Serotonin decreases alveolar epithelial fluid transport via a direct inhibition of the epithelial sodium channel.
Hypoxia and epithelial stretch that are commonly observed in patients with acute lung injury have been shown to promote the release of serotonin (5-hydroxytryptamine, 5-HT) in vitro. However, whether 5-HT contributes to the decrease of alveolar epithelial fluid transport, which is a hallmark of lung injury, is unknown. Thus, we investigated the effect of 5-HT on ion and fluid transport across the alveolar epithelium. 5-HT caused a dose-dependent inhibition of the amiloride-sensitive current across primary rat and human alveolar epithelial type II cell monolayers, but did not affect Na(+)/K(+) ATPase function. Furthermore, we found that the 5-HT induced inhibition of ion transport across the lung epithelium was receptor independent, as it was not prevented by the blockade of 5-HT2R (5-HT receptor 2), 5-HT3R (5-HT receptor 3), or by pretreatment with an intracellular calcium-chelating agent, BAPTA-AM (1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl) ester). In addition, the stimulation of 5-HT1R (5-HT receptor 1), 5-HT2R (5-HT receptor 2), 5-HT4R (5-HT receptor 4), and 5-HT7R (5-HT receptor 7) failed to reproduce the 5-HT effect on amiloride-sensitive sodium transport. We ascertained that 5-HT directly inhibited the function of rat alphabetagamma epithelial sodium channel (ENaC), as determined by heterologous expression of rat ENaC in Xenopus oocytes that do not express endogenous ENaC nor 5-HT receptors (5-HTR). Exposure of mice to hypoxia for 1 hour induced a 30% increase of 5-HT secretion into the distal airways of mice. Finally, the intratracheal instillation of 5-HT inhibited the amiloride-sensitive fraction of alveolar fluid clearance in mice. Together, these results indicate that 5-HT inhibits the amiloride-sensitive fraction of the alveolar epithelial fluid transport via a direct interaction with ENaC, and thus can be an endogenous inhibitor of this ion channel.
Goolaerts
Arnaud
A
Department of Anesthesia, Room 3C-38, San Francisco General Hospital, 1001 Potrero Avenue, San Francisco, CA 94110, USA. goolaertsa@anesthesia.ucsf.edu
Roux
Jérémie
J
Ganter
Michael T
MT
Shlyonsky
Vadim
V
Chraibi
Ahmed
A
Stéphane
Renauld
R
Mies
Frédérique
F
Matthay
Michael A
MA
Naeije
Robert
R
Sariban-Sohraby
Sarah
S
Howard
Marybeth
M
Pittet
Jean-Francois
JF
D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D002460:Cell Line; D045744:Cell Line, Tumor; D053503:Epithelial Sodium Channels / Q000378:metabolism*; D004848:Epithelium / Q000473:pathology; D006801:Humans; D000860:Hypoxia; D007477:Ions / Q000378:metabolism; D008168:Lung / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D009865:Oocytes / Q000378:metabolism; D018408:Patch-Clamp Techniques; D011650:Pulmonary Alveoli / Q000378:metabolism*; D051381:Rats; D012701:Serotonin / Q000378:metabolism*; D014132:Trachea / Q000378:metabolism; D014981:Xenopus
D053503:Epithelial Sodium Channels; D007477:Ions; D012701:Serotonin; D000584:Amiloride