The mechanism by which cyclic adenosine diphosphate ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) mobilize intracellular Ca 2؉ Intracellular Ca 2ϩ signals are initiated by Ca 2ϩ release from intracellular stores, and traditionally, the sarco/endoplasmic reticulum (S/ER) 2 has been considered to be the major releasable store. From this store, Ca 2ϩ may be released through the opening of inositol 1,4,5-trisphosphate receptors (IP 3 Rs) and/or ryanodine receptors (RyRs), the two groups of intracellular Ca 2ϩ release channels located on S/ER membranes. It is generally accepted that of the recognized Ca 2ϩ -mobilizing second messengers, inositol 1,4,5-trisphosphate facilitates this process by activating IP 3 Rs (1).By contrast, the mechanism by which the pyridine nucleotides cyclic adenosine diphosphate ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) (2) mobilize intracellular Ca 2ϩ stores remains controversial. Although a wealth of evidence across a variety of cell types (3-9) supports the original proposal that cADPR activates RyRs (10), studies on reconstituted RyRs in lipid bilayers have failed to conclusively demonstrate direct regulation of these channels by cADPR (11), and it has been suggested that cADPR may initiate Ca 2ϩ signals via RyRs and IP 3 Rs by promoting Ca 2ϩ uptake into the S/ER by S/ER Ca 2ϩ ATPases (SERCA) (12)(13)(14). This proposal has not been effectively countered by studies on ventricular myocytes, which exclusively express RyR2. This is due to the fact that the principal regulatory effect of cADPR with respect to RyR2 is to increase the sensitivity of this RyR subtype to Ca 2ϩ -induced Ca 2ϩ release (8, 9), and in light of the fact that the sensitivity of RyRs to Ca 2ϩ -induced Ca 2ϩ release may be augmented by an increase in Ca 2ϩ concentration within the cytoplasm and/or S/ER lumen (15).The mechanism by which NAADP triggers intracellular Ca 2ϩ release has also been hotly debated. We recently identified a family of two-pore domain channels (TPC1-3, TPCN1-3 for gene name) as endolysosome-targeted, NAADP-gated Ca 2ϩ release channels (16, 17), and our findings have since been confirmed by others (18,19