Origin and fate of rat plasma cholesterol in vivo. Modelling of cholesterol movements between plasma and organs.

A cholesterol system model was developed in the rat following a single injection of red cells containing free (unesterified) [3H]cholesterol. The radioactivity of free and esterified cholesterol in the different parts of the system was measured during the 48 h following tracer introduction. The model consisted of seven compartments (red cell free cholesterol, plasma and liver free and esterified cholesterol, total cholesterol in the rapidly and slowly exchangeable carcass pools). The model was validated by the similarity between simulated and experimental values during the 48 h following tracer introduction. Both the fractional rate of cholesterol esterification in the plasma (0.44 h-1) and liver (0.01 h-1) and the fractional exchange rate of free cholesterol from the plasma towards the various organs (particularly 3 h-1 towards the liver for a total of 7 h-1) can be estimated with this model. The results show that cholesterol movements between the plasma and the different organs take place mainly through intense free cholesterol exchanges, resulting in a low net flux.

The measurement in vivo of the rate of unesterified cholesterol exchange between rat plasma and erythrocytes.

In order to investigate the rate of unesterified cholesterol exchange between plasma and erythrocytes in vivo, cholesterol labelling in rats was achieved in one of the following ways: intravenous injection of cholesterol-labelled erythrocytes, subcutaneous injection of labelled acetate, feeding of labelled cholesterol. The specific activity of the unesterified cholesterol was measured at intervals up to 24 h and a kinetic analysis of the data was performed. It assumes that both the cholesterol in the erythrocytes and the unesterified cholesterol in the plasma were homogeneous pools. The rate constants obtained for the movements of unesterified cholesterol from erythrocytes to plasma and from plasma to erythrocytes were not significantly different in the three labelling conditions (mean values: 0.26 and 1.5 h-1, respectively).