Effects of pH on excitation and contraction in frog twitch muscle

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Abstract
The electrical and mechanical behaviour of frog twitch muscle in response to changes in membrane potential has been examined over a wide range of hydrogen ion concentration (pH 3.0–11.0). The changes in resting and action potentials, twitches, and maximum potassium-induced contractures (K contractures) were remarkably small when the pH was varied between 5.0 and 10.0. The time course of action potentials generally displayed small graded changes with variation in pH, possibly as the result of changes in surface potential.The amplitude of twitches and maximum K contractures was substantially decreased when pH was reduced to 4.0 or raised to 11.0 without significant alteration in membrane resting potential or consistent suppression of excitation, but maximum caffeine-induced contractures were unchanged. Replacement of chloride with perchlorate promptly antagonized the depressant effects of pH extremes (4.0, 11.0) on both twitch amplitude and maximum K-contracture tension. Acid-induced reductions in maximum K-contracture tension also were partially antagonized by increased calcium concentration. The onset and recovery from the contraction-depressant effects of pH extremes were too slow to be explained by the titration of groups immediately accessible at the membrane surface but too rapid to be accounted for by changes in intracellular pH. Thus, excitation and contraction apparently were uncoupled by sufficient alteration in extracellular pH. Changes in external pH had little effect on the impairment of maximum K contractures by media lacking divalent cations, or on the restoration of such responses by perchlorate except at very alkaline pH (10.0–11.0).The threshold for K contractures was reduced at pH 11.0, but otherwise was little affected by variation in pH at normal concentrations of divalent cations. Altered pH did not modify the usual effects of increased calcium concentration on the relation between potassium concentration and K-contracture tension. When K contractures were maintained by perchlorate in the absence of divalent cations, hydrogen ions displayed calcium-like actions on the relation between external K concentration ([K]0) and K-contracture tension, and also on the time course of submaximum K contractures. These observations are compatible with similar effects of hydrogen and calcium ions on surface potential.The problem of identifying putative charged groups which might influence the linkage between contractile responses and changes in membrane potential is discussed.