Receptor-mediated inositol phospholipid hydrolysis in astrocytes

Astrocyte-enriched cultures of the neonatal rat cortex were incubated for 24 h with [3H]inositol to prelabel the membrane inositol phospholipids. Exposure of the cultures to either noradrenaline or carbachol in the presence of Li+ produced a time- and dose-dependent accumulation of intracellular [3H]inositol phosphates. The separation of the individual inositol phosphates formed in response to receptor stimulation revealed that the major 3H-metabolite accumulated under these conditions was inositol monophosphate but that at least some of this was due to the initial formation of inositol trisphosphate. The use of selective receptor antagonists showed that noradrenaline- and carbachol-induced [3H]inositol phosphate accumulation was the result of the activation of alpha 1-adrenoceptors and muscarinic acetylcholine (probably of the M1 subtype) receptors respectively. Agonist-evoked [3H]inositol phosphate accumulation were found to be additive but the simultaneous addition of agonists and the Ca2+ ionophore A23187, which also promoted inositol phospholipid hydrolysis, was not. Agonist-induced [3H]inositol phosphate accumulation was only partially dependent on extracellular Ca2+, whilst that elicited by A23187 was entirely Ca2+-dependent. The results suggest that alpha 1-adrenoceptors and muscarinic acetylcholine receptors in these cultures are present either on the same cells and linked to separate inositol lipid pools or associated with different subpopulations of astrocytes in these cultures. Moreover, inositol lipids other than phosphatidylinositol 4,5-bisphosphate may be hydrolysed in response to agonist stimulation.