Expression and distribution of vasoactive intestinal polypeptide receptor VPAC2 mRNA in human airways

Vasoactive intestinal polypeptide (VIP) is a putative neurotransmitter of the inhibitory non-adrenergic noncholinergic nervous system and influences many aspects of mammalian airway function. VIP binds to two G-protein-coupled VPAC receptors that are highly homologous structurally but distinguished by their different affinities for peptide analogues of VIP. As VIP binding sites in the respiratory tract have only been examined by ligand binding and cytochemical techniques, we studied the distribution of the mRNA that encodes the inducible receptor subtype VPAC(2) in the human respiratory tract. Northern blots demonstrated the expression of VPAC(2) mRNA in human airways and other tissues. A human-specific VPAC(2) cRNA probe was used to detect VPAC(2) mRNA expression in human lung by nonradioactive in situ hybridization. In larger airways, positive VPAC(2) mRNA signals were localized to tracheal and bronchial ciliated epithelial cells. There was also marked staining of mucous and serous cells of submucosal glands. No signals were obtained in airway and vascular smooth muscle myocytes and endothelial cells. In peripheral lung tissues, VPAC(2) mRNA expression was localized to epithelial cells of the bronchioles. Specific staining was detected in immune cells and alveolar macrophages. In summary, VPAC(2) is localized in airway epithelial, glandular, and immune cells of the lung but not in airway and vascular smooth muscle. The absence of VPAC(2) mRNA in vascular and airway smooth muscle myocytes may indicate that the effects of VIP on vasodilation and bronchodilation are mediated by VPAC(1) or undefined receptors. However, a paracrine modulation of the two most prominent effects of VIP in the respiratory tract by VPAC(2) cannot be excluded.