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)P₃) receptor (InsP₃R). The compound shares with Ins(1,4,5)P₃ those structural elements essential for binding to the InsP₃R. However, its adenosine 2'-phosphate moiety has no counterpart in the Ins(1,4,5)P₃ molecule. To determine whether its unique structure conferred a distinctive biological activity, we characterized the adenophostin-induced Ca²⁺ signal in Xenopus oocytes using the Ca²⁺-gated Cl^- current assay. In high concentrations, adenophostin A released Ca²⁺ from Ins(1,4,5)P₃- sensitive stores and stimulated a Cl^- current that depended upon the presence of extracellular Ca²⁺. We used this Cl^- current as a marker of Ca²⁺ influx. In low concentrations, however, adenophostin A stimulated Ca²⁺ influx exclusively. In contrast, Ins(1,4,5)P₃ and (2-hydroxyethyl)- α-D-glucopyranoside 2',3,4-trisphosphate, an adenophostin A mimic lacking most of the adenosine moiety, always released intracellular Ca²⁺ before causing Ca²⁺ influx. Ins(1,4,5)P₃ could still release Ca²⁺ during adenophostin A-induced Ca²⁺ influx, confirming that the Ins(1,4,5)P₃- sensitive intracellular Ca²⁺ stores had not been emptied. Adenophostin- and Ins(1,4,5)P₃-induced Ca²⁺ influx were not additive, suggesting that both agonists stimulated a common Ca²⁺ entry pathway. Heparin, which blocks binding to the InsP₃R, prevented adenophostin-induced Ca²⁺ influx. These data indicate that adenophostin A can stimulate the influx of Ca²⁺ across the plasma membrane without inevitably emptying the Ins(1,4,5)P₃-sensitive intracellular Ca²⁺ stores.