Effects of secretagogues on net and unidirectional liquid fluxes across porcine bronchial airways

Abstract

Rates of liquid secretion and absorption across the bronchopulmonary airways are important for regulating airway surface liquid volume and maintaining mucociliary transport. The current study demonstrates the feasibility of measuring not just net liquid movements but unidirectional liquid movements across isolated intact bronchi from swine. Airways were liquid filled to assess both net liquid movements, and, in the presence of NPPB to selectively inhibit secretion, unidirectional absorption. Unidirectional liquid secretion rates were determined by subtraction. For comparison, net liquid movements were assessed in air-filled airways in parallel. In the absence of secretagogues, unidirectional absorption was observed (4.63 ± 0.53 μl·cm−2·h−1) with little unidirectional secretion (1.42 ± 0.36 μl·cm−2·h−1). ACh, substance P (SP), and vasoactive intestinal peptide (VIP) all induced unidirectional secretion (10.64 ± 1.52 μl·cm−2·h−1, 14.16 ± 1.39 μl·cm−2·h−1, and 4.25 ± 0.25 μl·cm−2·h−1, respectively) without affecting unidirectional absorption. Net liquid secretion in air-filled airways was close to that in liquid-filled airways except with VIP. VIP induced net secretion in air-filled airways (4.44 ± 1.26 μl·cm−2·h−1), but negligible net change in liquid movement occurred in liquid-filled airways. This effect was likely to have been caused by the higher solid content of the VIP-induced mucous liquid (3.98 ± 0.26%) compared with the ACh- and SP-induced liquid (2.06 ± 0.07% and 2.15 ± 0.07%, respectively). We conclude that this technique allows important quantitative distinctions to be made between liquid secretion and absorption in intact bronchial airways.