Abstract—
cADP-Ribose (cADPR) is a novel endogenous messenger that is believed to mobilize Ca
2+
from ryanodine-sensitive Ca
2+
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 Ca
2+
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 Ca
2+
sparks. These effects on local release events were accompanied by a significant increase in sarcoplasmic reticulum (SR) Ca
2+
content. In comparison, sensitization of ryanodine receptors (RyRs) by caffeine, a true RyR agonist, caused a rapid (<1 second) and transient potentiation of Ca
2+
sparks followed by a decrease in SR Ca
2+
content. When the increase in the SR load was prevented by partial inhibition of the SR Ca
2+
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 Ca
2+
uptake by cardiac SR microsomes. Our results suggest that the primary target of cADPR is the SR Ca
2+
uptake mechanism. Potentiation of Ca
2+
release by cADPR is mediated by increased accumulation of Ca
2+
in the SR and subsequent luminal Ca
2+
-dependent activation of RyRs.
