title: Relationships between phosphoinositide and calcium responses to muscarinic agonists in astrocytoma cells.
abstract: Activation of muscarinic receptors in human astrocytoma (1321N1) cells stimulates phosphoinositide metabolism and calcium mobilization. The muscarinic effect on phosphoinositide turnover is evidenced by increased formation of [3H]inositol 1-phosphate (Ins1P) and by increased [3H]inositol incorporation into PtdIns. The muscarinic effect on calcium mobilization is seen as a large increase in undirectional 45Ca2+ efflux from cells equilibrated with 45Ca2+ and a small increase in unidirectional 45Ca2+ influx. A series of muscarinic agonists was used to explore the relationship between phosphoinositide metabolism and unidirectional 45Ca2+ efflux. The maximal increases in [3H]Ins1P formation produced by carbachol and acetylcholine are similar and are much larger than those caused by oxotremorine and pilocarpine. The effects of these agonists on 45Ca2+ efflux are similar: carbachol and acetylcholine cause equivalent maximal increases in the rate of 45Ca2+ efflux whereas oxotremorine and pilocarpine cause submaximal 45Ca2+ efflux responses. The Kact values of carbachol and acetylcholine for stimulation of [3H]Ins1P formation are 40 microM and 1.5 microM, respectively. These values are only 2- to 3-fold higher than the respective Kact values for stimulating 45Ca2+ efflux. The finding that each of the muscarinic agonists tested has nearly identical efficacy and similar potency for stimulating [3H]Ins1P formation and 45Ca2+ efflux supports the idea that hormonal stimulation of phosphoinositide hydrolysis leads to calcium mobilization.
mesh_terms: D001254:Astrocytoma / Q000378:metabolism*; D001285:Atropine / Q000494:pharmacology; D001693:Biological Transport, Active / Q000187:drug effects; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002217:Carbachol / Q000494:pharmacology; D002460:Cell Line; D006801:Humans; D007700:Kinetics; D010716:Phosphatidylinositols / Q000378:metabolism*; D011976:Receptors, Muscarinic / Q000502:physiology*; D014403:Tubocurarine / Q000494:pharmacology
chemical_list: D010716:Phosphatidylinositols; D011976:Receptors, Muscarinic; D000001:Calcimycin; D001285:Atropine; D002217:Carbachol; D002118:Calcium; D014403:Tubocurarine