title: Receptor-mediated metabolism of the phosphoinositides and phosphatidic acid in rat lacrimal acinar cells.
abstract: The metabolism of the inositol lipids and phosphatidic acid in rat lacrimal acinar cells was investigated. The muscarinic cholinergic agonist methacholine caused a rapid loss of 15% of [32P]phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] and a rapid increase in [32P]phosphatidic acid (PtdA). Chemical measurements indicated that the changes in 32P labelling of these lipids closely resembled changes in their total cellular content. Chelation of extracellular Ca2+ with excess EGTA caused a significant decrease in the PtdA labelling and an apparent loss of PtdIns(4,5)P2 breakdown. The calcium ionophores A23187 and ionomycin provoked a substantial breakdown of [32P]PtdIns(4,5)P2 and phosphatidylinositol 4-phosphate (PtdIns4P); however, a decrease in [32P]PtdA was also observed. Increases in inositol phosphate, inositol bisphosphate and inositol trisphosphate were observed in methacholine-stimulated cells, and this increase was greatly amplified in the presence of 10 mM-LiCl; alpha-adrenergic stimulation also caused a substantial increase in inositol phosphates. A23187 provoked a much smaller increase in the formation of inositol phosphates than did either methacholine or adrenaline. Experiments with excess extracellular EGTA and with a protocol that eliminates intracellular Ca2+ release indicated that the labelling of inositol phosphates was partially dependent on the presence of extracellular Ca2+ and independent of intracellular Ca2+ mobilization. Thus, in the rat lacrimal gland, there appears to be a rapid phospholipase C-mediated breakdown of PtdIns(4,5)P2 and a synthesis of PtdA, in response to activation of receptors that bring about an increase in intracellular Ca2+. The results are consistent with a role for these lipids early in the stimulus-response pathway of the lacrimal acinar cell.
mesh_terms: D000818:Animals; D002118:Calcium / Q000494:pharmacology; D066298:In Vitro Techniques; D007295:Inositol Phosphates / Q000378:metabolism; D007476:Ionophores / Q000494:pharmacology; D007765:Lacrimal Apparatus / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D008094:Lithium / Q000494:pharmacology; D008297:Male; D016210:Methacholine Chloride; D008688:Methacholine Compounds / Q000494:pharmacology; D010712:Phosphatidic Acids / Q000378:metabolism*; D010716:Phosphatidylinositols / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains
chemical_list: D007295:Inositol Phosphates; D007476:Ionophores; D008688:Methacholine Compounds; D010712:Phosphatidic Acids; D010716:Phosphatidylinositols; D016210:Methacholine Chloride; D008094:Lithium; D002118:Calcium