Kinetics of endothelin-1 binding in the dog liver microcirculation in vivo.

Endothelin-1 (ET-1) is a 21-amino acid peptide produced by vascular endothelial cells that acts as a potent constrictor of hepatic sinusoids. Hepatic binding of tracer 125I-labeled ET-1 was investigated in anesthetized dogs with the multiple-indicator dilution technique with simultaneous measurements of unlabeled immunoreactive ET-1 plasma levels. Despite 80% binding to albumin, tracer 125I-ET-1 was avidly extracted by the liver, with only 15 ± 6% of the peptide surviving passage through the organ. Exchange of ET-1 between plasma and binding sites, probably located on the surface of liver cells, was quantitatively described by a barrier-limited, space-distributed variable transit time model. Reversible and irreversible parallel binding sites were found. Reversible and irreversible plasma clearances of unbound 125I-ET-1 were 0.084 ± 0.033 ml ⋅ s-1 ⋅ g liver-1 and 0.17 ± 0.09 ml ⋅ s-1 ⋅ g liver-1, respectively, and the dissociation rate constant for reversible binding was 0.24 ± 0.12 s-1. The specific ETA receptor antagonist BMS-182874 did not modify binding to either site. The nonspecific ETA/ETBantagonist LU-224332 dose-dependently reduced irreversible binding only. ET-1 levels in the hepatic vein were significantly lower than in the portal vein but were not different from those in the hepatic artery. The ratio between hepatic vein and portal vein levels (0.64 ± 0.31) was considerably higher than survival fractions, suggesting a substantial simultaneous release of newly synthesized or stored ET-1 by the liver. These results demonstrate both substantial clearance and production of ET-1 by the intact liver. Hepatic ET-1 clearance is mediated by the ETB receptor, with the presence of reversible, nonspecific ET-1 binding at the liver surface.