Insulin is released from pancreatic β-cells in a Ca²⁺-dependent manner as a result of depolarisation of the cell membrane. Hyperinsulinism in infancy (HI) has been shown in many cases to be caused by defects in the genes that encode K_ATP channels – ABCC8 and KCNJ11, and this leads to constantly depolarised membrane potential in HI β-cells. Since such cells would have a greater than normal need to maintain intracellular Ca²⁺ homeostasis, we investigated the handling of [Ca²⁺]_c in islets from control adult cadavers (n = 20 islets from 6 donors), and patients undergoing partial or subtotal pancreatectomy for HI (with informed consent and local ethical committee approval).

Islets were maintained in RPMI medium containing 5 mM glucose on poly-D-lysine-coated coverslips and were loaded with fura-2 AM prior to microfluorimetry procedures. HI islets were subdivided according to whether they came from patients with defects in K_ATP channels (Type 1 and Type 2 HI-K_ATP, 15 patients, n = 87 islets), from patients where there were no defects in K_ATP channels (Type 3, 2 patients, n = 8 islets) or from patients with an atypical histology (2 patients, n = 8 islets). Data are presented as means ± S.E.M. and were analysed using Student’s unpaired t tests and Mann-Whitney tests with P < 0.05 being significant.

We found that basal [Ca²⁺]_c levels were 99 ± 9 nM in control islets and 92 ± 10 nM in Type 3 HI islets. However, they were significantly higher in both the HI-K_ATP and atypical islets being 143 ± 5 nM (P < 0.001) and 199 ± 15 nM (P < 0.001), respectively. Treatment of the islets with a Ca²⁺-free medium containing 1 mM EGTA caused a greater fall in [Ca²⁺]_c in HI-K_ATP, Type 3 and atypical HI islets than controls -14 ± 5, ns; -19 ± 3, P < 0.03; -27 ± 6, P < 0.01 vs. -8 ± 2 nM). Release of Ca²⁺ from internal stores was provoked using a mixture of ATP, UTP and acetylcholine (0.1 mM), in the continued absence of [Ca²⁺]_o in order to stimulate capacitative Ca²⁺ entry following reintroduction of external Ca²⁺. This was found to stimulate a significantly higher rise in [Ca²⁺]_c in HI-K_ATP and atypical when compared to Type 3 or controls islets (62 ± 6, P < 0.03; 94 ± 16, P < 0.003; 44 ± 10, ns; vs. 40 ± 6 nM, respectively). Subsequent to these manoeuvres, experiments were carried out in the presence of thapsigargin (100 nM) to block the reuptake of Ca²⁺ into intracellular stores, and evoked changes in [Ca²⁺]_c were only statistically significantly different in the atypical islets which showed the largest rise in [Ca²⁺]_c when compared to control values (Δ[Ca²⁺]_c = 225 ± 37 vs. 144 ± 17). When calculated, the rates of rise of [Ca²⁺]_c seen under capacitive influx conditions were higher than controls for the HI-K_ATP, Type 3 and atypical islets, but reached significance only in the latter; 0.99 ± 0.15 (n = 18) vs. 1.16 ± 0.13 (n = 55), 1.27 ± 0.37 (n = 8) and 1.95 ± 0.37 (n = 8), P < 0.03, respectively.

These results show that whilst [Ca²⁺]_c in HI-K_ATP islets is elevated as a consequence of K_ATP channel defects, the dynamics of [Ca²⁺]_c handling are broadly similar to controls. Islets from patients in which hyperinsulinism is unrelated to K_ATP channel defects differ little from controls, whilst islets from patients with an atypical histological origin show evidence of a perturbation of Ca²⁺ storage dynamics which may be related to the pathogenesis of this novel form of HI.