We studied the effects of hyperglycemia on ␤-cell death and mass in syngeneically transplanted islets. Six groups of STZ-induced diabetic C57BL/6 mice were transplanted with 100 syngeneic islets, an insufficient ␤-cell mass to restore normoglycemia. Groups 1, 2, and 3 remained hyperglycemic throughout the study. Groups 4, 5, and 6 were treated with insulin from day 7 before transplantation to day 10 after transplantation. After insulin discontinuation, group 6 mice achieved definitive normoglycemia. Grafts were harvested at 3 (groups 1 and 4), 10 (groups 2 and 5), and 30 (groups 3 and 6) days after transplantation. On day 3, the initially transplanted ␤-cell mass (0.13 ؎ 0.01 mg) was dramatically and similarly reduced in the hyperglycemic and insulintreated groups (group 1: 0.048 ؎ 0.002 mg; group 4: 0.046 ؎ 0.007 mg; P < 0.001). Extensive islet necrosis (group 1: 30.7%; group 4: 26.8%) and increased ␤-cell apoptosis (group 1: 0.30 ؎ 0.05%; group 4: 0.42 ؎ 0.07%) were found. On day 10, apoptosis remained increased in both hyperglycemic and insulin-treated mice (group 2: 0.44 ؎ 0.09%; group 5: 0.48 ؎ 0.08%) compared with normal pancreas (0.04 ؎ 0.03%; P < 0.001). In contrast, on day 30, ␤-cell apoptosis was increased in grafts exposed to sustained hyperglycemia (group 3: 0.37 ؎ 0.03%) but not in normoglycemic mice (group 6: 0.12 ؎ 0.02%); ␤-cell mass was selectively reduced in islets exposed to hyperglycemia (group 3: 0.046 ؎ 0.02 mg; group 6: 0.102 ؎ 0.009 mg; P < 0.01). In summary, even in optimal conditions, ϳ60% of transplanted islet tissue was lost 3 days after syngeneic transplantation, and both apoptosis and necrosis contributed to ␤-cell death. Increased apoptosis and reduced ␤-cell mass were also found in islets exposed to chronic hyperglycemia, suggesting that sustained hyperglycemia increased apoptosis in transplanted ␤-cells.

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Increased ␤-cell sensitivity to glucose precedes the loss of glucose-induced insulin secretion in diabetic animals. Changes at the level of ␤-cell glucose sensor have been described in these situations, but it is not clear whether they fully account for the increased insulin secretion. Using a euglycemic-normolipidemic 60% pancreatectomized (60%-Px) mouse model, we have studied the ionic mechanisms responsible for increased ␤-cell glucose sensitivity. Two weeks after Px (Px14 group), Px mice maintained normoglycemia with a reduced ␤c e l l mass (0.88 ± 0.18 mg) compared with control mice (1.41 ± 0.21 mg). At this stage, the dose-response curve for glucose-induced insulin release showed a signific a n t displacement to the left (P < 0.001). Islets from the Px14 group showed oscillatory electrical activity and cytosolic Ca 2 + ([ C a 2 + ] i) oscillations in response to glucose concentrations of 5.6 mmol/l compared with islets from the control group at 11.1 mmol/l. All the above changes were fully reversible both in vitro (after 48-h culture of islets from the Px14 group) and in vivo (after regeneration of ␤-cell mass in islets studied 60 days after Px). No significant differences in the input resistance and ATP inhibition of ATP-sensitive K + (K AT P) channels were found between ␤-cells from the Px14 and control groups. The dose-response curve for glucoseinduced MTT (C,N-d i p h e n y l-N-4,5-dimethyl thiazol 2 yl tetrazolium bromide) reduction showed a significant displacement to the left in islets from the Px14 group (P < 0.001). These results indicate that increased glucose sensitivity in terms of insulin secretion and Ca 2 + s i gnaling was not due to intrinsic modifications of K AT P channel properties, and suggest that the changes are most likely to be found in the glucose metabolism.