Role of superficially membrane-bound calcium on excitation-contraction coupling in frog skeletal muscle.
Single fibers isolated from frog semitendinosus muscle continued to twitch for 5-8 min in calcium-free Ringer solution containing 1 mM ethylene glycol bis (beta-aminoethyl ether)-N, N'-tetraacetic acid (EGTA). Even after twitching was completely abolished, the tension of potassium contracture was not depressed, although the time course was markedly shortened. The resting potential recorded from single fibers 10 min after immersion in EGTA-Ringer solution decreased slightly. These fibers failed to generate action potential, however, in a whole sartorius muscle the self-exchangeable calcium was not significantly reduced by 1mM  EGTA, while it was reduced by about 27% during the potassium contracture induced in the EGTA solution. From these results, it is suggested that the calcium bound on the surface sites of the transverse tubular membrane and the sarcolemma can serve as a trigger for the release of calcium from the sarcoplasmic reticulum and can produce a resting potential and maintain relatively long duration excitability under the condition of extracellular calcium deficiency.
D000200:Action Potentials; D000818:Animals; D002118:Calcium / Q000502:physiology*; D002413:Cations, Divalent; D004558:Electric Stimulation; D004594:Electrophysiology; D066298:In Vitro Techniques; D008839:Microelectrodes; D009119:Muscle Contraction; D009132:Muscles / Q000502:physiology*; D011188:Potassium / Q000502:physiology; D011898:Ranidae
D002413:Cations, Divalent; D011188:Potassium; D002118:Calcium