Studies of substrate regulation in fasting

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Abstract
The effect of glucose infusion on rates of lipolysis were studied in a group of chair-trained papio baboons that had been prepared for chronic intravenous and intracarotid infusion. All studies were carried out after a 24 hr period of fasting and when the animals were fully awake. After a control interval of 1 hr, a glucose infusion was begun either intravenously or intra-arterially. The infusion was continued at a constant rate for 2 hr and then changed directly to the alternate route and continued an additional 2 hr. Blood samples were collected at 30-min intervals for glucose, free fatty acid (FFA), glycerol, insulin, and in some studies, growth hormone (GH) determination. When glucose doses less than 0.5 mg/kg per min were used, no change in the products of lipolysis was noted during either venous or carotid administration, and glucose and insulin levels remained stable or fell gradually. With doses of glucose between 0.5 and 0.6 mg/kg per min, a greater fall in both FFA and glycerol was noted during carotid administration. No definite changes in plasma glucose or insulin levels were noted during either infusion period. These changes in lipolysis were noted regardless of the sequence of infusion, and a similar differential suppression of FFA was noted during a 24 hr period of carotid glucose administration. When doses of glucose larger than 0.6 mg/kg per min were used, inhibition of lipolysis was noted during both phases of infusion. No definite change in GH levels was noted during the periods of fasting, and the levels of the hormone did not appear to be related to changes in glucose, insulin, or FFA levels. These data provide additional evidence for the presence in the central nervous system of a glucose-sensitive center which alters lipolytic rates independently of insulin and GH, probably by altering sympathetic tone to adipose tissue.