Skeletal muscle fibre swelling contributes to force depression in rats and humans: a mechanically-skinned fibre study

This study investigated the effects of fibre swelling on force production in rat and human skinned muscle fibres, using osmotic compression to reverse the fibre swelling. In mechanically-skinned fibres, the sarcolemma is removed but normal excitation–contraction coupling remains functional. Force responses in mechanically-skinned fibres were examined with and without osmotic compression by polyvinylpyrrolidone 40\ua0kDa (PVP-40) or Dextran 500\ua0kDa (dextran). Fibre diameter increased to 116 ± 2% (mean ± SEM) when rat skinned type II fibres were immersed in the standard intracellular solution, but remained close to the in situ size when 3% (mass/volume) PVP-40 or 4% Dextran were present. Myofibrillar Ca sensitivity, as indicated by pCa (−\ua0log[Ca] at half-maximal force), was increased in 4% Dextran (0.072 ± 0.007 pCa shift), but was not significantly changed in 3% PVP-40. Maximum Ca-activated force increased slightly to 103 ± 1% and 104 ± 1% in PVP-40 and Dextran, respectively. Both tetanic and depolarization-induced force responses in rat skinned type II fibres, elicited by electrical stimulation and ion substitution respectively, were increased by ~ 10 to 15% when the fibres were returned to their normal in situ diameter by addition of PVP-40 or Dextran. Interestingly, the potentiation of these force responses in PVP-40 was appreciably greater than could be explained by potentiation of myofibrillar function alone. These results indicate that muscle fibre swelling, as can occur with intense exercise, decreases evoked force responses by reducing both the Ca-sensitivity of the contractile apparatus properties and Ca release from the sarcoplasmic reticulum