Yc. Jolly et al., TRANSIENT ACTIVATION OF CALCIUM-DEPENDENT PHOSPHOLIPASE-A(2) BY INSULIN SECRETAGOGUES IN ISOLATED PANCREATIC-ISLETS, Biochemistry, 32(45), 1993, pp. 12209-12217
Arachidonic acid is believed to be an important and necessary mediator
of insulin secretion by beta-cells of islets of Langerhans, and it ma
y regulate intracellular Ca2+ homeostasis. Insulin secretagogues, such
as glucose and the muscarinic agonist carbachol, stimulate arachidoni
c acid accumulation, although the mechanisms involved are controversia
l: carbachol is believed to stimulate phospholipase A2, while glucose-
induced arachidonic acid release is the result of diacylglycerol hydro
lysis [Konrad, R. J., et al. (1992) Biochem. J. 287, 283-290]. In insu
lin-secreting clonal beta-cells RINm5F, HIT-T15, and beta-TC3, Ca2+-in
dependent phospholipase A2 was mainly cytosolic, while in islets it wa
s equally distributed between a crude membrane fraction and the cytoso
l. A membrane-associated Ca2+-dependent phospholipase A2 was found to
be stimulated by millimolar Ca2+ concentrations, while a cytosolic Ca2
+-dependent activity was activated by micromolar Ca2+ levels. In order
to determine whether phospholipase A2 was activated in insulin secret
ion, we assessed whether pretreatment of intact islets with secretagog
ues affected phospholipase A2 activity, which was subsequently measure
d in membrane or cytosolic fractions. The combination of glucose and c
arbachol transiently activated Ca2+-dependent (but not Ca2+-independen
t) phospholipase A2 activity at 10 min, which corresponded to the peak
of arachidonic acid release. No effect was seen with either agonist a
lone. Our results indicate that activation of Ca2+-dependent phospholi
pase A2 may be due to agonist-induced increases in intracellular Ca2+.
We suggest that activation of islet Ca2+-dependent phospholipase A2 m
ay be important in a distal process of insulin secretion, such as secr
etory granule exocytosis.