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The breakdown of phosphatidylinositol in myoblasts stimulated to fuse by the addition of Ca2+.

ABSTRACT: 1. The fusion of chick-embryo myoblasts to produce myotubes was studied. The myoblasts were grown for 50 h in medium containing 10--20 microM-Ca2+; during this period they achieve fusion competence. 2. A rapid breakdown of phosphatidylinositol is also observed on addition of 1.4 mM-Ca2+ to these cells. This Ca2+ concentration also stimulates rapid myoblast fusion. 3. The breakdown is complete within 15 min and shows the same dependence on Ca2+ concentration as the fusion process. 4. Fusion-incompetence myoblasts and cells where fusion is inhibited by sodium butyrate exhibit no phosphatidylinositol breakdown on Ca2+ addition. 5. The Ca2+ ionophore A23187 inhibits the Ca2+-stimulated breakdown by about 50%, but has no effect on fusion. 6. A concomitant increase in 1,2-diacylglycerol labelled and fall in phosphatidylinositol labelling was observed when the lipids were labelling with [14C]glycerol on increasing the Ca2+ concentration in the medium to 1.4 mM. 7. We propose that the breakdown of phosphatidylinositol with a resultant increase in 1,2-diacylglycerol content of the cell membrane promotes myoblast fusion.

SUBMITTER: Wakelam MJ

PROVIDER: S-EPMC1158168 | biostudies-other | 1982 Mar

REPOSITORIES: biostudies-other

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Project description:Rat hepatocytes rapidly incorporate [32P]Pi into phosphatidylinositol 4-phosphate (PtdIns4P) and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2]; their monoester phosphate groups approach isotopic equilibrium with the cellular precursor pools within 1 h. Upon stimulation of these prelabelled cells with Ca2+-mobilizing stimuli (V1-vasopressin, angiotensin, alpha 1-adrenergic, ATP) there is a rapid fall in the labelling of PtdIns4P and PtdIns(4,5)P2. Pharmacological studies suggest that each of the four stimuli acts at a different population of receptors. Insulin, glucagon and prolactin do not provoke disappearance of labelled PtdIns4P and PtdIns(4,5)P2. The labelling of PtdIns4P and PtdIns(4,5)P2 in cells stimulated with vasopressin or angiotensin initially declines at a rate of 0.5-1.0% per s, reaches a minimum after 1-2 min and then returns towards the initial value. The dose-response curves for the vasopressin- and angiotensin-stimulated responses lie close to the respective receptor occupation curves, rather than at the lower hormone concentrations needed to evoke activation of glycogen phosphorylase. Disappearance of labelled PtdIns4P and PtdIns(4,5)P2 is not observed when cells are incubated with the ionophore A23187. The hormone-stimulated polyphosphoinositide disappearance is reduced, but not abolished, in Ca2+-depleted cells. These hormonal effects are not modified by 8-bromo cyclic GMP, cycloheximide or delta-hexachlorocyclohexane. The absolute rate of polyphosphoinositide breakdown in stimulated cells is similar to the rate previously reported for the disappearance of phosphatidylinositol [Kirk, Michell & Hems (1981) Biochem. J. 194, 155-165]. It seems likely that these changes in polyphosphoinositide labelling are caused by hormonal activation of the breakdown of PtdIns(4,5)P2 (and may be also PtdIns4P) by the action of a polyphosphoinositide phosphodiesterase. We therefore suggest that the initial response to hormones is breakdown of PtdIns(4,5)P2 (and PtdIns4P?), and that the simultaneous disappearance of phosphatidylinositol might be a result of its consumption for the continuing synthesis of polyphosphoinositides.

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The breakdown of phosphatidylinositol in myoblasts stimulated to fuse by the addition of Ca2+.

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