G protein effects on Ca2+ release and excitation-contraction coupling in skeletal muscle fibers.

Skeletal muscle excitation-contraction (EC) coupling may involve secondary mechanisms, such as those involving G proteins. The aim of this study was to identify possible G protein effects on sarcoplasmic reticulum (SR) Ca2+ release, in general, and on voltage-dependent EC coupling, in particular. Effects of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) were studied using a single peeled rabbit skeletal muscle fiber preparation that is capable of releasing SR Ca2+ in response to transverse tubule (TT) depolarization. Because of possible nonspecific and residual effects of 200 microM GTP gamma S, a lower concentration of 50 microM GTP gamma S was used to stimulate G proteins in the peeled fiber system. Under conditions for steady-state (resting) polarization of TT, GTP gamma S rarely elicited SR Ca2+ release. When the TTs are in steady-state (resting) depolarization, 50 microM GTP gamma S or GTP elicited SR Ca2+ release and associated tension transients in only 69% of fibers tested. In contrast GTP gamma S always augmented Ca2+ release during TT depolarization-induced EC coupling. These results indicate the presence of at least two excitatory G proteins for SR Ca2+ release, only one of which is a modulator, albeit nonessential, of peeled fiber EC coupling.