinity inactivating binding site. Two other peptides (p(681-690) and p(681-685)) that contained an RKRRK-motif and have previously been suggested in in vitro studies to be important for skeletal-type E-C coupling, failed to specifically stimulate skeletal CRCs. Noteworthy, p(671-690), p(681-690) and p(681-685) induced similar subconductances and long-lasting channel closings in skeletal and cardiac CRCs, indicating that these peptides interact in an isoform-independent manner with the CRCs.