Here we investigate how ß-adrenergic stimulation of the heart alters regulation of ryanodine receptors (RyRs) by intracellular Ca(2+) and Mg(2+) and the role of these changes in SR Ca(2+) release. RyRs were isolated from rat hearts, perfused in a Langendorff apparatus for 5 min and subject to 1 min perfusion with 1 µM isoproterenol or without (control) and snap frozen in liquid N2 to capture their phosphorylation state. Western Blots show that RyR2 phosphorylation was increased by isoproterenol, confirming that RyR2 were subject to normal ß-adrenergic signaling. Under basal conditions, S2808 and S2814 had phosphorylation levels of 69% and 15%, respectively. These levels were increased to 83% and 60%, respectively, after 60 s of ß-adrenergic stimulation consistent with other reports that ß-adrenergic stimulation of the heart can phosphorylate RyRs at specific residues including S2808 and S2814 causing an increase in RyR activity. At cytoplasmic [Ca(2+)] <1 µM, ß-adrenergic stimulation increased luminal Ca(2+) activation of single RyR channels, decreased luminal Mg(2+) inhibition and decreased inhibition of RyRs by mM cytoplasmic Mg(2+). At cytoplasmic [Ca(2+)] >1 µM, ß-adrenergic stimulation only decreased cytoplasmic Mg(2+) and Ca(2+) inhibition of RyRs. The Ka and maximum levels of cytoplasmic Ca(2+) activation site were not affected by ß-adrenergic stimulation. Our RyR2 gating model was fitted to the single channel data. It predicted that in diastole, ß-adrenergic stimulation is mediated by 1) increasing the activating potency of Ca(2+) binding to the luminal Ca(2+) site and decreasing its affinity for luminal Mg(2+) and 2) decreasing affinity of the low-affinity Ca(2+)/Mg(2+) cytoplasmic inhibition site. However in systole, ß-adrenergic stimulation is mediated mainly by the latter.