Proceedings of The Physiological Society

Newcastle University (2009) Proc Physiol Soc 16, PC25

Poster Communications

Active Cl- and alveolar fluid transport under β-adrenergic stimulation, in the rat lung

D. Alexandrou1, A. Benjamin2, D. Walters2

1. Medway Maritime Hospital, Kent, United Kingdom. 2. Clinical Developmental Sciences, St George's, University of London, London, United Kingdom.

  • The effect of blocking Cl- and activating Na+ conductance. C = control, N = NPPB, T = terbutaline and A = amiloride. *P = 0.03 (n = 4), **P = 0.05 (n = 3), ***P = 0.005 (n = 3); all compared to corresponding controls by paired t-test.

The pulmonary epithelium is lined by a thin layer of liquid, and as there is no net liquid transport at rest, there must be a dynamic balance between absorption and secretion. Active Na+ transport underlies the resting absorptive process (1), and β-adrenergic stimulation is able to increase absorption of alveolar fluid markedly (2). Two mechanisms have been implicated in this process: the first involves direct stimulation of Na+ conductance across the apical membrane of the epithelium (3); the second involves a role for active, transepithelial Cl- transport, resulting in hyperpolarization of the apical membrane of the epithelial cell and thus an increased driving force for Na+ uptake (4). Chloride channels with the characteristics of CFTR have been detected in alveolar type I and II cells, and the role of CFTR in the β-agonist-mediated alveolar fluid clearance was explored in studies with ΔF508 transgenic mice (5). The primary goal of this study was to test the hypothesis that the β-adrenergic stimulation of alveolar fluid clearance in the mature lung occurs as a result of activation of absorptive Cl- channels. We performed experiments, using the in situ perfused rat lung. Male Wistar rats, aged 10-16 weeks, bred in house, were anaesthetized with a solution of fentanyl citrate with fluasinone and midazolam (2.7 intraperitoneally) and ventilated. Following median sternotomy the pulmonary artery and left atrium were catheterized. The lungs were perfused in situ with a modified Ringer’s solution containing 3% albumin. A known amount of an impermeant tracer (125I albumin) was added to the lung instillate (LL) and from its change in concentration over time the rate of net transepithelial liquid movement (Jv) was calculated. Positive values of Jv indicate net secretion, negative values net absorption. The β2-agonist terbutaline (100μM into perfusate) induced an increase of 138.8% in the rate of absorption (from -0.39±0.08 to -0.94±0.12, mean±SEM, p=0.05 by paired t-test, n=3) and the chloride channel blocker NPPB (100μM into the LL) induced an increase in the rate of absorption of 160% (from -0.18 ± 0.06 to -0.47±0.09, p=0.03 by paired t-test, n=4) (Figure 1). Simultaneous administration of terbutaline and NPPB resulted in an increase in absorption of 281.2% (from -0.33±0.19 to -1.25±0.12, p=0.005 by paired t-test, n=3) (Figure 1). Amiloride (100μM) given into the LL after NPPB or terbutaline caused the cessation of absorption. These results suggest that the β-agonist stimulated alveolar fluid clearance is not mediated via activation of absorptive Cl- channels, but rather via direct activation of amiloride-blockable Na+ conductance. They also suggest the functional presence of secretory Cl- channels in the alveolar epithelium, both in the resting state and under β-agonist stimulation.

Where applicable, experiments conform with Society ethical requirements