The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP/protein kinase A (PKA) regulated Cl- channel that plays an important role in maintaining salt and fluid balance in airway and also regulates a number of transport proteins, e.g. ENaC (1). In cystic fibrosis (CF), which is caused by mutations in CFTR, there is abnormal Ca2+ homeostasis in airway cells (2). Given the role CFTR plays as a channel regulator, cation currents in normal (16HBE14o-) and cystic fibrosis (CFBE41o-) airway epithelial cells were examined. Whole cell ruthenium red-sensitive currents (IRR) were measured in 16HBE14o- and CFBE41o- human airway cells under control conditions or after cAMP stimulation. The bath contained high NaCl and the pipette CsAsp. A ramp protocol clamped potential between +60 to -100mV. Na:Ca selectivity was investigated by measuring the shift in reversal potential (Vrev) of IRR when the extracellular solution was changed from low (2mM) to high Ca2+ (20mM). Under this condition a positive shift in Vrev indicated that the current was more selective for Ca2+ over Na+. Statistical significance was tested using Students t-test and assumed at the 5% level. Under control conditions switching from low to high bath Ca2+ shifted the Vrev of the IRR in CFBE41o- cells (-3.22 ± 1.6 mV to -0.02 ± 0.7 mV, n=6) but not in 16HBE14o- cells (-0.94 ± 2.5 mV to -0.83 ± 3.0 mV, n=7). On cAMP stimulation both cell types demonstrated significant and comparable shifts in Vrev when bath Ca2+ was increased, (-7.77 ± 3.25 mV to -3.13 ± 2.24 mV for 16HBE14o-, n=7 and -3.28 ± 1.40 mV to -0.31 ± 0.56 mV for CFBE41o-, n=15). Calcium imaging experiments further confirmed that in 16HBE14o- cells, cAMP stimulation caused a rise in intracellular Ca2+. Since 16HBE14o- cells show greater Ca2+ selectivity after cAMP stimulation, it was hypothesised that this could be due to a PKA mediated effect. Day matched 16HBE14o- cells were stimulated in the absence or presence of the PKA inhibitor H-89. In the absence of H-89 a positive shift in Vrev on increasing bath Ca2+ was observed (2.12 ± 1.57 mV to 13.5 ± 2.35 mV, n=16). In contrast, pre-incubation with H-89 before cAMP stimulation prevented the shift in Vrev (1.25 ± 1.36 mV to 0.92 ±1.88 mV, n=17). These data indicate that in normal airway cells cAMP stimulation increases the Ca2+ selectivity of cation channels, leading to a rise in intracellular Ca2+. Such a rise in Ca2+ would be expected to activate Ca2+ activated Cl- channels and support the CFTR mediated secretion of Cl- by these cells. This increase in Ca2+ selectivity was also dependent on PKA, suggesting a role for phosphorylation in this process. The higher Ca2+ selectivity in CF airway cells in un-stimulated conditions suggests that CFTR may regulate Ca2+ permeable channels in airway.