EFFECTS OF BLOOD-GLUCOSE CONCENTRATION ON THERMOGENESIS AND GLUCOSE DISPOSAL DURING HYPERINSULINEMIA

1. The thermogenic and cardiovascular responses to glucose ingestion or infusion are altered by undernutrition. These changes may be due, in part, to alterations in the blood glucose concentration. This study investigates the effect of variation in the blood glucose concentration on the thermogenic and cardiovascular responses to a hyperinsulinaemic glucose clamp. 2. Ten healthy post-absorptive subjects (six male, aged 21-30 years) were studied on two occasions. Baseline measurements were made for 30 min, followed by a 90 min continuous insulin infusion (100 m-units min-1 m-2), during which the arterialized venous blood glucose concentration was maintained at 4.5 mmol/l (euglycaemia) or 3.5 mmol/l (mild hypoglycaemia). Measurements were made of glucose infusion rate and respiratory gas exchange from which glucose storage and oxidation and metabolic rate were calculated. 3. During the final 30 min of each hyperinsulinaemic clamp, the rates of glucose disposal (53.5 ± 1.4 versus 38.2 ± 1.0 μmol min-1 kg-1, P < 0.01) and glucose storage (32.4 ± 1.1 versus 22.8 ± 1.2 μmol min-1 kg-1, P < 0.05), and the increase in metabolic rate (0.44 ± 0.08 versus 0.11 ± 0.09 kJ min, P < 0.01), were all greater with a blood glucose concentration of 4.5 mmol/l than of 3.5 m,mol/l. 4. Plasma noradrenaline rose during the clamp to significantly higher levels at a blood glucose concentration of 4.5 mmol/l (1.18 ± 0.06 nmol/l) than at a blood glucose concentration of 3.5 mmol/l (1.02 ± 0.06 nmol/l, P < 0.05), although the absolute changes were not significantly different (+ 0.17 ± 0.06 and + 0.14 ± 0.05 nmol/l at blood glucose concentrations of 4.5 and 3.5 mmol/l, respectively). Plasma adrenaline did not change at a blood glucose concentration of 4.5 mmol/l but increased slightly (to 0.36 + 0.04 nmol/l, P < 0.05) at a blood glucose concentration of 3.5 mmol/l. During hyperinsulinaemia, heart rate and forearm blood flow rose and diastolic blood pressure fell similarly (P < 0.01) at both blood glucose concentrations. 5. Mild hypoglycaemia reduces the thermogenic response to glucose infusion to a greater extent than the reduction in storage, but does not alter the noradrenaline or cardiovascular responses to hyperinsulinaemia. Thus, the cardiovascular and noradrenaline responses during the infusions may be linked and dependent upon the hyperinsulinaemia, whereas some other aspect of glucose metabolism influences the thermogenic response.