Human big endothelin (big ET) and endothelin-1 (ET-1) induce similar increases in left ventricular systolic pressure in the anesthetized rabbit. Unlike ET-1, human big ET does not induce an initial transient hypotension. Human big ET (3 nmol/kg) inhibits ADP-induced platelet aggregation ex vivo by 60% whereas ET-1 at 1 nmol/kg inhibits platelet aggregation by more than 80%. The C-terminal fragment, big ET[22-38] (3 nmol/kg), has no antiaggregatory properties. Inhibition of ex vivo platelet aggregation by human big ET and ET-1 was not seen in rabbits pretreated with indomethacin (5 mg/kg). Human big ET (10(-7) M) or ET-1 (2.5 x 10(-9)-10(-8) M) induced the release of prostacyclin (PGI2) from rabbit, guinea pig, and rat lungs. Phosphoramidon (50 microM, infused 45 min prior to and during administration of peptides) inhibited the prostanoid-releasing properties of human big ET without affecting the release induced by ET-1. Intravascular administration of human big ET (1 nmol/kg) significantly increased the circulating levels of immunoreactive ET-1 (ir-ET-1) for 30 min whereas administration of ET-1 at the same concentration increased the plasma level of ir-ET-1 for 5 min only. Our results suggest that human big ET is converted to ET-1 in the rabbit in vivo. We further suggest that to induce the release of prostanoids in perfused lungs, human big ET needs to be converted to ET-1 by a phosphoramidon-sensitive endothelin-converting enzyme (ECE).