| Abstract: | We used monolayer cultured rat hepatocytes as an experimental model to study the secretion of the newly synthesized cholesterol by the liver. Cellular cholesterol was labeled by exposing cultured hepatocytes to 14C]acetate prior to the study of secretion. Secretion of the newly synthesized cholesterol was measured by extracting cholesterol in the culture medium and assaying for the radioactivity of 14C]cholesterol. We found that: (a) cultured hepatocytes could secrete newly synthesized cholesterol in serum-free medium; (b) secreted 14C]cholesterol was bound to macromolecule(s) and the secretion rate was not affected by cycloheximide for up to 5 h; (c) serum added to the culture medium greatly enhanced hepatic cholesterol secretion; (d) serum high-density lipoproteins were most effective, lipoprotein-deficient serum (d greater than 1.21) less effective in stimulating cholesterol secretion, whereas low-density and very-low-density lipoproteins had little effect; (e) when the serum-free culture medium was fractionated by ultracentrifugation, a major portion of the secreted 14C]cholesterol was found in the high-density lipoprotein fraction; (f) part of the medium 14C]cholesterol also turned up in the high-density lipoprotein fraction when lipoprotein-deficient serum was added as the acceptor; (g) secreted 14C]cholesterol was found only in free form, although some of the cellular 14C]cholesterol was found as esters. |
