Processing of cholecystokinin by isolated liver cells

Although hepatic uptake of cholecystokinin (CCK) has been demonstrated, the liver cell involved and the mechanism of uptake remain unclear. We have used dispersed rat hepatocytes, Kupffer cells, and hepatic endothelial cells to characterize uptake and metabolism of radiolabeled CCK peptides. Only rat hepatocytes showed significant uptake of ¹²⁵I-labeled cholecystokinin octapeptide (¹²⁵I-CCK-8). Peptide specificity of uptake by hepatocytes was similar to that seen in the isolated perfused rat liver, with extraction of ¹²⁵I-CCK-8 being sevenfold greater than that of ¹²⁵I-CCK-33. Uptake was saturable, as 10^-4 M CCK-4 inhibited uptake of ¹²⁵I-CCK-8 by 85%. Uptake was rapid, temperature dependent, and extensive and was decreased by metabolic inhibition, a proteolytic enzyme (trypsin), organic anions (sulfobromophthalein and taurocholic acid), and an inhibitor of anion transport 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid. In addition, uptake was dependent on extracellular anions but not on extracellular sodium, calcium, or magnesium. After uptake, hepatocytes released radiolabel in a time- and temperature-dependent manner, predominantly in metabolized forms. Thus the hepatocyte is the liver cell that extracts CCK by an active, anion-dependent process. The characteristics of the uptake process resemble those described for organic anions and small, cyclic peptides and suggest that small, linear peptides may undergo hepatocyte extraction by a similar mechanism.