Inositol phospholipid hydrolysis in rat cerebral cortical slices: II. Calcium requirement.
The calcium requirement for agonist-dependent breakdown of phosphatidylinositol and polyphosphoinositides 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 EC50 for Ca2+ for histamine-stimulated [3H]inositol phosphate accumulation was 80 microM. Noradrenaline-stimulated breakdown was antagonised by manganese (IC50 1.7 mM), but not by the calcium channel blockers nitrendipine or nimodipine (30 microM). The calcium ionophore A23187 stimulated phosphatidylinositol/polyphosphoinositide hydrolysis with an EC50 of 2 microM, and this response was blocked by EGTA. Omission of Ca2+ or preincubation with EGTA or Mn2+ (EC50 = 230 microM) greatly enhanced the incorporation of [3H]inositol into phospholipids. The IC50 for Ca2+ in inhibiting incorporation was 25 microM. 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.
D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002122:Calcium Chloride / Q000494:pharmacology*; D002217:Carbachol / Q000494:pharmacology; D002540:Cerebral Cortex / Q000187:drug effects / Q000378:metabolism*; D004533:Egtazic Acid / Q000494:pharmacology; D006632:Histamine / Q000494:pharmacology; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D009638:Norepinephrine / Q000494:pharmacology; D010716:Phosphatidylinositols / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D014702:Veratrine / Q000494:pharmacology
D010716:Phosphatidylinositols; D000001:Calcimycin; D004533:Egtazic Acid; D006632:Histamine; D002217:Carbachol; D014702:Veratrine; D002122:Calcium Chloride; D011188:Potassium; D009638:Norepinephrine