NO donors were found to reduce the rate of Ca 2+ release from isolated skeletal muscle sarcoplasmic reticulum (SR) and the open probability of single ryanodine receptor Ca 2÷ release channels (RyRCs) in planar lipid bilayers, and these effects were prevented by the NO quencher hemoglobin and reversed by 2-mercaptoethanol. Ca 2÷ release assessed in skeletal muscle homogenates was also reduced by NO that was generated in situ from L-arginine by endogenous, nitro-L-arginine methylester-sensitive NO-synthase. The effect of NO on the RyRC might explain NO-induced depression of contractile force in striated muscles and, since both RyRC isoforms and NOS isoenzymes are ubiquitous, may represent a wide-spread feedback mechanism in Ca 2+ signaling; i.e. Ca-dependent activation of NO production and NO-evoked reduction of Ca 2÷ release from intracellular Ca 2÷ stores.

We used the planar lipid bilayer method to study single ryanodine receptor Ca2+ release channels (RyRCs) from fast skeletal muscle of the rabbit. We found that changes in membrane voltage directly induced gating transitions of the RyRC: (i) in the steady state, even at activating Ca2+ concentrations (20 μm), at a constant membrane potential the channels resided in a low open probability (Po) state (inactivated‐, I‐mode), and (ii) upon abrupt changes of voltage, the apparent inactivation of the RyRCs was relieved, resulting in a rapid and transient increase in Po. The magnitude of the Po increase was a function of both the duration and the amplitude of the applied prepulse, but was independent of the channel activity during the prepulse. The voltage‐induced Po increase probably involved major conformational changes of the channel, as it resulted in substantial alterations in the gating pattern of the channels: the voltage change‐induced increase in Po was accompanied by the rapid appearance of two types of channel activity (high (H) and low (L) open probability modes). The response of the RyRC to voltage changes raises the interesting possibility that the activation of RyRC in situ might involve electrical events, i.e. a possible dipole‐dipole coupling between the release channel and the voltage sensor.

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