Positive cooperativity of ryanodine binding to the calcium release channel of sarcoplasmic reticulum from heart and skeletal muscle.

Ryanodine is a specific ligand for the calcium release channel which mediates calcium release in excitation-contraction coupling in muscle. In this study, ryanodine binding in sarcoplasmic reticulum from heart muscle and skeletal muscle is further compared and correlated with function. The new findings include the following: (1) Two types of binding, high affinity (KD1 approximately 5-10 nM) and low affinity (KD2 approximately 3 microM), can now be discerned for the skeletal muscle receptor. KD1 is approximately the same as and KD2 of similar magnitude to that previously reported for heart. (2) The dissociation rates for the high-affinity binding have been directly measured for both heart and skeletal muscle (t1/2 approximately 30-40 min). These rates are more rapid than previously reported (t1/2 approximately 14 h). (3) KD1's obtained from the ratio of the dissociation and association rate constants agree with the dissociation constant measured by equilibrium binding Scatchard analysis. (4) Ryanodine binding to the low-affinity site can be correlated with a decrease in the dissociation rate constant (k-1) of the high-affinity site, and thereby in the apparent dissociation constant (KD1). The inhibition constant (KI) for inhibiting the high-affinity off rate obtained from a double-reciprocal plot of the change in off rate vs [ryanodine] is practically the same in heart (0.66 microM) and skeletal muscle (0.64 microM) and in the range of the KD2. The binding of cold ryanodine to the low-affinity site appears to lock the bound [3H]ryanodine onto the high-affinity site rather than to exchange with it. Thus, in this sense, the ryanodine receptor exhibits "positive cooperativity".(ABSTRACT TRUNCATED AT 250 WORDS)