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.