MECHANISMS OF SODIUM AND CHLORIDE TRANSPORT ACROSS EQUINE TRACHEAL EPITHELIUM

Equine tracheal epithelium, stripped of serosal muscle, mounted in Ussing chambers, and bathed in plasmalike Ringer solution generates a serosa-positive transepithelial potential of 10-22 mV and a short-circuit current (I(sc)) of 70-200-mu-A/cm2. Mucosal amiloride (10-mu-M) causes a 40-60% decrease in I(sc) and inhibits the net transepithelial Na flux by 95%. Substitution of Cl with gluconate resulted in a 30% decrease in basal I(sc). Bicarbonate substitution with 20 mM N-2-hydroxyethylpiperazine-N' -2-ethanesulfonic acid decreased the I(sc) by 21%. The Cl-dependent I(sc) was inhibited by serosal addition of 1 mM amiloride. Bicarbonate replacement or serosal amiloride (1 mM) inhibits the net Cl flux by 72 and 69%, respectively. Bicarbonate replacement significantly reduces the effects of serosal amiloride (1 mM0 on I(sc), indicating its effect is HCO3 dependent. Addition of 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP; M 100-mu-M) causes a 40% increase in I(sc). This effect is inhibited by subsequent addition of 10-mu-M serosal bumetanide. Bumetanide (10-mu-M) reduces net Cl secretion following stimulation with 9-BrcAMP (100-mu-M). Serosal addition of BaCl2 (1 mM) causes a reduction in I(sc) equal to that following Cl replacement in the presence or absence of 100-mu-M cAMP. These results suggest that 1) Na absorption depends on amiloride-inhibitable Na channels in the apical membrane, 2) Cl influx across the basolateral membrane occurs by both a Na-H/Cl-HCO3 parallel exchange mechanism under basal conditions and by a bumetanide-sensitive Na-(K?)-Cl cotransport system under cAMP-stimulated conditions, and 3) basal and cAMP-stimulated Cl secretion depends on Ba-sensitive K channels in the basolateral membrane.