REGULATION BY CALCIUM OF THE NITRIC-OXIDE CYCLIC-GMP SYSTEM IN CEREBELLAR GRANULE CELLS AND ASTROGLIA IN CULTURE

Ca2+ entry induced by N-methyl-D-aspartate (NMDA) in neurons and by no radrenaline (NA) in astrocytes is known to increase intracellular cycl ic GMP (cGMP) levels through stimulation of the Ca2+-dependent nitric oxide synthase type I (NOS-I). The possibility that Ca2+ entry could a lso down-regulate intracellular cGMP by activating a Ca2+/calmodulin-d ependent phosphodiesterase (CaM-PDE) has been investigated here in pri mary cultures enriched in granule neurons or in astroglia from rat cer ebellum, We show that the same agonists that stimulate nitric oxide (N O) formation (NMDA and NA at 100 mu M) and the Ca2+ ionophore A23187 ( 10 mu M) decrease cGMP generated in response to direct stimulation of soluble guanylyl cyclase (sGC) by NO donors in both cell types. This e ffect requires extracellular Ca2+ and is prevented by the calmodulin i nhibitor W7 (100 mu M). Membrane depolarization, manipulations of the Na+ gradient, and intracellular Ca2+ mobilization also decrease NO don or-induced cGMP formation in granule cells, In astroglia Ca2+ entry ad ditionally down-regulates cGMP generated by stimulation of the particu late GC by atrial natriuretic peptide (ANF). Decreases in cGMP produce d by A23187 were more pronounced in the absence than in the presence o f the PDE inhibitor 3-isobutyl-1-methylxanthine (IBMX; 1 mM), indicati ng that a CaM-PDE was involved, We also show that astroglial cells can accumulate similar amounts of cGMP than neurons in response to NO don ors when IBMX is present but much lower levels in its absence. This ma y result from a lower ratio of sGC to PDE activities in astroglia, (C) 1997 Wiley-Liss, Inc.