Evidence for Na+-glucose cotransporter in type I alveolar epithelium.

Functional evidence of Na(+)-glucose cotransport in rat lung has been provided by Basset et al. (J. Physiol. 384:325-345, 1987). By autoradiography [(3)H]phloridzin binding has been found confined to alveolar epithelial type II cells in mouse and rabbit lungs (Boyd, J. Physiol. 422: 44P, 1990). In this research we checked by immunofluorescence whether Na(+)-glucose cotransporter (SGLT1) is also expressed in alveolar type I cells. Lungs of anesthetized rats and lambs were fixed by paraformaldehyde, perfused in pulmonary artery, or instilled into a bronchus, respectively. Tissue blocks embedded in paraffin or frozen were sectioned. Two specific anti-SGLT1 antibodies for rat recognizing aminoacid sequence 402-420, and 546-596 were used in both species. Bound primary antibody was detected by secondary antibody conjugated to fluorescein isothiocianate or Texas red, respectively. In some sections cellular nuclei were also stained. In rats alveolar type I cells were identified by fluorescent Erythrina cristagalli lectin. Sections were examined by confocal laser-scanning microscope. Both in rats and lambs alveolar epithelium was stained by either antibody; no labeling occurred in negative controls. Hence, SGLT1 appears to be also expressed in alveolar type I cells. This is functionally relevant because type I cells provide 95-97% of alveolar surface, and SGLT1, besides contributing to removal of lung liquid under some circumstances, keeps low glucose concentration in lining liquid, which is useful to prevent lung infection.