Differential effects of lidocaine and mexiletine on relaxations to ATP-sensitive K+ channel openers in rat aortas

Background: In cardiac myocytes, lidocaine reduces but mexiletine increases adenosine triphosphate (ATP)-sensitive K+ currents, suggesting that these class Ib antiarrhythmic drugs may differentially modify the activity of ATP-sensitive K+ channels. The effects of lidocaine and mexiletine on arterial relaxations induced by K+ channel openers have not been studied. Therefore, the current study was designed to evaluate whether lidocaine and mexiletine may produce changes in relaxations to the ATP-sensitive K+ channel openers cromakalim and pinacidil in isolated rat thoracic aortas. Methods: Rings of rat thoracic aortas without endothelia were suspended for isometric force recording. Concentration-response curves were obtained in a cumulative fashion. During submaximal contractions to phenylephrine(3 x 10(-7) M), relaxations to cromakalim(10(-7) to 3 x 10(-5) ri), pinacidil(10(-7) to 3 x 10(-5) M), or diltiazem (10(-7) to 3 x 10(-4) M) were obtained. Lidocaine (10(-5) to 3 x 10(-4) M), mexiletine (10(-5) to 10(-4) M) or glibenclamide (5 x 10(-6) M) was applied 15 min before addition of phenylephrine. Results: During contractions to phenylephrine, cromakalim and pinacidil induced concentration-dependent relaxations, A selective ATP-sensitive K+ channel antagonist, glibenclamide (5 x 10(-6) M), abolished these relaxations, whereas it did not alter relaxations to a voltage-dependent Ca2+ channel inhibitor, diltiazem (10(-7) to 3 x 10(-4) hz), Lidocaine (more than 10(-5) M) significantly reduced relaxations to cromakalim or pinacidil in a concentration-dependent fashion, whereas Lidocaine (3 x 10(-4) M) did not affect relaxations to diltiazem, In contrast, mexiletine (more than 10(-5) M) significantly augmented relaxations to cromakalim or pinacidil, Glibenclamide (5 x 10(-6) M) abolished relaxations to cromakalim or pinacidil in arteries treated with mexiletine (10(-4) hi), Conclusions: These results suggest that lidocaine impairs but mexiletine augments vasodilation mediated by ATP-sensitive K+ channels in smooth muscle cells.