Frequency-dependent and proarrhythmogenic effects of FK-506 in rat ventricular

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 mumol/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 (I-NaCa)]. In indo-1-loaded cells, FK-506 generated doublets of Ca2+ transients associated with increased diastolic Ca2+ in one-half of the cells. FK-506 reversibly decreased the L-type Ca2+ current (I-CaL) by 25%, although high-frequency-dependent facilitation of ICaL persisted, and decreased three distinct K+ currents: delayed rectifier K+ current ( I-K; > 80%), transient outward K+ current ( < 20%), and inward rectifier K+ current (I-K1; > 40%). A shift in the reversal potential of I-K1 (- 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 I-CaL 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 I-K1 and I-CaL. Alterations in Ca2+ release and I-NaCa may contribute to FK-506-induced AP prolongation and EADs in addition to the permissive role of I-CaL facilitation at high rates of stimulation.