Ca²⁺ signalling and membrane current activated by cADPr in starfish oocytes

Cyclic ADP-ribose (cADPr) is a second messenger that regulates intracellular free [Ca²⁺] ([Ca²⁺]_i) in a variety of cell types, including immature oocytes from the starfish Astropecten auranciacus. In this study, we employed confocal laser scanning microscopy and voltage clamp techniques to investigate the source of the cADPr-elicited Ca ²⁺ wave originating from the cortical Ca²⁺ patches we have described previously. The Ca²⁺ swing was accompanied by a membrane current with a reversal potential of ≈+20 mV. Decreasing external Na⁺ almost abolished the current without affecting the Ca ²⁺ response. Removal of extracellular Ca²⁺ altered neither the Ca²⁺ transient nor the ionic current, nor did the holding potential exert any effect on the Ca²⁺ wave. Both the Ca ²⁺ response and the membrane current were abolished when BAPTA, ruthenium red or 8-NH₂-cADPr were preinjected into the oocytes, while perfusion with ADPr did not elicit any [Ca²⁺]_i increase or ionic current. However, elevating [Ca²⁺]_i by uncaging Ca²⁺ from nitrophenyl- (NP-EGTA) or by photoliberating inositol 1,4,5-trisphosphate (InsP₃) induced an ionic current with biophysical properties similar to that elicited by cADPr. These results suggest that cADPr activates a Ca²⁺ wave by releasing Ca²⁺ from intracellular ryanodine receptors and that the rise in [Ca²⁺] _i triggers a non-selective monovalent cation current that does not seem to contribute to the global Ca²⁺ elevation.