Comparison of calcium release from sarcoplasmic reticulum of slow and fast twitch muscles

Summary

The mechanism of Ca²⁺ release from the sarcoplasmic reticulum (SR) of slow and fast twitch muscle was compared by examining biochemical characteristics, ryanodine binding. Ca²⁺ efflux, and single Ca²⁺ channel properties of SR vesicles. Although many features of the Ca²⁺ release channel were comparable, two functional assays revealed remarkable differences. The comparable properties include: a high molecular weight protein from both types of muscle was immunologically equivalent, and Scatchard analysis of [³H]ryanodine binding to SR showed that theK _d was similar for slow and fast SR. In the flux assay the sensitivity to the agonists caffeine, doxorubicin, and Ca²⁺ and the antagonists Mg²⁺, ruthenium red, and tetracaine differed only slightly. When SR vesicles were incorporated into lipid bilayers, the single-channel conductances of the Ca²⁺ release channels were indistinguishable. The distinguishing properties are: When Ca²⁺ release from passively⁴⁵Ca²⁺-loaded SR were monitored by rapid filtration, the initial rates of Ca²⁺ release induced by Ca²⁺ and caffeine were three times lower in slow SR than in fast SR. Similarly, when Ca²⁺ release channels were incorporated into lipid bilayers, the open probability of the slow SR channel was markedly less, mainly due to a longer mean closed time. Our results indicate that slow and fast muscle have ryanodine receptors that are biochemically analogous, yet functional differences in the Ca²⁺ release channel may contribute to the different time to peak contraction observed in intact slow and fast muscles.