Influence of osmotic compression on calcium activation and tension in skinned muscle fibers of the rabbit

Single skinned muscle fibers were osmotically compressed back to and below their in situ size by addition of a large, random-coil polymer (Deytran T500;\(\bar M_{\text{N}}\)= 180,000;\(\bar M_{\text{W}}\) = 461,000) to the bathing medium. Maximal Ca²⁺-activated tension in fibers swollen (zero Dextran, fiber width 21% above in situ) or near in situ size (5% Dextran, in g/100 ml final solution) was similar, but compression to 86% of in situ width with 10% Dextran decreased maximal force by 15% relative to polymer-free control. While the relative tension-pCa relation in 0 and 10% Dextran was similar, with a pCa of 6.37 required for 50% activation, that in 5% Dextran was more sensitive to Ca²⁺, with a pCa₅₀ of 6.66. We feel these effects are most likely due to changes in interfilament spacing with compression and that alterations in Ca²⁺-sensitivity might be explained by changes in cross-bridge angle or in the concomitant attachment-detachment rate constants which would be expected to influence the troponin-Ca²⁺ binding equilibrium, as has been proposed by others.