Cholinoceptor regulation of cyclic AMP levels in bovine adrenal medullary cells

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Abstract
1 The regulation of adenosine 3′:5′-cyclic monophosphate (cyclic AMP) levels by cholinoceptors has been studied in cultured bovine adrenal medullary cells. 2 Acetylcholine (100 μm), nicotine (10 μm) and dimethylphenylpiperazinium (20 μm) each increased cellular cyclic AMP levels 2 to 4 fold over 5 min in the absence of phosphodiesterase inhibitors. The muscarinic agonist acetyl-β-methylcholine (100 μm) had no effect either on its own or on the response to nicotine. The responses to acetylcholine and nicotine were unaffected by atropine (1 μm) but were abolished by mecamylamine (5 μm). 3 Cellular cyclic AMP increased transiently during continuous exposure to nicotine (1–20 μm), with the largest response seen after 5 min, a smaller response after 20 min, and no change in cyclic AMP levels seen after 90 or 180 min. The maximal response after 5 min stimulation was seen with 5- 10 μm nicotine and the EC50 was about 2 μm. In contrast, extracellular cyclic AMP levels did not change after 5 or 20 min stimulation with nicotine, but increased slightly after 90 min and further after 180 min. 4 The cellular cyclic AMP response to nicotine (10 μm) was unchanged or weakly enhanced in the presence of the unselective phosphodiesterase inhibitor, isobutylmethylxanthine, and was unchanged in the presence of rolipram. Nicotine did not interact synergistically with low concentrations of forskolin. The response was however completely abolished in the absence of extracellular Ca2+. 5 The nicotinic cyclic AMP response was almost abolished by sphingosine (30 μm), which did not inhibit the cyclic AMP response to phorbol-12,13-dibutyrate (PDB). The nicotinic response was reduced by 55% by the calmodulin antagonist W7 at 3 μm, and by >90% at 10 μm W7. At these concentrations, W7 had no effect on the cyclic AMP response to 100 nm PDB. The nicotinic response was also selectively abolished by another calmodulin antagonist, trifluoperazine (10 μm). 6 The results indicate that nicotinic cholinoceptors can increase cyclic AMP production in chromaffin cells by a mechanism that does not directly involve Gs, that is dependent on extracellular Ca2+ and that is sensitive to the calmodulin antagonists W7 and trifluoperazine. We propose that a Ca2+/calmodulin-sensitive adenylate cyclase may mediate the nicotinic cyclic AMP response in bovine chromaffin cells.

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