Aim: 

Molecular evidence for Na⁺- glucose cotransporter (SGLT1) in rabbit pleural mesothelium has been recently provided, confirming earlier functional findings on solute-coupled liquid absorption from rabbit pleural space. In this research we checked whether SGLT1 is also expressed in sheep pleural mesothelium, and in a cell line of human pleural mesothelium, i.e. species with thick visceral pleura which receives blood from systemic circulation, but drains it into pulmonary veins. Moreover, since in sheep, at variance with rabbit, there is no functional evidence of a solute-coupled liquid absorption by the pleural mesothelium, immunofluorescence experiments were carried out in lambs, to check whether SGLT1 is expressed in the apical membrane of mesothelium.

Methods: 

The expression of Na⁺-glucose cotransporter (SGLT1) was identified by Western blot assays on total protein extract of scraped mesothelium from visceral or parietal pleura, and from the cell line MeT 5A (ATCC: CRL-9444). Immunolocalization of SGLT1 was performed examing the pleural surface for fluorescence with a confocal laser-scanning microscope.

Results: 

Immunoblot assays on total protein extract of scraped visceral and parietal mesothelium of lambs and adult sheep, and of human mesothelial cell line showed SGLT1 specific bands. Moreover, confocal immunofluorescence images showed that SGLT1 is located in apical membrane.

Conclusion: 

These findings provide evidence that a solute-coupled liquid absorption should also occur from pleural space of species with thick visceral pleura. Because of this protein-free liquid entering interstitium between visceral mesothelium and capillaries, inherent Starling forces should be different than hitherto considered, and visceral pleura capillaries could absorb liquid even in these species.