METABOLIC-ACTIVATION OF CA2-INDEPENDENT PHOSPHOINOSITIDE HYDROLYSIS IN BETA-CELLS AND ITS ROLE IN THE CONTROL OF INSULIN-SECRETION()

Recent studies have led to the proposal that the oxidative metabolism of glucose leads to the generation of messengers, in addition to ATP, that are important in the ability of changes in extracellular glucose concentration to stimulate insulin secretion from pancreatic beta-cell s. In particular, there is now evidence that glucose induces both a Ca 2+-dependent and Ca2+-independent increase in phosphoinositide (PI) hy drolysis. To explore the relationship between oxidative metabolism and PI hydrolysis, we examined the effect of low concentrations (2.5 mM) of alpha-ketoisocaproate (KIC) and monomethylsuccinate (MMSucc) either alone or in combination on insulin secretion and PI hydrolysis in iso lated rat pancreatic islets incubated with either no glucose, 5 mM glu cose, or 20 mM glucose. A combination of KIC and MMSucc leads to a mar ked increase in largely (80%) Ca2+-independent PI hydrolysis in either the absence or presence of 5 mM glucose. When glucose is absent, this combination of substrates induces a very small and transient first ph ase of insulin secretion but no significant second phase of secretion. In the presence of 5 mM glucose, either KIC or MMSucc alone induces a first phase of insulin secretion with a peak secretory fate 10-fold g reater than the basal rate but only a small second phase of secretion approximately 5-fold above control. However, in the presence of 5 mM g lucose, the combination of KIC plus MMSucc induces a large biphasic in crease in insulin secretion: peak first-phase secretion is increased 3 0-fold, and second-phase 40-fold. These response are comparable to tho se induced by 20 mM glucose and are completely inhibited by 0.5 mu M n itrendipine. In contrast, KIC plus MMSucc do not enhance the insulin s ecretory response induced by 20 mM glucose. Previous data showed that when 20 mM glucose acts, the resulting increase in PI hydrolysis is on ly partially Ca2+ dependent. A reanalysis of these data shows that rai sing the glucose concentration from 5 to 7 mM causes a 2-fold increase in Ca2+-independent PI hydrolysis, and a further increase to 20 mM le ads to a further 2-fold increase in Ca2+-dependent PI hydrolysis. Thes e data show that these two pathways are regulated by different ranges of glucose concentration. They raise the interesting possibility that these distinct pathways have different signaling functions. In particu lar, raising the glucose concentration from 5 to 7 mM is known to alte r the responsiveness of beta-cells to a variety of neurohumoral agonis ts and to tolbutamide. At 5 mM glucose, tolbutamide induces a very sma ll second phase of insulin secretion, but at 7 mM glucose it induces a n increase comparable to that seen in response to 20 mM glucose. Tolbu tamide has a very small effect on CA(2+)-dependent PI hydrolysis in is lets incubated with 5 mM glucose but a large effect in islets incubate d with 7 mM glucose. Thus, the Ca2+-independent increase in PI hydroly sis induced by a rise in glucose concentration from 5 to 7 mM glucose is at least a marker for a switch in beta-cell responsiveness and may be the switch that brings about this change.