Characterization of neurokinin effects and receptor selectivity in human isolated bronchi

Abstract

Sensitive afferent nerves and the neurokinins they release upon activation are considered to be important in controlling bronchomotor tone. Human isolated bronchi respond to neurokinin A (NKA), substance P (SP), and neurokinin B (NKB) with dose-dependent contractions. The order of potency of the three natural neurokinins is NKA > SP > NKB, suggesting the presence of NK-2 receptors. To further characterize the neurokinin receptors in human bronchi, we used selective agonists for each receptor type (i.e., NK-1, NK-2, and NK-3). In fact, NK-1 selective compounds, [Pro9]SP(1-11)sulfone and [β-Ala4,Sar9]SP(4-11)sulfone, did not induce significant contractions up to 10-5M. Similarly, the selective agonist for the NK-3 receptor, [MePhe7]NKB(4-10), was almost inactive. However, the NK-2 selective fragment [Nle10]NKA(4-10) was a potent stimulant. The negative log of the peptide concentration that caused 50% of maximal effect (pD2) was 6.99 for NKA and 6.12 for [Nle10]NKA(4-10). Removal of the epithelium significantly enhanced the contractile responses to the three neurokinins and also to the NK-2 selective agonist. Phosphoramidon, an enkephalinase inhibitor, was more potent than epithelium removal in enhancing the contractile responses to these agonists. However, epithelium removal and phosphoramidon did not increase the weak responses to the NK-1 and NK-3 selective compounds. In the presence of phosphoramidon, removal of the epithelium slightly enhanced the contractile responses to NKA and [Nle10]NKA(4-10) but not to SP and NKB. These results suggest that in human bronchi (1) the receptors involved in the neurokinin-induced contractions belong to the NK-2 type, (2) the epithelium exerts and inhibitory effect on these contractions, (3) bronchial enkephalinase is an important modulator of neurokinin-induced effects, and (4) the epithelial inhibitory effect could be attributed in part to an enkephalinase activity.