Effect of luminal and extravesicular Ca²⁺ on NAADP binding and release properties

Nicotinic acid adenine dinucleotide phosphate (NAADP) has been shown to be a powerful Ca²⁺ release agent in numerous systems, including echinoderms, plants, and mammalian cells. NAADP has been shown to release Ca²⁺ via a separate mechanism to IP₃ and ryanodine receptors, and specific binding sites have recently been characterised. However, functional studies have shown that there is a functional interplay between the NAADP-sensitive mechanism and the other two. In particular, it appears that activation of the NAADP receptor might act as a trigger to facilitate responses from IP₃ and ryanodine receptors. To further characterise this interplay, we have investigated the effects of luminal and cytosolic Ca²⁺ on the NAADP receptor in sea urchin egg homogenates. We report that neither cytosolic nor luminal Ca²⁺ appears to influence NAADP binding. Conversely, emptying of stores significantly amplifies NAADP-induced fractional Ca²⁺-release, providing a mechanism of self-adjustment independent of store loading.