Interactions between intracellular Ca²⁺ stores: Ca²⁺ released from the NAADP pool potentiates cADPR-induced Ca²⁺ release

Cells possess multiple intracellular Ca²⁺-releasing systems. Sea urchin egg homogenates are a well-established model to study intracellular Ca²⁺ release. In the present study the mechanism of interaction between three intracellular Ca²⁺ pools, namely the nicotinic acid adenine dinucleotide phosphate (NAADP), the cyclic ADP-ribose (cADPR) and the inositol 1′,4′,5′-trisphosphate (IP₃)-regulated Ca²⁺ stores, is explored. The data indicate that the NAADP Ca²⁺ pool could be used to sensitize the cADPR system. In contrast, the IP₃ pool was not affected by the Ca²⁺ released by NAADP. The mechanism of potentiation of the cADPR-induced Ca²⁺ release, promoted by Ca²⁺ released from the NAADP pool, is mediated by the mechanism of Ca²⁺-induced Ca²⁺ release. These data raise the possibility that the NAADP Ca²⁺ store may have a role as a regulator of the cellular sensitivity to cADPR.