Abstract
Heparin inhibits inositol trisphosphate receptors, particularly in smooth muscle, but its effect on skeletal muscle is controversial. Our study showed that heparin induced a decrease in the amplitude of 10 mM caffeine-induced contracture in slow and fast saponin-skinned fibres. Moreover, measurements on Triton X-100-skinned fibres in soleus muscle showed that heparin alone decreased maximal Ca 2+-activated tension and Ca 2+ sensitivity of contractile proteins, whereas no significant effect was observed in extensor digitorum longus muscle. However, in the presence of caffeine, heparin decreased maximal Ca 2+-activated tension in both muscles. It would appear that the heparin-induced decrease in the amplitude of caffeine contracture in rat skeletal muscle was not related to a direct inhibition of Ca 2+ release from sarcoplasmic reticulum but to a desensitising effect of heparin and caffeine on myofilaments.
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