Abstract

Adenophostin-A, a novel compound isolated from cultures ofPenicillium brevicompactum, has been shown to stimulate Ca²⁺ release from inositol-1,4,5-trisphosphate (IP₃)-sensitive Ca²⁺ stores in microsomal preparations, permeabilized cells, and lipid vesicles containing purified IP₃ receptor. The purpose of the current study was to compare the effects of adenophostin-A and IP₃ on Ca²⁺ release from stores and Ca²⁺ influx in intact Xenopus laevis oocytes. Ca²⁺ influx though store-operated Ca²⁺ channels and Ca²⁺release from stores were monitored by measuring two Ca²⁺-activated Cl⁻ currents that can be used as real-time indicators of Ca²⁺ release and Ca²⁺ influx (I_Cl-1 and I_Cl-2, respectively). We find that high concentrations (final intraoocyte concentrations of 5–10 μm) of adenophostin-A and IP₃ stimulate a large Ca²⁺ release from stores (as measured by I_Cl-1) followed by Ca²⁺ influx (as measured by I_Cl-2). Low concentrations (∼50 nm) of IP₃ stimulate oscillations in Ca²⁺ release without stimulating Ca²⁺ influx. In contrast, low concentrations of adenophostin-A can stimulate Ca²⁺ influx without stimulating a large Ca²⁺ release. However, Ca²⁺ influx did not occur in the complete absence of Ca²⁺ release. Therefore, it is unlikely that adenophostin-A directly stimulates store-operated Ca²⁺ channels. We hypothesize that adenophostin-A releases Ca²⁺ from a subpopulation of stores that is tightly coupled to store-operated Ca²⁺ channels.