Endothelin association with the cultured rat kupffer cell: Characterization and regulation

Circulating endothelin (ET) levels are elevated in conditions such as endotoxemia, hepatic ischemia-reperfusion injury, or orthotopic liver transplantation, and this potent peptide may contribute to hepatic pathophysiology. We measured the surface binding of [¹²⁵I]ET-1 to rat Kupffer cells in primary culture at 4°C; the apparent dissociation constant (K_d) was 270 pmol/L, and the apparent B_max was 3,000 receptors/cell. At 37°C, total association (surface binding plus internalization) was much greater than at 4°C, indicating that internalization of the receptor-ligand complex is rapid; the apparent K_d was 30 pmol/L, comparable with other reports for hepaticderived cells. Studies using [¹²⁵i]ET-1, [¹²⁵I]ET-3, and specific ET (ant)agonists showed that Kupffer cells possess predominantly ET_B type receptors. Prior treatment with 500 pmol/L unlabeled endothelin rapidly (<15 minutes) occluded 60% of subsequent [¹²⁵1]ET association; using 5 nmol/L unlabeled ET, this occlusion occurred within 1 minute. [¹²⁵1]ET association with Kupffer cells was unaffected by short-term (≈-1 hour) treatment with cyclic adenosine monophosphate (cAMP), but long-term (20 hour) treatment resulted in a twofold increase in [¹²⁵1]ET association with no change in the apparent K_d. Stimulation of protein kinase C in Kupffer cells by phorbol 12-myristate acetate had a dual regulatory effect on [¹²⁵I]ET association. Short-term (<1 hour) treatment with phorbol 12-myristate acetate decreased [¹²⁵1]ET-3 association by 50%, whereas prolonged treatment (20 hour) increased association twofold. In both cases, the apparent K_d for [¹²⁵1]-endothelin was unaltered.