Endothelin‐1 inhibits platelet aggregation in vivo: a study with 111indium‐labelled platelets

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Abstract
1 A non-invasive technique for the scintigraphic determination of 111indium-labelled platelet aggregation stimulated with submaximal doses of adenosine diphosphate (ADP, 56 μg kg−1 i.v.), collagen (100 μg kg−1 i.v.), platelet-activating factor (PAF, 0.1 μg kg−1 i.v.) or thrombin (18 iu kg−1 i.v.) was used to investigate the platelet-inhibitory effects of endothelin 1 (ET-1) in anaesthetized rabbits in vivo. 2 ET-1 (1 nmol kg−1 i.v.) inhibited ADP-stimulated platelet aggregation in vivo; a maximum inhibition of 78% of the control value was reached at 3 min, with 45% inhibition at 15 min, and a return to control values at 30 min after injection of the peptide. 3 ET-1 (1 nmol kg−1 i.v.) inhibited in vivo platelet aggregation in response to collagen or PAF by 86% and 52%, respectively, but had no effect on thrombin-induced platelet aggregation. 4 Indomethacin (5 mg kg−1 i.v.) abolished the ET-1-induced inhibition of ADP-stimulated platelet aggregation and significantly potentiated and prolonged the pressor response brought about by ET-1. 5 In conclusion, the data demonstrate that ET-1 potently inhibits platelet aggregation in the anaesthetized rabbit in vivo by releasing a hypotensive and anti-aggregatory cyclo-oxygenase product, presumably prostacyclin, into the circulation.