Effects of estradiol on cardiac ion channel currents

Abstract

Steroids are known to exert direct and indirect effects on cardiovascular functions, and women have been found to be more susceptible to QT prolongation than men. Although many clinical studies have been performed, the effects of steroids on cardiac repolarization are not yet fully understood. We examined the effects of 17-beta-estradiol (estradiol) on the major cardiac currents that are correlated to clinical observations of arrhythmias. Effects on the two major currents responsible for repolarization of the cardiac action potential (mediated by the human ether-à-gogo related gene (HERG) product), and by the potassium channel Q1 (KCNQ1) co-expressed with the potassium channel accessory subunit E1 (KCNE1) were examined, as well as effects on the sodium inward current (mediated by the sodium channel 5A (SCN5A) and generating the rapid upstroke of the action potential). A concentration-dependent effect of estradiol on the KCNQ1/KCNE1-mediated potassium current was observed. The half-maximal inhibition concentration (IC₅₀) of estradiol on the KCNQ1/KCNE1 ion channel was calculated to 1.13 ± 0.23 μM. The HERG-mediated potassium and the SCN5A-mediated sodium currents, however, were only slightly reduced by estradiol at concentrations of up to 30 μM. This suggests that alterations of the cardiac action potentials by steroids may be mediated by interaction with the KCNQ1/KCNE1 ion channel.

Introduction

Torsades de pointes is a type of life-threatening arrhythmia, preceded by a markedly prolonged QT interval in the electrocardiagram. Women have been found to be more susceptible to developing this kind of ventricular tachycardia than men (Makkar et al., 1993, Kawasaki et al., 1995). A role of steroids, particularly of estrogens, has therefore been proposed to contribute to the prolonged repolarization phase observed in women. However, in spite of a greater sensitivity of women to QT-prolonging drugs (Rodriguez et al., 2001), an effect of estradiol on the QT interval could not be clinically shown (Hulot et al., 2003, Nowinski et al., 2002). The higher risk of females to develop torsades de pointes is not yet fully understood.

The action potential of the human heart is governed by the flux of sodium, calcium and potassium ions through a variety of ion channels of different biophysical properties (Netzer et al., 2003). The currents through the human ether-à-gogo related gene (HERG) product and the potassium channel Q1 (KCNQ1) co-expressed with potassium channel accessory subunit E1 (KCNE1) coded ion channels are of major importance for the duration of the repolarization phase, and thereby for the length of the QT interval. While HERG forms the alpha subunit of the rapidly activating I_Kr channel, a coassembly of KCNQ1 and KCNE1 forms the slowly activating I_Ks channel. Malfunction of one or both of these ion channels can lead to a delayed repolarization and thus to a prolonged QT interval. Such a prolonged QT interval has been observed as a congenital form (arising from mutations in KCNQ1 (denoted LQT1), KCNE1 (LQT5), or HERG (LQT2)), and as a drug-induced form caused by compounds interacting with one of these channels (reviewed by, e.g., Ackerman and Clapham, 1997, Keating and Sanguinetti, 2001, Roden, 2004). So far, nearly all cases of drug-induced QT prolongation have been correlated to a blockade of the HERG potassium channel (reviewed by, e.g., Roden, 2004). It has been discussed in the literature, whether HERG requires beta-subunits (like KCNE2) to form the native I_Kr channel (Abbott et al., 1999). A detailed analysis performed by Weerapura et al. (2002) showed that the biophysical properties of HERG were altered when it was co-expressed with KCNE2. These alterations were shown to not affect the physiologically relevant outward current of the ion channel. Also the pharmacological effects of compounds on HERG remained unaltered when HERG was co-expressed with KCNE2 (Weerapura et al., 2002).

The SCN5A-mediated sodium current gives rise to the rapid upstroke at the beginning of the action potential. Malfunction of the SCN5A ion current inactivation has particularly been observed in a congenital form and has also been correlated to a prolonged QT interval (denoted LQT3) and clinical observations of cardiac arrhythmias.

To elucidate gender differences in the cardiac repolarization phase, we examined the effects of estradiol on the two major ion channels responsible for terminating the action potential of the human heart, and on the SCN5A mediated sodium current.