Apical sodium-sugar transport in pulmonary epithelium in situ

Expression profiling and immunolocalization of Na+-D-glucose-cotransporter 1 in mice employing knockout mice as specificity control indicate novel locations and differences between mice and rats

The expression and localization of sodium-D-glucose cotransporter SGLT1 (SLC5A1), which is involved in small intestinal glucose absorption and renal glucose reabsorption, is of high biomedical relevance because SGLT1 inhibitors are currently tested for antidiabetic therapy. In human and rat organs, detailed expression profiling of SGLT1/Sglt1 mRNA and immunolocalization of the transporter protein has been performed. Using polyspecific antibodies and preabsorption with antigenic peptide as specificity control, in several organs, different immunolocalizations of SGLT1/Sglt1 between human and rat were obtained. Because the preabsorption control does not exclude cross-reactivity with similar epitopes, some localizations remained ambiguous. In the present study, we performed an immunocytochemical localization of Sglt1 in various organs of mice. Specificities of the immunoreactions were evaluated using antibody preabsorption with the Sglt1 peptide and the respective organs of Sglt1 knockout mice. Because staining in some locations was abolished after antibody preabsorption but remained in the knockout mice, missing staining in knockout mice was used as specificity criterion. The immunolocalization in mouse was identical or similar to rat in many organs, including small intestine, liver, and kidney. However, the male-dominant renal Sglt1 protein expression in mice differed from the female-dominant expression in rats, and localization in lung, heart, and brain observed in rats was not detected in mice. In mice, several novel locations of Sglt1, e.g., in eyes, tongue epithelial cells, pancreatic ducts, prostate, and periurethral glands were detected. Using end-point and quantitative RT-PCR in various organs, different Sglt1 expression in mice and rats was confirmed.

Insulin signaling via the PI3-kinase/Akt pathway regulates airway glucose uptake and barrier function in a CFTR-dependent manner

Cystic fibrosis-related diabetes is the most common comorbidity associated with cystic fibrosis (CF) and correlates with increased rates of lung function decline. Because glucose is a nutrient present in the airways of patients with bacterial airway infections and because insulin controls glucose metabolism, the effect of insulin on CF airway epithelia was investigated to determine the role of insulin receptors and glucose transport in regulating glucose availability in the airway. The response to insulin by human airway epithelial cells was characterized by quantitative PCR, immunoblot, immunofluorescence, and glucose uptake assays. Phosphatidylinositol 3-kinase/protein kinase B (Akt) signaling and cystic fibrosis transmembrane conductance regulator (CFTR) activity were analyzed by pharmacological and immunoblot assays. We found that normal human primary airway epithelial cells expressed glucose transporter 4 and that application of insulin stimulated cytochalasin B-inhibitable glucose uptake, consistent with a requirement for glucose transporter translocation. Application of insulin to normal primary human airway epithelial cells promoted airway barrier function as demonstrated by increased transepithelial electrical resistance and decreased paracellular flux of small molecules. This provides the first demonstration that airway cells express insulin-regulated glucose transporters that act in concert with tight junctions to form an airway glucose barrier. However, insulin failed to increase glucose uptake or decrease paracellular flux of small molecules in human airway epithelia expressing F508del-CFTR. Insulin stimulation of Akt1 and Akt2 signaling in CF airway cells was diminished compared with that observed in airway cells expressing wild-type CFTR. These results indicate that the airway glucose barrier is regulated by insulin and is dysfunctional in CF.

Localizations of Na(+)-D-glucose cotransporters SGLT1 and SGLT2 in human kidney and of SGLT1 in human small intestine, liver, lung, and heart

Novel affinity-purified antibodies against human SGLT1 (hSGLT1) and SGLT2 (hSGLT2) were used to localize hSGLT2 in human kidney and hSGLT1 in human kidney, small intestine, liver, lung, and heart. The renal locations of both transporters largely resembled those in rats and mice; hSGLT2 and SGLT1 were localized to the brush border membrane (BBM) of proximal tubule S1/S2 and S3 segments, respectively. Different to rodents, the renal expression of hSGLT1 was absent in thick ascending limb of Henle (TALH) and macula densa, and the expression of both hSGLTs was sex-independent. In small intestinal enterocytes, hSGLT1 was localized to the BBM and subapical vesicles. Performing double labeling with glucagon-like peptide 1 (GLP-1) or glucose-dependent insulinotropic peptide (GIP), hSGLT1 was localized to GLP-1-secreting L cells and GIP-secreting K cells as has been shown in mice. In liver, hSGLT1 was localized to biliary duct cells as has been shown in rats. In lung, hSGLT1 was localized to alveolar epithelial type 2 cells and to bronchiolar Clara cells. Expression of hSGLT1 in Clara cells was verified by double labeling with the Clara cell secretory protein CC10. Double labeling of human heart with aquaporin 1 immunolocalized the hSGLT1 protein in heart capillaries rather than in previously assumed myocyte sarcolemma. The newly identified locations of hSGLT1 implicate several extra renal functions of this transporter, such as fluid absorption in the lung, energy supply to Clara cells, regulation of enteroendocrine cells secretion, and release of glucose from heart capillaries. These functions may be blocked by reversible SGLT1 inhibitors which are under development.

Nasal potential difference to detect Na+ channel dysfunction in acute lung injury

Pulmonary fluid clearance is regulated by the active transport of Na(+) and Cl(-) through respiratory epithelial ion channels. Ion channel dysfunction contributes to the pathogenesis of various pulmonary fluid disorders including high-altitude pulmonary edema (HAPE) and neonatal respiratory distress syndrome (RDS). Nasal potential difference (NPD) measurement allows an in vivo investigation of the functionality of these channels. This technique has been used for the diagnosis of cystic fibrosis, the archetypal respiratory ion channel disorder, for over a quarter of a century. NPD measurements in HAPE and RDS suggest constitutive and acquired dysfunction of respiratory epithelial Na(+) channels. Acute lung injury (ALI) is characterized by pulmonary edema due to alveolar epithelial-interstitial-endothelial injury. NPD measurement may enable identification of critically ill ALI patients with a susceptible phenotype of dysfunctional respiratory Na(+) channels and allow targeted therapy toward Na(+) channel function.

Expression of Na+-glucose cotransporter (SGLT1) in visceral and parietal mesothelium of rabbit pleura

Indirect evidence for a solute-coupled liquid absorption from rabbit pleural space indicated that it should be caused by a Na(+)/H(+)-Cl(-)/HCO(3)(-) double exchanger and a Na(+)-glucose cotransporter [Agostoni, E., Zocchi, L., 1998. Mechanical coupling and liquid exchanges in the pleural space. In: Antony, V.B. (Ed.), Clinics in Chest Medicine: Diseases of the Pleura, vol. 19. Saunders, Philadelphia, pp. 241-260]. In this research we tried to obtain molecular evidence for Na(+)-glucose cotransporter (SGLT1) in visceral and parietal mesothelium of rabbit pleura. To this end we performed immunoblot assays on total protein extracts of scraped visceral or parietal mesothelium of rabbits. These showed two bands: one at 72kDa (m.w. of SGLT1), and one at 55kDa (which should also provide Na(+)-glucose cotransport). Both bands disappeared in assays in which SGLT1 antibody was preadsorbed with specific antigen. Molecular evidence for Na(+)/K(+) ATPase (alpha1 subunit) was also provided. Immunoblot assays for SGLT1 on cultured mesothelial cells of rabbit pleura showed a band at 72kDa, and in some cases also at 55kDa, irrespectively of treatment with a differentiating agent. Solute-coupled liquid absorption hinders liquid filtration through parietal mesothelium caused by Starling forces, and favours liquid absorption through visceral mesothelium caused by these forces.

Glucose transport in the lung and its role in liquid movement

Glucose concentration in the liquid present in the alveolar/airway lumen is the consequence of the balance between removal by lung epithelial cells and entry from the plasma or lung interstitium through the paracellular pathway. Glucose removal is mediated by active, Na(+) -dependent, cotransport and results in transepithelial Na(+) transport and liquid absorption in animals with significant rates of luminal glucose uptake and when luminal glucose concentration is high enough. Cotransport kinetics predicted a low luminal glucose concentration at the steady state, and foetal lung fluid and adult alveolar epithelial lining fluid glucose concentrations were indeed found lower than plasma. When luminal glucose concentration is low, the glucose-dependent part of transepithelial Na(+) transport is abated and alveolar liquid clearance reduced. A means to refuel this mechanism of liquid absorption would be to increase glucose entry in alveolar spaces through an increase in paracellular permeability. This hypothesis was modelled, and experimental data were found to acceptably agree with predictions.

Mechanisms of pulmonary edema clearance

The mechanisms of pulmonary edema resolution are different from those regulating edema formation. Absorption of excess alveolar fluid is an active process that involves vectorial transport of Na+out of alveolar air spaces with water following the Na+osmotic gradient. Active Na+transport across the alveolar epithelium is regulated via apical Na+and chloride channels and basolateral Na-K-ATPase in normal and injured lungs. During lung injury, mechanisms regulating alveolar fluid reabsorption are inhibited by yet unclear pathways and can be upregulated by pharmacological means. Better understanding of the mechanisms that regulate edema clearance may lead to therapeutic interventions to improve the ability of lungs to clear fluid, which is of clinical significance.

Factors determining the appearance of glucose in upper and lower respiratory tract secretions

OBJECTIVES

(a). To describe the glucose content of normal human airways secretions; (b). to observe the effects of hyperglycemia and airways inflammation on airways glucose.

DESIGN

Observational studies.

SETTINGS

(a). St George's Hospital Medical School; (b). diabetes mellitus outpatient clinics; (c). adult general intensive care unit.

PATIENTS

Nineteen healthy volunteers, 24 volunteers with acute rhinitis, 20 patients with diabetes mellitus, and 60 patients admitted to a general adult intensive care unit.

MEASUREMENTS

(a). Non-ventilated patients: simultaneous measurement of blood and nasal glucose concentrations; (b). ICU patients: simultaneous blood, nasal, and endotracheal (ET) glucose concentrations.

RESULTS

Nasal glucose was undetectable in all healthy volunteers. Glucose was detected in 12/24 volunteers with acute viral rhinitis [1 (1-2) mmol l(-1)] and 18/20 people with diabetes [4 (2-7) mmol l(-1)]. Glucose was detected in the ET secretions of 31/60 ventilated patients on ICU. Patients with ET glucose had significantly higher blood glucose (9.8+/-0.4 mmol l(-1)) than patients without ET glucose (7.2+/-0.3 mmol l(-1), P<0.001), and all patients with blood glucose >10.1 mmol l(-1) had glucose in ET secretions. Enteral nutrition did not affect the presence or concentration of glucose in ET secretions.

CONCLUSIONS

Glucose is not normally present in airways secretions, but appears where hyperglycaemia or epithelial inflammation occur. The detection of glucose cannot reliably be used to detect enteral feed aspiration.

Invited review: Active fluid clearance from the distal air spaces of the lung

Active ion transport drives iso-osmolar alveolar fluid clearance, a hypothesis originally suggested by in vivo studies in sheep 20 yr ago. Over the last two decades, remarkable progress has been made in establishing a critical role for active sodium transport as a primary mechanism that drives fluid clearance from the distal air spaces of the lung. The rate of fluid transport can be increased in most species, including the human lung, by cAMP stimulation. Catecholamine-independent mechanisms, including hormones, growth factors, and cytokines, can also upregulate epithelial fluid clearance in the lung. The new insights into the role of the distal lung epithelium in actively regulating lung fluid balance has important implications for the resolution of clinical pulmonary edema.

Lung epithelial fluid transport and the resolution of pulmonary edema

The discovery of mechanisms that regulate salt and water transport by the alveolar and distal airway epithelium of the lung has generated new insights into the regulation of lung fluid balance under both normal and pathological conditions. There is convincing evidence that active sodium and chloride transporters are expressed in the distal lung epithelium and are responsible for the ability of the lung to remove alveolar fluid at the time of birth as well as in the mature lung when pathological conditions lead to the development of pulmonary edema. Currently, the best described molecular transporters are the epithelial sodium channel, the cystic fibrosis transmembrane conductance regulator, Na+-K+-ATPase, and several aquaporin water channels. Both catecholamine-dependent and -independent mechanisms can upregulate isosmolar fluid transport across the distal lung epithelium. Experimental and clinical studies have made it possible to examine the role of these transporters in the resolution of pulmonary edema.

Glucose transporter gene expression in freshly isolated and cultured rat pneumocytes

Alveolar epithelium in situ takes up luminal glucose by cotransport with sodium. Cultured alveolar type II pneumocytes have only sodium-independent glucose uptake. It is unclear which isoforms are responsible for glucose transport in these cells and why sodium-glucose cotransport activity disappears during culture. GLUT1, GLUT4, GLUT5 and SGLT1 mRNA were detected in freshly isolated rat alveolar type II cells by reverse transcriptase-polymerase chain reaction. We show that SGLT1 mRNA was 90% lower in cells cultured in plastic wells for 2 or 4 days than in freshly isolated cells. mRNAs coding for the facilitated transporters were reduced from 40% (GLUT1) and 75% (GLUT4 and GLUT5) in cultured cells. Cells cultured at the air-liquid interface better preserved their phenotype as attested by significantly higher surfactant-associated protein mRNA levels. However, these cells had no higher GLUT1 and SGLT1 gene expression. Thus, alveolar type II cells lose sodium-glucose cotransport activity in part because of a decrease in mRNA levels. These changes in gene expression and/or mRNA stability may be an additional consequence of the shift towards the type I cell phenotype observed in cultured type II pneumocytes.

Alveolar sodium and liquid transport in mice

We have developed a simple isolated lung preparation for measurement of liquid and solute fluxes across mouse alveolar epithelium. Liquid instilled into air spaces was absorbed at the rate (J(w)) of 3.7 +/- 0.32 ml x h(-1) x g dry lung wt(-1) x J(w) was significantly depressed by ouabain (P < 0.001) and amiloride (P < 0.001). Omission of glucose from the instillate or addition of the Na(+)-glucose cotransport inhibitor phloridzin did not affect J(w). However, the low epithelial lining fluid glucose concentration (one-third that of plasma), the larger-than-mannitol permeability of methyl-alpha-D-glucopyranoside, and the presence of Na(+)-glucose cotransporter SGLT1 mRNA in mouse lung tissue suggest that there is a Na(+)-glucose cotransporter in the mouse alveolar-airway barrier. Isoproterenol stimulated J(w) (6.5 +/- 0.45 ml x h(-1) x g dry lung wt(-1); P < 0.001), and this effect was blocked by amiloride, benzamil, ouabain, and the specific beta(2)-adrenergic antagonist ICI-118551 but not by atenolol. Similar stimulation was obtained with terbutaline (6.4 +/- 0.46 ml x h(-1) x g dry lung wt(-1)). Na(+) unidirectional fluxes out of air spaces varied in agreement with J(w) changes. Thus alveolar liquid absorption in mice follows Na(+) transport via the amiloride-sensitive pathway, with little contribution from Na(+)-glucose cotransport, and is stimulated by beta(2)-adrenergic agonists.

Expression of type II Na-P(i) cotransporter in alveolar type II cells

Type II Na-P(i) cotransporters (type IIa and type IIb) represent apically located Na-P(i) cotransporters in epithelia of proximal tubules (type IIa) and small intestine (type IIb). Here we provide evidence that the type IIb (but not the type IIa) Na-P(i) cotransporter is also expressed in the lung. With the use of immunohistochemistry, location of the type IIb protein was found exclusively in the apical membrane of type II cells of the alveolar epithelium. Such a location of the type IIb cotransporter suggests an involvement in the reuptake of phosphate necessary for the synthesis of surfactant. A possible regulation of the abundance of the type IIb cotransporter in the lung was studied after adaptation of mice to a low-P(i) diet. After a chronic adaptation to a low-P(i) diet, no changes in the type IIb protein and the type IIb transcript were observed. These results exclude dietary intake of phosphate as a regulatory factor of the type IIb Na-P(i) cotransporter in alveolar type II cells.

Active sodium and chloride transport across the isolated rabbit conjunctiva

The bulbar-palpebral conjunctiva from albino rabbits was dissected as a cylinder and cut longitudinally to convert it to a flat epithelium that was mounted as a partition between Using-type chambers, exposing 0.38 cm2 of cross-sectional area. The tissue was bathed with a modified Tyrode's solution at 37 degrees C, pH 7.5. The tear-facing side (apical) was 14.6 +/- 1.5 mV negative relative to the basolateral side. Transepithelial resistance was 1.23 +/- 0.01 K omega.cm2 and the short-circuit current (Isc) was 14.4 +/- 1.3, microA/cm2. Sixty percent of the Isc could be accounted for by a Na(+)-dependent, bumetanide-inhibitable Cl- transport directed towards the apical side. The remainder of the Isc reflected a Na+ absorptive process at the apical surface that was amiloride resistant. Evidence was obtained that a likely contributor to this activity is an electrogenic Na(+)-glucose co-carrier. The Cl-dependent Isc was stimulated by forskolin and epinephrine. Permeabilization of the apical membrane with amphotericin B evinced a current carried by a basolateral Na+:K+ pump. An effect by heptanol suggested that part of the Isc traverse the epithelium via gap junctions. Our results imply that transport processes at the conjunctiva could influence the composition of the tear film.

Sodium-coupled transport of glucose by plasma membranes of type II pneumocytes

Sodium-dependent absorption of alveolar fluid promotes efficient gas exchange. In animal models, alveolar glucose stimulates phlorizin-sensitive, Na(+)-dependent fluid absorption. It is hypothesized that Na+/glucose cotransporters are localized to apical membranes of type II pneumocytes. Enriched apical and basolateral plasma membrane vesicles were isolated from adult bovine type II pneumocytes. Uptakes of 22Na+ and [3H]glucose by enriched apical and basolateral vesicles were monitored over time. Following addition of external glucose (75 mM), 22Na+ uptake by mannitol-loaded, apically-enriched vesicles was significantly increased over controls. Substitution of interior-negative charge gradients for internally directed Na+ gradients increased glucose-dependent Na+ uptakes even greater. By contrast, external glucose did not significantly promote 22Na+ uptake by enriched basolateral vesicles. External Na+ (75 mM) significantly increased [3H]glucose uptakes by enriched apical vesicles with evidence of overshoot. Phlorizin (100 microM) inhibited both glucose-coupled 22Na+ uptakes and Na(+)-coupled [3H]glucose uptakes. These observations support localization of electrogenic, Na+/glucose cotransporters to enriched apical membranes of mature type II pneumocytes.

Identification of nonselective cation channels in cultured adult rat alveolar type II cells

There is evidence supporting the role of active transport of Na+ in the resolution of pulmonary edema, but the exact cellular mechanism(s) underlying this process remain unknown. This study demonstrated the presence of ion channels on adult rat alveolar type II cells that might be associated with this active transport of Na+. Patch-clamp techniques were used to characterize a nonselective cation channel in adult rat alveolar type II epithelial cells held in culture for 24 to 72 h. Single-channel currents were recorded from inside-out, cell-free membrane patches. The most common type of single channel had a linear slope conductance of 20.4 +/- 0.6 pS (n = 22) in symmetrical NaCl (150 mM) solutions. The channel was approximately equally permeable to Na+ and K+ ions (PK/PNa = 1.15) and was highly selective for cations (PCl/PNa < 0.05). Channel activity was Ca(2+)-dependent, and it required at least 10 microM Ca2+ on the cytosolic side of an inside-out patch to activate the channel. Amiloride (1 to 10 microM), a Na+ channel blocker in epithelial tissue, reduced the steady-state open probability of the channel 10-fold but had no significant effect on the magnitude of the single-channel conductance. Single channels with similar properties were not found in cultured rat alveolar macrophages. The possible role of this amiloride-sensitive, nonselective cation channel in Na+ transport and lung liquid clearance is discussed.

Liquid flow across the epithelium of the artificially perfused lung of fetal and postnatal sheep

1. The lungs of five fetal (133-140 days gestation) and thirty-four postnatal (2-240 days) sheep were artificially perfused in situ with warmed and oxygenated sheep blood. In postnatal animals the airspace of the lung was filled with liquid similar in composition to fetal lung liquid. In fetal and postnatal animals luminal liquid volume was measured by the impermeant tracer technique. 2. Under resting conditions the pulmonary epithelium of fetal animals secreted liquid at a mean (+/- S.E.M.) rate of 2.0 (+/- 0.4) ml (kg body weight)-1 h-1, those of postnantal animals absorbed liquid at -1.8 (+/- 0.2) ml (kg body weight)-1 h-1. 3. Addition of 2,4-dinitrophenol to achieve a concentration of 1.5 x 10(-3) M in the perfusing blood in postnatal animals caused complete cessation of liquid absorption. 4. Light and electron microscopic examination of the lung after periods of up to 6 h of artificial perfusion showed no evidence of epithelial damage. From 3 h onwards, liquid accumulation was evident in the perivascular spaces. 5. Addition of adrenaline to the perfusate in fetal animals caused absorption of liquid to occur at a mean rate of -2.9 (+/- 1.3) ml (kg body weight)-1 h-1. In postnatal animals adrenaline caused the rate of liquid absorption to increase from a mean rate of -1.4 (+/- 0.2) to -2.2 (+/- 0.3) ml (kg body weight)-1 h-1. 6. In the fetus addition of amiloride (0.8 x 10(-4) M) to the luminal fluid blocked adrenaline-induced liquid absorption and caused secretion to occur at 1.3 (+/- 0.3) ml (kg body weight)-1 h-1. 7. In postnatal animals the response to amiloride was age dependent. In newborn lambs (2-14 days) amiloride blocked liquid absorption and caused secretion of liquid to occur in seven out of eight animals at a mean rate of 0.9 (+/- 0.3) ml (kg body weight)-1 h-1 (n = 8). In older animals (15-240 days) the characteristic response to amiloride was slowing of the rate of liquid absorption (mean rate of absorption,-0.2 (+/- 0.09) ml (kg body weight)-1 h-1, n = 18) with liquid secretion being seen in only three of eighteen animals.(ABSTRACT TRUNCATED AT 400 WORDS)

GLUT 1-glucose transporter protein in adult and fetal mouse lung

We observed approximately 45-50 kD GLUT 1 protein in mouse lung homogenates and demonstrated a greater abundance in fetus compared to adult. In situ immunohistochemical analysis demonstrated GLUT 1 expression only in the perineural sheath of nerves. While the trapped fetal red blood cells expressed GLUT 1 abundantly, adult red blood cells were devoid of GLUT 1. No GLUT 1 was evident in fetal and adult lung alveolar and bronchiolar epithelial cells, vascular endothelial cells and the lung mesenchymal elements. Thus, GLUT 1 is not the major lung glucose transporter.

Development of the lung liquid reabsorptive mechanism in fetal sheep: synergism of triiodothyronine and hydrocortisone

1. Thyroidectomy was performed on twelve fetal sheep between 111 and 115 days gestation. Measurement of fetal lung liquid secretion and absorption rates (Jv) were made at rest and during short (45 min) and long (5 h) infusions of adrenaline (0.5 micrograms/min) in a total of thirty-seven experiments, some in the absence of triiodothyronine (T3) and hydrocortisone and some at set times after the administration of the two hormones. 2. T3 was given either as an I.V. infusion (60 micrograms/24 h) or as a bolus of 30 micrograms; hydrocortisone was given as an infusion of 10 mg/24 h. Both hormones were administered together. 3. Before T3 and hydrocortisone were given short infusions of adrenaline had no effect on Jv but 4 h after exposure to the hormones secretion rate was reduced to near zero (Jv = -0.5 +/- 1.6 ml/h, n = 4) by adrenaline; after 24 h of hormone exposure, absorption of fetal lung liquid was produced by adrenaline (Jv = -3.6 +/- 2.2 ml/h, n = 4) which was even greater after 72 h, (Jv = -11.2 +/- 2.2 ml/h, n = 4). 4. During long infusions of adrenaline when T3 and hydrocortisone were given at the start of the experiment, an effect on lung liquid secretion was evident at 2 h and absorption was produced at 4 h (Jv = -4.2 +/- 2.5 ml/h, n = 3). The effect was significantly different from control long infusions of adrenaline performed the previous day in the absence of hormones. 5. After 24 or 48 h of stopping T3 and hydrocortisone administration, adrenaline no longer produced absorption of lung liquid, indicating that the effect of the two hormones was reversible within 24-48 h. 6. The protein synthesis inhibitor cycloheximide put into lung liquid (4 x 10(-5) to 3 x 10(-4) M) blunted the effect of the hormones at 4 h and prevented absorption of lung liquid at 24 h. Jv during adrenaline was -3.6 +/- 1.5 ml/h in control experiments but was +3.3 +/- 0.9 ml/h after cycloheximide, n = 4, P < 0.01. This indicated that the two hormones produced their effect through protein synthesis.

Evidence for a sodium-dependent sugar transport in rat tracheal epithelium

The presence of Na(+)-coupled sugar transport in rat trachea was investigated using the nonmetabolizable glucose analogs methyl alpha-glucopyranoside and 3-O-methylglucose. The rates of disappearance from tracheal instillates and the tissue uptake of these analogs were compared with those of L-glucose. Experiments were performed in vivo, using a cross-circulation preparation, and in vitro, on tracheal strips. The analog methyl alpha-glucopyranoside was removed in vivo from the tracheal lumen faster than L-glucose. The cellular uptake in vivo or in vitro was determined by lysing the cells lining the tracheal lumen with detergents. This uptake was inhibited by luminal glucose, phloridzin and Na+ substitution with choline. The transport rate of 3-O-methylglucose was very low and thus discouraged inhibition experiments. These results indicate the presence of a Na+/sugar cotransport system in rat trachea. The effects of luminal interactions suggest that the cotransport is located in the apical membrane of the tracheal epithelium. It resembles that previously described in the rat alveolar epithelium, but apparently differs from that found in the fetal sheep lung in which a significant 3-O-methylglucose cotransport with Na+ has been described.