Phosphorylation of phosphatidylinositol by transverse tubule vesicles and its possible role in excitation‐contraction coupling

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Abstract
Phosphorylation of phosphatidylinositol to phosphatidylinositol 4‐monophosphate and to phosphatidyl‐inositol 4,5‐bisphosphate was demonstrated in transverse‐tubule membranes isolated from frog skeletal muscle using [γ‐32P]ATP as substrate. At millimolar concentrations of Mg2+ both phosphorylation reactions were completed within 15 s at 25°C. Isolated sarcoplasmic reticulum vesicles phosphorylated phosphatidyl‐inositol to phosphatidylinositol 4‐phosphate with a lower specific activity than the transverse tubules, and lacked the ability to produce phosphatidylinositol 4,5‐bisphosphate. These findings show, for the first time, that isolated transverse‐tubule membranes carry out one of the steps required to sustain a role for inositol trisphosphate as the physiological messenger in excitation‐contraction coupling in skeletal muscle. The finding that 0.5 mM tetracaine apparently inhibits the phosphorylation of phosphatidylinositol 4‐phosphate to phosphatidylinositol 4,5‐bisphosphate also supports a role for these intermediates in excitation‐contraction coupling.