Judith Brouillette

Functional properties of K + currents in adult mouse ventricular myocytes

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 Ca2+‐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+ channels in mouse atrium and ventricle. These findings provide new insights into the functional roles of IKur, Ito and Iss during action potential repolarization.

Effect of androgen deficiency on mouse ventricular repolarization

We previously demonstrated that female mouse ventricles have longer action potential durations (APDs) than males. This delayed repolarization results from a lower current density of the ultrarapid delayed rectifier K+ current (IK,ur) and a lower expression level of its underlying K+ channel (Kv1.5). To evaluate whether this sex difference could be attributable to the action of male sex hormones, we studied the effect of androgen deficiency on ventricular repolarization. We compared cardiac electrophysiological properties in castrated (orchiectomized; ORC) and control (CTL) male mice. Q‐Tc intervals as well as APDs measured at 20 %, 50 % and 90 % of repolarization were all significantly longer in ORC than in CTL. The current density of IK,ur was significantly lower in ORC than in CTL (at +50 mV, ORC: 29 ± 4 pA pF−1, n= 25; CTL: 48 ± 5 pA pF−1, n= 17; P= 0.006). In contrast, all the other K+ currents present in mouse ventricular myocytes were comparable between ORC and CTL. Moreover, results of Western blot analysis showed a lower expression level of Kv1.5 protein in ORC but no difference between the two groups for the other K+ channels studied. This study demonstrates that androgen deficiency leads to a reduction in the density of IK,ur and Kv1.5 in mouse ventricle, and consequently, to prolongation of APD and Q‐Tc interval. In conclusion, these findings strongly suggest that male sex hormones contribute to the sex difference that we previously reported in cardiac repolarization in adult mouse heart.

Reduction of ventricular sodium current in a mouse model of HIV.

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.

Estrogen regulation of the transient outward K+ current involves estrogen receptor α in mouse heart

BACKGROUND AND OBJECTIVE We have previously shown that androgens upregulate cardiac K(+) channels and shorten repolarization. However, the effects that estrogens (E2) and estrogen receptors (ER) might have on the various repolarizing K(+) currents and underlying ion channels remain incompletely understood. Accordingly, our objective was to verify whether and how E2 and its ERs subtypes influence these K(+) currents. METHODS AND RESULTS In order to examine the influence of E2 and ERs on K(+) currents we drastically lowered the E2 level through ovariectomy (OVX; 74% reduction vs CTL) and in parallel, we used female mice lacking either ERα (ERαKO) or ERβ (ERβKO). In OVX mice, results showed a specific increase of 35% in the density of the Ca(2+)-independent transient outward K(+) current (Ito) compared to CTL. Western blots showed increase in Kv4.2 and Kv4.3 sarcolemmal protein expression while qPCR revealed higher mRNA expression of only Kv4.3 in OVX mice. This upregulation of Ito was correlated with a shorter ventricular action potential duration and QTc interval. In ERαKO but not ERβKO mice, the mRNA of Kv4.3 was selectively increased. Furthermore, when ventricular myocytes obtained from ERαKO and ERβKO were cultured in the presence of E2, results showed that E2 reduced Ito density only in ERβKO myocytes confirming the repressive role of E2-ERα in regulating Ito. CONCLUSION Altogether, these results suggest that E2 negatively regulates the density of Ito through ERα, this highlights a potential role for this female hormone and its α-subtype receptor in modulating cardiac electrical activity.