Shortening induced deactivation of skinned fibres of frog and mouse striated muscle

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Abstract
The depressant effect of active shortening, previously established in intact muscle fibers, was studied during Ca-induced contractures of chemically skinned fibers from the semitendinosus muscle of R. temporaria and the psoas muscle of the mouse. The decrease in contractile activity was determined by comparing the rate of force redevelopment (at a given tension level) after a large (test) and a small (control) release step. Under standard experimental conditions (ionic strength: frog 135 mM, mouse 190 mM; Ca2+ 3.0 .mu.M; Mg2+: frog 25 .mu.M, mouse 100 .mu.M; MgATP2-: frog 1.0 mM, mouse 2.0 mM) active shortening of 0.15 .mu.m/sarcomere (in excess of control release) reduced the contractile activity by .apprx. 50% of the control in both frog and mouse muscle fibers. Full contractile activity was regained within < 4 s during isometric activity after the shortening phase. The depressant effect of shortening was steadily reduced, to almost complete disappearance of the effect, by increasing the free Ca concentration within the range 1.5-12.0 .mu.M. An increase in ionic strength from 105 to 235 mM reduced the depressant effect by .apprx. 40%. There was a progressive enhancement of the shortening effect as the Mg ion concentration was increased from 25 to 590 .mu.M. Interaction between the myosin cross-bridges and the thin filament during sarcomere shortening may lead to a decrease in troponin Ca binding, resulting in a temporary deactivation of the contractile system.