Effects of dihydropyridine receptor II-III loop peptides on Ca2+ release in skinned skeletal muscle fibers

In skeletal muscle fibers, the intracellular loop between domains II and II I of the alpha(1)-subunit of the dihydropyridine receptor (DHPR) may direct ly activate the adjacent Ca2+ release channel in the sarcoplasmic reticulum . We examined the effects of synthetic peptide segments of this loop on Ca2 + release in mechanically skinned skeletal muscle fibers with functional ex citation-contraction coupling. In rat fibers at physiological Mg2+ concentr ation ([Mg2+]; 1 mM), a 20-residue skeletal muscle DHPR peptide [A(S(20)); Thr(671)-Leu(690); 30 mu M], shown previously to induce Ca2+ release in a t riad preparation, caused only small spontaneous force responses in similar to 40% of fibers, although it potentiated responses to depolarization and c affeine in all fibers. The COOH-terminal half of A(S(20)) [A(S(10))] induce d much larger spontaneous responses but also caused substantial inhibition of Ca2+ release to both depolarization and caffeine. Both peptides induced or potentiated Ca2+ release even when the voltage sensors were inactivated, indicating direct action on the Ca2+ release channels. The corresponding 2 0-residue cardiac DHPR peptide [A(C(20)); Thr(793)-Ala(812)] was ineffectiv e, but its COOH-terminal half [A(C(10))] had effects similar to A(S(20)). I n the presence of lower [Mg2+] (0.2 mM), exposure to either A(S(20)) or A(C (10)) (30 mu M) induced substantial Ca2+ release. Peptide CS (100 mu M), a loop segment reported to inhibit Ca2+ release in triads, caused partial inh ibition of depolarization-induced Ca2+ release. In toad fibers, each of the A peptides had effects similar to or greater than those in rat fibers. The se findings suggest that the A and C regions of the skeletal DHPR II-III lo op may have important roles in vivo.