Effects of extracellular calcium ions on the elevation of guanosine 3′, 5′-monophosphate level in neuroblastoma cells

Abstract

The levels of guanosine 3′,5′-monophosphate in neuroblastoma N1E115 cells were increased 10- to 50-fold by depolarizing agents (110 mM KCl and 1 mM ouabain), 10 μM ionophore A23187, 1 mM sodium azide, 1 mM N-methyl- N′-nitro- N-nitrosoguanidine, 2 mM carbamylcholine and 10 μM prostaglandin E 1 in the medium containing 1.8 mM calcium. Removal of calcium from the incubation medium and addition of 3 mM ethyleneglycol-bis( ß-aminoethyl ether)- N, N, N′, N′-tetraacetic acid abolished responses of guanosine 3′,5′-monophosphate to depolarizing agents and reduced the basal level of guanosine 3′,5′-monophosphate to 10%. In contrast, guanosine 3′,5′-monophosphate levels still increased in the calcium-free medium in response to carbamylcholine, prostaglandin E 1 and ionophore A23187, although the absolute responses to these agents were reduced by 60–80%. The simultaneous addition of calcium with depolarizing agents to the calcium-free medium restored the responses to these agents, while the addition of calcium with carbamylcholine, prostaglandin E 1 or ionophore A23187 did not restore the responses. The combination of carbamylcholine with ionophore A23187 induced the same amount of increase of guanosine 3′,5′-monophosphate as that elicited by ionophore A23187 alone. In contrast, the combination of 110 mM KCl with carbamylcholine or ionophore A23187 resulted in a greater increase of guanosine 3′,5′-monophosphate than that induced by carbamylcholine, ionophore A23187 or 110 mM KCl alone, but less than additive.