Characterization of NK1 and NK2 tachykinin receptors in guinea‐pig and rat bronchopulmonary and vascular systems

Open Access
Publisher Website
Google Scholar
Abstract
1 NK1 and NK2 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 NK1 and NK2 receptors ([3H]-SP and [3H]-pNKA, respectively) did not label tachykinin receptors in homogenates of rat lungs or bronchi. In contrast, in the guinea-pig, [3H]-SP bound with high affinity to these tissues (KD = 0.23 ± 0.08 nm and 0.34 ± 0.05 nm, for lungs and bronchi, respectively). The total number of binding sites was 4.6 fold greater in bronchus (Bmax = 135 ± 27 fmol mg−1 protein) than in lung homogenates (Bmax = 29.3 ± 0.1 fmol mg−1 protein). Furthermore, this binding was markedly displaced by CP-96,345 (pKi = 9.5 ± 0.1) and RP 67580 (pKi = 7.6 ± 0.1), antagonists of NK1 receptors, slightly displaced by SR 48968 (pKi = 6.6 ± 0.1), but not affected by actinomycin D or L-659,877, antagonists of NK2 receptors. Specific binding of [3H]-pNKA, detected in guinea-pig bronchi (KD = 5.2 ± 0.1 nm, and Bmax = 203 ± 19 fmol mg−1 protein) but not in lungs, was similarly (40 to 53%) displaced by RP 67580 (1 μm), CP-96,345 (10 and 100 nm) or SR 48968 (10 and 100 nm). The displacement approximately doubled (87 to 91%) when SR 48968 (10 nm) was combined with either RP 67580 (1 μm) or CP-96,345 (10 nm), but not when RP 67580 was combined with CP-96,345. 3 In urethane-anaesthetized guinea-pigs, i.v. injections of the NK1 receptor agonists SP, [Pro9]-SP, [Sar9, Met(O2)11]-SP and septide, as well as the NK2 receptor agonists NKA and [Lys5, MeLeu9, NLeu10]-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 [Lys5, MeLeu9, NLeu10]-NKA(4–10) (EC50 = 0.38 ± 0.07 and 0.07 ± 0.02 μg kg−1, respectively). Interestingly, septide was 130 fold less potent than SP in displacing [3H]-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 NK1 receptor agonists. Conversely, the NK2 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 NK2 receptor agonists. 4 In pentobarbitone-anaesthetized rats, the NK1 and NK2 receptor agonists (0.01–4 μg kg−1, i.v.) produced dose-dependent hypotensive responses. The order of potency was SP = [Sar9, Met(O2)11]-SP = [Pro9]-SP > septide = NKA >[Lys5, MeLeu9, NLeu10]-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 NK1 and NK2 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 NK1 and NK2 receptors are both present in the guinea-pig bronchopulmonary system whereas only NK1 receptors are detectable in the rat vasculature under our experimental conditions. Furthermore, NK1 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.