| Objective:To investigate the effects and the possible mechanism(s) of diazepam (DZP) on isolated thoracic aorta rings pre-contracted by norepinephrine (NE) or potassium chloride (KCl) in adult rats and to discuss the possible mechanism(s).Methods:The rat was sacrificed by cervical vertebra luxation and the thoracic aorta was quickly isolated from each animal and placed in cold physiological salt solution (PSS), and the adherent fat and connective tissues were carefully removed. The vessel was then cut into rings about 2～3 mm in length. Two stainless-steel wires were passed through the lumen of each ring. One was fixed to the bottom of the bath chamber, the other was connected to an isometric force transducer to measure the tension of the ring. Rings were placed in a 10 ml organ chamber containing PSS, bubbled with 100% O2, at 37℃.The rings were stretched until they exerted an optimal basal tension of 2 g, and then were allowed to equilibrate for 1 h with the bath fluid being changed every 15 min. Isotonic tension of thoracic aortic rings pre-contracted by KC1 (30 mmol/L) or NE (10-6 mol/L) was recorded. The relaxant effects of DZP and effects of various drugs include NG-nitro-L-arginine methyl ester (L-NAME,10-4 mol/L), indomethacin (Indo,10-5 mol/L) and PK11195 (10-6 mol/L) were observed in the rings. The influence of tetraethylammonium (TEA,10-2 mol/L), 4-aminopyridine (4-AP,10-3 mol/L), glibenclamide (Gli,10-5 mol/L), barium chloride (BaCl2,10-3 mol/L) on the effects of DZP were also studied.Results:(1) DZP(10-6～3×10-4 mol/L) had no effect on the resting tone of isolated thoracic aorta rings of adult rats (P>0.05). (2) DZP(10-7～3×10-4 mol/L) concentration-dependently attenuated the contraction of adult rat aorta induced by KCl(30 mmol/L). The EC50 of DZP was (2.46±1.06)×10-5 mol/L, and the Emax was (117.2±6.67)% in isolated thoracic aorta rings of adult rats with intact endothelium. The EC50 of DZP was (3.80±0.90)×10-5 mol/L, and the Emax was (121.32±8.14)% with endothelium denuded. There was no significant difference between the rings with intact and denuded endothelium at the concentrations of DZP (10-7～3×10-4 mol/L), (P>0.05). (3) DZP(10-7～10-4 mol/L) concentration-dependently reduced the contraction of adult rat aorta induced by NE. The EC50 of DZP was (4.03±0.42)×10-5 mol/L, and the Emax was (101.48±5.57)% in isolated thoracic aorta rings of adult rats with intact endothelium. The EC50 of DZP was (5.46±0.45)×10-5 mol/L, and the Emax was (103.42±5.38)% in denuded endothelium isolated thoracic aorta rings of adult rats. There was no significant difference between the rings with intact and denuded endothelium at the concentrations of DZP (10-7～10-4 mol/L)(P>0.05). (4) When the contractions was induced by KCl, L-NAM,TEA,Gli and BaCl2 depressed DZP-induced relaxation at 3×10-5 mol/L (P<0.05), while Indo,4-AP and PK11195 had no significant inhibition on the vasorelaxant effects of DZP(P>0.05). L-NAME, TEA,4-AP, BaCl2, Indo, PK11195 and Gli could not attenuate the vasorelaxant effect of DZP at 3×10-4 mol/L. (5) When the contractions was induced by NE, L-NAME,TEA,4-AP,Gli and BaCl2 inhibited DZP-induced relaxation at 10-6 mol/L (P<0.05), while Indo and PK11195 had no significant inhibition on the vasorelaxant effects of DZP(P>0.05). L-NAME, Indo, PK11195/TEA,4-AP, Gli and BaCl2 could not attenuate the vascular relaxation effect of DZP at 3×10-4 mol/LConclusions:(1) DZP could produce remarkable vasodilatation on NE or KCl pre-contracted aorta rings in adult rats, while it does not affect the resting tone. (2) DZP at low concentrations relaxed aorta via endothelium-dependent mechanisms, probably by increasing NO release from the endothelium, but it not related to PGI2. (3) DZP at low concentrations relaxed aorta is not related to PBR. (4)Potassium channels on the vascular smooth muscle were involved in the vasorelaxant effect of DZP at low concentration. (5) None of ndothelium, PBR or potassium channels are involved in the vasorelaxant effect of DZP at high concentration.
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