Altered sarcoplasmic reticulum calcium transport in the presence of the heavy metal chelator TPEN

TPEN (N,N,N′,N′-tetrakis(2-pyridylmethyl)-ethylenediamine) is a membrane-permeable heavy-metal ion chelator with a dissociation constant for Ca2+ comparable to the Ca2+ concentration ([Ca2+]) within the intracellular Ca2+ stores. It has been used as modulator of intracellular heavy metals and of free intraluminal [Ca2+], without influencing the cytosolic [Ca2+] that falls in the nanomolar range. In our previous studies, we gave evidence that TPEN modifies the Ca2+ homeostasis of striated muscle independent of this buffering ability. Here we describe the direct interaction of TPEN with the ryanodine receptor (RyR) Ca2+ release channel and the sarcoplasmic reticulum (SR) Ca2+ pump (SERCA). In lipid bilayers, at negative potentials and low [Ca2+], TPEN increased the open probability of RyR, while at positive potentials it inhibited channel activity. On permeabilized skeletal muscle fibers of the frog, but not of the rat, 50 μM TPEN increased the number of spontaneous Ca2+ sparks and induced propagating events with a velocity of 273 ± 7 μm/s. Determining the hydrolytic activity of the SR revealed that TPEN inhibits the SERCA pump, with an IC50 = 692 ± 62 μM and a Hill coefficient of 0.88 ± 0.10. These findings provide experimental evidence that TPEN directly modifies both the release of Ca2+ from and its reuptake into the SR.