Differential effect of pH upon cyclic-ADP-ribose and nicotinate - adenine dinucleotide phosphate-induced Ca²⁺ release systems

We investigated the pH dependence and the effects of thimerosal and dithiothreitol (DTT) upon the Ca²⁺ release induced by cADP-ribose (cADPR) and nicotinate-adenine dinucleotide phosphate (NAADP) in sea urchin egg homogenates. Both Ca²⁺ release triggered by cADPR and the binding of [³H]cADPR to sea urchin egg homogenates were decreased by alkalization of the assay media from pH 7.2 to 8.9. In contrast, NAADP-triggered Ca²⁺ release was not influenced by changes in pH. The Ca²⁺ release induced by cADPR was potentiated by thimerosal and inhibited by DTT, but neither thimerosal nor DTT had any effect upon the Ca²⁺ release induced by NAADP. We conclude that cADPR-sensitive Ca²⁺-release mechanisms are dependent on pH of the assay media and are sensitive to thiol group modification. On the other hand, these functional properties are not shared by NAADP-regulated Ca²⁺ channels.