Secretion of hepatic lipase by perfused liver and isolated hepatocytes

Hepatic lipase is found in liver and in adrenal glands and ovaries. Because in adult rats, neither adrenals nor ovaries synthesize this enzyme, it is assumed that the liver is the origin of their hepatic lipase. Our aim was to study the secretion of hepatic lipase by the liver. We observed that plasma of both fed and fasted rats contained hepatic lipase activity. This activity was significantly correlated with that in the liver. Isolated livers, perfused with heparin-free medium, secreted fully active hepatic lipase to the perfusate. The addition of heparin resulted in a rapid and larger release of hepatic lipase to the perfusate. In isolated hepatocytes, heparin did not affect the secretion of hepatic lipase mass, although it increased the stability of the enzyme activity. To study the degradation of hepatic lipase by hepatocytes, protein synthesis was blocked with cycloheximide, and both secreted and intracellular hepatic lipases were analyzed by Western blotting. We observed that the amount of hepatic lipase secreted equaled the decrease of intracellular mass. The total mass of the enzyme (inside and outside the cells) remained constant, at least for 90 min. In the next experiment, 0.7 nM 125I-hepatic lipase was added to hepatocyte suspensions, and the appearance of trichloracetic acid-soluble products was analyzed. Only 12% of the radioactivity added was associated with the cells after 90 min of incubation, and less than 2% of the hepatic lipase added was degraded. Although the association was decreased in the presence of heparin, the amount of 125I-hepatic lipase degraded was not affected. Taking all these results into account, we propose a model for the continuous secretion of hepatic lipase by the liver.