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-contrac tion coupling. Single channel and [H-3]ryanodine binding studies with a ful l-length recombinant peptide (p(666-791)) confirmed that this region specif ically activates skeletal muscle Ca2+ release channels (CRCs). However, att empts to identify shorter domains of the II-III loop specific for skeletal CRC activation have yielded contradictory results. We assessed the specific ity 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+-activa tecl skeletal CRC in experiments using both mono and divalent ions as curre nt carriers. A third peptide, p(671-690) showed a bimodal activation/inacti vation behavior indicating a high-affinity activating and low-affinity inac tivating 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 stu dies 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 skele tal and cardiac CRCs, indicating that these peptides interact in an isoform -independent manner with the CRCs.