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Uptake and Utilization of Double‐Labeled High‐Density Lipoprotein Sphingomyelin in Isolated Brain Capillaries of Adult Rats

Abstract: Isolated rat brain capillaries were incubated in the presence of high‐density lipoprotein (HDL) containing [stearic acid‐14C, (methyI‐3H)choline]sphmgomyelim. This double‐labeled sphingomyelin was taken up in a concentration‐dependent manner. Cerebral capillary‐associated sphingomyelin had a 3H/14C ratio close to that of the incubation medium, a result indicating uptake of sphingomyelin without prior hydrolysis. TLC of lipid extracted from capillaries showed that part of the sphingomyelin (up to 40%) was hy‐drolyzed in the brain capillaries to ceramide and free fatty acids. The hydrolysis was proportional to the amount of in‐corporated sphingomyelin and reached a plateau when the HDL sphingomyelin concentration in the medium was 237 nmol/ml. The results of “pulse‐chase” experiments showed that the choline moiety of sphingomyelin was recovered in the incubation medium after the chase period and that there was no redistribution of liberated choline in phosphatidylcholine of capillaries.