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 Ca2+ 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 of Ca2+ mobilization is seen as a large increase in unidirectional 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 [activation constant] values of carbachol and acetylcholine for stimulation of [3H]Ins1P formation are 40 and 1.5 .mu.M, 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 Ca2+ mobilization.