The effects of over-expression of the FK506-binding protein FKBP12.6 on K+ currents in adult rabbit ventricular myocytes

This study examines the effects of the intracellular protein FKBP12.6 on action potential and associated K⁺ currents in isolated adult rabbit ventricular cardiomyocytes. FKBP12.6 was over-expressed by ~6 times using a recombinant adenovirus coding for human FKBP12.6. This over-expression caused prolongation of action potential duration (APD) by ~30%. The amplitude of the transient outward current (I _to) was unchanged, but rate of inactivation at potentials positive to +40 mV was increased. FKBP12.6 over-expression decreased the amplitude of the inward rectifier current (I _K1) by ~25% in the voltage range −70 to −30 mV, an effect prevented by FK506 or lowering intracellular [Ca²⁺] below 1 nM. Over-expression of an FKBP12.6 mutant, which cannot bind calcineurin, prolonged APD and affected I _to and I _K1 in a similar manner to wild-type protein. These data suggest that FKBP12.6 can modulate APD via changes in I _K1 independently of calcineurin binding, suggesting that FKBP12.6 may affect APD by direct interaction with I _K1.