Differential pharmacological profile of endothelin-1 and its precursor, big endothelin

In pentobarbital-anesthetized rats endothelin-1 (ET-1), endothelin-2 (ET-2), endothelin-3 (ET-3), and mouse ET-2 (mET-2), in contrast to human big ET-1 (h-big ET), administered as i.v. bolus injections (0.25 nmol/kg i.v.) produced rapidly appearing and short-lasting blood pressure decreases. This effect was markedly inhibited (80-100%) after an 8-min i.v. infusion (0.1 nmol/kg/min over 10 min) of any of the ET studied, but not by h-big ET, the precursor of ET-1. Similarly, in pithed rats given a 10 min i.v. infusion of an equipressor dose (0.1 nmol/kg/min) of ET-1 or h-big ET, the hypotensive effects of ET-1 were entirely blocked only in the group of animals pretreated with ET-1. In pithed rats, ET-1 (0.25 nmol/kg i.v.) and h-big ET (0.5 nmol/kg i.v.) produced equivalent maximal pressor responses and the same pattern of increase in systemic, hindquarter, and renal vascular resistance. However, ET-1 was three times more potent than h-big ET as a vasoconstrictor of the mesenteric bed. Also the pressor response to h-big ET, but not ET-1 (0.25 nmol/kg i.v.), was markedly inhibited by the metalloprotease inhibitor phosphoramidon (5 mg/kg i.v.). These results indicate that the hypotensive effects of ET isopeptides have a common mechanism because they elicit cross tachyphylaxis, although h-big ET did not inhibit the decrease in blood pressure produced by ET-1. A possible explanation for this finding is that h-big ET has intrinsic pressor activity but does not have affinity for receptors mediating the vasodepressor effects of ET isoforms. Alternatively, h-big ET is converted into ET-1 too slowly to yield biophase concentrations of ET-1 necessary for lowering blood pressure and developing tachyphylaxis to ET-isoform-induced hypotension. Finally, if the pressor effects of h-big ET are mediated by ET-1 formation, phosphoramidon can be considered as an inhibitor of the endothelin-converting enzyme.