Potentiation of calcium- and caffeine-induced calcium release by cyclic ADP-ribose

Cyclic ADP-ribose (cADPR) is a naturally occurring metabolite of NAD+ that is as potent as inositol 1,4,5-trisphosphate (IP3) in mobilizing Ca2+ in sea urchin eggs. Previous pharmacological evidence suggests that cADPR acts through a system similar to the Ca(2+)-induced Ca2+ release (CICR). Here I showed that in the presence of low concentrations of cADPR addition of Ca2+ to egg homogenates stimulated further release of Ca2+ in a concentration-dependent manner. In the absence of cADPR, no induced release was seen, and the added Ca2+ was, instead, sequestered by a thapsigargin-sensitive transport system. High concentrations of strontium (> 50 microM) could also induce Ca2+ release. The effective concentrations of Sr2+, however, were reduced 10-20-fold in the presence of low concentrations of cADPR. Barium, at up to 0.4 mM, did not stimulate Ca2+ release with or without cADPR. The potentiation between divalent cations and cADPR was mutual since the Ca2+ releasing activity of cADPR was also increased in the presence of strontium. Ionomycin and thapsigargin both released Ca2+ but neither potentiated Ca2+ release induced by divalent cations. Caffeine also released Ca2+ in a concentration-dependent manner, and its potency was greatly increased by low concentrations of cADPR, while no such simulation was seen with IP3. Conversely, low concentrations of caffeine that were not sufficient to release Ca2+ increased the effectiveness of cADPR 10-fold. Isocaffeine, an isomer of caffeine, was four to five times less effective, demonstrating the specificity of the caffeine effect. These results suggest that cADPR can function as an endogenous regulator of CICR in eggs.