Excitation-Contraction Coupling Is Not Affected by Scrambled Sequence in Residues 681–690 of the Dihydropyridine Receptor II-III Loop

A peptide corresponding to residues 681–690 of the II-III loop of the skeletal muscle dihydropyridine receptor α 1 subunit (DHPR, α 1S ) has been reported to activate the skeletal muscle ryanodine receptor (RyR1) in vitro . Within this region of α 1S , a cluster of basic residues, Arg 681 –Lys 685 , was previously reported to be indispensable for the activation of RyR1 in microsomal preparations and lipid bilayers. We have used an intact α 1S subunit with scrambled sequence in this region of the II-III loop (α 1S -scr) to test the importance of residues 681–690 and the basic motif for skeletal-type excitation-contraction (EC) coupling and retrograde signaling in vivo . When expressed in dysgenic myotubes (which lack endogenous α 1S ), α 1S -scr restored calcium currents that were indistinguishable, in current density and voltage dependence, from those restored by wild-type α 1S . The scrambled DHPR also rescued skeletal-type EC coupling, as indicated by electrically evoked contractions in the presence of 0.5 m m Cd 2+ and 0.1 m m La 3+ . Furthermore, the release of intracellular Ca 2+ , as assayed by the indicator dye, Fluo-3, had similar kinetics and voltage dependence for α 1S and α 1S -scr. These data suggest that residues 681–690 of the α 1S II-III loop are not essential in muscle cells for normal functioning of the DHPR, including skeletal-type EC coupling and retrograde signaling.