cADPR [cyclic ADPR (ADP-ribose)], NAADP (nicotinic acid-adenine dinucleotide phosphate) and ADPR belong to the family of adenine-containing second messengers. They are metabolically related and are all involved in the regulation of cellular Ca(2+) homoeostasis. Activation of specific plasma membrane receptors is connected to cADPR formation in many cell types and tissues. In contrast receptor-mediated formation of NAADP and ADPR has been shown only in a few selected cellular systems. The intracellular Ca(2+) channel triggered by cADPR is the RyR (ryanodine receptor); in the case of NAADP, both activation of RyR and a novel Ca(2+) channel have been proposed. In contrast, ADPR opens the non-specific cation channel TRPM2 [TRP (transient receptor potential) melastatin 2] that belongs to the TRP family of ion channels.
