Effects of dihydropyridine receptor II-III loop peptides on Ca(2+) release in skinned skeletal muscle fibers.
In skeletal muscle fibers, the intracellular loop between domains II and III of the alpha(1)-subunit of the dihydropyridine receptor (DHPR) may directly activate the adjacent Ca(2+) release channel in the sarcoplasmic reticulum. We examined the effects of synthetic peptide segments of this loop on Ca(2+) release in mechanically skinned skeletal muscle fibers with functional excitation-contraction coupling. In rat fibers at physiological Mg(2+) concentration ([Mg(2+)]; 1 mM), a 20-residue skeletal muscle DHPR peptide [A(S(20)); Thr(671)-Leu(690); 30 microM], shown previously to induce Ca(2+) release in a triad preparation, caused only small spontaneous force responses in approximately 40% of fibers, although it potentiated responses to depolarization and caffeine in all fibers. The COOH-terminal half of A(S(20)) [A(S(10))] induced much larger spontaneous responses but also caused substantial inhibition of Ca(2+) release to both depolarization and caffeine. Both peptides induced or potentiated Ca(2+) release even when the voltage sensors were inactivated, indicating direct action on the Ca(2+) release channels. The corresponding 20-residue cardiac DHPR peptide [A(C(20)); Thr(793)-Ala(812)] was ineffective, but its COOH-terminal half [A(C(10))] had effects similar to A(S(20)). In the presence of lower [Mg(2+)] (0.2 mM), exposure to either A(S(20)) or A(C(10)) (30 microM) induced substantial Ca(2+) release. Peptide C(S) (100 microM), a loop segment reported to inhibit Ca(2+) release in triads, caused partial inhibition of depolarization-induced Ca(2+) release. In toad fibers, each of the A peptides had effects similar to or greater than those in rat fibers. These findings suggest that the A and C regions of the skeletal DHPR II-III loop may have important roles in vivo.
Lamb
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School of Zoology, La Trobe University, Bundoora, Victoria 3083, Australia. zoogl@zoo.latrobe.edu.au
El-Hayek
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Ikemoto
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Stephenson
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D000595:Amino Acid Sequence; D000818:Animals; D002021:Buffers; D002024:Bufo marinus; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D020746:Calcium Channels, L-Type / Q000378:metabolism* / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D066298:In Vitro Techniques; D008274:Magnesium / Q000378:metabolism / Q000494:pharmacology; D008969:Molecular Sequence Data; D009119:Muscle Contraction / Q000187:drug effects; D018485:Muscle Fibers, Skeletal / Q000187:drug effects / Q000378:metabolism*; D018482:Muscle, Skeletal / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D010446:Peptide Fragments / Q000378:metabolism* / Q000494:pharmacology; D010726:Phosphodiesterase Inhibitors / Q000494:pharmacology; D011188:Potassium / Q000378:metabolism; D051381:Rats; D020318:Rats, Long-Evans; D012519:Sarcoplasmic Reticulum / Q000378:metabolism
D002021:Buffers; D020746:Calcium Channels, L-Type; D010446:Peptide Fragments; D010726:Phosphodiesterase Inhibitors; D002110:Caffeine; D008274:Magnesium; D011188:Potassium; D002118:Calcium