EFFECT OF HYPOXIA ON REFLEX RESPONSES OF TRACHEAL SUBMUCOSAL GLANDS

The effects of moderate sustained normocapnic hypoxia on tracheal subm ucosal gland reflex responses were studied. Experiments were performed in anesthetized, paralyzed, and mechanically ventilated dogs. The cha nges in the number of secreting glands and volume of secreted fluid in the subsequent period of time were recorded after 15-30 min of contro lled ventilation with room air [arterial PO2 (Pa-O2) 86 +/- 3 Torr], h ypoxic gas mixture (Pa-O2 49 +/- 4 Torr), or 100% O-2 (Pa-O2 339 +/- 3 9 Torr), under isocapnic and isohydric conditions. The hillocks method was used to quantify the changes in submucosal gland secretion. The c hanges in secretion 30 s after stimulation of pulmonary C-fiber recept ors by right atrial injection of capsaicin (10 mu g/kg; n = 10) were m arkedly lower during moderate hypoxia than in normoxia or hyperoxia. D ifferences in the number of liquid droplets and the volume of secreted fluid were statistically significant (P < 0.05 and P < 0.001, respect ively). Stimulation of airway rapidly adapting receptors by lung defla tion increased airway secretion; the number of ''hillocks'' and the vo lume of secreted fluid were lower in hypoxic than in hyperoxic state. Differences between response curves for the number of glands activated and secreted volume were statistically significant (P < 0.05 and P < 0.001). The number of glands activated by substance P given locally by arterial infusion was not affected by the state of oxygenation, but t he calculated volume of secreted fluid was lower during the hypoxic st ate than under hyperoxic condition (P = 0.05). When tracheal tension w as measured (n = 7), hypoxia-potentiated lung deflation induced trache al constriction (P < 0.05). These findings suggest that prolonged mode rate hypoxia triggers mechanisms that suppress reflex submucosal gland responses and potentiate tracheal smooth muscle reflex contraction.