Abstract

Tamoxifen, an estrogen receptor antagonist used in the treatment of breast cancer, inhibits the inward rectifier potassium current (I_K1) in cardiac myocytes by an unknown mechanism. We characterized the inhibitory effects of tamoxifen on Kir2.1, Kir2.2, and Kir2.3 potassium channels that underlie cardiac I_K1. We also studied the effects of 4-hydroxytamoxifen and raloxifene. All three drugs inhibited inward rectifier K⁺ 2.x (Kir2.x) family members. The order of inhibition for all three drugs was Kir2.3 > Kir2.1 ∼ Kir2.2. The onset of inhibition of Kir2.x current by these compounds was slow (T_1/2 ∼ 6 min) and only partially recovered after washout (∼30%). Kir2.x inhibition was concentration-dependent but voltage-independent. The time course and degree of inhibition was independent of external or internal drug application. We tested the hypothesis that tamoxifen interferes with the interaction between the channel and the membrane-delimited channel activator, phosphatidylinositol 4,5-bisphosphate (PIP₂). Inhibition of Kir2.3 currents was significantly reduced by a single point mutation of I213L, which enhances Kir2.3 interaction with membrane PIP₂. Pretreatment with PIP₂ significantly decreased the inhibition induced by tamoxifen, 4-hydroxytamoxifen, and raloxifene on Kir2.3 channels. Pretreatment with spermine (100 μM) decreased the inhibitory effect of tamoxifen on Kir2.1, probably by strengthening the channel's interaction with PIP₂. In cat atrial and ventricular myocytes, 3 μM tamoxifen inhibited I_K1, but the effect was greater in the former than the latter. The data strongly suggest that tamoxifen, its metabolite, and the estrogen receptor inhibitor raloxifene inhibit Kir2.x channels indirectly by interfering with the interaction between the channel and PIP₂.