Mechanisms responsible for the in vitro relaxation of a novel dibenzothiepine derivative (NSU-242) on tracheal and vascular smooth muscles.

In our previous general screening experiments, we found that NSU-242, a dibenzothiepine derivative (1-10 mg/kg), inhibited antigen-induced immediate asthmatic response in actively sensitized guinea pigs in a dose-dependent manner. The purpose of the present study was to assess the mechanism of the relaxing effect of NSU-242 on smooth muscle contractions in isolated smooth muscle tissues of the porcine trachea and rat aorta. NSU-242 administration resulted in a concentration-dependent inhibition of the tracheal-tissue contractions induced by carbachol and high K(+) and the aortic-tissue contractions induced by norepinephrine and high K(+). The IC(50) values of these inhibitions were 1-10 microM, and there was no selectivity for the type of stimulation. In tracheal tissue, the relaxations were accompanied by neither changes in cAMP nor changes in cGMP. Carbachol (1 microM) and high K(+) (59.2 mM) increased myosin light chain (MLC) phosphorylation in the trachea, and NSU-242 (3-30 microM) had no effect on the level of MLC phosphorylation. Furthermore, NSU-242 (300 microM) had no effect on contractions in membrane-permeabilized tracheal tissue. FITC-phalloidin staining of the actin fiber in cultured vascular smooth muscle cells (A7r5) indicated that NSU-242 (10-100 microM) altered the configuration of actin stress fiber in the cytosol. However, unlike cytochalasin D, NSU-242 did not inhibit actin polymerization as assessed by in vitro assay. These results suggest that NSU-242 inhibits smooth muscle contractions without any effect on the Ca(2+)-dependent MLC phosphorylation. NSU-242 may uncouple the force generated by the activated actomyosin interaction, possibly by modifying the actin assembly in smooth muscle cells without a direct effect on actin molecules.