MOST BASAL I-SC IN CALU-3 HUMAN AIRWAY CELLS IS BICARBONATE-DEPENDENTCL- SECRETION

Serous cells secrete antibiotic-rich fluid, but secretion is impaired in cystic fibrosis. We are investigating Calu-3 cells as a serous cell model. Basal short-circuit current (I-SC) in Calu-3 cells grown at ai r interface had a basal I-SC approximately six times larger than subme rged cultures (69 +/- 22 vs. 11 +/- 10 mu A/cm(2)). Basal I-SC in eith er condition was reduced only 7 +/- 5% by bumetanide and was unaffecte d by apical amiloride, 4,4'-diisothiocyanostilbene-2,2'-disulfonic aci d, 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS), or calixarene but was reduced 77 +/- 18% by N-phenylanthranilic acid. Three transport m echanisms accounted for almost all basal I-SC. The largest component i s HCO3--dependent Cl- secretion. Replacement of Krebs-Henseleit soluti on with N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid-buffered s olution and changing gassing from 95% O-2-5% CO2 to air reduced the ba sal I-SC by 61 +/- 10%. Acetazolamide decreased basal I-sc by 33 +/- 6 %, whereas acetazolamide + basolateral DNDS eliminated 42-58% of the b umetanide-insensitive basal I-SC. Neither DNDS nor acetazolamide had a ny effect when applied in HCO3--free solution. Apical phlorizin, a blo cker of Na+-glucose cotransport, eliminated one-half of the remaining I-sc. Cl- replacement with gluconate eliminated all I-SC except the ph lorizin-sensitive component. Unlike basal I-SC, 80 +/- 24% of stimulat ed I-SC was inhibited by bumetanide. Thus basal and stimulated secreti ons are mediated by different mechanisms.