NAADP activates a Ca 2+ current that is dependent on F‐actin cytoskeleton

Nicotinic acid adenine dinucleotide phosphate (NAADP) is involved in the Ca 2+ response observed at fertilization in several species, including starfish. In this study, we have employed Ca 2+ imaging and the single‐electrode voltage‐clamp technique to investigate whether the NAADP‐mediated Ca 2+ entry discovered in our laboratory in starfish oocytes was underlain by a membrane current and whether the response to NAADP required an intact cytoskeleton. Uncaging of preinjected NAADP evoked a cortical Ca 2+ flash that was followed by the spreading of the wave to the remainder of the cell. No Ca 2+ increase was detected in Ca 2+ ‐free sea water. Under voltage‐clamp conditions, the photoliberation of NAADP activated an inward rectifying membrane current, which reversed at potentials more positive than +50 mV and was abolished by removal of Ca 2+ but not of Na + . The current was affected by preincubation with verapamil, SK&F 96356, and thapsigargin but not by preinjection of heparin, 8‐NH 2 ‐ cyclic ADP‐ribose, or both antagonists. The membrane current and the Ca 2+ wave were inhibited by latrunculin‐A and jasplakinolide, which depolymerize and stabilize actin cytoskeleton, respectively. These data offer the first demonstration that NAADP initiates a Ca 2+ sweep by activating a Ca 2+ ‐permeable membrane current that requires an intact F‐actin cytoskeleton as other Ca 2+ ‐permeable currents, such as I CRAC and I ARC .