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1 Department of Endocrinology and Metabolism M, Aarhus University Hospital, 8000 Aarhus, Denmark; 2 Guilford, Connecticut 06437; 3 Novo Nordisk, 2880 Bagsvaerd, Denmark; and 4 Department of Medicine and National Science Foundation Center for Biological Timing, University of Virginia, Charlottesville, Virginia 22908
Insulin is largely secreted as serial
secretory bursts superimposed on basal release, insulin secretion is
regulated through changes of pulse mass and frequency, and the insulin
release pattern affects insulin action. Coordinate insulin release is
preserved in the isolated perfused pancreas, suggesting intrapancreatic coordination. However, occurrence of glucose concentration oscillations may influence the process in vivo, as it does for ultradian
oscillations. To determine if rapid pulsatile insulin
release may be induced by minimal glucose infusions and to define the
necessary glucose quantity, we studied six healthy individuals during
brief repetitive glucose infusions of 6 and 2 mg · kg
1 · min
1
for 1 min every10 min. The higher dose completely synchronized pulsatile insulin release at modest plasma glucose changes (~0.3 mM = ~5%), with large (~100%) amplitude insulin pulses at every single
glucose induction (n = 54) at a lag time of 2 min (P < 0.05), compared with small (10%) and rare (n = 3)
uninduced insulin excursions. The smaller glucose dose induced insulin
pulses at lower significance levels and with considerable breakthrough
insulin release. Periodicity shift from either 7- to 12-min or from 12- to 7-min intervals between consecutive glucose (6 mg · kg
1 · min
1)
infusions in six volunteers revealed rapid frequency changes. The
orderliness of insulin release as estimated by approximate entropy
(1.459 ± 0.009 vs. 1.549 ± 0.027, P = 0.016) was
significantly improved by glucose pulse induction (n = 6; 6 mg · kg
1 · min
1)
compared with unstimulated insulin profiles (n = 7). We
conclude that rapid in vivo oscillations in glucose may be an important regulator of pulsatile insulin secretion in humans and that the use of
an intermittent pulsed glucose induction to evoke defined and recurrent
insulin secretory signals may be a useful tool to unveil more subtle
defects in
-cell glucose sensitivity.
C-peptide; oscillations; fasting;
-cell
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