Distribution of substance P binding sites in equine airways

Autoradiography with [I-125]-Bolton Hunter substance P ([I]-BHSP) was used to detect substance P binding sites in the equine lung. Specific m-BHSP bin ding sites were very dense over small bronchial vessels, tracheobronchial g lands and airway epithelium in large and small airways. The density of [I]- BHSP binding sites over airway smooth muscle was much lower than in the pre ceding tissues. Competition with an excess of either a specific neurokinin 1 receptor agonist, or a specific neurokinin 2 receptor agonist indicated t hat most specific [I]-BHSP binding sites in the equine lung represent neuro kinin 1 receptors, The receptor-mediated effects of substance P in the equi ne lung are most likely to involve regulation of vascular tone and airway s ecretions based upon the density of specific m-BHSP binding sites in these tissues. Activation of intrapulmonary afferent nerves containing Substance P by noxious stimuli such as inhaled allergens or irritants may lead to inc reased mucus secretion and decreased airway diameter due to vascular conges tion.