Diazepam Attenuates Vascular Contractions In Old Rat Aorta

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 old rats.Methods: The old 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 2g, and then were allowed to equilibrate for 1 hour with the bath fluid being changed every 15 minutes. Isotonic tension of thoracic aortic rings pre-contracted by NE (10-6 mol/L) or KCl (30 mmol/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 effects of DZP on the extracellular Ca2+-dependent contraction and intracellular Ca2+-dependent contractions were also studied.Results: (1) DZP(10-6～3×10-4 mol/L) had no effect on the resting tone(P>0.05). (2) DZP(10-6～10-4 mol/L) concentration-dependently attenuated the contraction induced by NE. The EC50 of DZP was (3.45±0.81)×10-6 mol/L, and the Emax was (102.90±4.03) %. (3) DZP(10-6～10-4 mol/L) concentration-dependently reduced the contractions of denuded endothelium aortic rings which induced by NE. There was significant difference between the rings with intact and denuded endothelium at low concentrations of DZP (10-6, 3×10-6, 10-5, 3×10-5 mol/L), (P<0.01). (4) When the contractions was induced by NE, the vasodilation of DZP at 3×10-6 mol/L could be inhibited by L-NAME (P<0.01), but not by Indo and PK11195. (5) DZP (10-6～3×10-4 mol/L) concentration-dependently reduced the contractions of old rat aorta induced by KCl at 30 mmol/L. The EC50 of DZP was (3.05±0.47)×10-5 mol/L, and the Emax was (106.64±2.77) %. (6) DZP (10-6～3×10-4 mol/L) concentration-dependently reduced the contractions of denuded endothelium aortic rings which induced by KCl. There were significant differences between the rings with intact and denuded endothelium at low concentrations of DZP (10-6, 3×10-6, 10-5, 3×10-5 mol/L), (P<0.05). (7) When the contractions was induced by KCl, L-NAME diminished DZP-induced relaxation at 3×10-5 mol/L (P<0.05), but not at 3×10-4 mol/L. Indo and PK11195 had no significant inhibition on the vasorelaxant effects of DZP. (8) DZP at 10-4 mol/L attenuated contractions induced by adding NE and CaCl2 in the Ca2+-free medium.Conclusions: DZP could produce remarkable vasodilatation on NE or KCl pre-contracted aorta rings in old rats, while it does not affect the resting tone. DZP at low concentrations relaxed aorta via endothelium-dependent mechanisms, probably by increasing NO release from the endothelium. The relaxant mechanism of DZP at high concentrations may be concerned with the inhibition of extracellular Ca2+ influx and intracellular Ca2+ release.