The present study deals with the pharmacological effects of the sesquiterpene alcohol (-)-α-bisabolol on various smooth-muscle preparations from rats. Under resting tonus, (-)-α-bisabolol (30-300 µmol/L) relaxed duodenal strips, whereas it showed biphasic effects in other preparations, contracting endothelium-intact aortic rings and urinary bladder strips, and relaxing these tissues at higher concentrations (600-1000 µmol/L). In preparations precontracted either electromechanically (by 60 mmol/L K(+)) or pharmacomechanically (by phenylephrine or carbachol), (-)-α-bisabolol showed only relaxing properties. The pharmacological potency of (-)-α-bisabolol was variable, being higher in mesenteric vessels, whereas it exerted relaxing activity with a lesser potency on tracheal or colonic tissues. In tissues possessing spontaneous activity, (-)-α-bisabolol completely decreased spontaneous contractions in duodenum, whereas it increased their amplitude in urinary bladder tissue. Administered in vivo, (-)-α-bisabolol attenuated the increased responses of carbachol in tracheal rings of ovalbumin-sensitized rats challenged with ovalbumin, but was without effect in the decreased responsiveness of urinary bladder strips in mice treated with ifosfamide. In summary, (-)-α-bisabolol is biologically active in smooth muscle. In some tissues, (-)-α-bisabolol preferentially relaxed contractions induced electromechanically, especially in tracheal smooth muscle. The findings from tracheal rings reveal that (-)-α-bisabolol may be an inhibitor of voltage-dependent Ca(2+) channels.

Previous findings enable us to hypothesize that (-)-α-bisabolol acts as inhibitor of voltage-dependent Ca(2+) channels in smooth muscle. The current study was aimed at consolidating such hypothesis through the recording of isometric tension, measurement of intracellular Ca(2+) as well as discovery of channel target using in silico analysis. In rat aortic rings, (-)-α-bisabolol (1-1000 µM) relaxed KCl- and phenylephrine-elicited contractions, but the IC50 differed significantly (22.8 [17.6-27.7] and 200.7 [120.4-334.6] µM, respectively). The relaxation of phenylephrine contractions remained unaffected by l-NAME, indomethacin, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, tetraethylammonium, glibenclamide or KT-5720. Under Ca(2+)-free conditions, (-)-α-bisabolol did not alter the contractions evoked by phenylephrine or caffeine whereas it reduced those evoked by CaCl2 in KCl-, but not in PHE-stimulated preparations. Furthermore, it did not significantly alter the contractions evoked by phorbol 12,13-dibutyrate or induced by the extracellular Ca(2+) restoration in cyclopiazonic acid-treated preparations. In mesenteric rings loaded with Fluo-4 AM, (-)-α-bisabolol blunted the tension and the cytosolic levels of Ca(2+) in response to K(+) but not to norepinephrine. Silico docking analysis of the Cavβ2a subunit of voltage-dependent Ca(2+) channel indicated putative docking sites for (-)-α-bisabolol. These findings reinforce the ability of (-)-α-bisabolol to inhibit preferentially contractile responses evoked by Ca(2+) influx through voltage-dependent Ca(2+) channels.

Previously, we showed that 1‐nitro‐2‐phenylethene, a nitrostyrene derivative of 1‐nitro‐2‐phenylethane, induced vasorelaxant effects in rat aorta preparations. Here, we studied mechanisms underlying the vasorelaxant effects of its structural analog, trans‐4‐chloro‐β‐nitrostyrene (T4CN), in rat aortic rings. Increasing concentrations of T4CN (0.54‐544.69 µm) fully and similarly relaxed contractions induced by phenylephrine (PHE, 1 µm) or KCl (60 mm) in endothelium‐intact aortic rings with IC50 values of 66.74 [59.66–89.04] and 79.41 [39.92–158.01] µm, respectively. In both electromechanical and pharmacomechanical couplings, the vasorelaxant effects of T4CN remained unaltered by endothelium removal, as evidenced by the IC50 values (108.35 [56.49–207.78] and 65.92 [39.72–109.40] µm, respectively). Pretreatment of endothelium‐intact preparations with L‐NAME, ODQ, glibenclamide, or TEA did not change the vasorelaxant effect of T4CN. Under Ca2+‐free conditions, T4CN significantly reduced the phasic contractions induced by caffeine or PHE, as well as the contractions due to exogenous CaCl2 in aortic preparations stimulated with PHE (in the presence of verapamil). These results suggest that in rat aortic rings, T4CN induced vasorelaxation independently from the activation of soluble guanylate cyclase/cGMP pathway, an effect that may be related to the electrophilicity of the substituted chloro‐nitrostyrene. This vasorelaxation seems to involve inhibition of both calcium influx from the extracellular milieu and calcium mobilization from intracellular stores mediated by IP3 receptors and by ryanodine‐sensitive Ca2+ channels.

The aim of the present study was to investigate the vascular effects of the E-isomer of methyl cinnamate (E-MC) in rat isolated aortic rings and the putative mechanisms underlying these effects. At 1-3000 μmol/L, E-MC concentration-dependently relaxed endothelium-intact aortic preparations that had been precontracted with phenylephrine (PHE; 1 μmol/L), with an IC50 value (geometric mean) of 877.6 μmol/L (95% confidence interval (CI) 784.1-982.2 μmol/L). These vasorelaxant effects of E-MC remained unchanged after removal of the vascular endothelium (IC50 725.5 μmol/L; 95% CI 546.4-963.6 μmol/L) and pretreatment with 100 μmol/L N(G) -nitro-l-arginine methyl ester (IC50 749.0 μmol/L; 95% CI 557.8-1005.7 μmol/L) or 10 μmol/L 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (IC50 837.2 μmol/L; 95% CI 511.4-1370.5 μmol/L). Over the concentration range 1-3000 μmol/L, E-MC relaxed K(+) -induced contractions in mesenteric artery preparations (IC50 314.5 μmol/L; 95% CI 141.9-697.0 μmol/L) with greater potency than in aortic preparations (IC50 1144.7 μmol/L; 95% CI 823.2-1591.9 μmol/L). In the presence of a saturating contractile concentration of K(+) (150 mmol/L) in Ca(2+) -containing medium combined with 3 μmol/L PHE, 1000 μmol/L E-MC only partially reversed the contractile response. In contrast, under similar conditions, E-MC nearly fully relaxed PHE-induced contractions in aortic rings in a Ba(2+) -containing medium. In preparations that were maintained under Ca(2+) -free conditions, 600 and 1000 μmol/L E-MC significantly reduced the contractions induced by exogenous Ca(2+) or Ba(2+) in KCl-precontracted preparations, but not in PHE-precontracted preparations (in the presence of 1 μmol/L verapamil). In addition, E-MC (1-3000 μmol/L) concentration-dependently relaxed the contractions induced by 2 mmol/L sodium orthovanadate. Based on these observations, E-MC-induced endothelium-independent vasorelaxant effects appear to be preferentially mediated by inhibition of plasmalemmal Ca(2+) influx through voltage-dependent Ca(2+) channels. However, the involvement of a myogenic mechanism in the effects of E-MC is also possible.

Trans‐4‐methoxy‐β‐nitrostyrene (T4MN) induced more potent vasorelaxant effects in resistance arteries from hypertensive rats than its parent drug, β‐nitrostyrene 1‐nitro‐2‐phenylethene (NPe). To better understand the influence of insertion of the electron‐releasing methoxy group in the aromatic ring of NPe, we investigated vasorelaxant effects of T4MN in isolated pulmonary artery and compared them with those of NPe in view of the potential interest of T4MN in pulmonary arterial hypertension. T4MN and NPe both caused concentration‐dependent vasorelaxation in pulmonary artery rings pre‐contracted with either phenylephrine (1 µmol/L) or KCl (60 mmol/L), an effect unaffected by endothelium removal. In endothelium‐intact preparations pre‐contracted with phenylephrine, the vasorelaxant effect of T4MN was more potent than that of NPe. However, unlike NPe, this effect was significantly reduced following pretreatment with 1H‐[1,2,4]oxadiazolo[4,3‐a]quinoxalin‐1‐one (ODQ) (10 µmol/L, a guanylate cyclase inhibitor) or tetraethylammonium (5 mmol/L, a potassium channel blocker). T4MN abolished the CaCl2‐induced contractions in pulmonary artery preparations stimulated with phenylephrine (PHE) under Ca2+‐free conditions in the presence of verapamil, to preferentially activate receptor‐operated calcium channels. From these findings, we propose that T4MN evokes endothelium‐independent vasorelaxant effects in isolated rat pulmonary artery, partially by inhibiting Ca2+ influx through L‐type Ca2+ channels, as well as by activating soluble guanylate cyclase and potassium channels. The present results suggest the therapeutic potential of T4MN in treating pulmonary arterial hypertension.

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