Heterogeneity of tachykinin receptors in the rabbit lung

The tachykinins, substance P (SP) and neurokinin A (NKA), are agonists for the NK₁ and NK₂ receptors, respectively. Tachykinins have various respiratory effects, including bronchoconstriction. This study characterizes tachykinin binding sites in the rabbit lung. We hypothesize that (2[¹²⁵I]iodohistidyl¹)Neurokinin A ([¹²⁵I]NKA) interacts with NK₁ and NK₂ binding sites in the rabbit lung. The K(d) determined from saturation isotherms was 0.69 x// 1.14 nM (geometric mean x// SEM) and the B(max) was 4.15 ± 0.22 femtomole/mg protein (arithmetic mean ± SEM). Competitive inhibition studies with NKA, SP and various selective tachykinin agonists showed the rank order of potency: [β-Ala⁸]-Neurokinin A 4-10 = SP >> NKA >> [Sar⁹,Met(O₂)¹¹]-Substance P. [β-Ala⁸]-Neurokinin A 4-10, a selective NK₂ agonist, and SP inhibition of [¹²⁵I]NKA binding were best described using a two-site model. Competitive inhibition studies using the selective nonpeptide NK₂ antagonist (SR 48968) and the selective nonpeptide NK₁ antagonist (CP 96,345) revealed K(i)'s of 5.5 nM and 8.1 nM, respectively. Our data therefore suggest that [¹²⁵I]NKA binds to both the NK₁ and NK₂ receptors in the lung.