G-protein dependent potentiation of calcium release from sarcoplasmic reticulum of skeletal muscle

Abstract Skinned fibre experiments were conducted to determine if guanine nucleotide-binding proteins play a role in excitaion-contraction coupling of skeletal muscle. By itself, the GTP-γS, a non hydrolysable GTP analogue was unable to induce calcium release from the sarcoplasmic reticulum, even at concentrations as high as 500 μM. However, calcium- or caffeine-induced calcium releases were enhanced by GTP-γS in micromolar concentrations. This response was blocked by GDP-βS or Pertussis toxin. 32 P-ADP-ribosylation catalysed by Pertussis toxin, radiolabelled G-protein α subunits in the range of 40 kDa on membrane subcellular fractions of rat skeletal muscle. Using Western blot with antibodies raised against the bovine transducin, G-proteins were identified in frog and rat skeletal muscle subcellular fractions. In most of the muscle fractions (plasma membrane, T-tubules, triads, sarcoplasmic reticulum), the anti-β subunit antibodies recognized a 36 kDa protein which comigrated with transducin β subunit. It appears therefore that some of the G-proteins identified by ADP-ribosylation or immunostaining in several subcellular fractions from skeletal muscle, are implicated in the modulation of calcium release from sarcoplasmic reticulum. These results suggest that a Pertussis toxin sensitive G-protein is present at the loci of E–C coupling, and that it serves to regulate the calcum release.