P. Detimary et al., 2 SITES OF GLUCOSE CENTRAL OF INSULIN RELEASE WITH DISTINCT DEPENDENCE ON THE ENERGY-STATE IN PANCREATIC B-CELLS, Biochemical journal, 297, 1994, pp. 455-461
The energy state of pancreatic B-cells may influence insulin release a
t several steps of stimulus-secretion coupling. By closing ATP-sensiti
ve K+ channels (K+-ATP channels), a rise in the ATP/ADP ratio may regu
late the membrane potential, and hence Ca2+ influx. It may also modula
te the effectiveness of Ca2+ on its intracellular targets. To assess t
he existence of these two roles and determine their relative importanc
e for insulin release, we tested the effects of azide, a mitochondrial
poison, on mouse B-cell function under various conditions. During sti
mulation by glucose alone, when K+-ATP channels are controlled by cell
ular metabolism, azide caused parallel, concentration-dependent (0.5-5
mM), membrane repolarization, decrease in cytosolic Ca2+ concentratio
n [Ca2+](i) and inhibition of insulin release. When K+-ATP channels we
re closed pharmacologically (by tolbutamide in high glucose), azide di
d not repolarize the membrane or decrease [Ca2+](i), and was much less
effective in inhibiting insulin release. A similar resistance to azid
e was observed when K+-ATP channels were opened by diazoxide, and high
K+ was used to depolarize the membrane and raise [Ca2+](i). In contra
st, azide similarly decreased ATP levels and increased ADP levels, the
reby lowering the ATP/ADP ratio under all conditions. In conclusion, l
owering the ATP/ADP ratio in B-cells can inhibit insulin release even
when [Ca2+](i) remains high. However, this distal step is much more re
sistant to a decrease in the energy state of B-cells than is the contr
ol 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.