Nicotinate adenine dinucleotide phosphate (NAADP) triggers a specific calcium release system in sea urchin eggs

Transient fluxes of intracellular ionized calcium (Ca²⁺) from intracellular stores are integral components of regulatory signaling pathways operating in numerous biological regulations, including in early stages of egg fertilization. Therefore, we explored whether NADP, which is rapidly generated by phosphorylation of NAD upon fertilization may, directly or indirectly, exert a regulatory role as a trigger of Ca²⁺ release from intracellular stores in sea urchin eggs. NADP had no effect, but we found that the deamidated derivative of NADP, nicotinate adenine dinucleotide phosphate (β-NAADP), is a potent and specific stimulus (ED₅₀ 16 nM) for Ca²⁺ release in sea urchin egg homogenates. NAADP triggers the Ca²⁺ release via a mechanism which is distinct from the well-known Ca²⁺ release systems triggered either by inositol-1,4,5-triphosphate (IP₃) or by cyclic adenosine diphospho-ribose (cADPR). The NAADP-induced release of Ca²⁺ is not blocked by heparin, and antagonist of IP₃, or by procaine or ruthenium red, antagonists of cADPR. However, it is selectively blocked by thionicotinamide. NADP which does not inhibit the actions of IP₃ or cADPR. NAADP produced by heating of NADP in alkaline (pH = 12) medium or synthetized enzymatically by nicotinic acid-NADP reaction catalyzed by NAD glycohydrolase have identical properties. The results presented herein thus describe a novel endocellular Ca²⁺-releasing system controlled by NAADP as a specific stimulus. The NAADP-controlled Ca²⁺ release system may be an integral component of multiple intracellular regulations occurring in fertilized sea urchin eggs, which are mediated by intracellular Ca²⁺ release, and may also have similar role(s) in other tissues.