Effect of 4-aminopyridine and verapamil on the inhibitory action of normorphine on the guinea-pig ileum

Prejunctional modulation of the nitrergic innervation of the canine ileocolonic junction via potassium channels

1. The effects of different K+ channel blockers were studied on nitric oxide (NO)-mediated non-adrenergic non-cholinergic (NANC) relaxations in the canine ileocolonic junction. 2. The non-selective blockers of K+ channels, 4-aminopyridine (4-AP) and tetraethylammonium (TEA) and the blocker of large conductance Ca(2+)-activated K+ channels, charybdotoxin, potently enhanced the NANC relaxations induced by low frequency stimulation. The blocker of small conductance Ca(2+)-activated K+ channels, apamin, had no effect on electrically-induced NANC relaxations. 3. NANC nerve-mediated relaxations induced by adenosine 5'-triphosphate (ATP), acetylcholine (ACh) and gamma-aminobutyric acid (GABA) were significantly enhanced by 4-AP and charybdotoxin but not by apamin. TEA significantly enhanced the NANC relaxations in response to GABA and ATP while that in response to ACh was abolished. 4. None of the K+ channel blockers had an effect on the dose-response curve to NO, on the noradrenaline-induced contraction or on the relaxation to nitroglycerine (GTN). 5. From these results we conclude that inhibition of prejunctional K+ channels increases the nitrergic relaxations induced by electrical and chemical receptor stimulation of NANC nerves and thus suggests a regulatory role for these prejunctional K+ channels in the release of NO from NANC nerves in the canine ileocolonic junction.

Effects of 3,4-diaminopyridine and tetraethylammonium on the pre- and post-junctional alpha-adrenoceptor mediated inhibitory actions of noradrenaline in the guinea-pig ileum

The effects of potassium channel blockers, 3,4-diaminopyridine (DAP) and tetraethylammonium (TEA) were studied on the pre- and post-junctional alpha 2-adrenoceptor mediated effects of noradrenaline in the guinea-pig proximal ileum. Both DAP (4 to 500 mumol l-1) and TEA (0.3 to 3 mmol l-1) transiently increased the basal tension of the ileum. However, DAP also increased the amplitude of the smooth muscle twitches evoked by transmural nerve stimulation, whereas TEA marginally depressed them. Atropine (2 mumol l-1) antagonized the contractions induced by DAP but did not affect the similar effects of TEA. On the other hand, DAP restored the smooth muscle twitches depressed by atropine, while TEA did not. DAP, in a concentration-dependent manner, reduced or abolished the prejunctional inhibitory alpha 2-adrenoceptor mediated effect of noradrenaline, whereas TEA (up to 3 mmol l-1) was almost ineffective. The postjunctional inhibitory alpha 2-adrenoceptor mediated effect of noradrenaline was attenuated even at the smallest TEA concentration used (0.3 mmol l-1) and its postjunctional stimulatory alpha 1-adrenoceptor mediated effect was unmasked. However, DAP, was only marginally effective, even at the highest concentrations used (100 and 500 mumol l-1). From these results it would appear that in both the pre- and post-junctional inhibitory alpha 2-adrenoceptor mediated actions of noradrenaline in the guinea-pig ileum the primary step might be an increased potassium conductance. However, the potassium channels on the neuronal and the smooth muscle membrane have different sensitivities to DAP and TEA.