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

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Abstract
Y.‐P. Lou, P. Delay‐Goyet& J. M. Lundberg. 1991. Selective inhibition by dactinomycin of NANC sensory bronchoconstriction and [125I]NKA binding due to NK‐2 receptor antagonism.Acta Physiol Scand144, 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 × 10‐5m)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(10‐6m)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 × 10‐6m)and 66% of [125I]neurokinin A total binding was inhibited by 10‐4M dactinomycin. In the rat colon, [125I]neurokinin A binding to neurokinin‐2 sites on circular smooth muscle was inhibited by dactinomycin with an IC50value of 7.9 × 10‐6M. Dactinomycin failed to reduce increased nerve‐evoked contractions or those caused by Nle10neurokinin A (4–10)per sein 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.