In the present study, we explored the impact of gender on the transcript expression levels of the major cardiac ion channels, calcium handling proteins, and transcription factors in the failing and nonfailing human heart. We found gender-specific differences in relative expression levels of ion channel subunits, such as Kv4.3, KChIP2, Kv1.5, and Kir3.1 in the LA, but no significant gender-specific differences in relative expression levels of these subunits in the LV. Analyses of transmural heterogeneities in transcript expression levels in the LV, however, revealed a heterogeneity of expression across the LV wall in of the Ito,slow pore-forming subunit, Kv1.4, as well as with the Ito,fast accessory subunit, KChIP2. In the nonfailing heart, Kv1.4 exhibited higher expression in the epicardium of the nonfailing heart, whereas KChIP2 exhibited a stronger expression in the epicardium across gender and disease state.
Until recently, there has been very limited data available regarding global gender dependent cardiac ion channel gene expression in the human heart. A 2010 study by Gaborit, et al 
explored gender dependent differences in the nonfailing human ventricle (left and right ventricles) and, unlike in our investigation, found significantly lower expression levels in the female LV of key ion channels and accessory proteins important in cardiac repolarization. Their patient population consisted of 10 men and 10 women with average ages of 38±12 years and 43±13 years, respectively. Our patient population, on the other hand, consisted of both failing and nonfailing human hearts with significantly older average ages (failing females, 54±9 years; failing males, 56±9 years; nonfailing females, 51±11 years, failing males, 50±18 years). The variability of qPCR data has shown to be significantly increased in aged animals 
and humans 
, which in our case may mask any intrinsic gender dependent expression differences in the LV in our study.
The use of an older patient population may, however, be more representative with respect to the gender dependent analysis of LA targets important in the generation and maintenance of AF, as AF is relatively uncommon before the age of 60 and 10% of the overall population develops AF by 80 years of age 
. In addition, 1 in 4 heart failure patients develop AF within five years of diagnosis 
, so it is likely that relevant atrial remodeling processes 
are well underway in our patient population. Interestingly, although overall men are more likely to develop heart failure 
, women diagnosed with heart failure are approximately 1.5 times more likely to develop AF than their male counterparts 
. Additionally, the overall higher atrial expression levels of almost all transcripts analyzed in this study () in the failing male hearts, as compared to failing female hearts, may mask any intrinsic gender-based differences.
Interpretation of these results, however, with respect to both the LA and LV must be considered in light of the available clinical information, as shown in and . As described in the Results section, we identified one patient (failing heart no. 7) with a history of AF. This patient exhibited high expression levels of Kv
4.3, KChIP2, Kir
1.5, and Kv
1.5 when compared with all other failing LA specimens of both genders, as shown in . This sample did, however display a relatively lower expression level of Cav
1.2 as compared with other specimens in the group, which is likely related to the patient’s four-month history of permanent AF prior to transplantation. It should also be noted that, prior to diagnosis of AF, this patient had been previously diagnosed with ventricular arrhythmias which concurrently played a role in the progression of heart failure and overall arrhythmia history. As previously reported, in human AF the primarily physiological remodeling results from changes in ICa,L
and significant reductions of current density have only been reported after 18 months of persistent AF 
. In addition, as shown in , we observed no remarkable expression patterns in key repolarization targets in the LV of failing hearts with and without a history of ventricular tachyarrhythmias, which may in part be due to the individual etiologies of heart failure, which must be elucidated further in future studies.
Prolongation of the cardiac action potential is a hallmark of heart failure 
. Furthermore, heterogeneous distribution of repolarization transmurally across the ventricular wall manifests as electrophysiological heterogeneities which may also be pro-arrhythmic 
. In our recent study, we provide direct experimental evidence of transmural heterogeneites in action potential waveforms in the human LV wedge preparation 
. In the present study, as shown in , we explored the potential molecular mechanisms of this functional heterogeneity. The data reveal expression gradients in KChIP2 expression across the transmural wall consistent with differences in Ito
. We were unable, however, to pinpoint other channels potentially contributing to action potential heterogeneity as the transcripts encoding the other ion channels prominent in repolarization were found to be more homogenously expressed across the LV wall. In addition, we did not find connexin 43 transcript expression to be heterogeneously expressed, although previous reports have demonstrated significantly lower epicardial connexin 43 protein expression in both failing and nonfailing hearts in our previous study 
. Patterns of molecular heterogeneity directly affecting action potential durations may either be due to the presence of smaller “islands” of differential gene expression and function 
not captured in this study or post-translational protein modifications 
not assessed in our transcript level investigation.
The wealth of information gathered from large-scale gene expression studies such as this and others 
, not only provides insight into the gender dependent, pathological, or regional changes in ion channel expression, but also provides solid physiological data that can be incorporated into computational models. Furthermore, access to detailed clinical information, such as that presented this study, will both assist in interpretation of the molecular mechanisms involved in heart failure and arrhythmogenesis, as well as contribute to the development of tools for patient-specific computational models of cardiac electrophysiology. Constructing models of comprehensive human electromechanical activity requires a thorough understanding of the distribution of ion channels, calcium handling proteins, and other transporters in various regions of the heart, as well as their intrinsic modifications due to disease and gender as we have investigated in this study. Our colleagues Walmsley, et al in their paper “mRNA expression levels in failing human hearts predict cellular electrophysiological remodeling using a population-based simulation study” in this issue of the journal, present a first step towards patient-specific modeling of the action potential and calcium transient based on the comprehensive set of molecular data presented here.
The study of human hearts, while providing a unique opportunity to explore both the structural and functional mechanisms pertaining to arrhythmias, heart failure, and other pathological conditions, is limited by number of explanted hearts available. Nonfailing, donor hearts are especially precious and cannot be truly considered normal, control hearts due to that fact that they have been rejected for transplantation due to, in some cases, cardiac reasons. In addition, the underlying patient circumstances (i.e. medication, previous cardiac surgery, etc) cannot be controlled for. Thus, the hearts presented in this study represent the best available specimens.
Epidemiologically, it is clear there is a gender disparity in arrhythmia incidence. To attempt to elucidate the underlying molecular mechanisms of these differences, we have explored the gender dependent expression of various ion channels, calcium handling proteins, and other transcription factors in the failing and nonfailing human heart in this study. While we observed significant gender dependent differences in atrial tissues, we did not however observe clear patterns of gender dependent expression in the ventricles. Our data do, however, emphasize the importance of addressing such confounding factors as gender, age, and disease progression when exploring ionic remodeling processes in the human heart and in constructing patient-specific models of cardiac electrophysiology.