N-acetyl-L-cysteine and its derivatives activate a Cl- conductance in epithelial cells

N-Acetyl-L-cysteine (NAC) is a widely used mucolytic drug in patients with a variety of respiratory disorders including cystic fibrosis (CF). The beneficial effects of NAC are empirical and the exact mechanism of action in the airways remains obscure. In the present study we examined the effects on whole-cell (we) conductance (G(m)) and voltage (V-m) of NAC and the congeners S-carboxymethyl-L-cysteine (CMC) and S-carbamyl-L-cysteine (CAC) and L-cysteine in normal and CF airway epithelial cells. L-Cysteine (1 mmol/l) had no detectable effect. The increase in G(m)(Delta G(m)) by the other compounds was concentration dependent and was (all substances at 1 mmol/l) 3.8 +/- 1.4 nS (NAC; 11), 4.2 +/- 1.0 nS (CMC; n = 16) and 3.8 +/- 1.6 nS (CAC; n = 18), respectively. The changes in G(m) were paralleled by an increased depolarization (Delta V-m) when extracellular Cl- concentration was reduced to 34 mmol/l: under control conditions = -4.1 +/- 2.1 versus 10.2 +/- 2.1 mV in the presence of NAG, CMC, CAC (n = 36). In the presence of NAG, CMC and CAC, the reduction in Cl- concentration was paralleled by a reduction of G(m) by 2.1 +/- 0.4 nS (n = 35), indicating that all substances acted by increasing the Cl- conductance. Analysis of intracellular pH did not reveal any changes by any of the compounds (1 mmol/l). A Cl- conductance was also activated in HT29, colonic carcinoma and CF tracheal epithelial (CFDE) cells but not in CFPAC-1 cells, which do not express detectable levels of Delta F508-CFTR, suggesting that the presence of CFTR may be a prerequisite for the induction of Cl- currents. Next we examined the ion currents in Xenopus oocytes microinjected with CFTR-cRNA. Water-injected oocytes did not respond to activation by forskolin and 3-isobutyl-1-methylxanthine (IBMX) (Delta Gm = 0.08 +/- 0.04 mu S; n = 10) and no current was activated when these oocytes were exposed to NAC or CMC. In contrast, in CFTR-cRNA-injected oocytes G(m) was enhanced when intracellular adenosine 3',5'-cyclic monophosphate (cAMP) was increased by forskolin and IBMX (G(m) = 4.5 +/- 1.3 mu S; n = 8). G(m) was significantly increased by 0.74 +/- 0.2 mu S (n = 11) and 0.46 +/- 0.1 mu S (n = 10) when oocytes were exposed to NAC and CMC, respectively (both 1 mmol/l). In conclusion, NAC and its congeners activate Cl- conductances in normal and CF airway epithelial cells and hence induce electrolyte secretion which may be beneficial in CF patients. CFTR appears to be required for this response in an as yet unknown fashion.