Atypical effect of minoxidil sulphate on guinea pig airways

The effects of minoxidil sulphate, an "atypical" K(ATP) channel opener, and bimakalim, a benzopyran-type classical K(ATP) channel opener, on guinea pig airways in vitro and in vivo and on isolated portal veins from rats and guinea pigs were compared. Minoxidil sulphate inhibited the spontaneous activity of isolated guinea pig and rat portal vein preparations with pD2 values of 7.83+/-0.08 and 7.14+/-0.03, respectively (Emax=100% in both preparations). Bimakalim caused a more potent inhibition with pD2 values of 8.80+/-0.05 and 8.20+/-0.04, respectively (Emax=100% in both preparations). Minoxidil sulphate reduced the spontaneous tone of isolated guinea pig tracheal rings with a pIC50 value of 3.92+/-0.02 and the same efficacy as isoprenaline. Bimakalim was more potent (pIC50=7.25+/-0.02) but less efficacious (Emax=75% of the Emax of isoprenaline). The airway relaxant effect of bimakalim, but not minoxidil sulphate, was antagonised by glibenclamide (pA2=7.50) at concentrations above 0.1 microM. Bombesin-induced bronchoconstriction in anaesthetised, ventilated, normoreactive guinea pigs (measured as increase in total lung resistance) was dose-dependently reversed by intratracheally (i.t.) administered bimakalim (ED50=4 microg/kg; Emax=92% of maximally possible inhibition), but not by minoxidil sulphate, at doses up to 1 mg/kg i.t. In the same animals, following i.t. administration of higher doses, both minoxidil sulphate and bimakalim reduced blood pressure. Airways hyperreactivity to histamine induced by acute treatment of guinea pigs with immune complex was dose-dependently reversed by bimakalim (ED50=0.5 microg/kg i.t., Emax=100%). This effect was antagonised by glibenclamide (30 mg/kg i.v.). Minoxidil sulphate had a biphasic effect on airways hyperreactivity: at 1 microg/kg i.t., airways hyperreactivity was augmented, whereas at doses above 3.2 microg/kg i.t. it caused reversal of airways hyperreactivity. Both of the effects of minoxidil sulphate were insensitive to glibenclamide (30 mg/kg i.v.). It is concluded that the pharmacological profile of minoxidil sulphate in guinea pig airways is completely different from that of classical K(ATP) channel openers such as bimakalim. Minoxidil sulphate is either only weakly active or even inactive at K(ATP) channels in guinea pig airways or interacts with these channels in a different manner. The current results are consistent with there being differences between the K(ATP) channels in airways and blood vessels.