Overstimulation and beta-cell function.

Previous and present evidence ascribes an important role to overstimulation of beta-cells for the secretory abnormalities associated with type 2 diabetes. The abnormality most clearly linked to overstimulation is the elevated ratio of circulating proinsulin to insulin. Evidence obtained in human pancreatic islets suggests that aberrations in insulin oscillations that occur in type 2 diabetes could at least in part be linked to abnormalities in cytoplasmic Ca2+ oscillations induced by overstimulation. Furthermore, in a transplantation model, we have obtained evidence for long-lasting, perhaps irreversible, effects of overstimulation, implying that this is a causative factor for the well-recognized deterioration of insulin secretion with increasing duration of type 2 diabetes. The mechanisms behind the effects of overstimulation are only partly clarified, but it is clear that reduced insulin secretion after overstimulation is only partly explained by decreased insulin stores. In cultured human pancreatic islets, overstimulation by high glucose leads to a rise in cytoplasmic Ca2+ levels, which persists after normalization of the glucose levels. Persistent elevation of cytoplasmic Ca2+ may trigger apoptosis, thus participating in long-term irreversible deterioration of beta-cell function. These data provide sufficient rationale for clinical studies to test the beneficial effects of relative beta-cell rest in type 2 diabetic patients.