ADENOPHOSTIN-A CAN STIMULATE CA2-TRISPHOSPHATE-SENSITIVE CA2+ STORES IN THE XENOPUS OOCYTE( INFLUX WITHOUT DEPLETING THE INOSITOL 1,4,5)

Adenophostin A possesses the highest known affinity or the inositol 1, 4,5-trisphosphate (Ins(1,4,5)P-3) receptor (InsP(3)R). The compound sh ares with Ins(1,4,5)P-3 those structural elements essential far bindin g to the InsP(3)R. However, its adenosine 2'-phosphate moiety has no c ounterpart in the Ins(1,4,5)P-3 molecule. To determine whether its uni que structure conferred a distinctive biological activity, we characte rized the adenophostin-induced Ca2+ signal in Xenopus oocytes using th e Ca2+-gated Cl- current assay. In high concentrations, adenophostin A released Ca2+ from Ins(1,4,5)P-3-sensitive stores and stimulated a Cl - current that depended upon the presence of extracellular Ca2+. We us ed this Cl- current as a marker of Ca2+ influx. In low concentrations, however, adenophostin A stimulated Ca2+ influx exclusively. In contra st, Ins(1,4,5)P-3 and (2-hydroxyethyl)-alpha-D-glucopyranoside 2',3,4- trisphosphate, an adenophostin A mimic lacking most of the adenosine m oiety, always released intracellular Ca2+ before causing Ca2+ influx. Ins(1,4,5)P-3 could still release Ca2+ during adenophostin A-induced C a2+ influx, confirming that the Ins(1,4,5)P-3-sensitive intracellular Ca2+ stores had not been emptied. Adenophostin- and Ins(1,4,5)P-3-indu ced Ca2+ influx were not additive, suggesting that both agonists stimu lated a common Ca2+ entry pathway. Heparin, which blocks binding to th e InsP(3)R, prevented adenophostin-induced Ca2+ influx. These data ind icate that adenophostin A can stimulate the influx of Ca2+ across the plasma membrane without inevitably emptying the Ins(1,4,5)P-3-sensitiv e intracellular Ca2+ stores.