Effects of dihydropyridine receptor II-III loop peptides on Ca(2+) release in skinned skeletal muscle fibers

Inskeletal muscle fibers, the intracellular loop between domains II andIII of the ₁-subunit of the dihydropyridine receptor (DHPR) may directly activate the adjacent Ca²⁺ releasechannel in the sarcoplasmic reticulum. We examined the effects ofsynthetic peptide segments of this loop on Ca²⁺ release inmechanically skinned skeletal muscle fibers with functional excitation-contraction coupling. In rat fibers at physiological Mg²⁺ concentration ([Mg²⁺]; 1 mM), a20-residue skeletal muscle DHPR peptide[A_S(20);Thr⁶⁷¹-Leu⁶⁹⁰; 30 µM], shown previously toinduce Ca²⁺ release in a triad preparation, caused onlysmall spontaneous force responses in ~40% of fibers, although itpotentiated responses to depolarization and caffeine in all fibers. TheCOOH-terminal half of A_S(20)[A_S(10)] induced much larger spontaneousresponses but also caused substantial inhibition of Ca²⁺release to both depolarization and caffeine. Both peptides induced orpotentiated Ca²⁺ release even when the voltage sensors wereinactivated, indicating direct action on the Ca²⁺ releasechannels. The corresponding 20-residue cardiac DHPR peptide [A_C(20);Thr⁷⁹³-Ala⁸¹²] was ineffective, but itsCOOH-terminal half [A_C(10)] had effects similar to A_S(20). In the presence of lower[Mg²⁺] (0.2 mM), exposure to eitherA_S(20) or A_C(10) (30 µM) induced substantial Ca²⁺ release. PeptideC_S (100 µM), a loop segment reported to inhibit Ca²⁺ release in triads, caused partial inhibition ofdepolarization-induced Ca²⁺ release. In toad fibers, eachof the A peptides had effects similar to or greater than those in ratfibers. These findings suggest that the A and C regions of the skeletalDHPR II-III loop may have important roles in vivo.