On the natriuretic effect of verapamil: inhibition of ENaC and transepithelial sodium transport

The natriuretic effect of Ca2+ channel blockers has been attributed to hemodynamic changes and to poorly defined direct tubular effects. To test the possibility that verapamil may inhibit Na+ reabsorption at the distal tubule, its effect on transepithelial Na+ transport in aldosterone-stimulated A6 cells was determined. Cells were grown on permeable supports, and short-circuit current (I-sc) measured in an Ussing chamber was used as a surrogate marker for transepithelial Na+ transport. Application of 300 muM verapamil to the apical side inhibited I-sc by 77% and was nearly as potent as 100 muM amiloride, which inhibited I-sc by 87%. Verapamil-induced inhibition of I-sc was accompanied by a significant increase in transepithelial resistance, suggesting blockade of an apical conductance. Its action on transepithelial Na+ transport does not appear to occur through inhibition of L-type Ca2+ channels, since I-sc was unaffected by removal of extracellular Ca2+. Verapamil also does not appear to inhibit I-sc by modulating intracellular Ca2+ stores, since it fails to inhibit transepithelial Na+ transport when added to the basolateral side. The effect on Na+ transport is specific for verapamil, since nifedipine, Ba2+, 4-aminopyridine, and charybdotoxin do not significantly affect I-sc. A direct effect of verapamil on the epithelial Na+ channel (ENaC) was tested using oocytes injected with the alpha-, beta-, and gamma-subunits. We conclude that verapamil inhibits transepithelial Na+ transport in A6 cells by blocking ENaC and that the natriuresis observed with administration of verapamil may be due in part to its action on ENaC.