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 Ca2+ release from and the Ca2+ uptake into the sarcoplasmic reticulum (SR) of cultured C2C12 skeletal muscle cells. Low concentrations (50 μM) of the drug evoked Ca2+ transients in ∼60% of C2C12 myotubes, while at high concentrations (500 μM) it significantly reduced the size of both depolarization-and caffeine-induced Ca2+ transients, decreased the rate constant of decay and the calculated pump activity but failed to induce Ca2+ transients. Experiments at low extracellular [Ca2+] revealed that it is the total rather than free TPEN concentration that is responsible for the observed effects. TPEN does not modify Ca2+ release by Zn2+ chelation, as evidenced by the unaltered effect seen after the removal of Zn2+ from the extracellular space of the cells by chelating with EDPA. These findings provide experimental evidence that TPEN directly modifies both the release of Ca2+ from the SR and its removal from the myoplasm.