Decavanadate displaces inositol 1,4,5-trisphosphate (IP3) from its receptor and inhibits IP3 induced Ca2+ release in permeabilized pancreatic acinar cells

Inositol 1,4,5-trisphosphate (IP3) induced Ca2+ release in
digitonin permeabilized rat pancreatic acinar cells is specifically
inhibited by decavanadate. The Ca2+ release induced with 0.18 μM
IP3 is half maximally inhibited with approximately 5 μM
decavanadate. Complete inhibition is achieved with around 20 μM
decavanadate. Removal of decavanadate from the permeabilized cells
fully restores sensitivity towards IP3, indicating the
reversibility of the inhibition. Oligovanadate, which inhibits ATP
dependent Ca2+ uptake into intracellular stores, does not influence
IP3 induced Ca2+ release. In order to reveal the mechanism
underlying the effects of the different vanadate species, binding
of IP3 to the same cellular preparations was investigated. We found
that binding of IP3 to a high affinity receptor site (Kd approx.
1.2 nM) could be abolished by decavanadate but not by
oligovanadate. With 0.5 μM decavanadate, IP3 binding was half
maximally inhibited. A similar potency of decavanadate was also
found with adrenal cortex microsomes which bind IP3 with the same
affinity (Kd approx. 1.4 nM) as permeabilized pancreatic acinar
cells. Labelled IP3 was displaced from these subcellular membranes
with similar kinetics by unlabelled IP3 and decavanadate. The data
suggest that the inhibitory action of decavanadate on IP3 induced
Ca2+ release is a consequence of its effect on binding of IP3 to
its receptor. EGTA, ethylene-glycol-bis
(2-aminoethylether)-N,N,N′,N′-tetraacetic acid; EDTA,
ethylenediaminetetraacetic acid; PEG, polyethylene glycol; IP3,
inositol 1,4,5-trisphosphate; MOPS, morpholinopropane sulfonic
acid; HEPES, N-(2-hydroxyethyl)-piperazine-N′-2-ethanesulfonic
acid; Tris, tris(hydroxymethyl)-aminomethane