Aequorin-calcium transients in frog twitch muscle fibres.
Intracellular Ca2+ transients, evoked either by action potentials or depolarizing clamp pulses, were studied in frog sartorius muscle fibres injected with aequorin. The time course of the Ca2+ transients became shorter as the temperature was increased. The half rise time and decay time constants showed straight lines between 3 and 30 degrees C in Arrhenius plots, with a Q10 of 2.5 and 2.3 respectively. The potential dependence of the Ca2+ transient was examined under voltage clamp. The peak light amplitude reached a plateau at around +50 mV, suggesting that Ca2+ release continues beyond the potential level at which contraction was saturated. During a prolonged depolarization, the Ca2+ transient gradually declined. The time course of decline became faster when long depolarizing pulses were repeated, or when the temperature was increased. The Q10 for half duration of the Ca2+ transient evoked by prolonged depolarization was 2.2. A Ca2+ transient could be evoked in Ca2+-free Ringer solution containing EGTA. Formamide, which is known to abolish excitation-contraction coupling, also abolished the Ca2+ transient. During maintained depolarization, the time integral of the Ca2+ transient was larger for larger depolarizations, suggesting that the total amount of Ca2+ released was greater for the more intense depolarization. The decline of the Ca2+ transient during maintained depolarization is probably due to inactivation of excitation-contraction coupling rather than the depletion of intracellular Ca2+ stores. These findings support the view that in frog skeletal muscle fibres the increase in intracellular Ca2+, caused by membrane depolarization, is produced by the release of Ca2+ from intracellular stores and that any influx of Ca2+ from the external medium does not contribute appreciably to the aequorin-Ca2+ transient.
D000200:Action Potentials; D000331:Aequorin; D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D005559:Formamides / Q000494:pharmacology; D066298:In Vitro Techniques; D008164:Luminescent Proteins; D008564:Membrane Potentials; D009119:Muscle Contraction / Q000187:drug effects; D009132:Muscles / Q000378:metabolism / Q000502:physiology*; D011896:Rana temporaria; D013696:Temperature; D013997:Time Factors
D005559:Formamides; D008164:Luminescent Proteins; C031066:formamide; D000331:Aequorin; D002118:Calcium