NAADP induces Ca(2+) oscillations via a two-pool mechanism by priming IP(3)- and cADPR-sensitive Ca(2+) stores

In sea urchin eggs, Ca(2+) mobilization by nicotinic acid adenine dinucleotide phosphate (NAADP) potently self-inactivates but paradoxically induces long-term Ca(2+) oscillations. We investigated whether NAADP-induced Ca(2+) oscillations arise from the recruitment of other Ca(2+) release pathways. NAADP, inositol trisphosphate (IP(3)) and cyclic ADP-ribose (cADPR) all mobilized Ca(2+) from internal stores but only NAADP consistently induced Ca(2+) oscillations. NAADP-induced Ca(2+) oscillations were partially inhibited by heparin or 8-amino-cADPR alone, but eliminated by the presence of both, indicating a requirement for both IP(3)- and cADPR-dependent Ca(2+) release. Thapsigargin completely blocked IP(3) and cADPR responses as well as NAADP-induced Ca(2+) oscillations, but only reduced the NAADP-mediated Ca(2+) transient. Following NAADP-mediated release from this Ca(2+) pool, the amount of Ca(2+) in the Ca(2+)-induced Ca(2+) release stores was increased. These results support a mechanism in which Ca(2+) oscillations are initiated by Ca(2+) release from NAADP-sensitive Ca(2+) stores (pool 1) and perpetuated through cycles of Ca(2+) uptake into and release from Ca(2+)-induced Ca(2+) release stores (pool 2). These results provide the first direct evidence in support of a two-pool model for Ca(2+) oscillations.