Cyclic ADP-ribose increases Ca2+ removal in smooth muscle

Ca2+ release via ryanodine receptors (RyRs) is vital in cell signalling and regulates diverse activities such as gene expression and excitation-contraction coupling. Cyclic ADP ribose (cADPR), a proposed modulator of RyR activity, releases Ca2+ from the intracellular store in sea urchin eggs but its mechanism of action in other cell types is controversial. In this study, caged cADPR was used to examine the effect of cADPR on Ca2+ signalling in single voltage-clamped smooth muscle cells that have RyR but lack FKBP12.6, a proposed target for cADPR. Although cADPR released Ca2+ in sea urchin eggs (a positive control), it failed to alter global or subsarcolemma [Ca2+ ](c), to cause Ca2+-induced Ca2+ release or to enhance caffeine responses in colonic myocytes. By contrast, caffeine (an accepted modulator of RyR) was effective in these respects. The lack of cADPR activity on Ca2+ release was unaffected by the introduction of recombinant FKBP12.6 into the myocytes. Indeed in western blots, using brain membrane preparations as a source of FKBP12.6, cADPR did not bind to FKBPs, although FK506 was effective. However, cADPR increased and its antagonist 8-bromo-cADPR slowed the rate of Ca2+ removal from the cytoplasm. The evidence indicates that cADPR modulates [Ca2+](c) but not via RyR; the mechanism may involve the sarcolemma Ca2+ pump.