The relation between carbohydrate extraction by the forearm and arterial free fatty acid concentration in man: I. Forearm work with nicotinic acid infusion

To see if the magnitude of carbohydrate extraction by working skeletal muscle in man is inversely correlated with the arterial free fatty acid (FFA) concentration as in the heart, eighteen healthy men were studied during dynamic forearm work with and without nicotinic acid. The extraction or release of glucose, lactate and pyruvate was determined by the simultaneous sampling of blood from the brachial artery (a) and a deep vein (dv) of the active forearm. Nicotinic acid decreased the arterial FFA concentration from 498 +/- 53 to 134 +/- 12 mumol per litre plasma and this caused a decrease in calculated extraction of FFA. However, it did not affect the extraction of glucose, which was of a magnitude similar to one third of the oxidative metabolism in both situations. One of the possible reasons of this difference compared to the human heart muscle is that the exercising skeletal muscle may utilize stored substrate to a greater extent, which makes possible shifts in substrate utilized for oxidation without changes in substrate extraction. Another reason may be that FFA utilization covers a far smaller proportion of oxidative metabolism in skeletal than in heart muscle already before nicotinic acid.