Studies of Substrate Regulation in Fasting: I. Evidence for Central Regulation of Lipolysis by Plasma Glucose Mediated by the Sympathetic Nervous System

Infusion of glucose into fasting normal subjects at rates of 20 to 100 mg. per minute caused a transient fall in arterial plasma free fatty acids. The arterial glucose was raised only 3 to 10 mg. per 100 ml. by these infusions and plasma immunoreactive insulin did not change. During the recovery phase after an initial response to glucose FFA were refractory to a second challenge with glucose. Fructose was less potent than glucose in inducing a fall in FFA. Similar responses to glucose could be obtained in insulin-dependent diabetics after priming with long-acting insulin providing further evidence that the response of lipolysis to glucose does not require an acute change in insulin. Infusion of glucose intra-arterially failed to affect the local release of FFA. Beta-adrenergic blockade and ganglionic blockade abolished the response of FFA to these doses of glucose. These data suggest but do not establish the hypothesis that sympathetic tone to adipose tissue is under continuous regulation by the central nervous system and that the concentration of arterial blood glucose is the signal for this substrate controlling system.