Redox-cycling of iron ions triggers calcium release from liver microsomes

Elevation of cytosolic calcium levels has been shown to occur via oxidation of critical protein thiols in liver microsomes. Elevated cytosolic Ca²⁺ may also result from activation of calcium releasing channels. In the presence of NADPH or ascorbic acid, iron ions produced a concentration- dependent release of calcium from liver microsomes. Under anaerobic conditions, the iron-induced release of calcium was inhibited, suggesting that a reaction of oxidation triggers the releasing process. The calcium releasing process at pH 7.0 appears to be highly sensitive to activation by iron ions, as effective concentrations (e.g., 2-5 μM) did not alter the Ca²⁺, Mg²⁺-ATPase or the phospholipid component of the microsomal membranes. Iron-induced Ca²⁺-release could occur under conditions in which there was no iron-induced microsomal lipid peroxidation. Under conditions of intense lipid peroxidation, PBN fully prevented the iron-induced accumulation of thiobarbituric reactive reagents without affecting the release of Ca²⁺, suggesting that lipid peroxidation is not the mechanism by which iron causes release of calcium. Trolox, GSH and high concentrations of ascorbate, however, strongly inhibited the iron-induced calcium release, most likely due to modulation of the Fe²⁺/Fe³⁺ ratio. While the IP₃ receptor system is considered to be the main regulator of calcium release, liver also contains a ryanodine-sensitive calcium releasing store. The iron-induced calcium release at pH 7.0 was blocked by ruthenium red, a specific inhibitor of the ryanodine receptor, and Fe²⁺ (but not Fe³⁺) decreased the binding of ryanodine, a specific ligand for the ryanodine-sensitive calcium channel. These results suggest that redox-cycling of iron ions results in an activation of a ryanodine-sensitive calcium channel. Activation of calcium releasing channels by iron may play a role in the evolution of various hepatic disorders that are associated with chronic iron overload in humans.