G-PROTEIN EFFECTS ON CA2-CONTRACTION COUPLING IN SKELETAL-MUSCLE FIBERS( RELEASE AND EXCITATION)

Skeletal muscle excitation-contraction (EC) coupling may involve secon dary mechanisms, such as those involving G proteins. The aim of this s tudy pras to identify possible G protein effects on sarcoplasmic retic ulum (SR) Ca2+ release, in general, and on voltage-dependent EC coupli ng, in particular. Effects of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) were studied using a single peeled rabbit skeletal muscle fi ber preparation that is capable of releasing SR Ca2+ in response to tr ansverse tubule (TT) depolarization. Because of possible nonspecific a nd residual effects of 200 mu M GTP gamma S, a lower concentration of 50 mu M GTP gamma S was used to stimulate G proteins in the peeled fib er 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 mu M GTP gamma S or GTP elic ited 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 t he presence of at least two excitatory G proteins for SR Ca2+ release, only one of which is a modulator, albeit nonessential, of peeled fibe r EC coupling.