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-mu-M IP3 is half maximally inhibited with approximately 5-mu-M decavanadate. Complete inhibition is achieved with around 20-mu-M 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 (K(d) approx. 1.2 nM) could be abolished by decavanadate but not by oligovanadate. With 0.5-mu-M 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 (K(d) 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.