Effect of different endothelin receptor antagonists and of the novel non‐peptide antagonist Ro 46‐2005 on endothelin levels in rat plasma

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Abstract
The goal of our study was to evaluate and compare the effects of receptor blockade with different endothelin (ET) receptor antagonists on plasma concentrations of ET‐1, big ET‐1 and ET‐3 in conscious rats. Ro 46‐2005 (10 mg/kg, i.v.), a novel non‐peptide antagonist of both ETA and ETB receptors, increased the concentrations of ET‐1 in plasma to 200 ± 13% of basal levels (P < 0.001). This effect was dose‐ and time‐dependent and reached a maximum at 15 min. Ro 46‐2005 had no effect on plasma concentrations of big ET‐1 and only a minor effect on those of ET‐3. In contrast to Ro 46‐2005, the selective peptide ETA antagonists BQ‐123 and FR‐139317 had no effect on plasma ET‐1 concentrations. The increase in plasma ET‐1 concentrations by Ro 46‐2005 was most likely not due to de novo synthesis, since big ET‐1 levels were not increased and peak levels were reached early after compound injection, but perhaps to displacement of ET‐1 from the ETB receptors.