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Patch clamp methods and reverse transcription-polymerase chain reaction (RT-PCR) were used to characterize an apical K+ channel in Calu-3 cells, a widely used model of human airway gland serous cells. In cell-attached and excised apical membrane patches, we found an inwardly rectifying K+ channel (Kir). The permeability ratio was PNa/PK = 0.058. In 30 patches with both cystic fibrosis transmembrane conductance regulator and Kir present, we observed 79 cystic fibrosis transmembrane conductance regulator and 58 Kir channels. The average chord conductance was 24.4 +/- 0.5 pS (n = 11), between 0 and -200 mV, and was 9.6 +/- 0.7 pS (n = 8), between 0 and 50 mV; these magnitudes and their ratio of approximately 2.5 are most similar to values for rectifying K+ channels of the Kir4.x subfamilies. We attempted to amplify transcripts for Kir4.1, Kir4.2, and Kir5.1; of these only Kir4.2 was present in Calu-3 lysates. The channel was only weakly activated by ATP and was relatively insensitive to internal pH. External Cs+ and Ba2+ blocked the channel with Kd values in the millimolar range. Quantitative modeling of Cl- secreting epithelia suggests that secretion rates will be highest and luminal K+ will rise to 16-28 mm if 11-25% of the total cellular K+ conductance is placed in the apical membrane (Cook, D. I., and Young, J. A. (1989) J. Membr. Biol. 110, 139-146). Thus, we hypothesize that the K+ channel described here optimizes the rate of secretion and is involved in K+ recycling for the recently proposed apical H+ -K+ -ATPase in Calu-3 cells.

We are testing the hypothesis that the malfunctioning of airway gland serous cells is a component of cystic fibrosis (CF) airway disease. CF is caused by mutations that disrupt CF transmembrane conductance regulator, an anion channel essential for proper fluid secretion in some epithelia. Submucosal glands supply most of the mucus in upper airways, and gland serous cells are the primary site of CF transmembrane conductance regulator expression in airways. We have discovered a major defect in CF glands by in situ optical monitoring of secretions from single human airway glands. CF glands did not secrete to agents that elevated [cAMP](i) (0 responses/450 glands, 8 subjects), whereas glands were responsive in all donor tracheas (605/827 glands, 15 subjects) and in bronchi from subjects who were transplanted because of other lung diseases (148/166 glands, n = 10). CF glands secreted to cholinergic stimulation, and serous cells were abundant in glands from all CF subjects. The complete absence of secretion to agents that elevate [cAMP](i) suggests that altered secretion of gland mucus could contribute to CF lung disease.