Novel Mechanisms Involved in Superoxide Anion Radical-Triggered Ca 2+ Release from Cardiac Sarcoplasmic Reticulum Linked to Cyclic ADP-Ribose Stimulation

Abstract

It has been suggested that cyclic adenosine 5′-diphosphoribose (cADPR) directly activates the cardiac isoform of the ryanodine receptor (RyR)/Ca²⁺ release channel. We have previously shown that selective activation of RyR/Ca²⁺ release channel by superoxide anion radical (O₂ ^. ¯) is dependent of the presence of calmodulin and identified calmodulin as a functional mediator of O₂ ^. ¯ -triggered Ca²⁺ release through the RyR/Ca²⁺ release channel of cardiac sarcoplasmic reticulum (SR). We now demonstrate that although the effect of O₂ ^. ¯ on Ca²⁺ efflux from RyR/Ca²⁺ release channel at higher concentrations ( >5 μM) is due to its ability to produce a loss in function of calmodulin thereby decreasing calmodulin inhibition, O₂ ^. ¯ radicals at lower concentrations (<5 μM) may be able to stimulate Ca²⁺ release only in the presence of calmodulin from the SR via increased cADPR synthesis; it is also shown that cADPR is a modulator that can activate the Ca²⁺-release mechanism when it is in a sensitized state by the presence of calmodulin, possibly, at physiological concentration. In addition, the SR vesicles immediately upon addition of cADPR, but not NAD⁺, did exhibit Ca²⁺ efflux stimulation. When heart homogenate was incubated with O₂ ^. ¯, conversion of NAD⁺ into cADPR was stimulated; the reduction of homogenate Ca²⁺ uptake (by increasing Ca²⁺ efflux through RyR/Ca²⁺ release channel) occurred. Thus O₂ ^. ¯ radical is responsible for cADPR formation from NAD⁺ in the cellular environment outside of the SR of heart muscle. The results presented here provide the first evidence of a messenger role for O₂ ^. ¯ radical in cADPR-mediated Ca²⁺ mobilization in myocardium.