Novel mechanisms involved in superoxide anion radical-triggered Ca2+ release from cardiac sarcoplasmic reticulum linked to cyclic ADP-ribose stimulation.

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 microM) is due to its ability to produce a loss in function of calmodulin thereby decreasing calmodulin inhibition, O2.- radicals at lower concentrations (<5 microM) 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.