Cadmium inhibits plasma membrane calcium transport

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Abstract
The interaction of Cd2+ with the plasma membrane Ca2+-transporting ATPase of fish gills was studied. ATP-driven Ca2+-transport in basolateral membrane (BLM) vesicles was inhibited by Cd2+ with anI 50 value of 3.0nm at 0.25 μm free Ca2+ using EGTA, HEEDTA and NTA to buffer Ca2+ and Cd2+ concentrations. The inhibition was competitive in nature since theK 0.5 value for Ca2+ increased linearly with increasing Cd2+ concentrations while theV max remained unchanged. The Ca2+ pump appeared to be calmodulin dependent, but we conclude that the inhibition by Cd2+ occurs directly on the Ca2+ binding site of the Ca2+-transporting ATPase and not via the Ca2+-binding sites of calmodulin. It is suggested that Cd2+-induced inhibition of Ca2+-transporting enzymes is the primary effect in the Cd2+ toxicity towards cells followed by several secondary effects due to a disturbed cellular Ca2+ metabolism. Our data illustrate that apparent stimulatory effects of low concentrations of Cd2+ on Ca2+-dependent enzymes may derive from increased free-Ca2+ levels when Cd2+ supersedes Ca2+ on the ligands.

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