Diazepam Attenuates Phenylephrine-Induced Contractions in Rat Aorta

Abstract

In this in vitro study we examined the effects of diazepam on a phenylephrine-induced contraction in rat aorta and determined the associated cellular mechanism focusing on the endothelium-derived vasodilators. The concentration-response curves for phenylephrine and potassium chloride were generated in the presence or absence of diazepam. Phenylephrine concentration-response curves were generated from the endothelium-intact rings pretreated independently with N(W)-nitro-L-arginine methyl ester, PK 11195, tetraethylammonium, and indomethacin in the presence or absence of diazepam. Diazepam (7 x 10(-7) M) attenuated the phenylephrine-induced contraction in the endothelium-intact rings, whereas a large dose (5 x 10(-6) M) of diazepam attenuated the phenylephrine-induced contraction in the aortic rings with or without the endothelium. A pretreatment with the N(W)-nitro-L-arginine methyl ester completely abolished the diazepam (7 x 10(-7) M)-induced attenuation of the phenylephrine concentration-response curve, as well as the diazepam (5 x 10(-6) M)-induced attenuation of the maximal contractile response to phenylephrine. The N(W)-nitro-L-arginine methyl ester (10(-4) M)-induced contraction was enhanced in the rings pretreated with diazepam (5 x 10(-6) M). These results indicate that a supraclinical concentration of diazepam attenuates phenylephrine-induced contraction by increasing endothelial nitric oxide activity and directly affecting vascular smooth muscle.

Indigo carmine enhances phenylephrine-induced contractions in an isolated rat aorta

“…A low-molecular-weight superoxide anion scavenger tiron abolished the indigo carmine-induced potentiation of contraction evoked by phenylephrine. These results, along with previous reports [6,7,[11][12][13], suggest that indigo carmine enhances the phenylephrine-induced contraction in the endothelium-intact rings via NO inactivation induced by indigo carmine-induced ROS production (Fig. 6).…”

“…Enhanced production of O 2 • -increases NO inactivation [11], which leads to a decrease in endotheliumdependent NO-mediated relaxation. Endothelial NO was reported to inhibit the contractile agonist (e.g., phenylephrine and norepinephrine)-induced contraction of the rat aorta [12,13]. A low-molecular-weight superoxide anion scavenger tiron abolished the indigo carmine-induced potentiation of contraction evoked by phenylephrine.…”

Involvement of endothelium-dependent and -independent mechanisms in midazolam-induced vasodilation

“…For example, the BDZ diazepam induces an endothelium-dependent vasodilation involving endothelial NO production in rodents. 18,19 In contrast, our results excluded NO involvement in the mechanism of the BDZ midazolam because the NOS inhibitor NGnitro-L-arginine methyl ester did not modify midazolam-induced vasodilation (Figure 3b). …”

“…In pilot experiments in which aortic rings were exposed to increasing doses of diazepam, we found a vasodilatory response that was comparable to that of midazolam (data not shown). In addition, other authors reported vasodilation induced by different BDZs, such as diazepam in rat aorta, 18,19 canine arteries, 9 and human umbilical arteries and veins; 4 tetrazepam in aorta and portal vein of rats; 10 flurazepam in canine arteries; 9 and clonazepam in human umbilical arteries. 4 Moreover, BDZs seem to have a generalized muscle relaxant effect as demonstrated by diazepam-and clonazepam-induced relaxation of uterine rings from rats 38 and by diazepam-and flurazepaminduced relaxation of isolated taenia coli and longitudinal muscle of guinea pigs.…”

“…15 Although specific stimulation of TSPO can induce vasodilation in some models, possibly by inhibiting voltageoperated calcium channels, 16,17 its involvement in BDZ-induced vasodilation appears to be controversial. 18,19 The aim of this study was to assess the vascular effects of BDZs and the possible mechanisms involved in their direct vascular action. We hypothesized that such mechanisms might involve endothelial-dependent and -independent pathways.…”