Dynamics and Regulation of Insulin Secretion in Pancreatic Islets from Normal Young Children

Abstract

Insulin secretion has only exceptionally been investigated in pancreatic islets from healthy young children. It remains unclear whether those islets behave like adult islets despite substantial differences in cellular composition and higher β-cell replication rates. Islets were isolated from 5 infants/toddlers (11-36 month-old) and perifused to characterize their dynamics of insulin secretion when subjected to various stimuli and inhibitors. Their insulin responses were compared to those previously reported for similarly treated adult islets. Qualitatively, infant islets responded like adult islets to stimulation by glucose, tolbutamide, forskolin (to increase cAMP), arginine and the combination of leucine and glutamine, and to inhibition by diazoxide and CaCl2 omission. This similarity included the concentration-dependency and biphasic pattern of glucose-induced insulin secretion, the dynamics of the responses to non-glucose stimuli and metabolic amplification of these responses. The insulin content was not different, but fractional insulin secretion rates were lower in infant than adult islets irrespective of the stimulus. However, the stimulation index was similar because basal secretion rates were also lower in infant islets. In conclusion, human β-cells are functionally mature by the age of one year, before expansion of their mass is complete. Their responsiveness (stimulation index) to all stimuli is not smaller than that of adult β-cells. Yet, under basal and stimulated conditions, they secrete smaller proportions of their insulin stores in keeping with smaller in vivo insulin needs during infancy.

Fig 1. Concentration-dependency of glucose-induced insulin secretion in perifused islets from human infants.

(A and B) The concentration of glucose (G in mmol/l) was increased and decreased as indicated, but the islets were not exposed to the whole range of concentrations. One group of islets was perifused in G0 for 60 min before the glucose concentration was increased stepwise to G10 and eventually decreased to G1 at 150 min. Another group from the same preparation was perifused in G7 for 60 min before the glucose concentration was increased stepwise to G30 and eventually decreased to G7. Insulin secretion rates in G7 and G10 from the two series, run in parallel, were similar and therefore averaged to obtain the full dose curve for each of the five islet preparations. Parallel experiments were done in the absence (A) or presence (B) of 1 μmol/l forskolin (Fk) in islets from the five infants. (C) Concentration-dependency curves expressed as percentages of insulin secretion rates in G30. Values are means ± SE for the five infant cases. (D) Total insulin secretion (without and with forskolin) was calculated between 0 and 240 min and is shown for each of the five infant cases identified by their age in months. Columns show means ± SE for the five infant cases and for previously reported results with 8 preparations of adult islets [33].

Fig 2. Dynamics of glucose-induced insulin secretion in perifused islets from human infants.

The experiments also tested the effects of drugs opening (diazoxide) or closing (tolbutamide) K_ATP channels and of an increase in islet cAMP by forskolin. (A) The concentration of glucose was changed between 1 and 15 mmol/l (G1, G15), and diazoxide (Dz, 100 μmol/l), tolbutamide (Tolb, 100 μmol/l), and forskolin (Fk, 1 μmol/l) were added and withdrawn as indicated. (B) The whole experiment was performed in the presence of 1 μmol/l forskolin. Values are means ± SE for the five infant cases. (C and D) Glucose-induced insulin secretion was expressed as a stimulation index (ratio G15/G1) during first phase (2–10 min) and second phase (20–30 min) of the response. Mean and individual values for islets from the five infants (identified by age in months) are compared with mean values for 14–16 islet preparations from normal adults [33].

Fig 3. Effects of amino acids and tolbutamide on insulin secretion in perifused islets from human infants.

(A) The experiments were done in the presence of 3 mmol/l glucose (G3) and 1 μmol/l forskolin (Fk) throughout. Leucine and glutamine (5 mmol/l each) were added at 0 min. Between 30 and 50 min, CaCl₂ was omitted and 100 μmol/l EGTA was added. (B) Insulin secretion induced by leucine + glutamine was expressed as a stimulation index for the whole response (0–30 min). Mean and individual values for islets from four infants (identified by age in months) are compared with mean values for 5 islet preparations from normal adults [33]. (C) Three pulses of 10 mmol/l arginine (Arg) were applied in G3 alone, G3 + 25 μmol/l tolbutamide (Tolb 25), or G15 + Tolb 25. (D) Islets were fully depolarized by 500 μmol/l tolbutamide (Tolb 500) in 1 mmol/l glucose (G1). The glucose concentration was then increased to 15 mmol/l (G15) between 30 and 70 min, and forskolin (1 μmol/l) was eventually added to G1 (in 3/5 cases only). Values are means ± SE for islet preparations from 4 (A, B and C) or 5 infants (D).