Calcineurin-independent inhibition of K_v1.3 by FK-506 (tacrolimus): A novel pharmacological property

The interaction of FK-506 with K_v1.3, stably expressed in Chinese hamster ovary cells, was investigated with the whole cell patch-clamp technique. FK-506 inhibited K_v1.3 in a reversible, concentration-dependent manner with an IC₅₀ of 5.6 μM. Rapamycin, another immunosuppressant, produced effects that were similar to those of FK-506 (IC₅₀ = 6.7 μM). Other calcineurin inhibitors (cypermethrin or calcineurin autoinhibitory peptide) alone had no effect on the amplitude or kinetics of K_v1.3. In addition, the inhibitory action of FK-506 continued, even after the inhibition of calcineurin activity. The inhibition produced by FK-506 was voltage dependent, increasing in the voltage range for channel activation. At potentials positive to 0 mV (where maximal conductance is reached), however, no voltage-dependent inhibition was found. FK-506 exhibited a strong use-dependent inhibition of K_v1.3. FK-506 shifted the steady-state inactivation curves of K_v1.3 in the hyperpolarizing direction in a concentration-dependent manner. The apparent dissociation constant for FK-506 to inhibit K_v1.3 in the inactivated state was estimated from the concentration-dependent shift in the steady-state inactivation curve and was calculated to be 0.37 μM. Moreover, the rate of recovery from inactivation of K_v1.3 was decreased. In inside-out patches, FK-506 not only reduced the current amplitude but also accelerated the rate of inactivation during depolarization. FK-506 also inhibited K _v1.5 and K_v4.3 in a concentration-dependent manner with IC₅₀ of 4.6 and 53.9 μM, respectively. The present results indicate that FK-506 inhibits K_v1.3 directly and that this effect is not mediated via the inhibition of the phosphatase activity of calcineurin.