Low density lipoprotein. A metabolic pathway for return of cholesterol to the splanchnic bed.

The mechanism(s) by which cholesterol returns to the splanchnic bed from peripheral tissues are not well understood. To study this phenomenon in fasting man, lipoproteins were isolated from plasma obtained from hepatic vein and aorta. Cholesterol content of each lipoprotein class was determined and arteriovenous (AV) differences could be calculated for each patient. The results in the first 24 patients indicated splanchnic secretion of very low density lipoprotein cholesterol (mean AV difference - 3 mg/100 ml, P < 0.01), but not significant AV difference for total cholesterol, high density lipoprotein cholesterol, or low density lipoprotein (LDL) B protein. In contrast, for LDL (d 1.006-1.063 g/ml), there was significant uptake of cholesterol across the AV bed +8 mg/100 ml, P < 0.0002). In a further 15 patients, similar samples were obtained and intermediate density lipoprotein isolated at d 1.006-1.019 g/ml and LDL at 1.019-1.063 g/ml. The AV difference previously noted could now be localized to the 1.019-1.063 cholesterol ester moiety (+8 mg/100 ml, P < 0.0005). In the final 14 patients, the LDL cholesterol AV difference was again confirmed and shown to be unrelated to heparin. As well, there was secretion of triglyceride in the hepatic vein LDL. These quantitative data obtained in man raise the possibility that LDL rather than high density lipoprotein transports cholesterol ester to the splanchnic bed.