Adenophostin A and inositol 1,4,5-trisphosphate differentially activate Cl- currents in Xenopus oocytes because of disparate Ca2+ release kinetics

Abstract

Depletion of endoplasmic reticulum Ca2+ stores induces Ca2+ entry from the extracellular space by a process termed 'store-operated Ca2+ entry' (SOCE). It has been suggested that the novel fungal metabolite adenophostin-A may be able to stimulate Ca2+ entry without stimulating Ca2+ release from stores. To test this idea further, we compared Ca2+ release, SOCE, and the stimulation of Ca2+-activated Cl- currents in Xenopus oocytes in response to inositol 1,4,5-trisphosphate (IP3) and adenophostin-A injection. IP3 stimulated an outward Cl- current, I(Cl1).s, in response to Ca2+ release from stores followed by an inward current, I(Cl2), in response to SOCE. In contrast, low concentrations of adenophostins (AdAs) activated I(Cl2) without activating I(Cl1-S), consistent with the suggestion that AdA can activate Ca2+ entry without stimulating Ca2+ release. However, when Ca2+entry has been stimulated by AdA, Ca2+ stores are largely depleted of Ca2+, as assessed by the inability of ionomycin to release additional Ca2+. The Ca2+ release stimulated by AdA, however, was 7 times slower than the release stimulated by IP3, which could explain the minimal activation of I(Cl1-S); when Ca2+ is released slowly, the threshold level required for I(Cl1-S) activation is not attained.