Characterization of NK 1 and NK 2 tachykinin receptors in guinea‐pig and rat bronchopulmonary and vascular systems

1 NK 1 and NK 2 tachykinin receptors were characterized in guinea‐pig and rat bronchopulmonary systems and in the vasculature of the rat by use of radioligand binding and/or functional studies. 2 The radioligands for NK 1 and NK 2 receptors ([ 3 H]‐SP and [ 3 H]‐pNKA, respectively) did not label tachykinin receptors in homogenates of rat lungs or bronchi. In contrast, in the guinea‐pig, [ 3 H]‐SP bound with high affinity to these tissues ( K D = 0.23 ± 0.08 n m and 0.34 ± 0.05 n m , for lungs and bronchi, respectively). The total number of binding sites was 4.6 fold greater in bronchus ( B max = 135 ± 27 fmol mg −1 protein) than in lung homogenates ( B max = 29.3 ± 0.1 fmol mg −1 protein). Furthermore, this binding was markedly displaced by CP‐96,345 (p K i = 9.5 ± 0.1) and RP 67580 (p K i = 7.6 ± 0.1), antagonists of NK 1 receptors, slightly displaced by SR 48968 (p K i = 6.6 ± 0.1), but not affected by actinomycin D or L‐659,877, antagonists of NK 2 receptors. Specific binding of [ 3 H]‐pNKA, detected in guinea‐pig bronchi ( K D = 5.2 ± 0.1 n m , and B max = 203 ± 19 fmol mg −1 protein) but not in lungs, was similarly (40 to 53%) displaced by RP 67580 (1 μ m ), CP‐96,345 (10 and 100 n m ) or SR 48968 (10 and 100 n m ). The displacement approximately doubled (87 to 91%) when SR 48968 (10 n m ) was combined with either RP 67580 (1 μ m ) or CP‐96,345 (10 n m ), but not when RP 67580 was combined with CP‐96,345. 3 In urethane‐anaesthetized guinea‐pigs, i.v. injections of the NK 1 receptor agonists SP, [Pro 9 ]‐SP, [Sar 9 , Met(O 2 ) 11 ]‐SP and septide, as well as the NK 2 receptor agonists NKA and [Lys 5 , MeLeu 9 , NLeu 10 ]‐NKA (4–10) (0.1–10 μg kg −1 , i.v.), dose‐dependently increased lung inflation pressure. The most potent of these peptides were septide and [Lys 5 , MeLeu 9 , NLeu 10 ]‐NKA (4–10) (EC 50 = 0.38 ± 0.07 and 0.07 ± 0.02 μg kg −1 , respectively). Interestingly, septide was 130 fold less potent than SP in displacing [ 3 H]‐SP from its binding sites in the guinea‐pig lung, whereas it was 14 fold more potent than SP as a bronchoconstrictor. RP 67580 (0.3–5 mg kg −1 , i.v.) and CP‐96,345 (0.01–3 mg kg −1 , i.v.) dose‐dependently reduced the bronchoconstriction produced by the NK 1 receptor agonists. Conversely, the NK 2 receptor antagonists actinomycin D (1–10 mg kg −1 , i.v.) and SR 48968 (0.03–0.3 mg kg −1 , i.v.) inhibited specifically the responses induced by NK 2 receptor agonists. 4 In pentobarbitone‐anaesthetized rats, the NK 1 and NK 2 receptor agonists (0.01–4 μg kg −1 , i.v.) produced dose‐dependent hypotensive responses. The order of potency was SP = [Sar 9 , Met(O 2 ) 11 ]‐SP = [Pro9]‐SP > septide = NKA >[Lys 5 , MeLeu 9 , NLeu 10 ]‐NKA (4–10) . RP 67580 (0.13–0.5 mg kg −1 , i.v.) and CP‐96,345 (0.5–2 mg kg −1 , i.v.) antagonized in a dose‐related manner (20 to 64%) the vascular effects of both NK 1 and NK 2 receptor agonists, whereas actinomycin D (3 mg kg −1 , i.v.) and SR 48968 (2 mg kg −1 , i.v.) did not. RP 67580 was approximately 4 times more potent than CP‐96,345. 5 These studies indicate that NK 1 and NK 2 receptors are both present in the guinea‐pig bronchopulmonary system whereas only NK 1 receptors are detectable in the rat vasculature under our experimental conditions. Furthermore, NK 1 receptors in the guinea‐pig bronchopulmonary system are pharmacologically distinct from those present in the rat vascular system, since both agonist potencies and antagonist affinities differ between the two species.