Tachykinin receptors mediating airway macromolecular secretion

Three tachykinin receptor types, termed NK1, NK2, and NK3, can be distinguished by the relative potency of various peptides in eliciting tissue responses. Airway macromolecular secretion is stimulated by the tachykinin substance P (SP). The purposes of this study were to determine the tachykinin receptor subtype responsible for this stimulation, and to examine the possible involvement of other neurotransmitters in mediating this effect. Ferret tracheal explants maintained in organ culture were labeled with 3H-glucosamine, a precursor of high molecular weight glycoconjugates (HMWG) which are released by airway secretory cells. Secretion of labeled HMWG then was determined in the absence and presence of the tachykinins SP, neurokinin A (NKA), neurokinin B (NKB), physalaemin (PHY), and eledoisin (ELE). All the tachykinins tested stimulated HMWG release to an approximately equal degree. Stimulation was concentration-related, with log concentrations giving half-maximal effects (EC50) as follows: SP -9.47, NKA -7.37, NKB -5.98, PHY -8.08, and ELE -7.68. This rank order of potency (SP greater than PHY greater than or equal to ELE greater than or equal to NKA greater than NKB) is most consistent with NK1 receptors. To evaluate the possible contribution of other mediators, tachykinin stimulation was examined in the presence of several receptor blockers. The potency of SP was not diminished by pretreatment with atropine, propranolol, or chlorpheniramine, and atropine actually increased the magnitude of the secretory response. The SP receptor antagonist [D-Arg1,D-Phe5, D-Trp7,9, Leu11]-SP blocked SP-induced secretion. These findings indicate that SP is a potent stimulus of airway macromolecular secretion. This effect occurs through the action of NK1 receptors, and is not dependent upon cholinergic, beta-adrenergic, or H-1 histamine receptors. The facilitation by atropine of SP stimulation suggests the existence of a mechanism of cholinergic inhibition of SP-induced stimulation.