Differences in the regulation of RyR2 from human, sheep, and rat by Ca2+ and Mg2+ in the cytoplasm and in the lumen of the sarcoplasmic reticulum

Regulation of the cardiac ryanodine receptor (RyR2) by intracellular Ca(2+) and Mg(2+) plays a key role in determining cardiac contraction and rhythmicity, but their role in regulating the human RyR2 remains poorly defined. The Ca(2+)- and Mg(2+)-dependent regulation of human RyR2 was recorded in artificial lipid bilayers in the presence of 2 mM ATP and compared with that in two commonly used animal models for RyR2 function (rat and sheep). Human RyR2 displayed cytoplasmic Ca(2+) activation (K(a) = 4 µM) and inhibition by cytoplasmic Mg(2+) (K(i) = 10 µM at 100 nM Ca(2+)) that was similar to RyR2 from rat and sheep obtained under the same experimental conditions. However, in the presence of 0.1 mM Ca(2+), RyR2s from human were 3.5-fold less sensitive to cytoplasmic Mg(2+) inhibition than those from sheep and rat. The K(a) values for luminal Ca(2+) activation were similar in the three species (35 µM for human, 12 µM for sheep, and 10 µM for rat). From the relationship between open probability and luminal [Ca(2+)], the peak open probability for the human RyR2 was approximately the same as that for sheep, and both were ~10-fold greater than that for rat RyR2. Human RyR2 also showed the same sensitivity to luminal Mg(2+) as that from sheep, whereas rat RyR2 was 10-fold more sensitive. In all species, modulation of RyR2 gating by luminal Ca(2+) and Mg(2+) only occurred when cytoplasmic [Ca(2+)] was <3 µM. The activation response of RyR2 to luminal and cytoplasmic Ca(2+) was strongly dependent on the Mg(2+) concentration. Addition of physiological levels (1 mM) of Mg(2+) raised the K(a) for cytoplasmic Ca(2+) to 30 µM (human and sheep) or 90 µM (rat) and raised the K(a) for luminal Ca(2+) to ~1 mM in all species. This is the first report of the regulation by Ca(2+) and Mg(2+) of native RyR2 receptor activity from healthy human hearts.