A. Cane et al., PHOSPHOLIPASE A2-DEPENDENT AND A2-INDEPENDENT PATHWAYS OF ARACHIDONATE RELEASE FROM VASCULAR SMOOTH-MUSCLE CELLS, Biochemical pharmacology, 53(3), 1997, pp. 327-337
[Arg(8)]vasopressin (AVP), through its V-1 receptor coupled to GTP-bin
ding proteins, and aluminum fluoride (AlF4-), which directly activates
GTP-binding proteins, induced the release of [H-3]arachidonate from p
relabeled A(7)r(5) vascular smooth muscle-like cells. Using fura-2-loa
ded cells, we observed that the release induced by AVP occurred concur
rently with calcium (Ca2+) mobilization from internal stores and entry
of external Ca2+, whereas AlF4--dependent arachidonate release was mu
ch slower and was not accompanied by intracellular Ca2+ mobilization.
Arachidonate transfer from phosphatidylcholine to phosphatidyl ethanol
amine was an early event for both agonists, but phosphatidylinositol h
ydrolysis was an early event for AVP-stimulated cells and a late event
for cells triggered with AlF4-. In addition, phospholipase inhibitors
had no effect on arachidonate release induced by AlF4-. We investigat
ed the enzymatic pathways involved in the releases of arachidonate, wh
ich occur in such different ways. Phospholipase A(2) activities were a
ssayed in a cell-free system with various substrates, which made it po
ssible to differentiate between cytosolic, secretory and Ca2+-independ
ent phospholipases A(2). THe specific activities were in the order alk
enyl-AA-GPE > acyl-AA-GPE > acyl-PIA-GPC in the presence of Ca2+. No s
ignificant activity was observed in the presence of Ca2+-chelators and
when dipalmitoyl-glycerophosphocholine was used as a substrate. Phosp
holipase A(2) activities did not change in homogenates from stimulated
cells related to control cells. However, phospholipase A(2) activity
increased in membrane fractions from AVP-stimulated cells. Imunodetect
ed phosphorylated and unphosphorylated forms of cytosolic phospholipas
e A(2) (cPLA(2)) also clearly increased in the membrane fractions of A
VP-stimulated cells, and only the unphosphorylated form of cPLA(2) was
present in AlF4--triggered cells. We conclude that phospholipase C an
d translocation of cPLA(2) can account for arachidonate release with A
VP stimulation, whereas neither phospholipase C nor any phospholipase
A(2) activity appears to be implicated in AlF4--dependent arachidonate
release. Copyright (C) 1997 Elsevier Science Inc.