Lipoprotein Lipase Activity and Uptake of Exogenous Triglycerides in Fat Cells of Different Size

 

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Abstract

In order to elucidate the rate of uptake of exogenous fatty acids in small and large adipose tissue fat cells different experiments were performed in vitro and in vivo. Lipoprotein lipase eluted with heparin in vitro or assayed directly on extracts of the tissue was determined in rat or human adipose tissue with varying fat cell size. Furthermore, lipoprotein lipase activity was determined in fat cells from the same tissue separated into size classes by a flotation technique. All experimental approaches indicated higher activity in large fat cells in comparison with small fat cells.

Experiments were also performed in vivo. Rats were given a food rich in linoleic acid. The increase in concentration of these fatty acids with time in epididymal fat pad triglycerides was taken as a measure of fatty acid uptake, and this uptake was increased in the large as compared with the small fat cells from the same tissue.

Labelled palmitic acid in light cream was taken perorally together with supper by patients operated on for gall bladder disease next morning. Radioactivity was higher in large than in small fat cells from the same abdominal subcutaneous adipose tissue sample.

In both these experiments in vivo the mechanism of uptake of labelled fatty acids is not known in detail but is probably occurring mainly as uptake of triglyceride fatty acids after hydrolysis by adipose tissue lipoprotein lipase or as free fatty acids hydrolysed elsewhere. It was considered likely that both these processes were increased in large fat cells in comparison with small fat cells.

It was concluded that the rate of uptake of exogenous fatty acids is increased in large fat cells in comparison with small as has previously been demonstrated for triglyceride synthesis, fatty acid reesterification and lipolysis. Enlarged fat cells thus constitute a metabolic subcompartment of adipose tissue with increased triglyceride turnover. The increased triglyceride turnover of enlarged fat cells is an energy wasting process which might contribute to regulation of fat cell size.

1 Present address: Clinica Medica Generale Dell'Universita di Padova, Padova, Italy