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 Ca 2+ -independent transient outward K + current, I to , can be separated from total outward current using an 'inactivating prepulse'. The rapidly activating, slowly inactivating delayed rectifier K + current, I Kur , can be isolated using submillimolar concentrations of 4-aminopyridine (4-AP). The remaining K + current, I ss , can be obtained by combining these two procedures: (i) inactivating I to and (ii) eliminating I Kur by application of low concentration of 4-AP. I ss activates relatively slowly and shows very little inactivation, even during depolarizations lasting several seconds. Our findings also show that the rate of reactivation of I to is more than 20-fold faster than that of I Kur . 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 I Kur , I to and I ss during action potential repolarization. The extensive efforts to develop transgenic mouse models of cardiovascular diseases have resulted in strong interest in defining the functional properties and molecular basis of the K + currents in mouse heart. Accordingly, a number of detailed studies focusing on K + currents in adult mouse ventricular myocytes have appeared (Benndorf et al. 1987;Wang & Duff, 1997;Barry et al. 1998;Babij et al. 1998;Zhou et al. 1998Zhou et al. , 2003 London et al. 1998a,b;Guo et al. 1999Guo et al. , 2000Wickenden et al. 1999; Xu et al. 1999a,b; DuBell et al. 2000;Jeron et al. 2000;Zaritsky et al. 2001;Kuo et al. 2001;Brunet et al. 2004;Bondarenko et al. 2004). In combination, these results provide a reasonably Judith Brouillette and Robert B. Clark contributed equally to this study consistent and quite detailed account of the number and type of time-and voltage-dependent K + currents expressed in adult mouse ventricle. Although a number of different strategies have been developed for separating the total K + current into individual K + conductance components, none of these approaches...

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.

We conclude that activation mapping using simultaneous recording from both epicardial and endocardial surfaces provides potentially important insights into the mechanisms of atrial conduction and arrhythmogenesis.

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.