Insulin treatment improves alveolar-capillary gas exchange in type 2 diabetes, one suggested mechanism being insulin-evoked stimulation of alveolar epithelial fluid absorption [1]. Studies in A6 (Xenopus renal epithelial) cells [e.g. 2] have shown that insulin can stimulate epithelial Na⁺ transport, in turn providing an osmotic driving force for increased fluid absorption. There is however little data regarding the effect of insulin on lung epithelial Na⁺ transport and hence this study aims to investigate the response to insulin in H441 distal lung epithelial cells. H441 cells were grown on permeable supports and after 8 days resistive monolayers were mounted in Ussing chambers. Data are mean ± S.E.M. analysed by Student’s paired t test (significant if P<0.05). In this laboratory, H441 cells are routinely cultured in the presence of insulin (0.87 µM) and so the effect of removing it from the culture medium was assessed. Indeed cells grown in the absence of insulin displayed altered basal properties relative to age-matched cells grown in its presence. The removal of insulin resulted in decreased potential difference from 8.43 ± 0.96 to 3.02 ± 0.51 mV (n=6, P<0.001), short circuit current (I_sc) was also reduced from 25.88 ± 3.49 to 14.59 ± 2.16 µA cm^-2 (n=6, P<0.001) and transepithelial resistance (R_t) fell from 321.83 ± 33.05 to 199.67 ± 9.18 Ω cm^-2 (n=6, P=0.019). As a result of these findings we investigated the effect of acute addition of insulin to cells cultured in its absence. 20nM (sufficient to stimulate Na⁺ transport in A6 cells ⁽²⁾) was added to the basolateral membrane under short circuit conditions and evoked an increase in I_sc of ~40% (13.23 ± 1.55 to 18.38 ± 1.97 µA cm^-2, n=6, P<0001). Studies in basolaterally permeabilised cells showed that the amiloride-sensitive apical Na⁺ conductance was also increased by 20 nM insulin (74.86 ± 14.87 µS cm^-2 vs control 51.46 ± 10.56 µS cm^-2, n=5, P=0.013) confirming the changes in I_sc represent increased Na⁺ influx through the apical membrane. Insulin-evoked Na⁺ transport in A6 cells is dependent on the activity of PI-3-Kinase [2]. Likewise in the present study the PI-3-Kinase inhibitor LY-294002 (LY) inhibited both basal and insulin-stimulated I_sc. Basal I_sc was reduced by ~65% (from 14.90 ± 1.35 to 4.99 ± 0.56 µA cm^-2) while the response to insulin was all but abolished (increase in I_sc of only 0.23 ± 0.17 µA cm^-2 after LY vs control response of 5.66 ± 0.76 µA cm^-2). Interestingly, previous experiments have shown LY had no effect on the stimulatory response to forskolin addition in H441 cells [3], indicating a distinction between the signalling pathways used by these two agents to stimulate Na⁺ transport. In summary insulin stimulates Na⁺ transport in human H441 lung cells, at least in part by increasing apical Na⁺ conductance. This response appears to be mediated via PI-3-Kinase.