The stimulation of inositol lipid metabolism that accompanies calcium mobilization in stimulated cells: defined characteristics and unanswered questions

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Abstract
It now appears to be generally agreed that the `phosphatidylinositol response', discovered in 1953 by Hokin & Hokin, occurs universally when cells are stimulated by ligands that cause an elevation of the ionized calcium concentration of the cytosol. The initiating reaction is almost certainly hydrolysis of an inositol lipid by a phosphodiesterase. Phosphatidylinositol, phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate all break down rapidly under such circum-stances. However, we do not yet know which of these individual reactions is most closely coupled to receptor stimulation, nor do we know where in the cell it occurs. With many stimuli, inositol phospholipid breakdown is closely coupled to occupation of receptors and appears not to be a response to changes in cytosol [Ca$^{2+}$]: this provoked the suggestion that it may be a reaction essential to the coupling between activation of receptors and the mobilization of Ca$^{2+}$ within the cell. In a few situations, however, it appears probable that inositol lipid breakdown can occur as a result of the rise in cytosol [Ca$^{2+}$] that follows receptor activation: such observations gave rise to the alternative opinion that inositol lipid breakdown cannot be related to stimulus-response coupling at calcium-mobilizing receptors. It now seems likely that these two views are too rigidly polarized and that some cells probably display both receptor-linked and Ca$^{2+}$-controlled breakdown of inositol lipids. Both may sometimes occur simultaneously or sequentially in the same cell.