Cardiac hypertrophy and prolongation of the cardiac action potential are hallmark features of heart disease. We examined the molecular mechanisms and the functional consequences of this action potential prolongation on calcium handling in right ventricular myocytes obtained from rats 8 weeks following ligation of the left anterior descending coronary artery (post‐myocardial infarction (MI) myocytes). Compared with myocytes from sham‐operated rats (sham myocytes), post‐MI myocytes showed significant reductions in transient outward K+ current (Ito) density (sham 19·7 ± 1·1 pA pF−1versus post‐MI 11·0 ± 1·3 pA pF−1; means ± s.e.m.), inward rectifier K+ current density (sham ‐13·7 ± 0·6 pA pF−1versus post‐MI ‐10·3 ± 0·9 pA pF−1) and resting membrane potential (sham ‐84·4 ± 1·3 mV versus post‐MI ‐74·1 ± 2·6 mV). Depressed Ito amplitude correlated with significant reductions in Kv4.2 and Kv4.3 mRNA and Kv4.2 protein levels. Kv1.4 mRNA and protein levels were increased and coincided with the appearance of a slow component of recovery from inactivation for Ito. In current‐clamp recordings, post‐MI myocytes showed a significant increase in [Ca2+]i transient amplitude compared with sham myocytes. Using voltage‐clamp depolarizations, no intrinsic differences in Ca2+ handling by the sarcoplasmic reticulum or in L‐type Ca2+ channel density (ICa,L) were detected between the groups. Stimulation of post‐MI myocytes with an action potential derived from a sham myocyte reduced the [Ca2+] transient amplitude to the sham level and vice versa. The net Ca2+ influx per beat via ICa,L was increased about 2‐fold in myocytes stimulated with post‐MI action potentials compared with sham action potentials. Our findings demonstrate that reductions in K+ channel expression in post‐MI myocytes prolong action potential duration resulting in elevated Ca2+ influx and [Ca2+]i transients.

The aim of the present study was to assess differences in transient outward potassium current ( I to) between the right ventricular free wall and the interventricular septum of the adult rat ventricle and to evaluate the relative contributions of Kv4.2, Kv4.3, and Kv1.4 to I to in these regions. The results show that I to is composed of both rapidly and slowly recovering components in the right wall and septum. The fast component had a significantly higher density in the right free wall than in the septum, whereas the slow component did not differ between the two sites. Kv4.2mRNA and protein levels were also highest in the right wall and correlated with I to density, whereas Kv4.3 was expressed uniformly in these regions. The kinetics of the rapidly recovering component of I to in myocytes was similar to that recorded in tsa-201 cells expressing Kv4.2 and Kv4.3 channels. Kv1.4 mRNA and protein expression correlated well with the density of the slowly recovering I to, whereas the recovery kinetics of the slow component were identical to Kv1.4 expressed in tsa-201 cells. In conclusion, expression of Kv1.4, Kv4.2, and Kv4.3 differs between regions in rat hearts. Regionally specific differences in the genetic composition of I to can account for the region-specific properties of this current.

1. In rat heart, three K¤ channel genes that encode inactivating transient outward (ITO)-like currents are expressed. During development the predominant K¤ channel mRNA species switches from Kv1.4 to Kv4.2 and Kv4.3. However, no functional correlate of this isoform switch has been reported. We investigated action potential characteristics and ITO in cultured neonatal rat ventricular myocytes and adult rat hearts. We further examined whether the changes in K¤ channel gene expression and the associated electrophysiology that occurs during development could be induced by thyroid hormone. 2. In myocytes isolated from right ventricle of adult rat heart, action potential duration was short and independent of rate of stimulation. The density of ITO was 21·5 ± 1·8 pA pF¢ (n = 21). Recovery from inactivation was best described by a single exponential (ôfast = 31·7 ± 2·7 ms, n = 13). The current remaining at the end of a 500 ms pulse (ISUS) was 6·2 ± 0·5 pA pF¢ (n = 19). 3. In contrast to adult cells, action potential duration was prolonged and was markedly rate dependent in cultured neonatal rat ventricular myocytes. The current density of ITO measured in cultured ventricular myocytes from 1-to 2-day-old rats was 10·1 ± 1·5 pA pF¢ (n = 17). The recovery from inactivation for ITO was best described by the sum of two exponentials (ôfast = 64·3 ± 8·8 ms, 54·4 ± 10·2 %; ôslow = 8216 ± 2396 ms, 37·4 ± 7·9 %; n = 5). ISUS was 4·4 ± 0·6 pA pF¢ (n = 17). Steady-state activation and inactivation were similar in adult and neonatal ventricular myocytes. 4. In neonatal myocytes treated with thyroid hormone, tri-iodothyronine (T3, 100 nÒ), action potential duration was abbreviated and independent of stimulation rate. Whilst T3 did not significantly increase ITO density (24·0 ± 2·9 pA pF¢; n = 21 in T3 treated cells cf. 20·1 ± 3·0 pA pF¢; n = 37 in untreated controls), the recovery from inactivation of ITO was accelerated (ôfast = 39·2 ± 3·6 ms, 82·2 ± 8·9 %, n = 9). T3 did however, increase ISUS current density (4·7 ± 0·77 pA pF¢; n = 37 and 7·0 ± 0·7 pA pF¢, n = 21, in control and T3 treated cells, respectively). 5. The effects of T3 (100 nÒ) were associated with a marked decrease in the expression of Kv1.4 at the mRNA and protein level, and an increase in the expression of Kv4.3 without changes in Kv4.2 mRNA levels. 6. The findings of the present study indicate that postnatal development involves a shortening of action potential duration and an increase in the density of ITO. Furthermore, we show that development is also associated with a loss of action potential rate dependence, and an acceleration in the rate of recovery of ITO. We propose that these functional effects occur as a consequence of the previously reported developmental Kv1.4 to Kv4.2ÏKv4.3 isoform switch. In cultured neonatal myocytes, T3 induced many of the electrophysiological and molecular changes that normally occur during postnatal development, suggesting that this hormone may play an important role in postnatal electrophysiological development.

Action potential (AP) prolongation typically occurs in heart disease due to reductions in transient outward potassium currents (Ito), and is associated with increased Ca2+ transients. We investigated the underlying mechanisms responsible for enhanced Ca2+ transients in normal isolated rat ventricular myocytes in response to the AP changes that occur following myocardial infarction. Normal myocytes stimulated with a train of long post‐myocardial infarction (MI) APs showed a 2.2‐fold elevation of the peak Ca2+ transient and a 2.7‐fold augmentation of fractional cell shortening, relative to myocytes stimulated with a short control AP. The steady‐state Ca2+ load of the sarcoplasmic reticulum (SR) was increased 2.0‐fold when myocytes were stimulated with trains of long post‐MI APs (111 ± 21.6 μmol l−1) compared with short control APs (56 ± 7.2 μmol l−1). Under conditions of equal SR Ca2+ load, long post‐MI APs still resulted in a 1.7‐fold increase in peak [Ca2+]i and a 3.8‐fold increase in fractional cell shortening relative to short control APs, establishing that changes in the triggering of SR Ca2+ release are largely responsible for elevated Ca2+ transients following AP prolongation. Fractional SR Ca2+ release calculated from the measured SR Ca2+ load and the integrated SR Ca2+ fluxes was 24 ± 3 and 11 ± 2 % following post‐MI and control APs, respectively. The fractional release (FR) of Ca2+ from the SR divided by the integrated L‐type Ca2+ flux (FR/∫FCa,L) was increased 1.2‐fold by post‐MI APs compared with control APs. Similar increases in excitation‐contraction (E‐C) coupling gains were observed establishing enhanced E‐C coupling efficiency. Our findings demonstrate that AP prolongation alone can markedly enhance E‐C coupling in normal myocytes through increases in the L‐type Ca2+ current (ICa,L) trigger combined with modest enhancements in Ca2+ release efficiency. We propose that such changes in AP profile in diseased myocardium may contribute significantly to alterations in E‐C coupling independent of other biochemical or genetic changes.

(Ito) has been shown to vary between different regions of the normal myocardium and to be reduced in heart disease. In this study, we measured regional changes in action potential duration (APD), I to, and intracellular Ca 2ϩ concentration ([Ca 2ϩ ]i) transients of ventricular myocytes derived from the right ventricular free wall (RVW) and interventricular septum (SEP) 8 wk after myocardial infarction (MI). At ϩ40 mV, I to density in sham-operated hearts was significantly higher (P Ͻ 0.01) in the RVW (15.0 Ϯ 0.8 pA/pF, n ϭ 47) compared with the SEP (7.0 Ϯ 1.1 pA/pF, n ϭ 18). After MI, I to density was not reduced in SEP myocytes but was reduced (P Ͻ 0.01) in RVW myocytes (8.7 Ϯ 1.0 pA/pF, n ϭ 26) to levels indistinguishable from post-MI SEP myocytes. These changes in I to density correlated with Kv4.2 (but not Kv4.3) protein expression. By contrast, Kv1.4 expression was significantly higher in the RVW compared with the SEP and increased significantly after MI in RVW. APD measured at 50% or 90% repolarization was prolonged, whereas peak [Ca 2ϩ ]i transients amplitude was higher in the SEP compared with the RVW in sham myocytes. These regional differences in APD and [Ca 2ϩ ]i transients were eliminated by MI. Our results demonstrate that the significant regional differences in I to density, APD, and [Ca 2ϩ ]i between RVW and SEP are linked to a variation in Kv4.2 expression, which largely disappears after MI. right ventricle; septum; heart disease; contraction THERE ARE MARKED DIFFERENCES in the action potential duration (APD) in different regions of the mammalian ventricle (4,15,18,39,58). This electrical heterogeneity in normal myocardium correlates with regional differences in the Ca 2ϩ -independent transient outward K ϩ current density (I to ) (5,15,18,34,35,43) as well as in gene expression of K ϩ channels (8,16,58). APD prolongation and reductions in I to density occur in rat heart after left anterior descending coronary artery ligation (2, 43, 58), aortic banding (5, 22, 55), as well as after treatment with either catecholamine (11) or monocrotaline (32, 33). Depending on the model, the extent of I to density changes in disease may not be uniform throughout the ventricle (2, 5, 11, 22, 55), thereby leading to possible losses of electrical heterogeneity and increased susceptibility to arrhythmias (3).Aside from electrical heterogeneity, regional differences in other myocardial properties also exist. For example, systolic intracellular Ca 2ϩ concentration ([Ca 2ϩ ] i ) is higher in the endocardium than in the epicardium (19,54), consistent with the notion that the endocardium may play a more important role in contraction compared with the epicardium. The underlying basis for the regional differences in contraction is currently unknown but may be related to a heterogeneous transmural expression of Ca 2ϩ handling proteins (26, 31). Alternatively, APD might also play a key role because the action potential profile is an important determinant of the inotropic state of the heart in both normal (7,42,54) and hy...

1. We examined the mechanisms for rate-dependent changes in twitch force duration by simultaneously measuring force and [Ca2+]i in rat cardiac trabeculae. 2. Peak force decreased when the rate of stimulation was increased from 0.2 to 0.5 Hz, whilst it increased from 1 to 2 Hz. Over the same range of frequencies, peak [Ca2+]i transients increased monotonically, whilst both force and [Ca2+]i transient duration were abbreviated. 3. Changes in peak force or peak [Ca2+]i transients were not responsible for the changes in force or [Ca2+]i transient duration. 4. The changes in twitch force and [Ca2+]i transient duration were completed roughly within one beat following an abrupt change in the rate of stimulation. 5. Rate-dependent changes resembled those observed with isoproterenol (isoprenaline) application. However, kinase inhibitors (i.e. K252-a, H-89, KN-62 and KN-93) had no effect on the rate-dependent changes of twitch force and [Ca2+]i transient kinetics, suggesting that protein kinase A (PKA), protein kinase PKG) and Ca2+-calmodulin-dependent protein kinase II (CaM/kinase II) were not responsible for these kinetic changes. 6. Despite the changes in twitch force and [Ca2+]i transient kinetics, the rate-limiting step for the rate-dependent force relaxation still resides at the level of the contractile proteins. 7. Our results suggest that rate-dependent changes in force and [Ca2+]i transients do not depend on PKA or CaM/kinase II activity but might result from intrinsic features of the contractile and/or Ca2+-handling proteins.

Background-Prolongation of the action potential duration (APD) and decreased transient outward K ϩ current (I to ) have been consistently observed in cardiac hypertrophy. The relation between electrical remodeling and cardiac hypertrophy in vivo is unknown. Methods and Results-We studied rat hearts subjected to pressure overload by surgical ascending aortic stenosis (AS) and simultaneously infected these hearts with an adenovirus carrying either the Kv4.

The aim of the present study was to compare the biophysical properties and Cd²⁺ sensitivity of Kv4.2 and Kv1.4 in Xenopus oocytes with those of native transient outward potassium currents in rat and rabbit ventricular myocytes. In Xenopus oocytes, Kv4.2 inactivated at hyperpolarized voltages (V_½^inact = –58.4 ± 0.96 mV, n = 12) and recovered from inactivation rapidly (time constant = 224 ± 23 ms, n = 3). Cd²⁺ induced large (approx. 30 mV with 500 μM Cd²⁺), concentration-dependent rightward shifts in Kv4.2 steady-state activation and inactivation. Kv1.4 inactivated over more depolarized voltages than Kv4.2 (V_½^inact = –49.3 ± 1.4 mV, n = 12). Recovery from inactivation of Kv1.4 was dominated by a large slow component (time constant = 9,038 ± 1,178 ms, n = 4). Cd²⁺ exerted only modest effects on Kv1.4 gating, with 500 μM Cd²⁺ shifting the voltage dependence of steady-state activation and inactivation by approximately 12 mV.We show that the biophysical properties and Cd²⁺ sensitivity of rat ventricular I_to resemble those of heterologously expressed Kv4.2. These findings support previous suggestions that Kv4.2 is an important molecular component of I_to in adult rat heart. In addition, our findings show that I_to in rabbit ventricular myocytes and Kv1.4-based currents in Xenopus oocytes share similar biophysical properties and sensitivity to Cd²⁺, suggesting that Kv1.4 may underlie I_to in rabbit ventricle. However, a number of discrepancies exist between the properties of native currents and their putative molecular counterparts, suggesting that additional proteins and/or modulatory factors may also play a role in determining the biophysical and pharmacological properties of these native currents.