Components of electrogenic transport in unstimulated equine tracheal epithelium

Basic components of unstimulated electrolyte transport across equine tracheal mucosa were characterized. After the tissue was mounted in Ussing chambers, both current and tissue resistance gradually increased for approximately 60 min before reaching stable values. Thereafter, under open-circuit conditions, the tissue had a resistance of 250 +/- 14 omega.cm2, generated a transepithelial potential difference of -34 +/- 1.7 (SE) mV (referenced to the serosal side) and an equivalent short-circuit current (Ieqsc) of -149 +/- 10.2 microA/cm2. Even though 10(-5) M amiloride reduced the current by approximately 65%, mucosal Na+ substitution with choline decreased the current significantly more (approximately 80%), indicating that part of the Na(+)-dependent current was amiloride insensitive. No current decrease occurred after serosal application of 10(-4) M bumetanide, which was expected to inhibit Na(+)-K(+)-2Cl(-)-mediated Cl- secretion, even though bilateral Cl- substitution with gluconate reduced Ieqsc by approximately 30 microA/cm2. Continuous short-circuit conditions caused a reversible fall in the short-circuit current that was inhibited by amiloride but not by Cl- depletion, suggesting that sustained short circuiting leads to a significant underestimation of the amiloride-sensitive Na+ transport. In the absence of Cl-, the response to amiloride was significantly smaller, which suggests that Cl- depletion also has an inhibitory effect on electrogenic, amiloride-sensitive Na+ absorption.