N,N,N',N'-Tetrakis(2-pyridylmethyl)-ethylenediamine (
TPEN), a transition-
metal chelator, was recently found to protect against
myocardial ischemia-
reperfusion injury. The goals of this study were to investigate the in vivo antiarrhythmic and antifibrillatory potential of
TPEN in rats and guinea pigs and to study the in vitro effects of
TPEN on
calcium homeostasis in cultured newborn rat cardiac cells in normoxia and
hypoxia. We demonstrated on an in vivo rat model of
ischemia-reperfusion that
TPEN abolishes
ventricular fibrillation incidence and mortality and decreases the incidence and duration of
ventricular tachycardia. To elucidate the mechanism of cardioprotection by
TPEN, contraction, synchronization, and intracellular
calcium level were examined in vitro. We have shown for the first time that
TPEN prevented the increase in intracellular Ca(2+) levels ([Ca(2+)](i)) caused by
hypoxia and abolished [Ca(2+)](i) elevation caused by high extracellular Ca(2+) levels ([Ca(2+)](o)) or by
caffeine. Addition of
TPEN returned synchronized beating of cardiomyocytes desynchronized by [Ca(2+)](o) elevation. To discover the mechanism by which
TPEN reduces [Ca(2+)](i) in cardiomyocytes, the cells were treated with
thapsigargin, which inhibits Ca(2+) uptake into the sarcoplasmic reticulum (SR).
TPEN successfully reduced [Ca(2+)](i) elevated by
thapsigargin, indicating that
TPEN did not sequester Ca(2+) in the SR. However,
TPEN did not reduce [Ca(2+)](i) in the Na(+)-free medium in which the
Na(+)/Ca(2+) exchanger was inhibited. Taken together, the results show that activation of sarcolemmal
Na(+)/Ca(2+) exchanger by
TPEN increases Ca(2+) extrusion from the cytoplasm of cardiomyocytes, preventing cytosolic Ca(2+) overload, which explains the beneficial effects of
TPEN on postischemic cardiac status.