Effects Of Losartan And Its Combination With Aspirin On Platelet Aggregation In Normal Rats In Vitro

Objective:Previous studies have demonstrated that losartan can reduce platelet aggregation. However,the mechanism for this effect of losartan is still not fully understood.The aim of this study was to investigate the effect of losartan and its combination with cyclooxygenase inhibitor aspirin on platelet aggregation and the content of plasma TXB₂ in normal adult rats in vitro.Methods:â‘ Platelet aggregation was measured by changes in the light transmission. Experiments were conducted in male rats,weighted 240-260g.The blood directly drawn from the abdominal aorta was centrifuged to obtain platelet-rich plasma(PRP)and platelet-poor plasma(PPP).PRP pretreated with losartan and aspirin was incubated at 37℃for 5 min.We recorded the platelet aggregation curve stimulated by ADP(5μmol/L).â‘¡Affer the platelet aggregation assay was performed,the medium was centrifuged again.The supernatant was recovered and quick-frozen at -20℃.The content of TXB₂ was determined by radioimmunoassay in 3 months.Results:The maximal platelet aggregation rate induced by ADP(5μmol/L)was 56.44±2.81%, and the content of plasma TXB₂ was 587.95±30.98ng/ml.Losartan(10^-8mol/L)did not change the platelet aggregation or the content of TXB₂ induced by ADP(P＞0.05).Losartan (10^-7-10^-5mol/L)significantly attenuated ADP-induced platelet aggregation in a dose-dependent manner.The inhabitation rates were 14.17%,22.61%and 29.85%(P＜0.01).Furthermore,the content of TXB₂ induced by ADP was significantly reduced by losartan in a dose-dependent manner,and the reduction rates were 18.47%,23.26%and 44.82%(P＜0.01)Aspirin (3×10^-4mol/L)significantly inhibited platelet aggregation(P＜0.01)by a degree of 26.02%and reduced the content of TXB₂(P＜0.01)by 45.24%.The rates had no significant difference versus losartan(10^-5mol/L)(P＞0.05).Losartan(10^-5mol/L)combined with aspirin(3×10^-4mol/L) diminished platelet aggregation by about 44.38%.The effect was significantly greater compared with losartan single(P＜0.01)or aspirin single(P＜0.01).Whereas no additional effects on the content of TXB₂ were observed by adding aspirin+losartan than when using aspirin and losartan separately.Conclusion:Losartan dose-dependently inhibited platelet aggregation and induced the content of TXB₂ in normal rats in vitro.Losartan and aspirin had synergistic action on platelet aggregation.Losartan decreased platelet aggregation by inducing the content of TXA₂ and some other ways. Background:Our previous studies had proved losartan decreased platelet aggregation by a thromboxane A₂-dependent mechanism.It is well known that losartan is used as an antihypertensive therapy in clinic.It has been noted that acute administration of losartan is more potent than the peptide AT receptor antagonist saralasin and angiotensin converting enzyme (ACE)inhibitors in lowering blood pressure in the same hypertensive model.These studies suggest that additional mechanisms may be involved in this effect of antihypertensive action of losartan besides its ability to antagonize the AT1 receptor.It is shown that losartan could inhibit TXA₂-induced vasoconstriction.We investigated the effect of losartan on TXA₂ through vascular ring reactivity.Objective:The main purpose of this study was to investigate the effects of losartan on the reactivity of the aortic rings in male wistar rats.Methods:Isolated aortic rings were prepared conventionally.The rings were suspended in organ chambers,bathed in HEPES solution at 37℃and gassed with 100%O₂,and the vascular tension was recorded by MS4000 multi-channel physiology signal acquisition and analysis computer system through tension transducers.The effects of losartan on rat isolated thoracic aortic rings' contraction induced by U46619,KCl or Pit were observed. In addition,the effects of the NO synthase inhibitor L-NAME and the cyclooxygenase inhibitor indomethacin on losartan were observed.Results:1,Losartan(3×10^-8-3×10^-5mol/L)concentration-dependently reduced the constriction induced by U46619(1μmol/L).The logEC₅₀of losartan was -5.94±0.03,and the Emax was 98.76±1.92%.2,0.1mmol/L L-NAME significantly shifted the concentration-response curves of losartan to the fight(logEC50:-5.63±0.02 vs -5.94±0.03,L-NAME vs losartan single,P＜0.01),without affecting the maximal response.3,Indomethacin(1μmol/L)did not change the relaxations of losartan(logEC₅₀:-5.76±0.02 vs -5.94±0.03;E_max:98.37±1.14%vs 98.76±1.92%;Indo vs losartan single,p＞0.05).4,Losartan(10^-6mol/L)did not significantly affect the constriction induced by KCl(60mmol/L)or Pit(2u/L).Conclusions:1,Losartan preferentially antagonized contraction induced by U46619 in concentration-dependent manner.2,The vasorelaxation of losartan was relevant to the production of nitrogen.3,The vasorelaxation of losartan was not relevant to the production of prostaglandin.4,Losartan showed no effect on KCl- and Pit-induced contraction in isolated thoracic aortic rings.