Angiotensin-(1-7) [Ang-(1-7)] is the bioactive peptide which may be responsible for some of the pharmacological effects of losartan. Our previous study has demonstrated the antithrombotic action of losartan in a model of experimental thrombosis. In the present study, we compared the antithrombotic action of losartan and Ang-(1-7). Acute (10 mg/kg, p.o.) and chronic (10 mg/kg, p.o., three weeks) losartan administration to spontaneously hypertensive rats (SHR) induced a decrease in thrombus weight (1.6 +/- 0.6 mg and 1.2 +/- 0.3 mg respectively vs. control 2.9 +/- 0.8 mg; p<0.05, p<0.05). A similar reduction was observed in two-kidney, one-clip hypertensive rats (2K-IC)receiving acute losartan administration (1.39 +/- 0.29 mg vs. 3.25 +/- 0.62 mg; p<0.01). Infusion of Ang-(1-7) to2K-lC rats also reduced the thrombus weight(1.01 +/- 0.34 mg, 1.23 +/- 0.38 mg and 2.17 +/- 0.36 mg for 1, 10, 100 pmol/kg/min, respectively vs. 3.58 +/- 0.6 mg control; p<0.01, p<0.01, p<0.05). Losartan produced a decrease in systolic blood pressure (BP) in SHR as well as in 2K-1C rats, while Ang-(1-7) infusion had no effect on BP. Acute losartan dosing to 2K-1C rats decreased platelet adhesion to fibrillar collagen(24.9 +/- 1.0% vs. control 31.5 +/- 1.1%, p<0.001). The incubation of platelet samples with Ang-(1-7) (10-6 and 10 5 M) also reduced adhesion to fibrillar collagen(38.4 +/- 0.1% and 33.8 +/- 0.8% respectively vs. control 40.0 +/- 0.6%; p<0.05, p<0.001). There were no apparent changes in prothrombin time, activated partial thromboplastin time and euglobulin clot lysis time in losartan and Ang-(1-7)-treated groups. We conclude that, like losartan, Ang-(1-7) is able to act as an antithrombotic agent.

No abstract

Clinical and experimental data have recently accumulated for antithrombotic action of angiotensin-converting enzyme inhibitors (ACE-1s). We have shown previously that captopril (which contains a thiol group in the moiety) exerts more pronounced antithrombotic activity than does an equipotent dose of enalapril (the drug devoid of the thiol group). To clarify the relative importance of the presence of the thiol group in the molecule versus angiotensin-converting enzyme (ACE) inhibitory properties in the antithrombotic action of captopril, rats were treated with captopril (5 mg/kg twice daily; CAP), epicaptopril (stereoisomer of captopril devoid of ACE-inhibitory properties; 5 mg/kg twice daily; EPI), N-acetylcysteine (3.75 mg/kg twice daily; ACC), enalapril (3 mg/kg once daily; ENA), or distilled water (VEH) for 10 days, per os. After ligation of the vena cava, the incidence of the venous thrombosis and/or the thrombus weight decreased significantly in all but the ENA-treated groups when compared with control rats. The effect of CAP, EPI, and ACC was accompanied by a marked reduction of euglobulin clot lysis time and, with the exception of ACC, by an increase in prothrombin time in the blood collected from the site of the thrombus formation. Antithrombotic activity of EPI was completely abolished by nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) or indomethacin, with the parallel reversal of fibrinolytic and coagulation parameters toward normal. Activated partial thromboplastin time, mean blood pressure, and bleeding time were not altered by either of the administered drugs. Thus, we demonstrated that thiol compounds exert antithrombotic activity by increasing fibrinolysis and/or suppression of the extrinsic pathway of the coagulation cascade in a nitric oxide/prostacyclin-dependent manner.

In previous studies, we have shown that losartan possesses nitric oxide-dependent antithrombotic properties in various models of hypertension in rats. It was demonstrated that stimulation of AT2-receptors plays an important role in the pharmacological effects of AT1-receptor antagonists. Thus, in this study, we examine the participation of AT2-receptors in the antithrombotic action of losartan in renal hypertensive rats on venous thrombosis induced by a two-hour ligation of the vena cava. Losartan administration(30 mg/kg, p.o.) resulted in a marked decrease in thrombus weight (by 85%, p<0.001). PD123319, an AT2-receptor antagonist (10 mg/kg, i.v.), administered concomitantly with losartan, abolished its antithrombotic effect, whilst it had no influence on thrombus weight when given alone. A significant decrease in systolic blood pressure was observed in animals given losartan. PD123319 administration didnot abolish this action of losartan and did not alter blood pressure when given alone. No changes in prothrombin time, activated partial thromboplastin time, or euglobulin clot lysis time were observed in animals administered losartan and/or PD123319.Similarly, primary haemostatics evaluated by bleeding time and platelet count did not change in any group of rats. In conclusion, we have shown that AT2-receptor stimulation is involved in the antithrombotic action of losartan in renal hypertensive rats.

No abstract