The Action of a Potassium Channel Activator, BRL 38227 (Lemakalim), on Human Airway Smooth Muscle

Potassium (K+) channels are present on airway smooth muscle cells, and their activation results in hyperpolarization and relaxation. Because these effects may have therapeutic relevance to asthma, we examined the activity of the active L-enantiomer of cromakalim, BRL 38227 (lemakalim), a selective K+ channel activator, against a variety of spasmogens in human bronchi in vitro. BRL 38227 produced relaxation of bronchi with either resting tone or tone induced by histamine, carbachol, neurokinin A, or KCl (20 mM) with an efficacy (%Emax) of 60 to 80% of that of isoproterenol and an EC50 (the concentration producing 50% of the maximal response) of 0.2 to 0.6 µM. However, BRL 38227 had a significantly lower potency and efficacy against 80 mM KCl than against the other spasmogens (%Emax, 12% of isoproterenol and EC50, 7.2 µM; p < 0.005 and p < 0.001, respectively), supporting the view that BRL 38227 acts on K+ channels. The D-enantiomer BRL 38226 was less potent (EC50, 2.6 µM) than BRL 38227 and produced only 43% of the isoproterenol relaxation. BRL 38227-induced relaxation was significantly inhibited by the ATP-sensitive K+ channel antagonist glibenclamide (0.1 and 1 µM), with a three-fold and eight-fold shift to the right of the dose-response curve, respectively. In the presence of a maximal relaxation induced by the calcium voltage-dependent channel antagonist verapamil, BRL 38227 was able to produce an additional 37% relaxation response. Thus, BRL 38227 is an effective relaxant of human airway smooth muscle, and this activity results from an action at K+ channels. Its role in the management of airway diseases such as asthma deserves further investigation.