Effect of potassium channel blockade and α₂‐adrenoceptor activation on the release of nitric oxide from non‐adrenergic non‐cholinergic nerves

Using a superfusion bioassay cascade, we studied the effect of K + channel blockers and α 2 ‐adrenoceptor agents on the release of a transferable factor, previously characterized as nitric oxide (NO) or a nitric oxide‐related substance (NO‐R), in response to non‐adrenergic non‐cholinergic (NANC) nerve stimulation in the canine ileocolonic junction (ICJ). The non‐selective K + channel blockers, 4‐aminopyridine (4‐AP, 50 μ m ) and tetraethylammonium (TEA, 1 m m ) and the more selective blocker of Ca 2+ ‐activated K + channels, charybdotoxin ( Leiurus quinquestriatus venom (LQV), 0.4 μg ml −1 ), significantly enhanced the release of NO‐R induced by low frequency stimulation (2–4 Hz). In the presence of 4‐AP and TEA, the release of NO‐R was nearly abolished by tetrodotoxin (2 μ m ), and by l ‐N G ‐nitroarginine ( l ‐NOARG, 0.1 m m ). Relaxations induced by direct injection of exogenous NO (5–50 pmol) or nitroglycerin (GTN, 10–30 pmol) onto the rabbit aortic detector ring were not affected. The α 2 ‐adrenoceptor agonist, UK‐14,304 (0.3 μ m ) inhibited the release of NO‐R induced by low (2–4 Hz), but not that induced by high (16 Hz), frequency stimulation. This inhibitory effect was completely reversed by the α 2 ‐adrenoceptor antagonist, yohimbine (0.3 μ m ). Neither UK‐14,304 nor yohimbine affected the relaxations induced by exogenous NO (5 pmol) or GTN (10 pmol) on the aortic detector ring. On the other hand, in the presence of the K + channel blockers 4‐AP (50 μ m ) or charybdotoxin (LQV, 0.4 μg ml −1 ), UK‐14,304 (0.3 μ m ) failed to inhibit the electrically‐induced release of NO‐R. From these results, we conclude that the electrically‐induced release of NO‐R from NANC nerves of the canine ICJ is enhanced by K + channel blockers but inhibited by α 2 ‐adrenoceptor activation. In addition, these results suggest that the prejunctional modulation of NO‐R release by α 2 ‐adrenoceptors may involve neuronal K + channels.