GTPγcauses contraction of skinned frog skeletal muscle via the DHP-sensitive Ca2+channels of sealed T-tubules
✍ Scribed by B. Somasundaram; R. T. Tregear; D. R. Trentham
- Publisher
- Springer
- Year
- 1991
- Tongue
- English
- Weight
- 822 KB
- Volume
- 418
- Category
- Article
- ISSN
- 0031-6768
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✦ Synopsis
We have investigated the involvement of Gproteins 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~S (50-500 gM) caused a strong, transient isometric contraction in this preparation. Reduction of ethylenebis(oxonitrilo)tetraacetete (EGTA) in the sealed T-tubules from 5 mM to 0.1 mM lowered the threshold to GTP~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~S action. The phenylalkylamine methoxyverapamil (D-600) did likewise, reversibly, at 10 ~ C. The guanosine diphosphate analogue, GDP,S, and procaine reversibly blocked the action of GTP~S; pertussis toxin also blocked it. Photolytic release of40-100 gM GTP~S within 0.1 s from S-caged GTP~S caused contraction after a latent period of 0.3-20 s. We conclude that GTP~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~S activates a Gprotein, 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.