Adenophostin A Can Stimulate Ca2+ Influx without Depleting the Inositol 1,4,5-Trisphosphate-sensitive Ca2+ Stores in the Xenopus Oocyte

Adenophostin A possesses the highest known affinity for the inositol 1,4,5-trisphosphate (Ins(1,4,5)P 3 ) receptor (InsP 3 R). The compound shares with Ins(1,4,5)P 3 those structural elements essential for binding to the InsP 3 R. However, its adenosine 2′-phosphate moiety has no counterpart in the Ins(1,4,5)P 3 molecule. To determine whether its unique structure conferred a distinctive biological activity, we characterized the adenophostin-induced Ca 2+ signal in Xenopus oocytes using the Ca 2+ -gated Cl − current assay. In high concentrations, adenophostin A released Ca 2+ from Ins(1,4,5)P 3 -sensitive stores and stimulated a Cl − current that depended upon the presence of extracellular Ca 2+ . We used this Cl − current as a marker of Ca 2+ influx. In low concentrations, however, adenophostin A stimulated Ca 2+ influx exclusively. In contrast, Ins(1,4,5)P 3 and (2-hydroxyethyl)-α- D -glucopyranoside 2′,3,4-trisphosphate, an adenophostin A mimic lacking most of the adenosine moiety, always released intracellular Ca 2+ before causing Ca 2+ influx. Ins(1,4,5)P 3 could still release Ca 2+ during adenophostin A-induced Ca 2+ influx, confirming that the Ins(1,4,5)P 3 -sensitive intracellular Ca 2+ stores had not been emptied. Adenophostin- and Ins(1,4,5)P 3 -induced Ca 2+ influx were not additive, suggesting that both agonists stimulated a common Ca 2+ entry pathway. Heparin, which blocks binding to the InsP 3 R, prevented adenophostin-induced Ca 2+ influx. These data indicate that adenophostin A can stimulate the influx of Ca 2+ across the plasma membrane without inevitably emptying the Ins(1,4,5)P 3 -sensitive intracellular Ca 2+ stores.