Inositol Phospholipid Hydrolysis in Rat Cerebral Cortical Slices: II. Calcium Requirement

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Abstract
The calcium requirement for agonist-dependent breakdown of phosphatidylinositol and polyphos-phoinositides has been examined in rat cerebral cortex. The omission of added Ca2+ from the incubation medium abolished [3H]inositol phosphate accumulation from prelabelled phospholipid induced by histamine, reduced that due to noradrenaline and 5-hydroxytryptamine, but did not affect carbachol-stimulated breakdown. EC50 values for agonists were unaltered in the absence of Ca2+. Removal of Ca2+ by preincubation with EGTA (0.5 mM) abolished all responses, but complete restoration was achieved by replacement of Ca2+. The EC50for Ca2+ for histamine-stimulated [3H]inositol phosphate accumulation was 80 μM. Noradrenaline-stimulated breakdown was antagonised by manganese (IC50 1.7 mM), but not by the calcium channel blockers nitrendipine or nimodipine (30 μM). The calcium ionophore A23187 stimulated phosphatidylinositol/polyphosphoinositide hydrolysis with an EC50 of 2 μM, and this response was blocked by EGTA. Omission of Ca2+ or preincubation with EGTA or Mn2+ (EC50= 230 μM) greatly enhanced the incorporation of [3H]inositol into phospholipids. The IC50for Ca2+ in inhibiting incorporation was 25 μM. The results show that different receptors mediating phosphatidylinositol/ polyphosphoinositide breakdown in rat cortex have quantitatively different Ca2+ requirements, and it is suggested that rigid opinions regarding phosphatidylinositol/ polyphosphoinositide breakdown as either cause or effect of calcium mobilisation in rat cortex are inappropriate.