Ion transport regulation of lung liquid secretion in foetal lambs

To test the hypothesis that liquid formation in the foetal lung reflects the balance between Cl^- secretion and Na⁺ absorption by the respiratory tract epithelium, we studied the independent and combined effects of selective ion transport inhibitors on basal production of lung liquid in foetal lambs. We prepared 19 foetal lambs (gestation 125 ± 4, term = 147 days) with chronic indwelling catheters for subsequent measurement of luminal liquid production over time (Jv). Using an impermeant tracer technique, we measured Jv before and after tracheal instillation of 2 different inhibitors of ion transport: bumetanide, a Na⁺-K⁺-2Cl^- co-transport inhibitor, and amiloride, a Na⁺ transport inhibitor. In 7 foetuses we sequentially added bumetanide (10^-4 M) and 2 different concentrations of amiloride (10^-6 M, 10^-4 M) to the liquid within the lung lumen. After we gave bumetanide, Jv decreased from 12 ± 4 ml/h to 0 ± 5 ml/h and subsequently increased during the 2 periods of amiloride exposure (10^-6 M: 6 ± 5 ml/h; 10^-4 M: 7 ± 7 ml/h). In 5 control studies we gave bumetanide, followed by only amiloride vehicle. Jv for all time periods in the control studies was similar to the experimental group, demonstrating no effect of amiloride. In 5 foetuses we administered the 2 concentrations of amiloride before bumetanide. There was no change in Jv with either concentration of amiloride (baseline: 13 ± 2 mvh; 10^-6 M amiloride: 15 ± 5 ml/h; 10^-4 M amiloride: 13 ± 6 ml/h). Simultaneous delivery of bumetanide (10^-4 M) and amiloride (10^-4 M) decreased Jv to 0 ± 2 ml/h. Thus, the inhibitory effect of bumetanide was the same, with or without amiloride present. Additional studies showed that bumetanide inhibition of Jv could not be explained by diuretic-induced changes in pulmonary vascular pressure or protein osmotic pressure in plasma. These results are consistent with a model of resting net production of luminal liquid that is governed by Na⁺-K⁺-2Cl^- cotransport, without detectable absorption of liquid through amiloride-sensitive Na⁺ pathways.