Effect of TPEN on the calcium release of cultured C2C12 mouse myotubes.

N,N,N',N'-tetrakis(2-pyridylmethyl)-ethilenediamine (TPEN) is a membrane permeable heavy metal chelator that has been used to study intracellular calcium homeostasis but its exact mode of action is still unresolved. Here we examine the effects of TPEN on the Ca(2+) release from and the Ca(2+) uptake into the sarcoplasmic reticulum (SR) of cultured C2C12 skeletal muscle cells. Low concentrations (50 microM) of the drug evoked Ca(2+) transients in approximately 60% of C2C12 myotubes, while at high concentrations (500 microM) it significantly reduced the size of both depolarization-and caffeine-induced Ca(2+) transients, decreased the rate constant of decay and the calculated pump activity but failed to induce Ca(2+) transients. Experiments at low extracellular [Ca(2+)] revealed that it is the total rather than free TPEN concentration that is responsible for the observed effects. TPEN does not modify Ca(2+) release by Zn(2+) chelation, as evidenced by the unaltered effect seen after the removal of Zn(2+) from the extracellular space of the cells by chelating with EDPA. These findings provide experimental evidence that TPEN directly modifies both the release of Ca(2+) from the SR and its removal from the myoplasm.