On the relationship between metabolic activity and cholesterol uptake by intima-media of the rabbit aorta

Abstract

The relationship between metabolic activity and cholesterol uptake by intima-media of the rabbit aorta has been studied by use of a method in vitro previously described. A highly significant, positive correlation was found between intima-media cholesterol uptake and metabolic activity as expressed by lactate and CO 2 production for control hemisegments showing the same percental variation in these two parameters. This relationsbip might also apply if cholesterol uptake was related only to the glycolytic pathway. CN − at a concentration of 0.5 mM provoked an increase in intima-media cholesterol uptake. It is therefore concluded that cholesterol uptake does not depend on energy from the oxidative pathway. Judged from the lactate production cyanide brought about an increase in glucose catabolism via the glycolytic pathway, even when the decrease in CO 2 production is taken into account. Intima-media cholesterol uptake could be lowered in the presence of glycolytic inhibitors when lactate production by the inhibited aortic hemisegments was not too low. At a low lactate production by the inhibited hemisegment an increase in intima-media cholesterol uptake was seen, but a flabby appearance of the aortic tissue after incubation in such cases and a decrease in temperature dependence for cholesterol uptake supported the suggestion of a breakdown of the endothelial membrane barrier at low lactate production. The extrapolated value for intima-media cholesterol uptake in controls at a lactate production of zero was equal to a value for a basic cholesterol uptake in experiments with F − (5 mM), where this inhibitor no longer was able to bring about an inhibition in cholesterol uptake. This finding is in accordance with a previous finding of a non-temperature-dependent part of cholesterol uptake during a 4-h incubation, viz. the initial binding phase. From previous observations on temperature dependence for the cholesterol transport across the intimal cell surface and from the results reported in this study it is concluded that the unidirectional transport of cholesterol from serum to intima-media under ‘normal’ conditions in vitro, where cholesterol molecules seem to be most impeded in their passage through the luminal endothelial cell membrane, is an energy requiring process. Active glycolysis probably provides the immediate energy supply required. The results lend support to a previously stated suggestion of a pinocytotic mechanism involved in the cholesterol transport across the endothelial cell membrane. It is emphasized, however, that only indirect proof exists for such a concept, for which a demonstration of labelled cholesterol in the vesicles of the endothelial cells is rather crucial.