Regulation of ENaC by extracellular proteases is abnormal in airway epithelia from βENaC-Tg mice. Freshly excised tracheal tissues from WT and βENaC-Tg mice were pretreated with luminal aprotinin, and amiloride-sensitive Isc was measured either in the presence of aprotinin alone (Aprotinin) or after the addition of luminal trypsin (+Trypsin). n = 9–14 mice/group. *, p < 0.05; **, p < 0.001 compared with mice of same genotype; †, p < 0.001 compared with WT mice; §, p < 0.001 compared with βENaC-Tg mice. Error bars, S.E.