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 Ca2+-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 As peptide [A(S)(D-R18)] with an extended helical Structure. The peptides added to the cytoplasmic side of channels in lipid bilayers at greater than or equal to 10 nM activated channels when the cytoplasmic [Ca2+] was 100 nM. but either inhibited on did not affect channel activity when the cytoplasmic [Ca2+] was 10 or 100 muM. Both activation and inhibition were independent of bilayer potential. Activation by A(S), but not by A(C) on A(S)(D-R18), was reduced at peptide concentrations > 1 muM in a voltage-dependent manner (at + 40 mV). In control experiments, channels were not activated by the scrambled A(S) sequence (A(S)S) or skeletal II-III loop peptide (NB). Resting Ca2+ release from cardiac sarcoplasmic reticulum was not altered by peptide A(S) but Ca2+ induced Ca2+ release was depressed. Resting and Ca2+-induced Ca2+ release were enhanced by both the native and modified As peptides. NMR revealed (i) that the structure of peptide A(C)(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.