Potentiation of Ca²⁺ release by cADP-Ribose in the heart is mediated by enhanced SR Ca²⁺ uptake into the sarcoplasmic reticulum

cADP-Ribose (cADPR) is a novel endogenous messenger that is believed to mobilize Ca²⁺ from ryanodine-sensitive Ca²⁺ stores. Despite intense research, the precise mechanism of action of cADPR remains uncertain, and experimental findings are contradictory. To elucidate the mechanism of cADPR action, we performed confocal Ca²⁺ imaging in saponin-permeabilized rat ventricular myocytes. Exposure of the cells to cADPR resulted in a slow (>2 minutes) and steady increase in the frequency of Ca²⁺ sparks. These effects on local release events were accompanied by a significant increase in sarcoplasmic reticulum (SR) Ca²⁺ content. In comparison, sensitization of ryanodine receptors (RyRs) by caffeine, a true RyR agonist, caused a rapid (< 1 second) and transient potentiation of Ca²⁺ sparks followed by a decrease in SR Ca²⁺ content. When the increase in the SR load was prevented by partial inhibition of the SR Ca²⁺ with thapsigargin, cADPR failed to produce any increase in sparking activity. cADPR had no significant impact on activity of single cardiac RyRs incorporated into lipid bilayers. However, it caused a significant increase in the rate of Ca²⁺ uptake by cardiac SR microsomes. Our results suggest that the primary target of cADPR is the SR Ca²⁺ uptake mechanism. Potentiation of Ca²⁺ release by cADPR is mediated by increased accumulation of Ca²⁺ in the SR and subsequent luminal Ca²⁺-dependent activation of RyRs.