Modulation of frog skeletal muscle Ca2+ release channel gating by anion channel blockers

Effects of niflumic acid and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) on frog skeletal muscle ryanodine receptors have been studied by incorporating sarcoplasmic reticulum vesicles into planar lipid bilayers. Niflumic acid increased the mean open probability (Po) at 10 microM and decreased Po at 100 microM with no change in open time constants, unitary conductance, and reversal potential. The Po was augmented by DIDS at 5-200 microM without affecting either unitary conductance or reversal potential. DIDS induced a new third open time constant, probably contributing to a long-lived open state. Channels modified by niflumic acid or DIDS still responded to Ca2+ release channel modulators. These results provide evidence that niflumic acid and DIDS modify the gating mechanism of ryanodine receptors without affecting binding sites to the modulators and the physical pathway of the conducting pore. p-Chloromercuriphenyl sulfonic acid (pCMPS) transiently increased the Po. The channel modified by DIDS responded to pCMPS, whereas that by ryanodine did not. The long open state of the channel induced by DIDS is produced by a quite different mechanism(s) from that by ryanodine. Contrary to cardiac ryanodine receptors, Po of skeletal muscle channels was independent of voltage after DIDS modification.