Phosphoinositide Hydrolysis and Insulin Release From Isolated Perifused Rat Islets: Studies With Glucose

The ability of glucose to promote the hydrolysis of pre I a be led [2-³H]inositol-containing phosphoinositides (PI) was assessed by measuring the efflux of ³H in response to glucose and the accumulation of labeled inositol phosphates. The inclusion of nonradioactive inositol (1 mM) in the perifusion medium dramatically improved our ability to monitor glucose-induced increases in ³H efflux. Efflux studies with this method revealed the following. 1) ³H efflux is significantly greater at 7 than at 2.75 mM glucose, and this parallels a small but significant increase in insulin secretion. 2) D-manno-Heptulose reduces ³H efflux with 7 mM glucose to a level approximating that seen in the presence of 2.75 mM glucose and has no effect on ³H efflux with 2.75 mM glucose. 3) In the presence of 20 mM glucose plus 1 mM inositol, ³H efflux is rapid and biphasic, a response that parallels the timing and amplitude of the biphasic pattern of insulin secretion. Direct measurements of labeled inositol and inositol phosphate levels in islets revealed the following. 4) After 50 min of perifusion with 2.75 or 7 mM glucose, labeled inositol phosphates were significantly greater with 7 mM glucose. 5) In response to 20 mM glucose alone, islet levels of free inositol, inositol monophosphate (IP,), and inositol bisphosphate (IP₂) increased. 6) In response to 20 mM glucose plus 1 mM cold inositol, islet levels of free inositol increased, whereas islet levels of IP₁, IP₂, and inositol trisphosphate (IP₃) were reduced compared with values obtained with 20 mM glucose alone. 7) In response to perifusion with 20 mM glucose, IP₃ accumulation was biphasic in nature, and this response precedes the biphasic pattern of both insulin output and ³H efflux by several minutes. These results suggest that PI hydrolysis in islets is tightly regulated by the ambient glucose level and that second-messenger signals generated by activation of this pathway may contribute to the biphasic pattern of glucose-induced insulin secretion.