Mécanismes électrophysiologiques responsables de l'augmentation de la fréquence cardiaque induite par les œstrogènes lors de la grossesse

An increased heart rate (HR) is observed in pregnant women. In fact, in the last trimester, in average, the HR increases by 15%, which is a known risk factor to developing cardiac arrhythmias or exacerbating pre-existing arrhythmias. This can lead to major consequences for both the mother and fetus. However, the mechanisms underlying this increased HR remain largely unexplored. Our laboratory recently demonstrate that pregnancy is associated with an increased density of the pacemaker current (If) and the L-type calcium current (ICaL) as well as changes in calcium homeostasis of mouse sinoatrial node (SAN) cells. Knowing that estrogens are increased during pregnancy and that these sex hormones can modify cardiac electrophysiological properties, we hypothesized that estrogens play a key role in the pregnancy-induced increased HR and regulate the SAN electrophysiological properties. Our research project aims to determine the role of 17β-estradiol (E2) on the pregnancy-induced increased HR, to determine if these effects are regulated through estrogen receptor alpha (ERα) and/or beta (ERβ) and to study the E2 underlying mechanisms on SAN electrophysiology. Non-pregnant female mice (2-4 months) lacking ERα (ERKOα) or ERβ (ERKOβ) received a chronic E2 treatment (30 μg twice daily for four days) mimicking E2 concentrations found in late pregnancy (23.3 ± 5.0 nM). Surface electrocardiogram analysis showed a significant increased HR in ERKOβ mice (ERKOβ: 511 ± 15 bpm; ERKOβ +E2: 580 ± 10 bpm; n = 10; p<0.001) following E2 administration. However, the HR remains unchanged in ERKOα mice (ERKOα: 520 ± 16 bpm; ERKOα +E2: 530 ± 21 bpm, n = 7, p = 0.114). Following E2 treatment, current-clamp method demonstrates an increase SAN cells automaticity in ERKOβ mice, resulting in an increase in the spontaneous action potential frequency (ERKOβ : 284 ± 24 bpm, n = 8; ERKOβ +E2 : 354 ± 23 bpm, n = 15, p = 0.0395), associated with a steeper diastolic depolarization slope (ERKOβ : 82 ± 12 mV/s, n = 8; ERKOβ +E2 : 140 ± 14 mV/s, n = 15, p < 0.003), a major determinant of cardiac automaticity. In line with these results, voltage-clamp data showed an increased If density in SAN cells of ERKOβ mice treated with E2 (at -90 mV: ERKOβ: -6.6 ± 0.7 pA/pF, n = 12-15; ERKOβ +E2: -11.0 ± 1.3 pA/pF, n = 9-11, p < 0.05). Nevertheless, If density was similar in E2-treated ERKOα mice. E2-treated nodal-like human-induced pluripotent stem cell-derived cardiomyocytes (N-hiPSC-CM) also showed an increased spontaneous action potential frequency (CTL : 69 ± 5 bpm, n = 12; +E2 : 99 ± 6 bpm, n = 14, p < 0.001) and If density (at -90 mV: CTL: -0.95 ± 0.14 pA/pF, n = 7-10; +E2: -1.62 ± 0.17 pA/pF, n = 13-14, p < 0.05). Following E2 administration, the rate of calcium transient was similar in SAN cells from ERKOα (139 ± 15, n = 13-14; +E2 : 142 ± 14, n = 15-16, p = ns) and ERKOβ (142 ± 11, n = 14-15; +E2 : 147 ± 13, n = 15-16, p = ns) mice. In line with these results, no modification was seen on ICaL density in E2-treated N-hiPSC-CM (at 0 mV: CTL: -14.0 ± 1.3 pA/pF, n = 12-13; +E2: -14.5 ± 1.4 pA/pF, n = 22, p = ns). In conclusion, the increased cardiac automaticity observed during pregnancy is, in part, explained by an increased If density. This mechanism is mediated by the E2-ERα pathway. In the other hand, calcium homeostasis changes detected during pregnancy appear to be mediated by an E2-independent mechanism. Finally, results obtained on N-hiPSC-CM are consistent with our observations on mouse SAN cells, demonstrating the human applicability of our results. This study provides novel insight on the effects of female sex hormones on the SAN functions. Ultimately, this information can lead to improved management of arrhythmias associated with female hormone fluctuations and/or pregnancy-induced arrhythmias.

Keyphrases: Pregnancy, Sinoatrial node, Cardiac automaticity, Estrogens, Estrogen receptor, Pacemaker current, L-type calcium current, Calcium homeostasis, Calcium transient, Nodal-like human-induced pluripotent stem cell-derived cardiomyocytes