Ca2+ release from sarcoplasmic reticulum of skinned fast- and slow-twitch muscle fibers.

We have performed a comparative study of Ca2+ release from the sarcoplasmic reticulum (SR) of chemically skinned fibers from rabbit fast- and slow-twitch skeletal muscle. Ca2+ fluxes have been indirectly monitored by following either tension development or the inhibition of net Ca2+-loading rate by a light-scattering method. Several drugs (Ca2+-release modulators) have been used to either trigger or block Ca2+ release. Our results indicate that caffeine, doxorubicin, and ryanodine activate Ca2+ release, whereas ruthenium red blocks Ca2+ release from both fast- and slow-twitch skinned fibers. Caffeine has greater affinity for slow SR, whereas doxorubicin, ruthenium red, and ryanodine have greater affinity for fast SR. Our results indicate that Ca2+-release mechanisms in fast and slow SR are homologous but not identical and that differences in twitch-contraction time might be also related to the inherent properties of the Ca2+-release mechanism.