Dysregulated Zn(2+) homeostasis impairs cardiac type-2 ryanodine receptor and mitsugumin 23 functions, leading to sarcoplasmic reticulum Ca(2+) leakage

Aberrant Zn(2+) homeostasis is associated with dysregulated intracellular Ca(2+) release, resulting in chronic heart failure. In the failing heart a small population of cardiac ryanodine receptors (RyR2) displays sub-conductance-state gating leading to Ca(2+) leakage from sarcoplasmic reticulum (SR) stores, which impairs cardiac contractility. Previous evidence suggests contribution of RyR2-independent Ca(2+) leakage through an uncharacterized mechanism. We sought to examine the role of Zn(2+) in shaping intracellular Ca(2+) release in cardiac muscle. Cardiac SR vesicles prepared from sheep or mouse ventricular tissue were incorporated into phospholipid bilayers under voltage-clamp conditions, and the direct action of Zn(2+) on RyR2 channel function was examined. Under diastolic conditions, the addition of pathophysiological concentrations of Zn(2+) (≥2 nm) caused dysregulated RyR2-channel openings. Our data also revealed that RyR2 channels are not the only SR Ca(2+)-permeable channels regulated by Zn(2+). Elevating the cytosolic Zn(2+) concentration to 1 nm increased the activity of the transmembrane protein mitsugumin 23 (MG23). The current amplitude of the MG23 full-open state was consistent with that previously reported for RyR2 sub-conductance gating, suggesting that in heart failure in which Zn(2+) levels are elevated, RyR2 channels do not gate in a sub-conductance state, but rather MG23-gating becomes more apparent. We also show that in H9C2 cells exposed to ischemic conditions, intracellular Zn(2+) levels are elevated, coinciding with increased MG23 expression. In conclusion, these data suggest that dysregulated Zn(2+) homeostasis alters the function of both RyR2 and MG23 and that both ion channels play a key role in diastolic SR Ca(2+) leakage.