Mechanism of fluoxetine block of cloned voltage-activated potassium channel Kv1.3

The effects of fluoxetine (Prozac), a widely used antidepressant drug, on K v1.3 stably expressed in Chinese hamster ovary cells were examined using th e whole-cell and excised inside-out configurations of the patch-clamp techn ique. In whole-cell recordings, fluoxetine accelerated the decay rate of in activation of Kv1.3 and thus decreased the current amplitude at the end of the pulse in a concentration-dependent manner with an IC50 value of 5.9 mu M. The inhibition displayed a weak voltage dependence, increasing at more p ositive potentials. Neither the activation nor the steady-state inactivatio n curve was affected by fluoxetine. In addition, fluoxetine reduced the tai l current amplitude and slowed the deactivation of the tail current, result ing in a crossover phenomenon. When applied to the internal side of the mem brane in inside-out recordings, the inhibition by fluoxetine was much faste r and more potent with an IC50 value of 1.7 mu M compared with whole-cell r ecordings. Norfluoxetine, the major metabolite of fluoxetine, also inhibite d Kv1.3 in a concentration-dependent manner (IC50 5 1.4 mu M) in whole-cell recordings. To check whether the fluoxetine-induced inhibition demonstrate d in cloned Kv1.3 could also be observed in native T lymphocytes, the effec ts of fluoxetine were investigated on human T lymphocytes. Fluoxetine also inhibited outward K+ current in human T lymphocytes. Our results indicate t hat fluoxetine produced a concentration- and voltage-dependent inhibition o f Kv1.3 that can be interpreted as an open channel block and that a binding site for fluoxetine is more accessible from the intracellular side.