Ryanodine as a probe for the functional state of the skeletal muscle sarcoplasmic reticulum calcium release channel

In this paper, we study the modulation of the rabbit fast twitch skeletal muscle calcium release channel by assaying the kinetics of [3H]ryanodine binding, 45Ca2+ flux, and single-channel activity. The effects of modulators of the Ca2+ release channel (confirmed here with both flux and single-channel data) were examined for effects on [3H]ryanodine binding to terminal cisternae vesicles. We find that activators of the release channel, such as adenine nucleotides (1 mM) and caffeine (1 mM), enhance the rate of association of [3H]ryanodine, whereas inhibitors, such as Mg2+ (1 mM) and ruthenium red (100 nM), decrease the rate of association. High concentrations of either ryanodine or ruthenium red, which close the channel, slow the dissociation of [3H]ryanodine, suggesting that at these concentrations the inhibitory effects of both ryanodine and ruthenium red occur as the result of binding at a site distinct from but interacting cooperatively with the high affinity site. Our data are consistent with a model in which the high affinity ryanodine binding site is within a conformationally sensitive area of the channel, such that conditions that open the channel (ATP, caffeine, etc.) enhance the rate at which [3H]ryanodine reaches its binding site and other conditions that close the channel (the binding of ryanodine and ruthenium red to a low affinity site) slow the dissociation of [3H]ryanodine from the high affinity site. Some conditions that inhibit channel activity (high concentrations of Mg2+ and Ca2+) slow association but do not affect dissociation of bound [3H]ryanodine, suggesting a completely different state of the channel from that which is inactive in the presence of high concentrations of ryanodine or ruthenium red. In summary, the functional state of the fast twitch skeletal muscle calcium release channel can be characterized by the changes in the kinetics of [3H]ryanodine binding. Different modulators (activators/inhibitors) affect different aspects of ryanodine binding (association/dissociation).