Previous studies on skinned muscle fibers have demonstrated a direct effect of elevated levels of H+ ion to depress force production; however, the molecular basis for this effect is presently unknown. Here, whole troponin complexes were removed from skinned single fiber preparations of rat slow-twitch and fast-twitch muscles, and the effect of H+ ions on the resultant Ca2+-insensitive force was examined. The effect of H+ ions to depress force was found to be virtually identical in untreated control fibers activated in the presence of Ca2+ and in fibers activated in the absence of Ca2+ by troponin removal. Thus, the effect of H+ ions to depress force occurs at a step in activation beyond the disinhibition of the thin filament by Ca2+, probably involving reductions in the number of attached cross-bridges or in the force per attachment.
Bibliographical noteFunding Information:
This work was supported by a National Institutes of Health grant (HL25861) to R.L. Moss and a National Institutes of Health post- doctoral fellowship (AR0781 1) to J.M. Metzger. Received for publication 15 April 1988 and in final form 22 August 1988.