Possible Mechanism of Benzodiazepine-Induced Relaxation of Vascular Smooth Muscle

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Abstract
This study investigated the mechanism of benzodiazepine-induced relaxation of vascular smooth muscle. The ability of several benzodiazepine and isoquinolinecarboxamide compounds, including a pair of enantiomers, to inhibit [3H]Ro5-4864 binding to the peripheral-type benzodiazepine binding site in rat aortic smooth muscle was compared with their relative ability to induce relaxation of rat aortic rings. The binding was performed in a membrane fraction obtained from a pellet centrifuged at 11,400 g and enriched with high-affinity [3H]Ro5-4864 binding. The rank order of potency (Ki) for inhibition of [3H]Ro5-4864 binding to isolated membranes was: (-)PK 14067 (6.4 .+-. 0.7 nM) = PK 11195 (6.6 .+-. 0.8 nM) > Ro5-4864 (17.6 .+-. 2.1 nM) > diazepam (600 .+-. 180 nM) = (+)PK 14068 (530 .+-. 70 nM) > clonazepam (14,300 .+-. 2,100 nM). However, micromolar concentrations of these agents were required to induce relaxation of rat aortic rings contracted with KCl and/or norepinephrine (NE). Moreover, the relaxations induced by these agents were not stereoselective. The rank order of potency (IC50) for relaxation of KCl-induced contracted muscle was: Ro5-4864 (6.6 .+-. 0.3 .mu.M) = PK 11195 (6.7 .+-. 0.9 .mu.M) = (-)PK 14067 (11.6 .+-. 0.7 .mu.M) = (+)PK 14068 (7.6 .+-. 1.1 .mu.M) > diazepam (47.4 .+-. 5.3 .mu.M) = clonazepam (47.5 .+-. 5.7 .mu.M). Further investigation of the mechanism of benzodiazepine-induced relaxation showed that (-)PK 14067 and (-)PK 14068 inhibited CaCl2-induced contractions. The benzodiazepines relaxed muscle contracted with KCl to a greater magnitude than those contracted with NE or prostaglandin F2.alpha. (PGF2.alpha.). These data suggest that the relaxant effects of these agents are unrelated to the high-affinity benzodiazepine binding site and are probably mediated through inhibition of the voltage-operated calcium channel.