Local and central reflex mechanisms in the neural control of airway microcirculation

Sensory and parasympathetic neural mechanisms play an essential role in the control of the microcirculation in the airway mucosa in response to inhalation of irritants and in local allergic reactions. Thus extravasation of plasma proteins is evoked by local release of peptides such as substance P (SP) from capsaicin-sensitive sensory nerves upon inhalation of cigarette smoke. Calcitonin gene-related peptide (CGRP) is another vasodilator agent which is also released from airway afferent nerves by capsaicin or antidromic nerve stimulation. Parasympathetic cholinergic and noncholinergic vasodilator mechanisms dominate in the nasal mucosa and trachea whereas the bronchial circulation is influenced mainly by local mechanisms involving mediator release from capsaicin-sensitive sensory nerves. The allergen-induced vasodilatation in the airway mucosa seems to a large extent to be caused by histamine which activates capsaicin-sensitive afferent nerves and evokes peptide release.