Relationships between phosphoinositide and calcium responses to muscarinic agonists in astrocytoma cells.

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.