INOSITOL 1,4,5-TRISPHOSPHATE RECEPTOR IN SKELETAL-MUSCLE - DIFFERENTIAL EXPRESSION IN MYOFIBERS

The role of inositol 1,4,5-trisphosphate as a second messenger in sign al transduction has been well established in many cell types. However, conflicting reports have led to a controversy regarding the role, if any, of inositol 1,4,5-trisphosphate signalling in skeletal muscle. In deed, expression of the inositol 1,4,5-trisphosphate receptor has not previously been demonstrated in skeletal muscle. In the present study we used in situ hybridization, immunohistochemistry, and [H-3]-inosito l 1,4,5-trisphosphate binding to demonstrate that rat skeletal muscle fibres contain inositol 1,4,5-trisphosphate receptors. RNAse protectio n and partial sequencing suggested that the inositol 1,4,5-trisphospha te receptors expressed in skeletal muscle was most similar to the non- neuronal form of the type 1 inositol 1,4,5-trisphosphate receptor. Whi le in situ hybridization showed inositol 1,4,5-trisphosphate receptor mRNA in all types of skeletal myofibres, immunodetectable inositol 1,4 ,5-trisphosphate receptor protein and specific [H-3]-inositol 1,4,5-tr isphosphate binding sites were preferentially expressed in slow oxidat ive (type I) and fast oxidative-glycolytic (type IIA) fibres, but not in fast glycolytic (type IIB) fibres. These findings indicate that an inositol 1,4,5-trisphosphate receptor is preferentially expressed in o xidative fibres of skeletal muscle.