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Glycerinated rabbit psoas fibers 5-19 days old were immersed in contracting solutions with and without varying concentrations of different sodium salts. Raising the pH of the solution in small steps identified a threshold pH above which the fibers rapidly developed maximum tension. In solutions with added chloride or acetate (O-O.15 M), threshold pH and maximum tension changed only slightly. In solutions with added nitrate, bromide, or iodide, fibers developed much less tension, according to the series: CH&OO-> Cl-> NOa-> Br-> I-. Addition of excess calcium to the last solutions produced no further tension. After 15 min in NO,, Br-, or I-solution, fibers partially regained tension when reimmersed in acetate but not chloride solution. Fibers that had developed tension in chloride solution lost some of it when reimmersed in nitrate, bromide, or iodide solution, according to the same anion series, out they retained more tension than they could develop initially in the latter solution.

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Resting and action potential parameters were measured from papillary muscle isolated from hibernating and control hamsters and from rats. The temperature range of the study was 12-38 degrees C. The decrease in resting membrane potential (Em) with decreasing temperature was significantly less in the hibernation preparations (HH), down to 20 degrees C, than in either the control hamsters or rats. Below 20 degrees C the declines in Em of all preparations were indistinguishable. Action potential magnitude was adequately maintained in HH to 12 degrees C while both control hamster and rat action potentials declined markedly as temperatures were reduced. Both types of hamster preparations showed greatly prolonged action potentials with reduced temperatures as contrasted to a limited prolongation of rat action potentials. The data are suggestive of a membrane modication in hibernation.