Alterations in action potential profile enhance excitation-contraction coupling in rat cardiac myocytes

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 micromol l(-1)) compared with short control APs (56 +/- 7.2 micromol 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/[integral]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.

Altered connexin expression in human congestive heart failure

Congestive heart failure is associated with a high risk of life-threatening ventricular re-entrant arrhythmias. Down-regulation of the principal gap-junctional protein of the ventricular myocytes, connexin43, has previously been implicated in arrhythmia in ischaemic heart disease, but it is not known whether connexin43 is similarly reduced in heart failure due to idiopathic dilated cardiomyopathy, whether disease-related connexin43 down-regulation occurs at the level of transcription or translation, or whether the expression of other connexin isotypes is altered in congestive heart failure. We therefore investigated the expression of the four connexins expressed in the heart-connexins 43, 40, 45 and 37-at the mRNA and protein levels in explanted hearts from transplant patients with end-stage heart failure (NYHA class 4) by immunoconfocal analysis, and northern and western blotting. Connexin43 mRNA and protein were markedly downregulated in the left ventricle in end-stage heart failure due both to ischaemic cardiomyopathy and idiopathic dilated cardiomyopathy. Connexin43 content was spatially heterogeneous in the diseased ventricle. Connexin40 mRNA was increased in the ischaemic group, more so in the left ventricle than the right. This correlated with an increased depth of connexin40 protein expression in myocytes at the endocardial surface. Connexin45 mRNA and protein, present only in very low quantities, followed a similar trend to connexin43, while connexin37 (exclusively expressed in endothelium) showed no change. Our findings show that congestive heart failure is associated with significantly reduced levels of the principal gap junction protein, connexin43, in the left ventricle, potentially contributing to enhanced arrhythmogenicity and contractile dysfunction. This down-regulation is due predominantly to a reduced transcript steady-state level. Elevated connexin40 may represent a compensatory response that improves the spread of depolarization in the otherwise compromised ischaemic ventricle.

Effect of Cd2+ on Kv4.2 and Kv1.4 expressed in Xenopus oocytes and on the transient outward currents in rat and rabbit ventricular myocytes

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(1/2)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 microM Cd2+), concentration-dependent rightward shifts in Kv4.2 steady-state activation and inactivation. Kv1.4 inactivated over more depolarized voltages than Kv4.2 (V(1/2)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 microM 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 Ito resemble those of heterologously expressed Kv4.2. These findings support previous suggestions that Kv4.2 is an important molecular component of Ito in adult rat heart. In addition, our findings show that Ito 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 Ito 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.

Rate-dependent changes of twitch force duration in rat cardiac trabeculae: a property of the contractile system

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.

The thyroid hormone analog DITPA restores I(to) in rats after myocardial infarction

Previous studies have established that reductions in repolarizing currents occur in heart disease and can contribute to life-threatening arrhythmias in myocardium. In this study, we investigated whether the thyroid hormone analog 3, 5-diiodothyropropionic acid (DITPA) could restore repolarizing transient outward K(+) current (I(to)) density and gene expression in rat myocardium after myocardial infarction (MI). Our findings show that I(to) density was reduced after MI (14.0 +/- 1.0 vs. 10.2 +/- 0.9 pA/pF, sham vs. post-MI at +40 mV). mRNA levels of Kv4.2 and Kv4.3 genes were decreased but Kv1.4 mRNA levels were increased post-MI. Corresponding changes in Kv4.2 and Kv1.4 protein were also observed. Chronic treatment of post-MI rats with 10 mg/kg DITPA restored I(to) density (to 15.2 +/- 1.1 pA/pF at +40 mV) as well as Kv4.2 and Kv1.4 expression to levels observed in sham-operated controls. Other membrane currents (Na(+), L-type Ca(2+), sustained, and inward rectifier K(+) currents) were unaffected by DITPA treatment. Associated with the changes in I(to) expression, action potential durations (current-clamp recordings in isolated single right ventricular myocytes and monophasic action potential recordings from the right free wall in situ) were prolonged after MI and restored with DITPA treatment. Our results demonstrate that DITPA restores I(to) density in the setting of MI, which may be useful in preventing complications associated with I(to) downregulation.

Relationship between K+ channel down-regulation and [Ca2+]i in rat ventricular myocytes following myocardial infarction

1. 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). 2. 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-1 versus 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-1 versus 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. 3. 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. 4. 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. 5. 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. 6. 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.

Regional contributions of Kv1.4, Kv4.2, and Kv4.3 to transient outward K+ current in rat ventricle

The aim of the present study was to assess differences in transient outward potassium current (Ito) 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 Ito in these regions. The results show that Ito 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. Kv4.2 mRNA and protein levels were also highest in the right wall and correlated with Ito density, whereas Kv4.3 was expressed uniformly in these regions. The kinetics of the rapidly recovering component of Ito 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 Ito, 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 Ito can account for the region-specific properties of this current.

The effect of lower body positive pressure on the cardiovascular response to exercise in sedentary and endurance-trained persons with paraplegia

Exercise intolerance in persons with paraplegia (PARAS) is thought to be secondary to insufficient venous return and a subnormal cardiac output at a given oxygen uptake. However, these issues have not been resolved fully. This study utilized lower-body positive pressure (LBPP) as an intervention during arm crank exercise in PARAS in order to examine this issue. Endurance-trained (TP, n = 7) and untrained PARAS (UP, n = 10) with complete lesions between T6 and T12, and a control group consisting of sedentary able-bodied subjects (SAB, n = 10) were tested. UP and TP subjects demonstrated a diminished cardiac output (via CO2 rebreathing) during exercise compared to SAB subjects. Peak oxygen uptake (O2peak) remained unchanged for all groups following LBPP. LBPP resulted in a significant decrease in heart rate (HR) in UP and TP (P < 0.05), but not SAB subjects. LBPP produced an insignificant increase in cardiac output (Q) and stroke volume (SV). The significant decrease in HR in both PARA groups may indicate a modest hemodynamic benefit of LBPP at higher work rates where circulatory sufficiency may be most compromised. We conclude that PARAS possess a diminished cardiac output during exercise compared to the able-bodied, and LBPP fails to ameliorate significantly their exercise response irrespective of the conditioning level. These results support previous observations of a lower cardiac output during exercise in PARAS, but indicate that lower-limb blood pooling may not be a primary limitation to arm exercise in paraplegia.

The role of action potential prolongation and altered intracellular calcium handling in the pathogenesis of heart failure

Action potential prolongation is a common finding in human heart failure and in animal models of cardiac hypertrophy. The mechanism of action potential prolongation involves altered expression of a variety of depolarising and hyperpolarising currents in the myocardium. In particular, decreased density of the transient outward potassium current seems to play a prominent role, regardless of species, precipitating factors or the severity of hypertrophy. The decreased density of the transient outward current appears to be caused by reduced transcription of Kv4.2 and Kv4.3 and may be caused in part by an inhibitory effect of alpha-adrenoceptor stimulation. During the early stage of the disease process, action potential prolongation may increase the amplitude of the intracellular calcium transient, causing positive inotropy. We argue therefore, that action prolongation may be a compensatory response which may acutely support the compromised cardiac output. In severe hypertrophy and end-stage heart failure however, despite continued action potential prolongation, the amplitude of the calcium transient becomes severely reduced. The mechanism underlying this event appears to involve reduced expression of calcium handling proteins, and these late events may herald the onset of failure. At present the events leading to the late changes in calcium handling are poorly understood. However, chronic activation of compensatory mechanisms including action potential prolongation may trigger these late events. In the present article we outline a hypothesis which describes a potential role for action potential prolongation, and the associated elevation in the levels of intracellular calcium, in maladaptive gene expression and the progression toward cardiac failure.

Effects of development and thyroid hormone on K+ currents and K+ channel gene expression in rat ventricle

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-1 (n = 21). Recovery from inactivation was best described by a single exponential (tau fast = 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-1 (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-1 (n = 17). The recovery from inactivation for ITO was best described by the sum of two exponentials (tau fast = 64.3 +/- 8.8 ms, 54.4 +/- 10.2%; tau slow = 8216 +/- 2396 ms, 37.4 +/- 7.9%; n = 5). ISUS was 4.4 +/- 0.6 pA pF-1 (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 nM), action potential duration was abbreviated and independent of stimulation rate. Whilst T3 did not significantly increase ITO density (24.0 +/- 2.9 pA pF-1; n = 21 in T3 treated cells cf. 20.1 +/- 3.0 pA pF-1; n = 37 in untreated controls), the recovery from inactivation of ITO was accelerated (tau fast = 39.2 +/- 3.6 ms, 82.2 +/- 8.9%, n = 9). T3 did however, increase ISUS current density (4.7 +/- 0.77 pA pF-1; n = 37 and 7.0 +/- 0.7 pA pF-1, n = 21, in control and T3 treated cells, respectively. 5. The effects of T3 (100 nM) 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 Kv4.2/Kv4.3 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.

Evaluation of S12363, a novel vinca alkaloid drug in the treatment of advanced malignant melanoma. A phase II study

We conducted a phase II study with the new vinca alkaloid derivative S12363 in ten patients with metastatic malignant melanoma. Six patients had been pre-treated with other cytotoxic agents and all had received Vindesine. Four patients had no prior systemic treatment before entry into the study. S12363 was well tolerated subjectively, the main toxicity being haematological. Despite encouraging in vitro observations, no objective responses to S12363 have been documented in these patients with the doses and schedule used in this study. S12363 appears to have no clinically useful activity in metastatic malignant melanoma.