The possible role of Ca2+ influx in alpha-1-adrenoceptor-stimulated [H-3]inositol phosphate ([H-3]InsP) formation was examined in primary cultures of glial cells from 1-day-old rat brain. The Ca2+ ionophore A23187 caused a concentration- and time-dependent increase in [H-3]InsP formation similar in magnitude to that caused by norepinephrine (NE). Responses to A23187 and NE were both completely dependent on extracellular Ca2+, with a similar concentration dependence. However, cadmium was more potent in blocking the response to A23187 than to NE. Lanthanum (1 mM) blocked the response to NE, although cobalt (5 mM) did not. The [H-3]InsP response to A23187 was not additive with the response to NE or to the muscarinic agonist carbachol, although responses to NE and carbachol were additive. Both A23187 and ionomycin inhibited the additive stimulation caused by a combination of NE and carbachol, and this inhibition was potentiated by cadmium. Ionomycin stimulated [H-3]InsP formation at concentrations lower than those inhibiting receptor-mediated responses, and this stimulation was not additive with responses to NE or carbachol. High-performance liquid chromatography separation showed similar patterns of [H-3]InsPs formed in response to both Ca2+ ionophore and receptor agonists. These results raise the possibility that receptor-activated Ca2+ influx may be involved in stimulation of [H-3]InsP formation in these cells.
