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The active transport of Na and Cl across bovine tracheal epithelium was studied in vitro by measuring 22Na and 36Cl fluxes under short-circuit conditions. Under basal conditions, both net Cl secretion and net Na absorption were observed: the sum of these two net fluxes accounted for 85% of the measured short-circuit current. The rate of spontaneous Cl secretion exceeded that of Na absorption by a factor of 2. Indomethacin, an inhibitor of endogenous prostaglandin production, decreased Cl secretion and increased Na absorption, reversing the direction of net transepithelial ion flow from secretion to absorption. The ratio of the change in each net ion flux was about 1:1. 50% of the basal net flux of Na was inhibited by amiloride (10(-4) M). The indomethacin-induced increase in the lumen-to-serosa flux of Na was entirely amiloride sensitive. An amiloride-insensitive fraction of this flux, of constant magnitude, was apparent in both control and indomethacin-treated tissues. The Na transport inhibitor had no effect on unidirectional or net Cl fluxes. Cl secretion was abolished by 4-methyl-diphenylamine-2'-carboxylic acid (50B). The Cl transport inhibitor had no effect on unidirectional or net Na fluxes. The results suggest that the rates of Na and Cl transport may be modulated in a reciprocal fashion by certain agents, which probably act through cyclic AMP, but that the two transport processes are not mutually interdependent in any simple, direct fashion. The lack of evidence for direct interaction between Na and Cl transport raises the possibility that there are separate absorptive and secretory cells in the tracheal epithelium, rather than a single transporting cell.

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