Effects of glucose, insulin and epinephrine on lactate metabolism by isolated adipocytes

Epididymal fat cells were incubated in vitro with either 2 mM or 6 mM [U-14C]lactate, and where appropriate, 6 mM glucose, 10 mU/ml insulin or 2 micrograms/ml epinephrine. Increasing lactate concentration promoted lactate carbon incorporation into specific products, and the presence of glucose significantly enhanced this effect. Significant lipogenic or lipolytic hormonal effects upon lactate metabolism were observed only in the presence of glucose, as insulin primarily increased lactate conversion to fatty acids while epinephrine promoted lactate oxidation. These data emphasize the importance of lactate as an alternative substrate for intermediary metabolism in adipocytes, and indicate that hormonal modulation of lactate metabolism is modest, but distinct, and requires the presence of physiological glucose concentrations.