Pisarri, Thomas E., and Gordon G. Giesbrecht. Reflex tracheal smooth muscle contraction and bronchial vasodilation evoked by airway cooling in dogs. J. Appl. Physiol. 82(5): 1566-1572, 1997.Cooling intrathoracic airways by filling the pulmonary circulation with cold blood alters pulmonary mechanoreceptor discharge. To determine whether this initiates reflex changes that could contribute to airway obstruction, we measured changes in tracheal smooth muscle tension and bronchial arterial flow evoked by cooling. In nine chloralose-anesthetized open-chest dogs, the right pulmonary artery was cannulated and perfused; the left lung, ventilated separately, provided gas exchange. With the right lung phasically ventilated, filling the right pulmonary circulation with 5°C blood increased smooth muscle tension in an innervated upper tracheal segment by 23 ± 6 (SE) g from a baseline of 75 g. Contraction began within 10 s of injection and was maximal at ~30s. The response was abolished by cervical vagotomy. Bronchial arterial flow increased from 8 ± 1 to 13 ± 2 ml/min, with little effect on arterial blood pressure. The time course was similar to that of the tracheal response. This response was greatly attenuated after cervical vagotomy. Blood at 20°C also increased tracheal smooth muscle tension and bronchial flow, whereas 37°C blood had little effect. The results suggest that alteration of airway mechanoreceptor discharge by cooling can initiate reflexes that contribute to airway obstruction.