2 SITES OF GLUCOSE CENTRAL OF INSULIN RELEASE WITH DISTINCT DEPENDENCE ON THE ENERGY-STATE IN PANCREATIC B-CELLS

The energy state of pancreatic B-cells may influence insulin release at several steps of stimulus-secretion coupling. By closing ATP-sensitive K+ channels (K+-ATP channels), a rise in the ATP/ADP ratio may regulate the membrane potential, and hence Ca2+ influx. It may also modulate the effectiveness of Ca2+ on its intracellular targets. To assess the existence of these two roles and determine their relative importance for insulin release, we tested the effects of azide, a mitochondrial poison, on mouse B-cell function under various conditions. During stimulation by glucose alone, when K+-ATP channels are controlled by cellular metabolism, azide caused parallel, concentration-dependent (0.5-5 mM), membrane repolarization, decrease in cytosolic Ca2+ concentration [Ca2+](i) and inhibition of insulin release. When K+-ATP channels were closed pharmacologically (by tolbutamide in high glucose), azide did not repolarize the membrane or decrease [Ca2+](i), and was much less effective in inhibiting insulin release. A similar resistance to azide was observed when K+-ATP channels were opened by diazoxide, and high K+ was used to depolarize the membrane and raise [Ca2+](i). In contrast, azide similarly decreased ATP levels and increased ADP levels, thereby lowering the ATP/ADP ratio under all conditions. In conclusion, lowering the ATP/ADP ratio in B-cells can inhibit insulin release even when [Ca2+](i) remains high. However, this distal step is much more resistant to a decrease in the energy state of B-cells than is the control of membrane potential by K+-ATP channels. Generation of the signal triggering insulin release, high [Ca2+](i), through metabolic control of membrane potential requires a higher global ATP/ADP ratio than does activation of the secretory process itself.