Regulated transport of Na⁺ is an important feature of the adult alveolar epithelial (ATII) cell and the mature fetal distal lung epithelial (FDLE) cell. We compared the bioelectric properties of ATII cells and FDLE cells which have been cultured in conditions that favour the development of the adult cell phenotype.

ATII cells were isolated from rats killed by intraperitoneal injection of pentobarbital (10 mg (kg body weight)^-1) and FDLE cells were isolated from fetuses (gestational age 20 days) that were removed from anaesthetised rats (3 % halothane, killed before regaining consciousness). Cells were isolated by enzymatic digestion and incubated at adult alveolar PO₂ (100 mmHg) in minimally defined serum-free media and were treated with dexamethasone (200 nM) and tri-iodo-thyronine (T₃; 10 nM). FDLE and ATII cells formed resistive monolayers at 48 and 96 h, respectively, which were mounted in Ussing Chambers. At 96 h the mean baseline PD, I_SC and R_t of ATII and FDLE cells were 1.95 ± 0.29 mV, 6.62 ± 0.83 µA cm^-2 and 337 ± 39 V cm^-2 (n = 11) and 3.09 ± 0.83 mV cm^-2, 7.92 ± 0.35 µA cm^-2 and 396 ± 110 V cm^-2 (n = 4), respectively (mean ± S.E.M.; Student’s paired t test). Addition of apical amiloride (10 µM) to both FDLE and ATII cells decreased basal I_SC by ~80 %, indicating predominantly sodium absorbing cells.

To establish the pharmacological properties of the ion channels underlying this Na⁺ transport, concentration-effect curves were constructed for Na⁺ channel antagonists. The rank order of potency amongst these compounds in both cell types was benzamil > amiloride > EIPA, suggesting that the highly selective epithelial Na⁺ channel (ENaC) is involved in Na⁺ transport across these cells.

We have previously (Collett et al. 2002) shown that addition of the β-adrenoceptor agonist isoprenaline to the basolateral membrane of FDLE cells at 48 h increases I_SC by ~60 % (n = 6, basal I_SC: 7.67 ± 0.63 µA cm^-2, peak increase I_SC: 12.07 ± 0.99 µA cm^-2, P < 0.05). This response is not evident when FDLE cells are maintained in culture for 96 h, basolateral isoprenaline induced no significant increase in I_SC (n = 4, basal I_SC: 8.76 ± 1.51 µA cm^-2, peak increase I_SC: 9.96 ± 1.25 µA cm^-2 P > 0.05). In ATII cells stimulation with basolateral isoprenaline does not induce an increase in I_SC, but a significant decrease in I_SC (approximately 35%) was observed (n = 5, basal I_SC: 6.48 ± 0.52 µA cm^-2, peak decrease I_SC: 4.27 ± 0.39 µA cm^-2, P < 0.05). Further studies have shown that in ATII cells cultured in serum-containing media (DMEM: FBS (10%)) for the initial 48 h in culture, the observed decrease in I_SC in response to stimulation with isoprenaline is abolished (basal I_SC: 8.72 ± 0.98 µA cm^-2, peak increase I_SC: 9.59 ± 1.03 µA cm^-2, P > 0.05, n = 7).