506 TACROLIMUS

Calcineurin-independent inhibition of Kv1.3 by FK-506 (Tacrolimus): A novel phar HS Ahn, SE Kim, BH Choi, JS Choi, MJ Kim, DJ Rhie, SH Yoon, YH Jo, MS Kim, KW Sung, OJ Kwon, SJ Hahn The interaction of FK-506 with Kv1.3, stably expressed in CHO cells, was investigated using the whole-cell patch-clamp technique. FK-506 inhibited Kv1.3 in a reversible, concentration-dependent manner with an IC50 of 5.6 microM. Rapamycin, another immunosuppressant, produced effects that were similar to those of FK-506 (IC50 = 6.7 microM). Other calcineurin inhibitors (cypermethrin or calcineurin autoinhibitory peptide) alone had no effect on the amplitude or kinetics of Kv1.3. In addition, the inhibitory action of FK-506 continued, even after the inhibition of the calcineurin activity. The inhibition produced by FK-506 was voltage-dependent, increasing in the voltage range for channel activation. At potential 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 Kv1.3. FK-506 shifted the steady-state inactivation curves of Kv1.3 in the hyperpolarizing direction in a concentration-dependent manner. The apparent dissociation constant (Ki) for FK-506 to inhibit Kv1.3 in the inactivated state was estimated from the concentration-dependent shift in the steady-state inactivation curve and was calculated to be 0.37 microM. Moreover, the rate of recovery from inactivation of Kv1.3 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 Kv1.5 and Kv4.3 in a concentration-dependent manner with an IC50 of 4.6 and 53.9 microM, respectively. The present results indicate that FK-506 inhibits Kv1.3 directly and that this effect is not mediated via the inhibition of the phosphatase activity of calcineurin. Key words: potassium channel, calcineurin inhibitor, immumosuppressant.