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Trabeculae from guinea-pig ventricles were bathed in a physiological saline, were stimulated electrically and were stretched until isometric force production was maximal. They were then 'skinned' by superfusion for 30 min with a solution containing the non-ionic detergent, Triton X-100. The steady isometric force generated by these skinned trabeculae was measured in solutions of different pCa (-log10 [Ca2+]) and salt concentration to investigate the influence of salt concentration on (i) the maximum Ca2+-regulated force (at pCa approximately 3.8), (ii) the resting force (at pCa greater than 8) and (iii) the pCa required for 50% Ca2+ activation (pCa50). The apparent affinity constants of EGTA (ethyleneglycol-bis-(beta-aminoethylether)-N,N,N',N'-tetraacetic acid) for Ca2+ were measured in the solutions of different salt composition by a pH-metric method and these constants were used in the calculation of solution pCa. Increases in ionic strength (gamma/2) (and osmolarity) over the range 0.1-0.4 mol/l produced by the addition of various monovalent Cl or K salts resulted in an approximately linear decrease in the maximum Ca2+-regulated force. The inhibitory actions of the different salts were similar despite the differences in size and structure of their constituent ions. Increases in solution osmolarity alone, produced by the addition of sucrose up to 0.6 mol/l, did not significantly alter the maximum Ca2+-regulated force. It was concluded that the influence of monovalent salts on the maximum Ca2+-regulated force was due entirely to the changes in ionic strength and was not related to changes in solution osmolarity or to specific ion effects. Resting force was little affected by changes in salt concentration at ionic strengths of 0.2 mol/l and above. At lower ionic strengths the muscles exhibited an elevated resting force and an inability to relax completely after a Ca2+ -regulated contracture. A decrease in ionic strength from 0.2 to 0.1 mol/l produced by a reduction in KCl concentration raised the pCa50 from 5.56 to 5.85, i.e. the Ca2+ sensitivity was increased. However, the same increase in pCa50 was observed when the [K+] was lowered at constant ionic strength, osmolarity and [Cl-] by the replacement of KCl with tetramethylammonium Cl or choline Cl. Therefore, the change in pCa50 induced by an alteration in KCl concentration was due entirely to a specific inhibitory action of K+ on the Ca2+ sensitivity of cardiac myofibrils and was unrelated to changes in ionic strength, osmolarity and [Cl-]. Substitution of KCl 0.1 mol/l in the solutions by NaCl did not significantly alter the pCa50.(ABSTRACT TRUNCATED AT 400 WORDS)

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