Background: The previous study by the authors showed that the class Ib antiarrhythmic drug lidocaine impairs but mexiletine augments vasorelaxation mediated by adenosine triphosphate-sensitive K+ channels. Lidocaine and mexiletine have different values of the negative logarithm of the drug-proton dissociation constant, indicating that the ion channel-blocking effects of these drugs under different pH levels may vary. However, the role of pH in the effects of lidocaine and mexiletine on vasodilation mediated by K+ channels has not been studied. Therefore, the current study was designed to examine whether the inhibition and augmentation of vasorelaxation in response to an adenosine triphosphate-sensitive K+ channel opener, levcromakalim, by the clinically relevant concentrations of lidocaine or mexiletine are modified by mild alkalinization or acidification in the isolated rat aorta.
Methods: Rings of the rat aorta without endothelium were suspended for isometric force recording. Three types of modified Krebs-Ringer solutions (pH 7.2, 7.4, and 7.6) were prepared by changing the composition of NaCl and NaHCO3. During contractions in response to phenylephrine (3 x 10(-7) M), relaxations in response to levcromakalim (10(-8) to 10(-5) M) were obtained. Lidocaine (10(-5) to 10(-4) M), mexiletine (10(-5) to 10(-4) M), or glibenclamide (10(-5) M) was applied 15 min before addition of phenylephrine.
Results: Relaxations in response to levcromakalim, which are abolished by the selective adenosine triphosphate-sensitive K+ channel antagonist glibenclamide (10(-5) M), were not different among the three pH groups. In the normal Krebs-Ringer solution of pH 7.4, lidocaine significantly reduced these relaxations in a concentration-dependent fashion. Alkalinization of pH 7.6 augmented the inhibitory effect of lidocaine on these relaxations, whereas acidification of pH 7.2 substantially abolished this effect. In contrast, mexiletine pH independently augmented relaxations in response to levcromakalim. Glibenclamide (10(-5) M) abolished these relaxations in arteries treated with mexiletine (10(-4) M) in any pH group.
Conclusions: These results suggest that even under conditions of such mild alkalosis or acidosis, vasorelaxation via adenosine triphosphate-sensitive K+ channels is dependent on pH in the presence of clinically relevant concentrations of lidocaine but not mexiletine.