Glucose concentrations are low or undetectable in animal airway secretions (Barker et al. 1989). Sodium-glucose co-transporters in pulmonary epithelium, identified by RT-PCR in rat and mouse and by functional studies in sheep, could actively maintain this low glucose concentration (Barker et al. 1989; Lee et al. 1994). In healthy humans glucose is undetectable in airway secretions, but is present in secretions from those with hyperglycaemia (King et al. 2001; Meguer et al. 2001). The dynamic relationship between blood and airway glucose has not previously been studied.

Twelve non-diabetic volunteers with healthy nasal mucosa underwent repeated measurement of nasal glucose (NG) during elevation of blood glucose by the hyperglycaemic clamp technique. Nasal glucose was measured using glucose oxidase indicator sticks wetted by secretions during a 30 s application to the nasal mucosa under direct vision. Glucose was measured in arterialised whole blood using a glucose oxidase-based analyser (Analox, UK).

In six subjects blood glucose was raised to 12 mmol l^-1 over 15 min. Glucose appeared in nasal secretions of all subjects within 20 min of commencing the infusion and 10 min of reaching the target blood sugar. NG decreased as the blood glucose fell on stopping the glucose infusion and was never higher than blood glucose. In a further six subjects blood glucose levels were raised from baseline by 1 mmol l^-1 increments and held at each level for 15 min before NG measurement. The blood glucose level at which nasal glucose appeared was 8.2 ± 1.0 mmol l^-1 (mean ± S.D.) (range 7.1-10.0).

Passive diffusion is likely to account for the rapid appearance of glucose in nasal secretions when blood glucose is raised. However, active transport must account for reabsorption of glucose against the concentration gradient as blood sugar falls. Our observation of a blood glucose threshold above which glucose appears in airway secretions is consistent with the presence of a saturable glucose transporter, such as a sodium- glucose co-transporter, actively reabsorbing glucose across human airway epithelium. Maintenance of glucose-free airway secretions may be important for pulmonary defence against infection.

All procedures accord with current local guidelines and the Declaration of Helsinki.