Neonatal monosodium glutamate treatment increases epididymal adipose tissue sensitivity to insulin in three-month old rats

We determined the response of glucose transport to insulin in isolated adipocytes and the lipogenic activity of insulin in fragments of epididymal adipose tissue obtained from male MSG-obese rats. Basal glucose transport rates (pmol 3 min-1 10(5) cells-1) were 100% higher in MSG than in control cells (3-month old male Wistar rats) pre-incubated for 30 min (P < 0.01). Nevertheless, when expressed as fmol 3 min-1 microns 2 cell surface area-1, transport rates were similar for the two groups (31.2 +/- 2.6 for MSG and 26.5 +/- 3.2 for controls, N = 7). No differences were observed in maximally insulin-stimulated glucose transport rates between groups (72.6 +/- 10.6 for MSG and 101.0 +/- 12.0 for controls, N = 7). In contrast, for adipocytes pre-incubated for 2 h, the basal uptake rates were 3.7 times higher and the maximal response to insulin was 103% higher in cells from MSG rats compared to control cells. These alterations in MSG rat adipocytes were accompanied by changes in cell sensitivity to insulin (EC50, 0.13 +/- 0.02 ng/ml for MSG vs 0.46 +/- 0.10 ng/ml for controls, P < 0.01). The rates of incorporation of labelled substrates (3H2O and 14C-glucose) into total lipids showed that in vitro lipogenesis was also 79% (3H2O) and 250% (14C-glucose) higher in MSG adipose tissue fragments. The MSG animals were consistently hyperinsulinemic. These data suggest that the obesity of 3-month old MSG rats is a metabolic alteration characterized by an enhanced adipocyte capacity to transport glucose and to synthetize lipids resulting in increased insulin sensitivity.