Effect of low cytoplasmic [ATP] on excitation-contraction coupling in fast-twitch muscle fibres of the rat.
In this study we investigated the roles of cytoplasmic ATP as both an energy source and a regulatory molecule in various steps of the excitation-contraction (E-C) coupling process in fast-twitch skeletal muscle fibres of the rat. Using mechanically skinned fibres with functional E-C coupling, it was possible to independently alter cytoplasmic [ATP] and free [Mg2+]. Electrical field stimulation was used to elicit action potentials (APs) within the sealed transverse tubular (T-) system, producing either twitch or tetanic (50 Hz) force responses. Measurements were also made of the amount of Ca2+ released by an AP in different cytoplasmic conditions. The rate of force development and relaxation of the contractile apparatus was measured using rapid step changes in [Ca2+]. Twitch force decreased substantially (approximately 30%) at 2 mm ATP compared to the level at 8 mm ATP, whereas peak tetanic force only declined by approximately 10% at 0.5 mm ATP. The rate of force development of the twitch and tetanus was slowed only slightly at [ATP] > or = 0.5 mm, but was slowed greatly (> 6-fold) at 0.1 mm ATP, the latter being due primarily to slowing of force development by the contractile apparatus. AP-induced Ca2+ release was decreased by approximately 10 and 20% at 1 and 0.5 mm ATP, respectively, and by approximately 40% by raising the [Mg2+] to 3 mm. Adenosine inhibited Ca2+ release and twitch responses in a manner consistent with its action as a competitive weak agonist for the ATP regulatory site on the ryanodine receptor (RyR). These findings show that (a) ATP is a limiting factor for normal voltage-sensor activation of the RyRs, and (b) large reductions in cytoplasmic [ATP], and concomitant elevation of [Mg2+], substantially inhibit E-C coupling and possibly contribute to muscle fatigue in fast-twitch fibres in some circumstances.
Department of Zoology, La Trobe University, Victoria, 3086, Australia.
Travis L
TL
Dutka
Graham D
GD
Lamb
D000200:Action Potentials / Q000502:physiology; D000241:Adenosine / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000008:administration & dosage / Q000378:metabolism* / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation; D066298:In Vitro Techniques; D008274:Magnesium / Q000378:metabolism; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology*; D018656:Muscle Fibers, Fast-Twitch / Q000378:metabolism / Q000502:physiology*; D018482:Muscle, Skeletal / Q000378:metabolism / Q000502:physiology*; D009994:Osmolar Concentration; D051381:Rats; D020318:Rats, Long-Evans
D000255:Adenosine Triphosphate; D008274:Magnesium; D000241:Adenosine; D002118:Calcium