The role of ENaC and CNG‐channels in AFC following continuous isoproterenol treatment

Abstract

It is well described that -adrenoceptor stimulation increases alveolar fluid clearance (AFC) in vivo, yet the pathways of sodium transport after continuous β-adrenoceptor stimulation are not as well described, even though stimulation of epithelial sodium channels (ENaC) have been suggested to play a central role. Rats were treated with the β-agonist isoproterenol (200 ìg/kg/h sc) for three days. Vehicle treated rats were used as controls. Western blotting on whole lung homogenates showed that isoproterenol treatment increased the abundance of both the α- and β-subunit of ENaC (2.23 ± 0.51 and 2.74 ± 0.55 of vehicle, resp), whereas the level of γ-ENaC was unchanged (1,20 ± 0,17 of vehicle). In parallel groups, AFC over a 1-hour period was determined in anesthetized artificially ventilated rats, by installing an isosmolar Ringer-solution containing 125I-marked albumin into the bronchial tree. The study showed that AFC was increased in isoproterenol treated rats (18,9% ± 1,4 % vs. 13,3 % ± 3,3 %). These AFC studies were repeated during condition where sodium transport though either ENaC or the cyclic nucleotide gated (CNG) channel were blocked by adding amiloride or l-cis diltiazem respectively to the instillate. Amiloride caused a net decrease of ~8 % in AFC in both groups, yet the net decrease when adding l-cis-diltiazem was 11,7 % in isoproterenol treated rats and 5,3 % in the vehicle treated rats. The results suggest that the increase in AFC after continuous β-agonist stimulation is due to an increase in sodium transport through the CNG-channel, while sodium transport though ENaC, despite an up regulation of both the α- and β-ENaC subunit, is unchanged. The project was supported by the Danish Heart Foundation