Molecular mechanisms of the reversal of imipramine-induced sodium channel blockade by alkalinization in human cardiac myocytes

Background: Alkalinizing agents reverse cardiotoxicity of a variety of sodium channel blockers, including tricyclic antidepressants, but their mechanisms of action are poorly understood. Purpose: To establish the mechanisms by which alkalinization diminishes the sodium channel blocking action of imipramine. Methods: The whole-cell voltage-clamp technique was used to measure I,, during a variety of depolarizing pulse protocols in isolated human atrial myocytes, in the presence and absence of imipramine. A three-state model was used to analyze state-dependent I,, block. Results: Imipramine (1 and 5 mu M) strongly inhibited I,,. Experimental data and piecewise exponential analysis suggested significant binding to both activated and inactivated states. Alkalosis antagonized imipramine-induced I,, blockade by increasing the unbinding rate, with intracellular alkalosis being more effective than extracellular alkalosis. The dissociation constant (K-d) for the inactivated state was increased from 0.55 to 1.40 mu M by extracellular alkalosis and to 2.51 mu M by intracellular alkalosis. Along with the reversal of drug-induced shifts in the inactivation curve, these data indicate that alkalosis on either side of the membrane antagonized drug interactions with the inactivated state. On the other hand, only intracellular alkalosis antagonized activated state block, increasing the K-d from 0.67 mu M to 2.18 mu M, while extracellular alkalosis left the activated state K-d unaltered at 0.67 mu M Conclusions: Alkalinization antagonizes the I-Na-blocking action of imipramine by promoting unbinding from the receptor. Intracellular alkalosis has a particularly important effect related to the activated-state interaction. The lipid-soluble, uncharged moiety appears to be a critical determinant of imipramine's ability to dissociate from the Na+ channel receptor. (C) 1998 Elsevier Science B.V. All rights reserved.