Clomipramine Induces the Opening of Na⁺ Channels in the Frog Skin Epithelium

Abstract

This paper studies the effect of clomipramine on the electrical parameters of the frog skin epithelium. We have found that clomipramine induces: 1) an increase in short circuit current that is equivalent to an increase in net Na π flux, 2) an increase in total transepithelial conductance, 3) a depolarisation of the apical barrier, and 4) a decrease in the fractional resistance of the apical membrane. These results suggest that clomipramine promotes an increase in the sodium permeability of the apical barrier by increasing either the concentration of amiloride-sensitive Na π channels or by increasing their individual conductance. Using kinetic models, values for the affinity constants for clomipramine and amiloride were estimated (K Clo Ω(18.1∫1.04)¿10 ª6 M; K Amil Ω56¿10 ª8 M). Identical studies were performed for eel intestinal epithelium and no significant changes on the short circuit current were observed.Clomipramine and imipramine are amphiphilic compounds with a hydrophobic tricyclic ring and a polar ammonium group, with a pka of 9.5 (Ullrich et al. 1993) and a partition coefficient of 3.88 and 4.62 respectively (Leo et al. 1971; Kodavanti & Mehendale 1990). In aqueous solution, imipramine forms micelles with a critical micelle concentration of approximately 150 mM (Srivastava et al.1982).Clomipramine and imipramine are denominated tricyclic antidepressants as they are used in the treatment of major depressions (Trimble 1990) and obsessive compulsive disorders (Pigott & Seay 1999;Freeman et al. 1994). These drugs act as inhibitors of serotonergic (Marcusson & Tiger 1988;Ramamoorthy et al. 1993a;Humphreys et al. 1994) and noradrenergic (Wyllie et al. 1985;Ramamoorthy et al. 1993b) systems and are easily absorbed and distributed in the organism. Clomipramine single oral dose administration (0.7 mg/kg) reaches a mean peak value of 43.8 ng/ml (0.139 mM) in 2 hr; long-term administration of clomipramine on obsessive compulsive disorders patients with a daily dosage of 239.4∫57 mg, presented a mean plasma concentration of 169.9∫102 ng/ml (Della-Corte et al. 1993). A significant correlation was found between the plasma concentration of clomipramine and the decrease of obsessive compulsive disorders symptoms to a subclinical level (Mavissakalian et al. 1990;Chen et al. 1998). In post mortem situations of several human cases it has been observed that clomipramine plasma concentration varies from 0.21 to 21.5 mg/l (0.66 to 68 mM) (Pounder & Jones 1990;McIntyre et al. 1994;Romano & Bono 1994). Post-mortem liver concentrations for 10 human cases varied from 7-320 mg/ Author for correspondence: Teresa F. Moura, Department of Chemistry, Faculty of Sciences and Technology, New University of Lisbon, Quinta da Torre, 2829-516 Caparica, Portugal (fax π351212948550, e-mail tfm/dq.fct.unl.pt).kg where in the two cases directly associated with clomipramine toxicity, the highest liver concentrations were observed (10 to 30 times greater than the others) (McIntyre et al. 1994). A wide range of effects due to clomipra...