Adipocytes, available energy and endocrine pancreas

Effects of beta-hydroxybutyrate, glycerol, and free fatty acid infusions on glucagon and epinephrine secretion in dogs during acute hypoglycemia

The importance of glucagon in the regulation of carbohydrate metabolism is clearly established. However, the role played by this hormone in the regulation of the overall fuel economy is less certain, particularly with respect to such nonglucose fuels as free fatty acids, glycerol, and ketoacids. In order to elucidate glucagon's role with respect to the latter substrates, dogs were infused with solutions of these three fuels, and their A-cell responses to concomitant insulin-induced hypoglycemia were studied. In addition, epinephrine levels were also monitored. It was found that while these infusions failed to suppress glucagon release, the ketoacid infusion did significantly reduce epinephrine secretion during the insulin-induced hypoglycemic period. It was therefore concluded that glucagon secretion under these experimental conditions is not responsive to prevailing non-glucose fuel levels. Indeed, these data suggest that the sympathetic nervous system may play an important role in the regulation of the over-all fuel economy.

The effect of glucose, insulin and adrenaline on glycerol metabolism in vitro in rat adipose tissue

The uptake and utilization of [1-14C]glycerol was determined in pieces of rat epididymal fat-pads incubated in Krebs--Ringer bicarbonate buffer containing albumin. Insulin (200 muunits/ml), adrenaline (epinephrine; 0.5 mug/ml) and glucose (0, 5, 15 and 20 mM) were added to the medium. Changes in the specific radioactivity of the tracer during the incubation were taken into account in calculating the rate of glycerol utilization. Adrenaline decreased glycerol uptake, whereas insulin plus adrenaline increased it. The rate of incorporation of glycerol into glycerides was decreased by adrenaline and insulin, singly or together. Insulin increased the rate of formation of CO2 and fatty acids from glycerol. The formation of CO2 and fatty acids was further enhanced by insulin plus adrenaline. The decrease in glycerol uptake induced by adrenaline, the decrease in incorporation of glycerol into glycerides induced by insulin and insulin plus adrenaline and the synthesis of fatty acids were dependent on the presence of glucose in the medium. Thus insulin and adrenaline act on glycerol utilization in adipose tissue and some of their effects are mediated by action on glucose metabolism, but others are independent of this.