CHOLESTEROL EFFLUX IN-VIVO FROM A DEPOT OF CATIONIZED LDL INJECTED INTO A THIGH MUSCLE OF SMALL RODENTS

We have developed a model system to measure quantitatively removal of cholesterol from a well-defined depot in vivo. To that end, lipoprotei ns were injected into the rectus femoris muscle of small rodents, usin g a 25 mu l Hamilton syringe and a 27-gauge needle. In most experiment s, the injected volume was 10 mu l containing 200 mu g of cholesterol. The lipoproteins tested were native or modified LDL labeled with trac e amounts of [H-3]free cholesterol ([H-3]FC). The amount of label or o f cholesterol mass recovered at various time intervals after injection was normalized to that found after 10 min (designated time 0). In mic e, the highest recovery of the [H-3]cholesterol 24 h after injection w as found with cationized LDL, and ranged between 78% and 84%, whereas retention of native LDL did not exceed 24%. Based on results of 9 expe riments with cationized LDL, the loss of [H-3]FC was mono-exponential between 1 and 14 days and the t(1/2) was about 4 days. The disappearan ce curve of cholesterol mass showed an initial slow and a later more r apid component, the latter with a t(1/2) of 4 days. The initial lag is most probably due to the presence of cholesteryl ester, which needs t o be hydrolyzed prior to egress. This assumption was verified by injec tion of cat-LDL labeled with [H-3]cholesteryl oleate and finding a sim ilar lag as well as evidence of [H-3]cholesteryl ester hydrolysis. His tological examination of the injected muscle 1-4 days after injection of cat LDL showed infiltration with mononuclear cells in an area limit ed to the site of injection. The presently described model system, whi ch mimics to some extent events occurring during atherogenesis, permit s quantitative evaluation of egress of deposited cholesterol and may a llow to study the role of HDL in such a process. (C) 1997 Elsevier Sci ence Ireland Ltd.