Effects of non-neural mediators and allergen on tracheobronchial blood flow

There are a large number of non-neural mediators released from a variety of cells in the airways. The release of many of these mediators is often enhanced during the inflammatory phase of airway diseases such as asthma. Therefore, they are likely to be important regulators of not only airway smooth muscle tone but also airway blood flow and microvascular permeability. Non-neural mediators have potent and in some cases (histamine, 5-hydroxytryptamine (5HT) complex actions on tracheobronchial blood flow. The blood flow effects of some of these mediators (histamine, 5HT, bradykinin) occur by activation of specific receptors, but may also involve secondary mediators (bradykinin). There are clear species differences in the responses of the tracheobronchial circulation to non-neural mediators. Allergen challenge of the airways changes not only airway smooth muscle tone but also tracheobronchial blood flow. These changes in blood flow (dilation or constriction) seem to be mediated, at least in the sheep, by antigen-induced release of a mixture of prostaglandins and leukotrienes. The type of vascular response observed with antigen challenge partly depends on the balance of mediators released, which again may depend on several factors including the species and the age of the animal.