Active transport of Na+ and Cl- across the canine tracheal epithelium in vitro.

The flow of water across the tracheobronchial epithelium is likely to be an important determinant of the efficiency of mucociliary clearance. Because bulk water flow has been shown to be coupled to net ion flux in several epithelia, experiments were performed to determine whether ions are transported actively by the canine tracheal epithelium. Electrical potential difference, short circuit current, and the unidirectional fluxes of positive sodium ions (Na+) and negative chloride ions (Cl-) were measured using an in vitro preparation of the posterior membranous portion of canine trachea. The values of the electrical parameters were potential difference, 30.7 +/- 2.7 mV (SE; n = 30), lumen negative to submucosa; short circuit current, 108 +/- 8 muA per cm2 (SE; n = 30). Bidirectional fluxes of Na+ and Cl- were measured both with and without continuous short circuiting of the epithelium. Under short circuit conditions there was a unidirectional flux of Cl- toward the lumen of 7.4 +/- 1.2 muEq per cm2 per hour (SE; n = 6) and toward the submucosa of 4.7 +/- 1.1 muEq per cm2 per hour (SE; n = 6), resulting in a calculated net flux of Cl- of 2.7 +/- 0.6 muEq per cm2 per hour toward the lumen. The unidirectional flux of Na+ toward the lumen was 1.3 +/- 0.5 muEq per cm2 per hour (SE; n = 8) and toward the submucosa was 2.1 +/- 0.7 muEq per cm2 per hour (SE; n = 8). This resulted in a calculated net flux of Na+ of 0.8 +/- 0.2 muEq per cm2 per hour toward the submucosa. These fluxes are consistent with active transport and together they account for the entire short circuit current. Under open circuit conditions, the calculated net fluxes of Cl- and Na+ were not significantly different from zero.