Nitric oxide, via activation of guanylyl cyclase, suppresses α2-adrenoceptor-mediated 5-hydroxytryptamine release from neuroendocrine epithelial cells of rabbit tracheae

Isolated tracheae of newborn rabbits were incubated In vitro and the outflow of 5-hydroxytryptamine (5-HT) was determined by HPLC with electrochemical detection, Evidence has previously been provided that this 5-HT outflow derives from neuroendocrine epithelial (NEE) cells of the airway mucosa. Phenylephrine, at a maximally effective concentration of 10 mu M, caused a transient increase in 5-HT outflow by about 250%, an effect mediated by alpha(2B)-adrenoceptors, as previously shown. The phenylephrine-induced 5-HT release remained unchanged in calcium-free medium, but was reduced by 75% when the tracheae were incubated in calcium-free medium which contained 0.5 mM EDTA, a treatment known to lower also intracellular calcium, The NO donor SNAP (S-nitroso-N-acetyl-penicillinamine, 10 mu M) almost completely inhibited phenylephrine-induced 5-HT release, The inhibitory effect of SNAP was prevented by ODQ, (1H-[1, 2, 4]oxadiazolo[4, 3-a]quinoxalin-1-one), an inhibitor of soluble guanylyl cyclase. In contrast, 5-HT release induced by depolarizing concentrations of potassium (45 mM), which was reduced by 96% in calcium-free medium, was not affected by SNAP, In conclusion, NO, via activation of soluble guanylyl cyclase, inhibits 5-HT release from NEE cells in a stimulus-dependent manner, alpha(2)-Adrenoceptor-mediated 5-HT release, which appears to be triggered by liberation of calcium from intracellular stores, is suppressed by NO, whereas high potassium-evoked 5-HT release which is triggered by calcium influx through voltage regulated channels, is not affected.