Ryanodine inhibits the Ca‐dependent K current after depletion of Ca stored in smooth muscle cells of the rabbit ileal longitudinal muscle

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Abstract
1 Effects of ryanodine on the membrane currents were investigated on dispersed smooth muscle cells of rabbit ileal longitudinal layer using voltage and patch clamp procedures. 2 With voltage clamp, membrane depolarization to 0mV from the holding potential of − 60 mV produced an inward Ca current (ICa) which was followed by transient and sustained outward currents (ITO and Iso, respectively). Prolonged depolarization of the membrane produced spontaneous oscillations of the outward current (oscillatory outward current; Ioo) on Iso. 3 Ryanodine (30 μm) modified neither the basal membrane current recorded at the holding potential (-60 mV) nor Iso. Ryanodine inhibited both ITO and Ioo in a concentration-dependent manner (IC50 = 5.5 and 4.5 μm, respectively, measured 12 min after application of ryanodine). These values were much higher than that observed in skeletal muscle for Ca release. 4 The time course of the ryanodine-induced inhibition of Ioo was slow and the inhibition was irreversible. Caffeine (3mm) enhanced the amplitudes of ITO and Ioo in the presence of Ca, and only transiently enhanced Ioo in the absence of Ca. However, following application of 10 μm ryanodine, 3 mM caffeine did not increase Ioo. 5 Ryanodine (3–30 μm) slightly enhanced the amplitude of ICa evoked by depolarization pulses at potentials more negative than 0 mV but not that induced by larger depolarizations (positive potentials). 6 With patch clamp procedure, single Ca-dependent K channel currents were recorded in cell free and cell attached configurations. Application of 30 μm ryanodine transiently enhanced the Ca-dependent K current without any detectable changes in the amplitude of the single channel current recorded in the cell attached condition. In the inside-out membrane patch, when the intracellular membrane side was superfused with 1 μm Ca buffered with 10 mM EGTA, bath application of 10 μm ryanodine had no effect on the Ca-dependent K current. 7 It was concluded that both ITO and Ioo are generated by Ca released from intracellular stores, mainly sarcoplasmic reticulum. Ryanodine appears to open irreversibly the Ca channel in the store and to inhibit the Ca-dependent K channel due to depletion of the stored Ca.

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