Selective inhibition by dactinomycin of NANC sensory bronchoconstriction and [125I] NKA binding due to NK-2 receptor antagonism

Y.-P. Lou, P. Delay -Goyet & J. M. Lundber g. 1991. Selective inhibition by dactinomycin of NANC sensory bronchoconstriction and [¹²⁵I]NKA binding due to NK-2 receptor antagonism. Acta Physiol Scand 144 , 221–231. Received 6 May 1991, accepted 13 September 1991. ISSN 0001–6772. Department of Pharmacology, Karolinska Institutet, Stockholm. In the present study, dactinomycin (m) inhibited the non-adrenergic, non-cholinergic bronchoconstriction upon antidromic vagal nerve stimulation (1 Hz for 1 min) in the isolated perfused guinea-pig lung by 84%. The release of calcitonin gene-related peptide was unchanged, however, suggesting a postjunctional action. Dactinomycin (10^-5, 5 × 10^-5 m) also reduced non-adrenergic non-cholinergic bronchial contractions (maximally by 75%) induced by electrical field stimulation or capsaicin, while the cholinergic component and non-adrenergic non-cholinergic relaxation remained intact. The neurokinin-2 receptor antagonist l-659 877 (10^-6 m) had a similar effect as dactinomycin, inhibiting the non-adrenergic non-cholinergic bronchial contractions by 69%, while the neurokinin-1 receptor antagonist CP-96,345 (10^-6m) had no effect. The bronchoconstriction evoked by neurokinin A, the selective neurokinin-2 receptor agonist Nle¹⁰neurokinin A (4–10) and capsaicin was markedly inhibited by dactinomycin while the contraction induced by substance P (SP), the selective neurokinin-1 receptor agonist Sar⁹Met(0₂)¹¹SP, endothelin-1 and acetyl-choline was not affected. In autoradiographic experiments on guinea-pig lung, [¹²⁵I]neurokinin A-labelled sections showed dense binding in the bronchial smooth muscle layer. Dactinomycin inhibited the specific binding of [²⁵1]neurokinin A in a concentration-dependent manner (IC₅₀= 6.3 × 10^-6 m) and 66% of [¹²⁵I]neurokinin A total binding was inhibited by 10^-4 M dactinomycin. In the rat colon, [¹²⁵I]neurokinin A binding to neurokinin-2 sites on circular smooth muscle was inhibited by dactinomycin with an IC₅₀ value of 7.9 × 10^-6 M. Dactinomycin failed to reduce increased nerve-evoked contractions or those caused by Nle¹⁰neurokinin A (4–10) per se in the rat vas deferens, which are considered to be mediated by neurokinin-2 receptor activation. In the rat portal vein, dactinomycin did not influence the contractions caused by the neurokinin-3 selective agonist Pro⁷neurokinin B. In conclusion, dactinomycin selectively inhibited neurokinin-2 receptor activation in guinea-pig lung and rat colon, but not in rat vas deferens, which may depend on the existence of different neurokinin-2 receptor subtypes. Neurokinin A is most likely the main endogenous excitatory non-adrenergic non-cholinergic transmitter in guinea-pig bronchi.