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The effects of cromakalim and glibenclamide on membrane properties and responses to acetylcholine of canine trachea were studied in the double sucrose gap to evaluate the presence and function of ATP-sensitive K+ channels. Cromakalim produced a concentration-dependent hyperpolarization of muscle membrane potential which at maximum brought the membrane potential near the potassium equilibrium potential. Current clamping by hyperpolarizing current to this equilibrium potential abolished the hyperpolarization but not the membrane resistance decrease to cromakalim. Glibenclamide had no effect on resting membrane properties but reduced or abolished effects of cromakalim. Another K+ channel antagonist, tetraethylammonium at 20 mM, also reduced the effects of cromakalim, but 4-aminopyridine (5 mM), Ba2+ (1 mM, and apamin (10−6M) had no antagonistic effect. The {EJP} produced on stimulation of cholinergic nerves sometimes increased just after cromakalim-induced hyperpolarization, but within 5–10 min as membrane resistance dramatically fell it was reduced, as was the depolarization to infused acetylcholine. Initially the reduction in {EJP} amplitude could be partially overcome by applying hyperpolarizing currents or by applying a second field stimulation; later the {EJP} was reduced further and was unaffected by these procedures. Even when depolarization to acetylcholine was markedly reduced, the contraction was not. Glibenclamide had no effects alone but antagonized all the effects of cromakalim. These results suggest that ATP-sensitive cromakalim activated K+ channels are present in canine trachea but are usually closed during resting conditions under our experimental conditions. When they are opened by cromakalim, they hyperpolarize to near EK, markedly decrease membrane resistance and reduce the depolarization response to acetylcholine, probably by short circuiting the acetylcholine-induced current. There also appear to be ATP-sensitive K+ channels in cholinergic nerves which, when activated, reduced acetylcholine output.
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