A single-cell model of phase-driven control of ventricular fibrillation frequency

Inward rectifier potassium channels control rotor frequency in ventricular fibrillation

Ventricular fibrillation (VF) is the most important cause of sudden cardiac death. While traditionally thought to result from random activation of the ventricles by multiple independent wavelets, recent evidence suggests that VF may be determined by the sustained activation of a relatively small number of reentrant sources. In addition, recent experimental data in various species as well as computer simulations have provided important clues about its ionic and molecular mechanisms, particularly in regards to the role of potassium currents in such mechanisms. The results strongly argue that the inward rectifier current, I(K1,) is an important current during functional reentry because it mediates the electrotonic interactions between the unexcited core and its immediate surroundings. In addition, I(K1) is a stabilizer of reentry due to its ability to shorten action potential duration and reduce conduction velocity near the center of rotation. Increased I(K1) prevents wave front-wave tail interactions and thus averts rotor destabilization and breakup. Other studies have shown that while the slow component of the delayed rectifier potassium current I(Ks) does not significantly modify rotor frequency or stability, it plays a major role in postrepolarization refractoriness and wave break formation. Therefore, the interplay between I(K1) and the rapid sodium inward current (I(Na)) is a major factor in the control of cardiac excitability and thus the stability and frequency of reentry, while I(Ks) is an important determinant of fibrillatory conduction.

Tamoxifen inhibits inward rectifier K+ 2.x family of inward rectifier channels by interfering with phosphatidylinositol 4,5-bisphosphate-channel interactions

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 approximately Kir2.2. The onset of inhibition of Kir2.x current by these compounds was slow (T(1/2) approximately 6 min) and only partially recovered after washout ( approximately 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 (PIP(2)). Inhibition of Kir2.3 currents was significantly reduced by a single point mutation of I213L, which enhances Kir2.3 interaction with membrane PIP(2). Pretreatment with PIP(2) significantly decreased the inhibition induced by tamoxifen, 4-hydroxytamoxifen, and raloxifene on Kir2.3 channels. Pretreatment with spermine (100 microM) decreased the inhibitory effect of tamoxifen on Kir2.1, probably by strengthening the channel's interaction with PIP(2). In cat atrial and ventricular myocytes, 3 microM 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 PIP(2).