Abstract
Under normoxic conditions, adipocytes in primary culture convert huge amounts of glucose to lactate and glycerol. This “wasting” of glucose may help to diminish hyperglycemia. Given the importance of insulin in the metabolism, we have studied how it affects adipocyte response to varying glucose levels, and whether the high basal conversion of glucose to 3-carbon fragments is affected by insulin. Rat fat cells were incubated for 24 h in the presence or absence of 175 nM insulin and 3.5, 7, or 14 mM glucose; half of the wells contained 14C-glucose. We analyzed glucose label fate, medium metabolites, and the expression of key genes controlling glucose and lipid metabolism. Insulin increased both glucose uptake and the flow of carbon through glycolysis and lipogenesis. Lactate excretion was related to medium glucose levels, which agrees with the purported role of disposing excess (circulating) glucose. When medium glucose was low, most basal glycerol came from lipolysis, but when glucose was high, release of glycerol via breakup of glycerol-3P was predominant. Although insulin promotes lipogenesis, it also limited the synthesis of glycerol-3P from glucose and its incorporation into acyl-glycerols. We assume that this is a mechanism of adipose tissue defense to avoid crippling fat accumulation which has not yet been described.
Highlights
White adipose tissue (WAT) is a disperse organ [1], distributed in a number of locations in which its basic energy storage activity [2] is complemented by many other physiological functions [3,4,5].In any case, its main role is to contribute to the defense of energy homoeostasis helping to control glucose [6], lipid [7], and amino acid [8] metabolism.Most studies on WAT metabolism have been focused on adipocytes, since they are considered the characteristic and defining cells of WAT
Lactate efflux was affected by initial glucose concentration, being its efflux uptake by adipocytes was slightly affected by initial glucose concentration showing a peak with greater at 7 and 14 mM glucose; Non-esterified fatty acids (NEFA) efflux decreased with glucose concentration
Lactate efflux was affected by initial glucose concentration, being its Insulin, increased lactate efflux, slightly decreased that of glycerol, and almost abolished NEFA
Summary
White adipose tissue (WAT) is a disperse organ [1], distributed in a number of locations in which its basic energy storage activity [2] is complemented by many other physiological functions [3,4,5]. Most studies on WAT metabolism have been focused on adipocytes, since they are considered the characteristic and defining cells of WAT. Adipose tissue structure is far more complex, containing a large number of cell types, mature adipocytes, in spite of them occupying most of the space because of their larger size [5]. Adipose tissue contains other cell types such as fibroblasts, histiocytes, mast cells, lymphocytes, granulocytes, blood cells, and nerve terminals
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