QUANTITATIVE ASPECTS OF LIPOLYSIS AND REESTERIFICATION IN HUMAN ADIPOSE TISSUE IN VITRO

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Abstract
The capacity of human adipose tissue specimens, obtained during surgery, to release glycerol and to reesterify free fatty acids was measured in vitro under different experimental conditions.Glycerol release was about 0.5 μmoles/g wet weight/h under basal conditions. As compared with the epididymal fat pad of 250–350 g rats, human subcutaneous adipose tissue showed a maximal lipolytic response to hormones which was only 1/15 in young patients and 1/50 in older patients. Isolated human fat cells were not different from intact tissue. Catecholamines gave higher lipolytic response than the peptide hormones ACTH, glucagon and growth hormone with dexamethasone.Reesterification of free fatty acids as measured by the balance method was low in these fasting patients undergoing surgery. It increased after addition of insulin or large amounts of glucose in vitro, or after glucose infusion into the patient before surgery. Free fat cells showed no difference compared with intact tissue. Omental fat reesterified more fatty acids than subcutaneous adipose tissue.By parallel measurements of reesterification with the balance method and with glyceride synthesis from labelled glucose it was possible to show that, in some cases, the latter method gave higher reesterification than the balance method. This indicates a significant contribution of fatty acids from partial hydrolysis in the lipolytic process.These results show that triglycerides in human subcutaneous adipose tissue have a much slower turnover than in the rat epididymal fat pad. This is more pronounced at a more advanced age, which may contribute to the greater tendency to obesity with increasing age. The results furthermore suggest that, in these specimens taken from operated patients after 18 hours fasting, in spite of the low lipolytic potential, reesterification of free fatty acids is so low that it cannot match lipolysis and fatty acid outflow increases. An alternative explanation is partial hydrolysis of triglycerides.