Losartan prevents thromboxane A₂/prostanoid (TP) receptor mediated increase in microvascular permeability in the rat

We explored the putative inhibitory effects of losartan, a potent nonpeptide, AT₁ receptor antagonist, against thromboxane A₂ (TxA₂)/prostanoid (TP) receptor-mediated transcapillary shift of plasma fluid and proteins. The effects of the TP receptor agonist U-46619 (1.25 or 10 μg/kg intravenously) on hematocrit (Hct), albumin extravasation (AE), and mean arterial pressure (MAP) were evaluated in anesthetized Sprague- Dawley rats. U-46619 dose-dependently increased Hct (by 4.5% ± 0.7% and 7.5% ± 1.0% at the low and high dose, respectively; both P < .05 v vehicle-infused group) and decreased MAP (by 7.9% ± 4.1% and 16.8% ± 5.7% at the low and high dose, respectively; P = NS and P < .05 v vehicle- infused group, respectively). In these experiments, using a quantitative Evans blue technique, we showed that U-46619 dose-dependently increased AE in kidney, lung, spleen, and testis (by ~31%, 172%, 52%, and 57% at the highest dose) but not in adipose tissue, brain, liver, mesentery, and skeletal muscle. In the heart, AE was maximally increased by the low dose of U-46619. The U-46619 (10 μg/kg)-induced increases in Hct and AE and decreases in MAP were blocked by pretreatment with the TP receptor antagonist SQ 29,548 (2.5 mg/kg intravenously + 2.5 mg/kg/h) and the high dose of losartan (40 mg/kg intravenously). The low dose of losartan (10 mg/kg intravenously) did not significantly alter the responses to U-46619 except for the AE, which was reduced in some but not all tissues. Furthermore, the U-46619-induced changes in Hct (+6.3% ± 1.7%), MAP (- 13.9% ± 8.4%) and AE were not affected in rats pretreated with the converting-enzyme inhibitor enalapril. Thus, selective activation of TP receptors by U-46619 induced plasma fluid and protein exudation; these responses were specifically attenuated by the relatively high dose of losartan, suggesting that this compound acts as a TP receptor antagonist in this experimental model.