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) (GTPγS) 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 μM GTPγS, a lower concentration of 50 μM GTPγS was used to stimulate G proteins in the peeled fiber system. Under conditions for steady state (resting) polarization of TT, GTPγS rarely elicited SR Ca²⁺ release. When the TTs are in steady- state (resting) depolarization, 50 μM GTPγS or GTP elicited SR Ca²⁺ release and associated tension transients in only 69% of fibers tested. In contrast, GTPγS 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.