Is a guanine nucleotide-binding protein involved in excitation-contraction coupling in skeletal muscle?

Plasma membrane depolarization causes skeletal muscle contraction by triggering Ca2+ release from an intracellular membrane network, the sarcoplasmic reticulum. A specialized portion of the sarcoplasmic reticulum, the terminal cisternae, is junctionally associated with sarcolemmal invaginations called the transverse tubules, but the mechanism by which the action potential at the level of the transverse tubules is coupled to Ca2+ release from the terminal cisternae is still mysterious. Here we show that: (i) GTP gamma S, a non-hydrolyzable analog of GTP, elicits isometric force development in skinned muscle fibre; (ii) GTP gamma S is unable to release CA2+ from isolated sarcoplasmic reticulum fractions; (iii) the threshold for tension development is shifted to higher GTP gamma S concentrations by pre-incubation with pertussis toxin. These results suggest that a GTP-binding protein is involved in coupling the action potential of transverse tubules to Ca2+ release from the terminal cisternae.