Key result
FK-506 prolonged action potentials, induced early afterdepolarizations in 59% of cells, and inhibited IK1 and ICaL in rat ventricular cells.
Why the study?
Does FK-506 alter action potentials and ionic currents in rat ventricular cells?
Does FK-506 alter action potentials and ionic currents in rat ventricular cells?
FK-506 induces complex proarrhythmogenic effects in rat ventricular cells, including action potential prolongation, early afterdepolarizations, and inhibition of multiple potassium and calcium currents.
Alerts to potential proarrhythmic risk with FK-506; leaves open clinical translation from rat myocytes.
FK-506, a widely used immunosuppressant, has caused a few clinical cases with QT prolongation and torsades de pointe at high blood concentration. The proarrhytmogenic potential of FK-506 was investigated in single rat ventricular cells using the whole cell clamp method to record action potentials (APs) and ionic currents. Fluorescence measurements of Ca2+ transients were performed with indo-1 AM using a multiphotonic microscope. FK-506 (25 micromol/l) hyperpolarized the resting membrane potential (RMP; -3 mV) and prolonged APs (AP duration at 90% repolarization increased by 21%) at 0.1 Hz. Prolongation was enhanced by threefold at 3.3 Hz, and early afterdepolarizations (EADs) occurred in 59% of cells. EADs were prevented by stronger intracellular Ca2+ buffering (EGTA: 10 vs. 0.5 mmol/l in the patch pipette) or replacement of extracellular Na+ by Li+, which abolishes Na+/Ca2+ exchange [Na+/Ca2+ exchanger current (INaCa)]. In indo-1-loaded cells, FK-506 generated doublets of Ca(2+) transients associated with increased diastolic Ca2+ in one-half of the cells. FK-506 reversibly decreased the L-type Ca2+ current (ICaL) by 25%, although high-frequency-dependent facilitation of ICaL persisted, and decreased three distinct K+ currents: delayed rectifier K+ current (IK; >80%), transient outward K+ current (<20%), and inward rectifier K+ current (IK1; >40%). A shift in the reversal potential of IK1 (-5 mV) accounted for RMP hyperpolarization. Numerical simulations, reproducing all experimental effects of FK-506, and the use of nifedipine showed that frequency-dependent facilitation of ICaL plays a role in the occurrence of EADs. In conclusion, the effects of FK-506 on the cardiac AP are more complex than previously reported and include inhibitions of IK1 and ICaL. Alterations in Ca2+ release and INaCa may contribute to FK-506-induced AP prolongation and EADs in addition to the permissive role of ICaL facilitation at high rates of stimulation.
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Fauconnier et al. (2004) studied this question. FK-506 was evaluated on Action potential duration and ionic currents. FK-506 prolonged action potentials, induced early afterdepolarizations in 59% of cells, and inhibited IK1 and ICaL in rat ventricular cells.
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