Glutamate-Stimulated, Guanine Nucleotide-Mediated Phosphoinositide Turnover in Astrocytes Is Inhibited by Cyclic AMP

Abstract

The potential for cross-talk between the adenyl cy-clase and phosphoinositide (PPI) lipid second messenger system was investigated in astrocytes cultured from neonatal rat brain. Glutamate-stimulated PPI turnover, measured by the formation of total inositol phosphates from myo -[ 3 H]inositoI-labeled lipids, was inhibited in a concentration-dependent manner by the elevation of intracellular cyclic AMP levels produced either by stimulation of the isoproter-enol receptor linked to adenyl cyclase or by its direct activation by forskolin. N 6 ,2′- O -Dibutyryl cyclic AMP, an analogue that can also activate cyclic AMP-dependent kinase, inhibited glutamate-stimulated PPI turnover in a concentration-dependent manner as well, a result suggesting that cyclic AMP-dependent kinase is involved in mediating the inhibition. Inclusion of an inhibitor of cyclic AMP-dependent kinase, l-(5-isoquinolinesulfonyl)-2 methylpiperazine dihy-drochloride or N -(2-guanidinoethyl)-5-isoquinolinesulfon-amide hydrochloride, blocked the cyclic AMP-mediated inhibition in a concentration-dependent manner, a finding further supporting this hypothesis. The site of inhibition of the phosphoinositol lipid pathway by cyclic AMP was probed using a digitonin-permeabilized cell system. Guanosine 5′- O -(3-thiotriphosphate), a nonhydrolyzable analogue of GTP, stimulated PPI turnover and potentiated glutamate-stimulated PPI turnover, and guanosine 5′- O -(3-thiodiphosphate) inhibited glutamate-stimulated PPI turnover in these cells, results providing evidence that glutamate receptors are coupled to phospholipase C by a guanine nucleotide binding protein in astrocytes. N 6 ,2′- O -Dibutyryl cyclic AMP and agents that elevate cyclic AMP levels inhibited the PPI turnover stimulated by guanosine 5′- O -(3-thiotriphosphate), as well as that potentiated by guanosine 5′- O -(3-thiotriphos-phate) in the presence of glutamate, results suggesting that the cyclic AMP-dependent inhibition occurs at or distal to the putative guanine nucleotide binding protein. Because basal PPI turnover was not altered by elevation of cyclic AMP levels, direct inhibition of phospholipase C is unlikely.