Sodium and chloride transport by the tracheal epithelium of fetal, new-born and adult sheep.

In vitro measurements were made of Na+ and Cl- isotopic fluxes across the tracheal epithelium of mature fetal lambs (130-143 days gestation), new-born lambs (up to 41 days of age) and adult sheep under conditions of continuous short circuiting. The effects of a variety of drugs were examined, but only in the case of amiloride and isoprenaline were observations made in all three groups. Experiments designed to elucidate the mechanism of basal Cl- secretion were performed in adult trachea only. Under resting conditions the net flux of Na+ from lumen to submucosa exceeds that of Cl- in the reverse direction in fetal and adult trachea. In the new-born the two fluxes are more or less equivalent in magnitude. In none of the three groups is the sum of ion fluxes significantly different from the short-circuit current (Isc). Removal of Na+ from, or addition of furosemide (10(-3) M) to, the solution bathing the submucosal surface of adult trachea has the effect of reducing Isc by an amount which approximates to the Cl- current (29%). At a concentration of 10(-4) M on the submucosal side of adult trachea, ouabain causes potential difference and Isc to fall to zero within 70 min of addition to the bathing solution. Nevertheless, there remains a significant net Na+ flux from submucosa to lumen. The addition of isoprenaline (10(-4) M) to the medium bathing the submucosal surface of both fetal and adult trachea causes an increase in the one-way flux of Cl- from submucosa to lumen with consequent increase in net Cl- flux towards the lumen. (The Na+ fluxes are unchanged.) However, in the adult the Cl- secretory response to isoprenaline is very much less and is not accompanied by an increase in electrical conductance. As judged by the change in Isc, all the post-natal fall in beta-agonist responsiveness takes place within the 3 week period following birth. Whereas, in the fetus, the effect of luminally applied amiloride on the Na+ fluxes is negligible, in the adult the one-way flux of Na+ from lumen to submucosa is reduced by 35% with a consequent 60% fall in net Na+ flux towards the submucosa.(ABSTRACT TRUNCATED AT 400 WORDS)