Endothelins (ETs) are a family of peptide hormones that act on G protein-coupled ET(A) and ET(B) receptors. ETs exert inotropic and chronotropic actions in the heart. Myocardial ischemia is associated with increased plasma levels of ET and cell swelling. We examined the effect of ETs on dog atrial swelling-induced chloride current (I(Cl,swell)). Whole-cell patch clamp was used; 10 nM ET-1 or ET-2 increased I(Cl,swell) by approximately twofold. ET-2 had no effect if I(Cl,swell) activation was prevented by hypertonic superfusate. Outward ET-2-induced current was blocked by 150 microM DIDS more effectively than inward current. Overnight pretreatment with phorbol 12-myristate 13-acetate (1.6 microM), pertussis toxin (100 ng/ml), or dialysis of the cell with 300 microM 2'-deoxyadenosine 3'-monophosphate, a P-site inhibitor of adenylyl cyclase, did not diminish the effect of ET-2. The effect of ET-2 was blocked by an ET(A1)- (BQ123), but not an ET(B)-selective (BQ788) antagonist. ET-2-induced currents were inhibited approximately 70% by PD 98059 (30 microM), a selective MAPK kinase (MEK) blocker. PD 98059 did not affect basal whole cell current or I(Cl,swell) before exposure to ET-2. The data suggest that MEK activity is not required for activation of atrial I(Cl,swell) but that ET-2 stimulates I(Cl,swell) by a MEK-dependent pathway.