Tachykinin receptors in rabbit airways — characterization by functional, autoradiographic and binding studies

1 In many species, both NK1 and NK2 tachykinin receptors appear to be important in mediating the contraction of airway smooth muscle. We have examined the distribution and characterization of receptors for tachykinins in rabbit airways using functional length tension studies, autoradiography and radioligand binding studies. 2 Contractile responses to tachykinins were elicited in four different areas of the respiratory tree — trachea, and three progressively more distal areas of the right bronchus. The NK2 receptor‐preferring agonists, neurokinin A (NKA), neuropeptide gamma (NPγ) and the NK2‐selective [Lys5 MeLeu9, Nle10]‐NKA(4–10) [NKA (4–10) analogue] produced similar contraction in all four areas. Substance P (SP) and the NK1‐selective [Sar9,Met(O2)11]‐SP (Sar‐SP) exhibited a marked location‐dependence in the magnitude of contraction, producing minimal contraction in the trachea and more proximal bronchi with contractions becoming progressively larger in the more distal airways. Senktide (which is selective for the NK3 receptor) produced negligible contraction in all areas. 3 The NK2‐selective antagonist, MDL29,913, was a weak antagonist of NKA and NKA(4–10) analogue. At a concentration of 2 μm, it produced a small but significant shift in the response curve to NKA and a greater shift (8 fold) in the curve to NKA(4–10) analogue, but it had no effect on responses to Sar‐SP. The non peptide NK, receptor antagonist, CP‐96,345, was also unexpectedly weak in this preparation. The pD2 value for Sar‐SP was decreased 27 fold by CP‐96,345 at a concentration of 1 μm, without alteration in the maximum response. 4 Autoradiographic binding sites to [125I]‐NKA were sparse over smooth muscle in proximal airway preparations and markedly increased in density in the more distal airways. There was negligible binding over vascular smooth muscle and epithelium. 5 Radioligand binding studies revealed binding to [125I]‐NKA which was 82% specific. The order of potency for inhibition of [125I]‐NKA binding was SP >= Sar‐SP > NKA = NPγ>CP‐96,345 > NKA (4–10) analogue > NKB ⋙ MEN 10207 (the NK2 subtype selective antagonist) > MDL 29,913 > senktide. This profile indicates binding predominantly to NK1 receptors. 6 These results suggest that there are at least two types of tachykinin receptors in rabbit airways, a population of NK1 receptors, the density of which is greatest in the periphery and, in addition, NK2 receptors which are uniformly distributed throughout the airways. These