Inositol 1,4,5-trisphosphate-sensitive Ca²⁺ release in rat fast- and slow-twitch skinned muscle fibres

Abstract.

Inositol 1,4,5-trisphosphate (InsP ₃), an intracellular messenger, induces Ca²⁺ release in various types of cells, particularly smooth muscle cells. Its role in skeletal muscle, however, is controversial. The present study shows that the application of InsP ₃ to rat slow- and fast-twitch saponin-skinned fibres induced contractile responses that were not related to an effect of InsP ₃ on the properties of the contractile proteins. The amplitude of the contractures was dependent upon the Ca²⁺-loading period, and was larger in slow- than in fast-twitch muscle. In both types of skeletal muscle, these responses, unlike caffeine contractures, were not inhibited by ryanodine (100 µM), but were abolished by heparin (20 µg·ml^–1). In soleus muscle, the concentration of heparin required to inhibit the response by 50% (IC₅₀) was 5.7 µg·ml^–1, a similar value to that obtained previously in smooth muscle. Furthermore, the results show that in slow-twitch muscle, the InsP ₃ contractures have a "bell-shaped" dependency on the intracellular Ca²⁺ concentration. These results show that InsP₃ receptors should be present in skeletal muscle. Thus, it is possible that InsP₃ participates in the regulation of sarcoplasmic reticulum Ca²⁺ release in skeletal muscle, particularly in slow-twitch fibres.