Effects of amiloride on alveolar epithelial PD and fluid composition in rabbits

To look for evidence of active absorption of Na+ in the alveolus in vivo in air-filled lungs, we measured [K+] and [Cl-] in the alveolar lining fluid and the potential difference (PD) across the alveolar epithelium by puncturing alveoli in lungs of anesthetized rabbits with nonselective and ion-selective microelectrodes. After intravenous doses of amiloride, the PD and [K+] decreased (-1.0 +/- 0.3 to -0.3 +/- 0.1 mV, 7.4 +/- 1.1 to 4.2 +/- 0.4 meq/l, P less than 0.001), but [Cl-] did not change (96 +/- 9, 94 +/- 4 meq/l). In another set of experiments, the PD was measured with microelectrodes filled with an electrolyte solution, and midway through each measurement some of the solution was injected into the alveolar lumen. Injecting the solution without amiloride did not alter the alveolar PD (-1.0 +/- 0.4 before and -1.1 +/- 0.5 after injection). The alveolar PD decreased to -0.1 +/- 0.2 mV after injecting the solution with 10(-5) M amiloride into the alveolar interior. These results support the hypothesis that alveolar epithelium in air-filled lungs actively absorbs sodium in vivo, which accounts for the majority of the transepithelial PD.