Search

Males and females exhibit intrinsic differences in the structure and function of the heart, while the prevalence and severity of cardiovascular disease vary in the two sexes. However, the mechanisms of this sex-based dimorphism are yet to be elucidated. Sex chromosomes and sex hormones are the main contributors to sex-based differences in cardiac physiology and pathophysiology. In recent years, the advances in induced pluripotent stem cell-derived cardiac models and multi-omic approaches have enabled a more comprehensive understanding of the sex-specific differences in the human heart. Here, we provide an overview of the roles of these two factors throughout cardiac development and explore the sex hormone signaling pathways involved. We will also discuss how the employment of stem cell-based cardiac models and single-cell RNA sequencing help us further investigate sex differences in healthy and diseased hearts. [ABSTRACT FROM AUTHOR]

Unlike artificial nanosystems, biological systems are ideally engineered to respond to their environment. As such, natural molecular buffers ensure precise and quantitative delivery of specific molecules through self-regulated mechanisms based on Le Chatelier's principle. Here, we apply this principle to design self-regulated nucleic acid molecular buffers for the chemotherapeutic drug doxorubicin and the antimalarial agent quinine. We show that these aptamer-based buffers can be programmed to maintain any specific desired concentration of free drug both in vitro and in vivo and enable the optimization of the chemical stability, partition coefficient, pharmacokinetics and biodistribution of the drug. These programmable buffers can be built from any polymer and should improve patient therapeutic outcome by enhancing drug activity and minimizing adverse effects and dosage frequency. Natural buffer molecules ensure the controlled and precise delivery of specific molecules in biology. Here, the authors replicate the natural buffer systems using aptamers with controlled binding efficiency to control and maintain the levels of free drugs both in vitro and in vivo. [ABSTRACT FROM AUTHOR]

Male sex is one of the most important risk factors of atrial fibrillation (AF), with the incidence in men being almost double that in women. However, the reasons for this sex difference are unknown. Accordingly, in this study, we sought to determine whether there are sex differences in intracellular Ca2+ homeostasis in mouse atrial myocytes that might help explain male predisposition to AF. AF susceptibility was assessed in male (M) and female (F) mice (4–5 months old) using programmed electrical stimulation (EPS) protocols. Males were 50% more likely to develop AF. The Ca2+ transient amplitude was 28% higher in male atrial myocytes. Spontaneous systolic and diastolic Ca2+ releases, which are known sources of triggered activity, were significantly more frequent in males than females. The time to 90% decay of Ca2+ transient was faster in males. Males had 54% higher Na+-Ca2+ exchanger (NCX1) current density, and its expression was also more abundant. L-type Ca2+ current (ICaL) was recorded with and without BAPTA, a Ca2+ chelator. ICaL density was lower in males only in the absence of BAPTA, suggesting stronger Ca2+-dependent inactivation in males. CaV1.2 expression was similar between sexes. This study reports major sex differences in Ca2+ homeostasis in mouse atria, with larger Ca2+ transients and enhanced NCX1 function and expression in males resulting in more spontaneous Ca2+ releases. These sex differences may contribute to male susceptibility to AF by promoting triggered activity. [ABSTRACT FROM AUTHOR]

Men have a higher risk of developing atrial fibrillation (AF) than women, though the reason for this is unknown. Here, we compared atrial electrical and structural properties in male and female mice and explored the contribution of sex hormones. Cellular electrophysiological studies revealed that action potential configuration, Na+ and K+ currents were similar in atrial myocytes from male and female mice (4–5 months). Immunofluorescence showed that male atrial myocytes had more lateralization of connexins 40 (63 ± 4%) and 43 (66 ± 4%) than females (Cx40: 45 ± 4%, p = 0.006; Cx43: 44 ± 4%, p = 0.002), with no difference in mRNA expression. Atrial mass was significantly higher in males. Atrial myocyte dimensions were also larger in males. Atrial fibrosis was low and similar between sexes. Orchiectomy (ORC) abolished sex differences in AF susceptibility (M: 65%; ORC: 38%, p = 0.050) by reducing connexin lateralization and myocyte dimensions. Ovariectomy (OVX) did not influence AF susceptibility (F: 42%; OVX: 33%). This study shows that prior to the development of age-related remodeling, male mice have more connexin lateralization and larger atria and atrial myocyte than females. Orchiectomy reduced AF susceptibility in males by decreasing connexin lateralization and atrial myocyte size, supporting a role for androgens. These sex differences in AF substrates may contribute to male predisposition to AF. [ABSTRACT FROM AUTHOR]

The sinus node (SAN) is the primary pacemaker of the human heart, and abnormalities in its structure or function cause sick sinus syndrome, the most common reason for electronic pacemaker implantation. Here we report that transcription factor GATA6, whose mutations in humans are linked to arrhythmia, is highly expressed in the SAN and its haploinsufficiency in mice results in hypoplastic SANs and rhythm abnormalities. Cell-specific deletion reveals a requirement for GATA6 in various SAN lineages. Mechanistically, GATA6 directly activates key regulators of the SAN genetic program in conduction and nonconduction cells, such as TBX3 and EDN1, respectively. The data identify GATA6 as an important regulator of the SAN and provide a molecular basis for understanding the conduction abnormalities associated with GATA6 mutations in humans. They also suggest that GATA6 may be a potential modifier of the cardiac pacemaker. [ABSTRACT FROM AUTHOR]

Aims During pregnancy, there is a significant increase in heart rate (HR) potentially associated with an increased risk of arrhythmias or exacerbation of pre-existing cardiac conditions endangering both mother and foetus. Calcium homeostasis plays an important role in regulating automaticity of the sinoatrial node (SAN); however, its contribution to the accelerated HR during pregnancy remains unknown. Methods and results Using murine SAN cells, we showed that pregnancy increased L-type Ca2+ current (I CaL) and CaV1.3 mRNA expression, whereas T-type Ca2+ current (I CaT) and its underlying channel were unchanged. Analysis of SAN intra-cellular Ca2+ oscillations showed that the rate of spontaneous Ca2+ transients was significantly higher in pregnant mice along with a higher mRNA expression of ryanodine receptor. Assessment of supra-ventricular arrhythmias using programmed electrical stimulation protocols on anaesthetized mice revealed higher susceptibility in pregnancy. Of note, the modifications associated with pregnancy were reversible following delivery. Furthermore, chronic administration of 17β-estradiol (E2) to nodal-like human-induced pluripotent stem cell-derived cardiomyocytes (N-hiPSC-CM), control mice, oestrogen-receptor-β knockout (ERKOβ) but not ERKOα mice, accelerated cardiac automaticity, recapitulating the pregnancy phenotype in both mouse and human SAN cell models. Conclusion Together, these results indicate that pregnancy considerably alters intra-cellular Ca2+ homeostasis sustaining faster HR during pregnancy. Importantly, these changes were dependent on an oestrogen receptor α (ERα) mechanism that resulted in increased I CaL and spontaneous Ca2+ release from the sarcoplasmic reticulum, highlighting a novel role for oestrogen in regulating HR. [ABSTRACT FROM AUTHOR]

The Role of KChIP2 in Heart Failure Introduction KV4 together with KV Channel-Interacting Protein 2 (KChIP2) mediate the fast recovering transient outward potassium current ( Ito,f) in the heart. KChIP2 is downregulated in human heart failure (HF), potentially underlying the loss of Ito,f. We investigated remodeling associated with HF hypothesizing that KChIP2 plays a central role in the modulation of outward K+ currents in HF. Methods and Results HF was induced by aortic banding in wild-type (WT) and KChIP2 deficient (KChIP2-/-) mice, evaluated by echocardiography. Action potentials were measured by floating microelectrodes in intact hearts. Ventricular cardiomyocytes were isolated and whole-cell currents were recorded by patch clamp. Left ventricular action potentials in KChIP2-/- mice were prolonged in a rate dependent manner, consistent with patch-clamp data showing loss of a fast recovering outward K+ current and upregulation of the slow recovering Ito,s and IKur. HF decreased all outward K+ currents in WT mice and did not change the relative contribution of Ito,f in WT mice. Compared to WT HF, KChIP2-/- HF had a larger reduction of K+-current density. However, the relative APD prolongation caused by HF was shorter for KChIP2-/- compared with WT, and the APs of the 2 HF mouse types were indistinguishable. Conclusion Ito,f is just one of many K+ currents being downregulated in murine HF. The downregulation of repolarizing currents in HF is accentuated in KChIP2-/- mice. However, the prolongation of APs associated with HF is less in KChIP2-/- compared to WT, suggesting other compensatory mechanism(s) in the KChIP2-/- mouse. [ABSTRACT FROM AUTHOR]

Transgenic mice that overexpress human type 1 angiotensin II receptor (AT1R) in the heart develop cardiac hypertrophy. Previously, we have shown that in 6-mo AT1R> mice, which exhibit significant cardiac remodeling, fractional shortening is decreased. However, it is not clear whether altered contractility is attributable to AT1R> overexpression or is secondary to cardiac hypertrophy/remodeling. Thus the present study characterized the effects of AT1R> overexpression on ventricular L-type Ca2+ currents (ICaL), cell shortening, and Ca2+ handling in 50-day and 6-mo-old male AT1R> mice. Echocardiography showed there was no evidence of cardiac hypertrophy in 50-day AT1R> mice but that fractional shortening was decreased. Cellular experiments showed that cell shortening, ICaL, and Cav1.2 mRNA expression were significantly reduced in 50-day and 6-mo-old AT1R> mice compared with controls. In addition, Ca2+ transients and caffeine-induced Ca2+ transients were reduced whereas the time to 90% Ca2+ transient decay was prolonged in both age groups of AT1R> mice. Western blot analysis revealed that sarcoplasmic reticulum Ca2+-ATPase and Na+/Ca2+ exchanger protein expression was significantly decreased in 50-day and 6-mo AT1R> mice. Overall, the data show that cardiac contractility and the mechanisms that underlie excitation-contraction coupling are altered in AT1R> mice. Furthermore, since the alterations in contractility occur before the development of cardiac hypertrophy, it is likely that these changes are attributable to the increased activity of the renin-angiotensin system brought about by AT1R> overexpression. Thus it is possible that AT1R> blockade may help maintain cardiac contractility in individuals with heart disease. [ABSTRACT FROM AUTHOR]

Aims Tamoxifen is a selective oestrogen receptor modulator widely used in the prevention and treatment of breast cancer. Women receiving long-term tamoxifen therapy do not experience cardiac arrhythmias although acute perfusion of tamoxifen has been shown to inhibit cardiac K+ currents. This observation suggests that chronic tamoxifen treatment does not negatively modulate cardiac K+ currents. Therefore, we investigated the chronic effects of tamoxifen on K+ currents and channels in mouse and guinea pig ventricles. Methods and results Female mice and guinea pigs were treated with placebo or tamoxifen pellets for 60 days. Voltage-clamp experiments showed that the density of the Ca2+-independent transient outward (Ito), the ultrarapid delayed rectifier (IKur), the steady-state (Iss), and the inward rectifier (IK1) K+ currents were increased in tamoxifen-treated mice ventricle. Western blot analysis revealed that protein expression of the underlying K+ channels Kv4.3 (Ito), Kv1.5 (IKur), Kv2.1 (Iss), and Kir2.1 (IK1) were significantly higher in the ventricle of tamoxifen-treated mice. Protein expression of the K+ channel subunits encoding IKr and IKs (ERG1, KCNQ1, and KCNE1) was also increased in tamoxifen-treated guinea pig ventricle. Conclusion Conditions with high oestrogen levels are associated with reduced K+ currents. Thus, conceivably, tamoxifen might prevent the inhibitory effects of oestrogen on K+ channels by blocking the oestrogen receptors, which would explain the reported increase in K+ currents. These findings could contribute to explain the absence of cardiac arrhythmia with long-term tamoxifen therapy. [ABSTRACT FROM AUTHOR]

Effect of HIV on Cardiac Sodium Current. Introduction: Cardiac arrhythmias have been reported in AIDS patients. Arrhythmias can arise from alterations in ventricular Na+ channel function. However, it is unknown whether HIV affects cardiac Na+ channel function. Therefore, the purpose of this study was to characterize the effect of HIV on ventricular Na+ current (INa) in a transgenic model of HIV (CD4C/HIV mice), which exhibit a severe AIDS-like disease. Methods and Results: Patch-clamp techniques were used to examine INa and action potentials (AP) in ventricular myocytes isolated from HIV and wild-type (WT) mice. In HIV myocytes peak INa was reduced (at −50 mV: HIV, −55.3 ± 4.3 pA/pF, n = 15; WT, −79.4 ± 5.2 pA/pF, n = 16, P < 0.05), whereas late INa was similar in both groups (HIV, −4.3 ± 0.4 pA/pF; WT, −4.4 ± 0.4 pA/pF, n = 22/group). AP amplitude (HIV 91.5 ± 4.7 mV, n = 12; WT 104.4 ± 3.1 mV, n = 15, P < 0.05) and the maximal velocity of the AP upstroke (Vmax; HIV, 57.2 ± 9.3 mV/ms, n = 12; WT, 113.5 ± 8 mV/ms, n = 15, P < 0.05) were decreased in HIV myocytes. ECG recordings revealed that the QRS complex was prolonged in HIV mice (HIV, 15.7 ± 0.2 ms, n = 22; WT, 14.1 ± 0.5 ms, n = 10, P < 0.05). The serum levels of interleukin-1β were elevated in HIV mice (HIV, 18.1 ± 3.1 pg/mL, n = 3; WT, 5.1 ± 1.1 pg/mL, n = 4, P < 0.05) in line with previous evidence that suggests that elevated levels of cytokines can affect cardiac ion currents. Conclusion: Overall, our observations suggest that elevated levels of proinflammatory cytokines in CD4C/HIV mice could alter Na+ channel function, thus altering cardiac depolarization and contribute to the generation of arrhythmias. (J Cardiovasc Electrophysiol, Vol. 21, pp. 916-922, August 2010) [ABSTRACT FROM AUTHOR]

Cardiac-specific overexpression of wild-type a w-adrenergic receptors (α1B-AR) in mice predisposes to dilated cardiomyopathy and sudden death. Although α-adrenergic stimulation is thought to contribute to induction of α1-rhythmias in heart failure, the electrophysiological consequences of chronic at-adrenergic activation have not been clearly defined. Thus we characterized ventricular repolarization and monitored incidence of spontaneous arrhythmias in end-stage heart failure α1B-AR mice (9-12 mo) and younger α1B-AR mice (2-3 mo) that do not present signs of heart failure. Compared with agedmatched controls, the corrected QT interval was 34% longer in the 9to 12-mo α1B-AR mice, and the action potential durations were also significantly prolonged in these mice. These changes were associated with a decrease in the density of the outward K+ currents, Ca2+-independent transient, ultrarapid delayed rectifier, and steady state (at +30 mV, reduction of 68, 64, and 41%, respectively), and underlying K+ channel expression. Electrocardiogram (ECG) recordings revealed that older α1B-AR mice exhibited spontaneous ventricular arrhythmias. The alterations in repolarization can contribute to these rhythm abnormalities and are likely caused by chronic α1B-AR activity. Additional data obtained in 2- to 3-mo α1B-AR mice clearly showed that electrical remodeling was already observed in younger transgenic animals. However, it appeared to be slightly less pronounced than in older mice. These results suggest that there are two waves of remodeling: one due to chronic α1B-AR activity, and a second due to heart failure. Taken together, these data provide strong evidence for a pathological role of chronic α1B-AR activity in the development of repolarization defects and ventricular arrhythmias. [ABSTRACT FROM AUTHOR]

The effects of sex hormones on the heart are dependent on the presence and distribution of sex steroid hormone receptors (SSHR) in cardiac tissue. This study used subcellular fractionation, Western blot analysis and densitometry to characterize the subcellular distribution and abundance of estrogen receptor (ER) α, ERβ and androgen receptor (AR) in atrial and ventricular tissue from male and female mice. The results showed that in both atrial and ventricular tissue ERα was primarily found in the sarcolemma, whereas ERβ and AR were predominantly located in the nucleus and cytosol. Interestingly, ERα expression was greater in the ventricles compared to the atria, whereas ERβ and AR expression were similar in both heart chambers. Furthermore, the distribution and abundance of SSHR in the atria and ventricles did not differ between sexes. This study also showed that a reduction in hormone levels (as a result of ovariectomy) resulted in a significant increase in the abundance of ERα in the ventricular sarcolemmal fraction. Overall, the results suggest ERα, ERβ and AR distribution and expression are not sex dependent in the mouse heart. However, it appears that ERα expression is chamber specific and that, in certain cases, hormone levels can modulate the subcellular location of SSHRs. Copyright © 2009 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

Aims: Mice with cardiac-specific overexpression of human angiotensin II type 1 receptor (AT1R) undergo cardiac remodelling and die prematurely of sudden death. Since excessive QT prolongation is a major risk factor for ventricular arrhythmias and sudden death, we hypothesize that chronic stimulation of AT1R might contribute to sudden death by promoting delayed repolarization and ventricular arrhythmias. [ABSTRACT FROM PUBLISHER]

To better understand the mechanisms that underlie cardiac repolarization abnormalities in the immature heart, this study characterized and compared K+ currents in mouse ventricular myocytes from day 1, day 7, day 20, and adult CD1 mice to determine the effects of postnatal development on ventricular repolarization. Current- and patch-clamp techniques were used to examine action potentials and the K+ currents underlying repolarization in isolated myocytes. RT-PCR was used to quantify mRNA expression for the K+ channels of interest. This study found that action potential duration (APD) decreased as age increased, with the shortest APDs observed in adult myocytes. This study also showed that K+ currents and the mRNA relative abundance for the various K+ channels were significantly greater in adult myocytes compared with day 1 myocytes. Examination of the individual components of total K+ current revealed that the inward rectifier K+ current (IK1) developed by day 7, both the Ca2+-independent transient outward current (Ito) and the steady-state outward K+ current (Iss) developed by day 20, and the ultrarapid delayed rectifier K+ current (IKur) did not fully develop until the mouse reached maturity. Interestingly, the increase in IKur was not associated with a decrease in APD. Comparison of atrial and ventricular K+ currents showed that Ito and IKur density were significantly greater in day 7, day 20, and adult myocytes compared with age-matched atrial cells. Overall, it appears that, in mouse ventricle, developmental changes in APD are likely attributable to increases in Ito, Iss, and IK1, whereas the role of IKur during postnatal development appears to be less critical to APD. [ABSTRACT FROM AUTHOR]

Abstract: Objective: Gender differences in mouse cardiac repolarization have been reported to be due to the stimulatory action of androgens on the ultrarapid delayed rectifier K+ current (IKur) and its underlying Kv1.5 channel. To confirm the regulation of ventricular repolarization by androgens, the present study compared two strains of mice (CD-1 and C57BL/6) that present different androgen levels. Methods and results: Measurement of testosterone levels in different strains of mice (CD-1, C57BL/6, C3H and FVB) revealed that male C57BL/6 mice had very low levels of testosterone, whereas males of the other strains displayed normal testosterone levels. Furthermore, whole-cell voltage clamp recordings in isolated ventricular myocytes showed that the current density of IKur in male C57BL/6 mice was similar to that in female mice but smaller with respect to male CD-1 mice. Androgen replacement in male C57BL/6 mice as well as in castrated male CD-1 mice shortened ventricular repolarization, increased IKur current density, and increased expression of Kv1.5 channels. Conclusion: Strain and gender differences observed in mouse cardiac repolarization can be explained by different androgen levels. As a consequence, androgens are major regulatory factors in cardiac repolarization and special attention should be paid to the hormonal status of the animal when studying hormonal regulation of cardiac repolarization. [Copyright &y& Elsevier]

Objectives and methods: This study examines postnatal development of action potential duration (APD) and voltage-dependent K+ currents in mouse atrial myocytes and compares the expression levels of corresponding K+ channels between adult and neonatal mouse atrial tissues. APD and K+ currents were compared between atrial myocytes isolated from postnatal Day-1, Day-7, Day-20, and adult mice. Results: All K+ currents examined underwent significant up-regulation during postnatal life in mouse atrium, resulting in a dramatic shortening of the APD. The ultrarapid delayed rectifier (IKur) was absent in the developing mouse heart and only contributed to repolarization in the adult mouse atrium, whereas the density of the other K+ currents increased earlier during the developmental period. Indeed, the major changes in the expression of the inward rectifier current (IK1) occurred within the first week of life, the density of the Ca2+-independent transient outward K+ current (Ito) gradually increased while the development of the steady-state outward K+ current (Iss) was completed within the first 3 weeks of life. Results of RNase protection assay and Western blot analysis confirmed that the postnatal development of the mouse atrial K+ currents correlates with an increase in expression levels of underlying K+ channel isoforms. Conclusion: These findings indicate that in mouse atrium, each K+ current exhibits a specific postnatal development, suggesting that regulatory factors taking place during development are major determinants of the functional role of K+ channels in cardiac repolarization. The mouse atrium is, therefore, a very interesting model to gain information on the mechanisms regulating K+ channel activity. [Copyright &y& Elsevier]

Although the K+ currents expressed in hearts of adult mice have been studied extensively, detailed information concerning their relative sizes and biophysical properties in ventricle and atrium is lacking. Here we describe and validate pharmacological and biophysical methods that can be used to isolate the three main time- and voltage-dependent outward K+ currents which modulate action potential repolarization. A a2+-independent transient outward K+ current, Ito, can be separated from total outward current using an 'inactivating prepulse'. The rapidly activating, slowly inactivating delayed rectifier K+ current, IKur, can be isolated using submillimolar concentrations of 4-aminopyridine (4-AP). The remaining K+ current, Iss, can be obtained by combining these two procedures: (i) inactivating Ito and (ii) eliminating IKur by application of low concentration of 4-AP. Iss activates relatively slowly and shows very little inactivation, even during depolarizations lasting several seconds. Our findings also show that the rate of reactivation of Ito is more than 20-fold faster than that of IKur. These results demonstrate that the outward K+ currents in mouse ventricles can be separated based on their distinct time and voltage dependence, and different sensitivities to 4-AP. Data obtained at both 22 and 32°C demonstrate that although the duration of the inactivating prepulse has to be adapted for the recording temperature, this approach for separation of K+ current components is also valid at more physiological temperatures. To demonstrate that these methods also allow separation of these K+ currents in other cell types, we have applied this same approach to myocytes from mouse atria. Molecular approaches have been used to compare the expression levels of different K +... [ABSTRACT FROM AUTHOR]

The effect of advancing age on the kinetics of the antiarrhythmic agent mexiletine was studied by comparing various kinetic parameters calculated after administration of a single oral dose of mexiletine hydrochloride to seven elderly and eight young healthy volunteers. The rate of absorption of the drug from the gastrointestinal tract was significantly slower in the elderly (1.37 ± 0.51 hr−1) than in the young group (2.25 ± 0.79 hr−1). The mean values for elimination half-life and oral clearance were 12.3 ± 3.7 hr and 10.3 ± 5.4 mL/min/kg respectively in the young group and 14.4 ± 4.5 hr and 8.5 ± 2.9 mL/min/kg respectively in the elderly group. Neither of these parameters was significantly different between the two groups. The amount of mexiletine eliminated in urine up to 48 hours postdose was identical in both groups and represented less than 5% of the administered dose. It is concluded that the age-related modifications in the kinetics of mexiletine are not clinically important during chronic administration of the drug. [ABSTRACT FROM AUTHOR]