EFFECTS OF PERCHLORATE ON MYOFIBRILLAR CALCIUM SENSITIVITY IN RAT SKINNED SKELETAL-MUSCLES

The effects of perchlorate (1-20 mM) on myofibrillar calcium responsiv eness have been tested in Triton X-100-skinned fibre bundles from rat soleus (slow-twitch) and extensor digitorum longus (fast-twitch) skele tal muscles. In extender digitorum longus and soleus, perchlorate dose -dependently shifted the pCa (-log [Ca2+])/tension relationship toward s lower free calcium concentration (sensitizing effect) and maximal te nsion was unchanged. The degree of sensitization was greater in extens or digitorum longus than in soleus bundles. Reversibility after exposu re to 12 mM perchlorate was complete in soleus but not in extensor dig itorum longus muscles. In fact, the 'return' pCa/tension relationship in extensor digitorum longus was shifted to higher free calcium concen tration (desensitizing effect) compared with control. Perchlorate (12 mM) also enhanced myofibrillar calcium responsiveness oi frog semitend inosus skinned skeletal fibres. Assuming a passive distribution of per chlorate across the sarcolemma, this sensitizing effect is probably no t involved in perchlorate-induced potentiation of contractile response s of intact muscles and thereby supports the specificity of perchlorat e as an agonist of the excitation/calcium release sequence in skeletal muscle fibres.