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I(-), CH(3)SO(4) (-), and ClO(4) (-), like other previously studied type A twitch potentiators (Br(-), NO(3) (-), SCN(-), and caffeine), lower the mechanical threshold in K depolarization contractures of frog skeletal muscle. In potentiated twitches, I(-), Br(-), CH(3)SO(4) (-), ClO(4), and SCN, as already reported for NO(3) (-) and caffeine, slightly shorten the latent period (L) and considerably increase the rate of tension development (dP/dt) during the first few milliseconds of the contraction period. Divalent cations (8 mM Ca(2+), 0.5-1.0 mM Zn(2+) and Cd(2+)) raise the mechanical threshold of contractures, and correspondingly affect the twitch by depressing the tension output, increasing L, and decreasing the early dP/dt, thus acting oppositely to the type A potentiators. These various results form a broad, consistent pattern indicating that electromechanical coupling in the twitch is conditioned by a mechanical threshold as it is in the contracture, and suggesting that the lower the threshold, in reference to the raised threshold under the action of the divalent cations, the more effective is a given action potential in activating the twitch as regards especially both its early rate and peak magnitude of tension development. The results suggest that the direct action by which the various agents affect the level of the mechanical threshold involves effects on E-C coupling processes of the T tubular and/or the sarcoplasmic reticulum which control the release of Ca for activating contraction.

1. Reversible changes in twitch and tetanus contractions of isolated frog muscle fibres were produced by varying the duration and strength of a transversely applied d.c. stimulus.2. When the stimulus strength was at least 1.1 times threshold the peak force of the contraction elicited was reduced by stimulus durations longer than 1 msec at 20 degrees C, and by durations longer than 3 msec at 2-5 degrees C.3. As the strength of a prolonged stimulus was progressively increased to 3 times rheobase or more, peak twitch and tetanus force declined to 40-70% of the amplitude with short (0.2-0.5 msec) stimuli. The time to peak of the twitch was slightly decreased and the first phase of relaxation was accelerated.4. Cine-micrography of fibres which were permitted to shorten under a light load revealed that the side of the fibre adjacent to the anode was unactivated under conditions which also reduced peak force output.5. With stimulus durations longer than 20 msec, peak force, time to peak, and relaxation time increased as the stimulus strength increased from 2.5 to 3 times rheobase. This might have been produced by a cathodal contracture following the action potential.6. The results are discussed in relation to previous studies of activation in striated muscle.

Diverse potentiators of contraction have basically identical, activestate mechanical effects, but act by different membrane-mediated electromechanical coupling mechanisms. The falling phase of the action potential is greatly prolonged by Zn(2+) (0.1 mM) and UO(2)(2+) (0.5 to 1 microM), neither of which affects the mechanical threshold. Caffeine (1 mM), like the lyotropic anions, acts conversely. Thus changes in the duration and mechanical threshold of the action potential determine independent electromechanical coupling processes which can act individually, or conjointly in the action of other potentiators, in determining the duration of the active state and thus the potentiation of twitch tension.

The membrane potential of frog single muscle fibers in solutions containing tetrodotoxin was controlled with a two-electrode voltage clamp. Local contractions elicited by 100-ms square steps of depolarization were observed microscopically and recorded on cinefilm. The absence of myofibrillar folding with shortening to striation spacings below 1.95 microm served as a criterion for activation of the entire fiber cross section. With depolarizing steps of increasing magnitude, shortening occurred first in the most superficial myofibrils and spread inward to involve axial myofibrils as the depolarization was increased. In contractions in which the entire fiber cross section shortened actively, both the extent of shortening and the velocity of shortening at a given striation spacing could be graded by varying the magnitude of the depolarization step. The results provide evidence that the degree of activation of individual myofibrils can be graded with membrane depolarization.