GTP gamma S causes contraction of skinned frog skeletal muscle via the DHP-sensitive Ca2+ channels of sealed T-tubules.
We have investigated the involvement of G-proteins in excitation-contraction coupling of fast-twitch skeletal muscle, using a fibre preparation designed to retain intact T-tubules and sarcoplasmic reticulum. The nonhydrolysable analogue of guanosine triphosphate, GTP gamma S (50-500 microM) caused a strong, transient isometric contraction in this preparation. Reduction of ethylene-bis(oxonitrilo)tetraacete (EGTA) in the sealed T-tubules from 5 mM to 0.1 mM lowered the threshold to GTP gamma S and removal of sodium reversibly raised it. The dihydropyridine (DHP) calcium channel antagonists nicardipine and nifedipine allowed a first contraction and then blocked subsequent GTP gamma S action. The phenylalkylamine methoxyverapamil (D-600) did likewise, reversibly, at 10 degrees C. The guanosine diphosphate analogue, GDP beta S, and procaine reversibly blocked the action of GTP gamma S; pertussis toxin also blocked it. Photolytic release of 40-100 microM GTP gamma S within 0.1 s from S-caged GTP gamma S caused contraction after a latent period of 0.3-20 s. We conclude that GTP gamma S can activate contraction in frog skeletal muscle via a route requiring both the integrity of the T-tubular DHP-sensitive calcium channel (DHPr) and the presence of sodium in the sealed T-tubules. We propose that in this preparation GTP gamma S activates a G-protein, which in turn activates the DHPr as a calcium channel and releases stored calcium from within the sealed T-tubule. Implications of these results for the excitation-contraction coupling mechanism in skeletal muscle are discussed.
Somasundaram
B
B
AFRC Institute of Animal Physiology, Babraham, Cambridge, UK.
Tregear
R T
RT
Trentham
D R
DR
D000818:Animals; D001665:Binding Sites; D001667:Binding, Competitive; D002110:Caffeine / Q000494:pharmacology; D015220:Calcium Channels / Q000378:metabolism*; D004095:Dihydropyridines / Q000037:antagonists & inhibitors / Q000494:pharmacology; D004533:Egtazic Acid / Q000494:pharmacology; D019204:GTP-Binding Proteins / Q000031:analogs & derivatives / Q000378:metabolism / Q000502:physiology*; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D007700:Kinetics; D008870:Microtubules / Q000378:metabolism*; D009119:Muscle Contraction / Q000502:physiology*; D009529:Nicardipine / Q000494:pharmacology; D037342:Pertussis Toxin; D010782:Photolysis; D011894:Rana pipiens; D011896:Rana temporaria; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
D015220:Calcium Channels; D004095:Dihydropyridines; D010566:Virulence Factors, Bordetella; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D002110:Caffeine; D004533:Egtazic Acid; C038806:1,4-dihydropyridine; D009529:Nicardipine; D037342:Pertussis Toxin; D019204:GTP-Binding Proteins