Mobilisation of intracellular Ca²⁺ stores plays a pivotal role in the regulation of arterial smooth muscle function, paradoxically during both contraction and relaxation. However, the underlying spatiotemporal pattern of Ca²⁺ signals may also contribute to the regulation of differential gene expression. Thus, it is important that we characterise the mechanisms that determine different signalling patterns. A significant advance in this respect was the realisation that different Ca²⁺ storing organelles may be selected by the discrete or co-ordinated actions of multiple Ca²⁺ mobilising messengers. When considering such messengers it is accepted that sarcoplasmic reticulum (SR) stores may be mobilised by inositol 1,4,5 trisphosphate (IP3). However, relatively little attention has been paid to the role of Ca²⁺ mobilising pyridine nucleotides in arterial smooth muscle, namely cyclic adenosine diphosphate-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP). We have found NAADP to be a powerful Ca²⁺ mobilising messenger in arterial smooth muscle (threshold ≤10 nM), and that it initiates global Ca²⁺ waves via a two-pool mechanism (Boittin et al. 2002). Initially, NAADP evokes highly localised intracellular Ca²⁺ signals, namely Ca²⁺ bursts, by mobilising a thapsigargin-insensitive intracellular Ca²⁺ store other than the SR. These Ca²⁺ bursts were shown to initiate subsequent, global Ca²⁺ waves and contraction of myocytes through the recruitment of ryanodine receptors (RyRs) on the SR via Ca²⁺-induced Ca²⁺ release (CICR; Boittin et al. 2002). By contrast, NAADP-induced Ca²⁺ bursts did not recruit IP3 receptors (IP3Rs) by CICR. Subsequent studies established that NAADP evokes Ca²⁺ bursts by mobilising a bafilomycin A1-sensitive, lysosome-related Ca²⁺ store. And that lysosomal stores facilitate the generation of a global Ca²⁺ wave by co-localising with a subpopulation of RyRs on the SR to comprise a highly specialized trigger zone for NAADP-dependent Ca²⁺ signalling. Importantly, Ca²⁺ signalling by NAADP was also found to be induced in an agonist-specific manner by the vasoconstrictor endothelin-1 (Kinnear et al. 2004). Much higher concentrations of cADPR (20-100 μM) are required to initiate Ca²⁺ signals in pulmonary arterial smooth muscle. In marked contrast to NAADP, however, threshold concentrations (20 μM) of cADPR were found to induce a sustained increase in intracellular Ca²⁺ concentration by mobilising a cyclopiazonic acid (CPA)-sensitive SR store proximal to the plasma membrane. Further investigation established that adenylyl cyclase coupled β-adrenoreceptors may mediate vasodilation by cADPR-dependent Ca²⁺ release via RyRs in the SR proximal to the plasma membrane, leading to subsequent BKCa-dependent hyperpolarization and vasodilation (Boittin et al. 2003). Paradoxically, we had previously shown that cADPR-dependent SR Ca²⁺ release via RyRs in a CPA-insensitive SR store underpinned pulmonary artery constriction by hypoxia (Dipp & Evans, 2001). When taken together, these findings suggest that cADPR-dependent Ca²⁺ release via RyRs could lead to stimulus-dependent relaxation or contraction in arterial smooth muscle. Given that RyR subtypes 1, 2 and 3 are present in vascular smooth muscle, this paradox may be explained if: (1) β-adrenoreceptor signalling targets PKA-dependent cADPR synthesis to a particular RyR subtype in the peripheral SR that is in close apposition to BKCa channels in the plasma membrane, (2) cADPR-dependent vasoconstriction results from the activation of a discrete RyR subtype localized in the central SR proximal to the contractile apparatus and (3) the peripheral and central SR represent functionally segregated compartments (Boittin et al. 2003). Considering the above, it seems likely that future studies may establish that NAADP and cADPR have a combinatorial role in mediating smooth muscle contraction. This is clear from the fact that cADPR may either activate RyRs or lower the threshold for CICR via RyRs (Galione et al. 1991). Thus, cADPR may also modulate the threshold for initiation of global Ca²⁺ signals by NAADP and / or the frequency of subsequent Ca²⁺ oscillations. By contrast to the effects of cADPR and NAADP, Ca²⁺ signalling by IP3 occurred in a manner independent of lysosomal Ca²⁺ stores. Moreover, IP3 was able to initiate a global Ca²⁺ wave in the absence of CICR via RyRs. Cells may therefore co-ordinate and restrict the relationship between lysosomal Ca²⁺ stores and Ca²⁺ release channels on the SR/ER in a manner suited to their function. These findings further advance our understanding of how the selection of different organnellar stores by the discrete or combinatorial effects of different Ca²⁺-mobilizing messengers may underpin differential Ca²⁺ signalling patterns.