Contribution of estrogen to the pregnancy-induced increase in cardiac automaticity

Highlights

• Higher SAN automaticity in E₂-treated ERKOβ mice is explained by larger I_f current.

• N-hiPSC-CM data indicate that E₂ regulates I_f in nodal cells of human origin.

• E₂ is a major hormone involved in the increased SAN automaticity during pregnancy.

• This work provides novel insight into mechanisms of action of E₂ on SAN function.

Abstract

Background

The heart rate progressively increases throughout pregnancy, reaching a maximum in the third trimester. This elevated heart rate is also present in pregnant mice and is associated with accelerated automaticity, higher density of the pacemaker current I_f and changes in Ca²⁺ homeostasis in sinoatrial node (SAN) cells. Strong evidence has also been provided showing that 17β-estradiol (E₂) and estrogen receptor α (ERα) regulate heart rate. Accordingly, we sought to determine whether E₂ levels found in late pregnancy cause the increased cardiac automaticity associated with pregnancy.

Methods and results

Voltage- and current-clamp experiments were carried out on SAN cells isolated from female mice lacking estrogen receptor alpha (ERKOα) or beta (ERKOβ) receiving chronic E₂ treatment mimicking late pregnancy concentrations. E₂ treatment significantly increased the action potential rate (284 ± 24 bpm, +E₂ 354 ± 23 bpm, p = 0.040) and the density of I_f (+52%) in SAN cells from ERKOβ mice. However, I_f density remains unchanged in SAN cells from E₂-treated ERKOα mice. Additionally, E₂ also increased I_f density (+67%) in nodal-like human-induced pluripotent stem cell-derived cardiomyocytes (N-hiPSC-CM), recapitulating in a human SAN cell model the effect produced in mice. However, the L-type calcium current (I_CaL) and Ca²⁺ transients, examined using N-hiPSC-CM and SAN cells respectively, were not affected by E₂, indicating that other mechanisms contribute to changes observed in these parameters during pregnancy.

Conclusion

The accelerated SAN automaticity observed in E₂-treated ERKOβ mice is explained by an increased I_f density mediated by ERα, demonstrating that E₂ plays a major role in regulating SAN function during pregnancy.

Introduction

During pregnancy, there are several important changes in cardiovascular function, including an increased resting heart rate [[1], [2], [3]]. Although necessary for the development of the fetus, these changes can create an environment favorable to arrhythmogenesis. Pregnancy is indeed associated with an increased incidence of various types of supraventricular arrhythmias such as ectopic beats, symptomatic palpitations, reentry arrhythmias and supraventricular tachycardia (SVT) [4,5]. The latter is reported to be the most common arrhythmia during pregnancy [4,6,7]. In fact, pregnancy is associated with a significant increase in the new onset of SVT and exacerbation of pre-existing SVT episodes [7,8]. Previous studies have reported that the increased heart rate observed in pregnancy may be involved in these arrhythmic events [2,[4], [5], [6], [7],[9], [10], [11], [12], [13], [14]]. Moreover, therapeutic levels of drugs may be difficult to maintain as pregnancy tends to decrease the concentration of drugs due to an increase in the volume of distribution and the metabolism of drugs [4,7,8]. This may also contribute to explain why women previously stable on therapy have breakthrough arrhythmias during pregnancy. These arrhythmias can pose a significant risk to the mother and fetus, depending on the severity of the symptoms and the risk associated with potentially recurrent arrhythmias throughout pregnancy. Furthermore, drugs used to suppress arrhythmias can adversely affect the health of the unborn child [7,8]. For these reasons, a better understanding of the underlying mechanisms of the chronotropic effect of pregnancy is needed.

The spontaneous electrical activity of sinoatrial node (SAN) cells, called pacemaker activity, is responsible for the control of heart rate. The spontaneous diastolic depolarization phase of the SAN action potential plays a major role in the pacemaker activity [15]. A complex set of ionic mechanisms present in each SAN cell control the automaticity of the heart. The hyperpolarization-activated current I_f (also called pacemaker current) and the L-type Ca²⁺ current (I_CaL) as well as the intracellular Ca²⁺ cycling mechanisms are among the most important ionic mechanisms that govern the diastolic depolarization of SAN cells [[15], [16], [17]].

Previously, we have reported that, as in women, pregnant mice have an increased heart rate and a greater vulnerability to supraventricular tachycardias [18,19]. Moreover, we discovered that pregnancy increased the density of I_f and I_CaL in SAN cells and also upregulates RyR2 expression, resulting in faster Ca²⁺ transient rate [18,19]. Together, these changes contribute to maintain an elevated heart rate during pregnancy.

17β-estradiol (E₂) is the most potent estrogen and is involved in many physiological processes. It is also the predominant estrogen during pregnancy [20]. Its levels gradually increase during pregnancy, peak during the last trimester, and decrease soon after delivery. It is therefore conceivable that the significant increase observed during pregnancy (up to 10 times) [19,21] could affect the heart rate [22]. In fact, recently, we provided strong evidence supporting a role for E₂ in the pregnancy-induced increased heart rate [19]. We first found that heart rate returns to control values shortly after delivery, which coincides with a significant decline in E₂ levels. Moreover, we showed that chronic administration of E₂ at levels found in late pregnancy accelerated heart rate in non-pregnant mice, as observed in pregnant mice [18]. In addition, using estrogen receptor α or β knockout (ERKOα and ERKOβ) female mice, we established that the estrogen receptor α (ERα) was involved in heart rate control. Accordingly, in this study, we sought to determine whether the changes observed during pregnancy on I_f and Ca²⁺ homeostasis were mediated by E₂ via ERα signaling.