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1 Diabetes Research Laboratory, School of Kinesiology, Simon Fraser University, Burnaby, British Columbia V5A 1S6; and 2 Division of Endocrinology, Department of Medicine, University of Alberta, Edmonton, Alberta, Canada T6G 2S2
To determine the
importance of insulin for glucose disposal during an intravenous
glucose tolerance test in rats, experiments were performed in four
cohorts of conscious unrestrained rats fasted overnight. In
cohorts
1-3,
a bolus of tracer
([3-3H]glucose, 50 µCi) was given alone, with glucose (0.3 g/kg) to induce an endogenous
insulin response (~1,100 pmol/l), or with exogenous insulin to give
physiological (1,700 pmol/l) or supraphysiological (12,000 pmol/l)
plasma levels. Raising plasma insulin within the physiological range
had no effect (P > 0.05), but supraphysiological levels
induced hypoglycemia (7.3 ± 0.2 to 3.6 ± 0.2 mmol/l) and increased [3H]glucose
disappearance rate (P < 0.001). In
cohort
4, a primed, continuous tracer
infusion was started 120 min before saline or glucose bolus injection.
[3H]glucose levels
fell 15-20%, and the disappearance rate rose 36%
(P < 0.05) after glucose injection.
These results indicate that in fasted rats a tracer bolus injection
protocol is not sufficiently sensitive to measure the physiological
effect of insulin released in response to a bolus of glucose because
this effect of insulin is small. Glucose itself is the predominant
mediator of glucose disposal after a bolus of glucose in the fasted rat.
glucose tolerance test; insulin action; glucose disposal
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