Triacylglycerol turnover in large and small rat adipocytes: effects of lipolytic stimulation, glucose, and insulin.

Rates of lipolysis and reesterification were determined under various conditions in adipocytes from epididymal fat pads of old, spontaneously obese rats and compared to cells from younger, leaner animals. No differences were observed in lipolytic responsiveness to several concentrations (2--50 microM) of the beta-adrenergic agent ritodrine compared to cells from younger, leaner rats. The large cells showed diminished rates of lipolysis, reesterification, and glyceride-glycerol synthesis from glucose of a submaximal but not maximal insulin concentrations, probably reflecting decreased large cell receptor numbers. In both cell types reesterification measured in the presence of ritodrine progressively rose with increasing concentrations of glucose in the medium. At each glucose concentration (0.5--25 mM) rates of reesterification were similar in each cell type. When maximal concentrations of insulin (5 nM) were also added, at low glucose concentrations (less than 5 mM) there was a similar increase in reesterification in large and small cells. No insulin effect in either cell type was observed at high concentrations of glucose. Although fatty acid synthesis from glucose in the large cells was markedly diminished, glyceride-glycerol synthesis was well maintained, correlating well with calculated reesterification rates. In fact, reesterification was found to be quantitatively very important in determining total triacylglycerol turnover in both cell types. High rates of reesterification might not only allow maintenance of triacylglycerol stores, but could also increase metabolic sensitivity to changes in hormonal or substrate concentrations.