COTRASPORTATORE NA+-GLUCOSIO NEL MESOTELIO PLEURICO E NELL'EPITELIO ALVEOLARE POLMONARE: ESPRESSIONE E CENNI DI REGOLAZIONE BETA ADRENERGICA

SGLT1 (SLC5A1) is a Na+–glucose cotransporter belonging to the SLC5 family, expressed in several absorbing epithelia. Indirect evidence for a solute-coupled liquid absorption from rabbit pleural space indicated that it should be due to a Na+/H+-Cl-/HCO3- double exchanger and a Na+-glucose cotransporter (Agostoni and Zocchi, Clin Chest Med 19:241-60, 1998). Furthermore, functional evidence of Na+-glucose cotransport in rat lung has been provided by Basset (Basset et al., J Physiol 384:325-345, 1987). By autoradiography [3H]phloridzin binding has been found confined to alveolar type II cells in mouse and rabbit lungs (Boyd, J Physiol 422: 44P, 1990). In the alveolar airspace the active Na+ transport is regulated by beta adrenergic receptors, particularly the β2 subtype (Mutlu et. al., Am J Respir Crit Care Med 170:1270–1275, 2004). Experimental data suggest that beta adrenergic agonists, working via the β2AR, accelerate clearance of excess fluid from the alveolar airspace; activation of β2AR seems to increase both Na+/K+-ATPase and ENaC function and expression (Pesce et. al., FEBS Letters 486:310-314, 2000; Chen et. al., Am J Physiol Lung Cell Mol Physiol 282:609-620, 2002). In literature there are no data regarding a beta adrenergic regulation of Na+–glucose cotransport in the respiratory system, but in two cases is reported a beta adrenergic stimulation of SGLT1, in rat small intestine (Ishikawa et al., Biochimica et Biophysica Acta 1357: 306–318, 1997) and in ruminal epithelium of sheep (Aschernbach et al., J. Nutr. 132: 1254–1257, 2002). In this research, we first tried to obtain molecular evidence for Na+-glucose cotransporter (SGLT1) in the respiratory system, particularly in pleural and alveolar epithelia. The expression of SGLT1 was identified by Western blot assays on total protein extracts of scraped mesothelium from visceral or parietal pleura, and from alveolar cells; in all experiments we obtained SGLT1 specific bands. Immunolocalization of SGLT1 was performed examining for fluorescence the pleural surface, the alveolar epithelium and isolated alveolar cells. Confocal immunofluorescence images of lamb pleural mesothelium showed that SGLT1 is located in the apical membrane; in alveolar sections of rats and lambs most alveolar surface was stained by anti-SGLT1 antibody; furthermore also alveolar type I and type II cells isolated from rat lung were stained by anti-SGLT1 antibody, so we can assume that SGLT1 is expressed on both types of alveolar cells. After that, we concentrated on beta adrenergic regulation of SGLT1; immunofluorescence assays were performed on A549 cells (alveolar epithelial cell line) treated for different times with the beta adrenergic agonist isoproterenol. Immunolabeling images showed an increased expression of the Na+-glucose cotransporter after treatment with the β-agonist isoproterenol; this increase appears to be reversible. These findings provide evidence at molecular level, in pleural and alveolar epithelium, for Na+-glucose cotransporter (SGLT1); in the respiratory system, it seems to be involved in solute-coupled liquid absorption from the pleural space and from the alveolar airspace. Like other channels and trasporters that play a role in the solute-coupled liquid absorption, SGLT1 appears to be upregulated by β-adrenergic agonists.