APICAL SODIUM-SUGAR TRANSPORT IN PULMONARY EPITHELIUM INSITU

The presence of an apical sodium-coupled transport system for D-glucose in lung alveolar epithelial cells has been demonstrated in lungs instilled with Ringer''s fluid and perfused with either blood or Ringer''s fluid (Basset et al. (1987) J. Physiol. 384, 325-345). The direction of transport is from alveoli towards interstitium. The characteristics of the system were evaluated in similar preparations by use of sugar analogues such as .alpha.-methyl-glucopyranoside, 2-deoxyglucose, 3-O-methylglucose and L-glucose. The main finding was the presence of a transport system for .alpha.-methylglucopyranoside and 2-deoxyglucose in the apical cell membrane. This system was unaffected by phloretin. Both .alpha.-methylglucopyranoside and 2-deoxyglucose transports were inhibited by phloridizin and by the presence of glucose (10-2 mol .cntdot. l-1). Competition was demonstrated between D-glucose and .alpha.-methylglucopyranoside or 2-deoxyglucose, but not for 3-O-methylglucose or L-glucose. 3-O-Methylglucose was cleared as slowly as L-glucose. The results comply partly with those known from intestinal epithelium and kidney proximal tubular epithelium, but the handling of 3-O-methylglucose was different. The relative transport rates of Na+ and glucose are compatible with a Na+:glucose coupling ratio larger than one.