In vivo effects of copper on the calcium homeostasis mechanisms of mussel gill cell plasma membranes

In vivo effects of Cu²⁺ on the Ca²⁺ homeostasis mechanisms of gill cell plasma membranes were assessed in mussels (Mytilus galloprovincialis) exposed for 1, 4, and 7 days to sublethal concentrations of Cu²⁺ (0.6 μM), by quantifying the activity of Ca²⁺ ATPase and Na⁺/K⁺ ATPase on a plasma membrane-enriched subcellular fraction. Enzyme activities showed an initial decrease reaching a minimum after four days of metal exposure (about 60-70% inhibition), followed by a recovery. Impairment of the Ca²⁺-ATPase activity was due to an inhibition of the phosphorylated intermediate formation. The inhibition of the plasma membrane Ca²⁺-ATPase activity is direct evidence of metal effects on Ca²⁺ homeostasis processes, while the inhibition of the Na⁺/K⁺ ATPase could also involve an impairment of the Na⁺/Ca²⁺ antiporter. Copper-induced enzyme deterioration probably derives from the high affinity of the metal for SH groups, but a role could also be played by metal-induced oxidative stress and consequent lipid peroxidation of cell membranes, as suggested by glutathione depletion and MDA increase, respectively. Recovery of enzyme functionality and glutathione levels, notwithstanding progressive Cu²⁺ accumulation in mussel gill tissue, suggests the activation of metal-induced cell detoxification systems.