CFTR as a Regulator of the Epithelial sodium Channel – What's the Problem?

The search for the molecular defect causing cystic fibrosis (CF) has taken many routes. In 1968, Schulz and Frömter suggested that in sweat gland ducts from CF patients "chloride permeability is decreased and sodium leak permeability is increased". The observation that the activity of the amiloride-sensitive, epithelial sodium (Na +) channel (ENaC) is increased in the airways of CF patients is also old: Knowles et al. (1981) made the important discovery that the electrical potential of CF nasal mucosa was twice as negative as that of controls. Knowles et al. (1981) postulated that the increased electrical potential of CF airways was due to increased Na + absorption from the airway surface liquid (ASL), resulting in increased fluid absorption and decreased volume of ASL. Quinton (1983) was the first to demonstrate convincingly that the main defect in sweat ducts from CF patients is a strongly reduced Cl-conductance. The indirect conclusions of Quinton (1983) from simple potential measurements were subsequently confirmed by Bijman and Frömter (1986). Using transepithelial potential and resistance measurements, Bijman and Frömter (1986) demonstrated that the transepithelial resistance is high in CF sweat ducts and not changed by chloride (Cl-) replacement, whereas in control ducts the transepithelial resistance is low and increases dramatically by Cl-replacement. Recently Quinton's group reported that the ENaC conductance in the sweat duct is enhanced by activation of cystic fibrosis transmembrane conductance regulator (CFTR), whereas in CF sweat ducts ENaC conductance is low and unresponsive to stimulation (Reddy at al., 1999). It is important to realize the "technicalities" of these demanding experiments: they were current-clamp experiments, i.e. current was clamped and potential measured. The conductance was inferred from current pulses. Because a large gradient of NaCl concentration was imposed across the epithelium, CFTR stimulation induced a voltage shift from ~+10 mV to ~+90 mV in the presence of amiloride and from ~–10 mV to ~+45 mV in its absence (see fig. 1 in Reddy et al., 1999). However, using a large NaCl concentration gradient, the conductance of the voltage-independent channels CFTR and ENaC is expected to be "voltage dependent" (from the Goldmann-Hodgkin-Katz equation, see e.g. Hille 1992). The theoretical current-voltage relationship for ENaC, derived for a Na + gradient of 150 mM to 10 mM, is shown in fig. 1. From this relationship it follows that the observed increase of ENaC conductance by CFTR activation can qualitatively be explained by the CFTR-induced voltage shift. However, …