Mechanical activation in slow and twitch skeletal muscle fibres of the frog.

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Abstract
Slow and twitch muscle fibers of the frog were studied with a two-micro-electrode point voltage-clamp method. Slow fibers were identified in pyriformis and cruralis muscles by their appearance in the light microscopy, electrical characteristics and rate of sarcomere shortening or of tension development. The relation between the amplitude and duration of threshold depolarizing pulses was determined in sartorius twitch and pyriformis slow fibers. Strength-duration relations for contractile activation are very similar in the 2 fiber types. The effect of a brief subthreshold pulse on the threshold voltage level decays with a half-time of 1-2 ms at 9.degree. C in both slow and twitch fibers. This fast decay, throught to reflect voltage-dependent deactivation of Ca2+ release following repolarization is followed by a slower decay of greatly different rates in the 2 fiber types. The slower components of decay might reflect the rate of background Ca2+ removal by the sarcoplasmic reticulum. Reducing external Ca2+ levels to about 0.1 .mu.M with 2.5 .mu.M-EGTA [ethylene glycyl-bis (.beta.-aminoethyl ether) N,N,N'',N''-tetraacetic acid] has no effect on the shapes of strength-duration curves for both slow and twitch fibers, suggesting that activator Ca2+ in both fiber types originates entirely from intracellular stores. Tonic contractions were studied using voltage-clamped short cruralis slow fibers at 20.degree. C. Reducing external Ca2+ to about 0.1 .mu.M had no effect on the steepness of the steady-state tension-voltage relation or on the ability of slow fibers to maintain maximal tension during long (200 s) depolarizations to membrane potentials of up to +50 mV. Functional similarities in activation kinetics of slow and twitch fibers are discussed in relation to the sensing of tubular membrane potential by the sarcoplasmic reticulum, to Ca2+ release from it and to possible mechanisms involved in these processes. Processes leading to the rapid turning on and off of Ca2+ release in response to changes in tubular membrane potential are probably similar in slow and twitch fibers. The apparent lack of voltage- and time-dependent inactivation of Ca2+ increase in slow fiber points to a major difference in the 2 types of muscle.