ROLE OF CAPILLARY ENDOTHELIUM IN CLEARANCE OF CHYLOMICRONS - MODEL FOR LIPID TRANSPORT FROM BLOOD BY LATERAL DIFFUSION IN CELL-MEMBRANES

The mechanism proposed for transport of fatty acids from chylomicrons to fat cells consists of conversion of chylomicron triglyceride by lipoprotein lipase to acyl lipids which can move by lateral diffusion in a continuous interface of cell membranes to fat cells where they are esterified to triglyceride and separated from the membrane interface. The surface film of chylomicrons becomes redundant when the triglyceride core is reduced by lipolysis. When this happens in capillaries of adipose tissue, cholesterol in the surface film may cross the capillary wall by lateral diffusion, also in a continuous interface of cell membranes, and contributes to membrane proliferation of fat cells during storage of triglyceride. Similar mechanisms are undoubtedly involved in uptake of triglyceride and cholesterol by lactating mammary gland, with incorporation of glyceride into milk triglyceride, and cholesterol into milk fat globule and cell membranes. Phospholipid may also be transferred to extrahepatic tissues by a process similar to that for cholesterol. Cholesteryl ester, in contrast, is not taken up by extrahepatic tissues. It is released to the blood stream as part of the chylomicron remnant and taken up by liver. It is likely that the mechanism proposed above for transfer of lipids in endothelium of adipose and mammary tissue would operate in endothelium of large blood vessels and heart as well as in the capillaries of these and other tissues.