Effects of perchlorate on myofibrillar calcium sensitivity in rat skinned skeletal muscles.

The effects of perchlorate (1-20 mM) on myofibrillar calcium responsiveness have been tested in Triton X-100-skinned fibre bundles from rat soleus (slow-twitch) and extensor digitorum longus (fast-twitch) skeletal muscles. In extensor digitorum longus and soleus, perchlorate dose-dependently shifted the pCa (-log[Ca2+])/tension relationship towards lower free calcium concentration (sensitizing effect) and maximal tension was unchanged. The degree of sensitization was greater in extensor digitorum longus than in soleus bundles. Reversibility after exposure to 12 mM perchlorate was complete in soleus but not in extensor digitorum longus muscles. In fact, the 'return' pCa/tension relationship in extensor digitorum longus was shifted to higher free calcium concentration (desensitizing effect) compared with control. Perchlorate (12 mM) also enhanced myofibrillar calcium responsiveness of frog semitendinosus skinned skeletal fibres. Assuming a passive distribution of perchlorate across the sarcolemma, this sensitizing effect is probably not involved in perchlorate-induced potentiation of contractile responses of intact muscles and thereby supports the specificity of perchlorate as an agonist of the excitation/calcium release sequence in skeletal muscle fibres.