Peptide fragments of the dihydropyridine receptor can modulate cardiac ryanodine receptor channel activity and sarcoplasmic reticulum Ca²⁺ release

We show that peptide fragments of the dihydropyridine receptor II-III loop alter cardiac RyR (ryanodine receptor) channel activity in a cytoplasmic Ca ²⁺-dependent manner. The peptides were A_c (Thr-793-Ala-812 of the cardiac dihydropyridine receptor), A_s (Thr-671-Leu-690 of the skeletal dihydropyridine receptor), and a modified A_s peptide [A_s(D-R18)], with an extended helical structure. The peptides added to the cytoplasmic side of channels in lipid bilayers at ≥ 10 nM activated channels when the cytoplasmic [Ca²⁺] was 100 nM, but either inhibited or did not affect channel activity when the cytoplasmic [Ca²⁺] was 10 or 100 μM. Both activation and inhibition were independent of bilayer potential. Activation by A_s, but not by A_c or A _S(D-R18), was reduced at peptide concentrations > 1 μM in a voltage-dependent manner (at + 40 mV). In control experiments, channels were not activated by the scrambled A_s sequence (A_SS) or skeletal II-III loop peptide (NB). Resting Ca²⁺ release from cardiac sarcoplasmic reticulum was not altered by peptide A_c, but Ca ²⁺-induced Ca²⁺ release was depressed. Resting and Ca ²⁺-induced Ca²⁺ release were enhanced by both the native and modified A_s peptides. NMR revealed (i) that the structure of peptide A_S(D-R18) is not influenced by [Ca²⁺] and (ii) that peptide A_c adopts a helical structure, particularly in the region containing positively charged residues. This is the first report of specific functional interactions between dihydropyridine receptor A region peptides and cardiac RyR ion channels in lipid bilayers.