Transient outward current in catecholamine-induced cardiac hypertrophy in the rat

Compensatory and decompensatory alterations in cardiomyocyte Ca2+ dynamics in hearts with diastolic dysfunction following aortic banding

Key points

At the cellular level cardiac hypertrophy causes remodelling, leading to changes in ionic channel, pump and exchanger densities and kinetics.

Previous studies have focused on quantifying changes in channels, pumps and exchangers without quantitatively linking these changes with emergent cellular scale functionality.

Two biophysical cardiac cell models were created, parameterized and validated and are able to simulate electrophysiology and calcium dynamics in myocytes from control sham operated rats and aortic‐banded rats exhibiting diastolic dysfunction.

The contribution of each ionic pathway to the calcium kinetics was calculated, identifying the L‐type Ca²⁺ channel and sarco/endoplasmic reticulum Ca²⁺ATPase as the principal regulators of systolic and diastolic Ca²⁺, respectively.

Results show that the ability to dynamically change systolic Ca²⁺, through changes in expression of key Ca²⁺ modelling protein densities, is drastically reduced following the aortic banding procedure; however the cells are able to compensate Ca²⁺ homeostasis in an efficient way to minimize systolic dysfunction.

Abstract

Elevated left ventricular afterload leads to myocardial hypertrophy, diastolic dysfunction, cellular remodelling and compromised calcium dynamics. At the cellular scale this remodelling of the ionic channels, pumps and exchangers gives rise to changes in the Ca²⁺ transient. However, the relative roles of the underlying subcellular processes and the positive or negative impact of each remodelling mechanism are not fully understood. Biophysical cardiac cell models were created to simulate electrophysiology and calcium dynamics in myocytes from control rats (SHAM) and aortic‐banded rats exhibiting diastolic dysfunction. The model parameters and framework were validated and the fitted parameters demonstrated to be unique for explaining our experimental data. The contribution of each ionic pathway to the calcium kinetics was calculated, identifying the L‐type Ca²⁺ channel (LCC) and the sarco/endoplasmic reticulum Ca²⁺‐ATPase (SERCA) as the principal regulators of systolic and diastolic Ca²⁺, respectively. In the aortic banding model, the sensitivity of systolic Ca²⁺ to LCC density and diastolic Ca²⁺ to SERCA density decreased by 16‐fold and increased by 23%, respectively, relative to the SHAM model. The energy cost of ionic homeostasis is maintained across the two models. The models predict that changes in ionic pathway densities in compensated aortic banding rats maintain Ca²⁺ function and efficiency. The ability to dynamically alter systolic function is significantly diminished, while the capacity to maintain diastolic Ca²⁺ is moderately increased.

At the cellular level cardiac hypertrophy causes remodelling, leading to changes in ionic channel, pump and exchanger densities and kinetics.

Previous studies have focused on quantifying changes in channels, pumps and exchangers without quantitatively linking these changes with emergent cellular scale functionality.

Two biophysical cardiac cell models were created, parameterized and validated and are able to simulate electrophysiology and calcium dynamics in myocytes from control sham operated rats and aortic‐banded rats exhibiting diastolic dysfunction.

The contribution of each ionic pathway to the calcium kinetics was calculated, identifying the L‐type Ca²⁺ channel and sarco/endoplasmic reticulum Ca²⁺ATPase as the principal regulators of systolic and diastolic Ca²⁺, respectively.

Results show that the ability to dynamically change systolic Ca²⁺, through changes in expression of key Ca²⁺ modelling protein densities, is drastically reduced following the aortic banding procedure; however the cells are able to compensate Ca²⁺ homeostasis in an efficient way to minimize systolic dysfunction.

Regulation of cardiac shal-related potassium channel Kv 4.3 by serum- and glucocorticoid-inducible kinase isoforms in Xenopus oocytes

The human cardiac transient outward potassium current I(to) is formed by co-assembly of voltage-dependent K(+) channel (Kv 4.3) pore-forming alpha-subunits with differently spliced K channel interacting protein (KChIP) accessory proteins. I(to) is of considerable importance for the normal course of the cardiac ventricular action potential. The present study was performed to determine whether isoforms of the serum- and glucocorticoid-inducible kinase (SGK) family influence Kv 4.3/KChIP2b channel activity in the Xenopus laevis heterologous expression system. Co-expression of SGK1, but not of SGK2 or SGK3, increased Kv 4.3/KChIP2b channel currents. The up-regulation of the current was not due to changes in the activation curve or changes of channel inactivation. The currents in oocytes expressing Kv 4.3 alone were smaller than those in Kv 4.3/KChIP2b expressing oocytes, but were still stimulated by SGK1. The effect of wild-type SGK1 was mimicked by constitutively active SGK1 (SGK1 S422D) but not by an inactive mutant (SGK1 K127N). The current amplitude increase mediated by SGK1 was not dependent on NEDD4.2 or RAB5, nor did it reflect increased cell surface expression. In conclusion, SGK1 stimulates Kv 4.3 potassium channels expressed in Xenopus oocytes by a novel mechanism distinct from the known NEDD4.2-dependent pathway.

Structure and function of Kv4-family transient potassium channels

Shal-type (Kv4.x) K(+) channels are expressed in a variety of tissue, with particularly high levels in the brain and heart. These channels are the primary subunits that contribute to transient, voltage-dependent K(+) currents in the nervous system (A currents) and the heart (transient outward current). Recent studies have revealed an enormous degree of complexity in the regulation of these channels. In this review, we describe the surprisingly large number of ancillary subunits and scaffolding proteins that can interact with the primary subunits, resulting in alterations in channel trafficking and kinetic properties. Furthermore, we discuss posttranslational modification of Kv4.x channel function with an emphasis on the role of kinase modulation of these channels in regulating membrane properties. This concept is especially intriguing as Kv4.2 channels may integrate a variety of intracellular signaling cascades into a coordinated output that dynamically modulates membrane excitability. Finally, the pathophysiology that may arise from dysregulation of these channels is also reviewed.

Role of PKC in autocrine regulation of rat ventricular K+ currents by angiotensin and endothelin

Transient and sustained K(+) currents were measured in isolated rat ventricular myocytes obtained from control, steptozotocin-induced (Type 1) diabetic, and hypothyroid rats. Both currents, attenuated by the endocrine abnormalities, were significantly augmented by in vitro incubation (>6 h) with the angiotensin-converting enzyme inhibitor quinapril or the angiotensin II (ANG II) receptor blocker saralasin. Western blots indicated a parallel increase in Kv4.2 and Kv1.2, channel proteins that underlie the transient and (part of the) sustained currents. Under diabetic and hypothyroid conditions, both currents were also augmented by an endothelin receptor blocker (PD142893) or by an endothelin-converting enzyme inhibitor. Kv4.2 density was also enhanced by PD142893. Incubation (>5 h) with the PKC inhibitor bis-indolylmaleimide augmented both currents, whereas the PKC activator dioctanoyl-rac-glycerol (DiC8) prevented the augmentation of currents by quinapril. DiC8 also prevented the augmentation of Kv4.2 density by quinapril. Specific peptides that activate PKC translocation indicated that PKC-epsilon and not PKC-delta is involved in ANG II action on these currents. In control myocytes, quinapril and PD142893 augmented the sustained late current but had no effect on peak current. It is concluded that an autocrine release of angiotensin and endothelin in diabetic and hypothyroid conditions attenuates K(+) currents by suppressing the synthesis of some K(+) channel proteins, with the effects mediated at least partially by PKC-epsilon.

Effects of sustained beta-adrenergic stimulation on ionic currents of cultured adult guinea pig cardiomyocytes

Short-term stimulation of beta-receptors is known to affect cardiac ion channels; however, the impact of longer-term stimulation on intrinsic channel function is poorly understood. To evaluate this, cultured guinea pig ventricular myocytes were exposed to isoproterenol (10 nM), vehicle, or isoproterenol plus propranolol (1 microM) for 48 h. Sustained exposure to isoproterenol decreased the density of the inward rectifier (I(K1)), slow delayed rectifier (I(Ks)), and L-type Ca2+ (I(Ca L)) currents, effects that were fully prevented by propranolol. Changes in K+ currents were prevented by the beta1-selective antagonist CGP-20712A, unaffected by the beta2-antagonist ICI-118,551, and mimicked by the membrane-permeable cAMP analog 8-bromo-cAMP. Isoproterenol did not alter the current-voltage relationship of the K+ currents but increased the density of T-type Ca2+ current (I(Ca T)) and thereby increased the proportion of the total Ca2+ current at more negative potentials. We conclude that sustained exposure to isoproterenol reduces I(K1), I(Ks), and I(Ca L) density and increases the density of I(Ca T). The direct ionic current remodeling effects of sustained beta-adrenoceptor stimulation resemble changes reported with heart failure and may be important in arrhythmogenic ionic remodeling.

Transmural differences in rat ventricular protein kinase C epsilon correlate with its functional regulation of a transient cardiac K+ current

The effects of PKC activation on a transient (It) and a sustained (Iss) cardiac K+ current and the subcellular distribution of the epsilon isoform of PKC (PKC(epsilon)) were compared in epicardial and endocardial regions of the rat ventricle. Activation of PKC(epsilon) with a diacylglycerol analogue (di-octanoyl-glycerol (DiC8), 20 (mu)M) leads to differential effects in epicardial and endocardial cells. In epicardial cells (n = 20) It and Iss are attenuated by 17.7 +/- 2.1 % and 11.9 +/- 3.1 %, respectively (means +/- S.E.M.). In endocardial cells It attenuation was significantly smaller (4.6 +/- 1.6 %, n = 14, P < 0.0005). Iss attenuation was similar to that in epicardial cells (10.5 +/- 3.8 %). PKC[epsilon] expression was measured by Western blotting. Calculated endocardial/epicardial ratios showed no regional differences in total protein extracts (1.04 +/- 0.11, mean +/- S.E.M, n = 4), but PKC[epsilon] distribution in the cytosolic fraction showed a marked difference, with significantly (P < 0.05) higher levels in endocardial extracts. The cytosolic endocardial/epicardial PKC[epsilon] ratio was 2.64 +/- 0.24 (n = 4), indicating a reduced amount of PKC[epsilon] in the membrane fraction of the endocardium. This could account for the reduced effect of DiC8 on It in endocardial myocytes. Under both hypothyroid and streptozotocin-induced diabetic conditions the difference in endocardial and epicardial cytosolic PKC[epsilon] levels was absent (ratios of 0.86 +/- 0.21 (n = 4) and 1.09 +/- 0.16 (n = 3), respectively; means +/- S.E.M.). Ratios in the total protein extracts were not significantly different from those in control conditions. The results show transmural differences in the functional effects of PKC(epsilon) activation on a cardiac K+ current, and in the subcellular distribution of PKC(epsilon). These differences are absent in diabetic and hypothyroid conditions.

Endurance training alters outward K+ current characteristics in rat cardiocytes

The effect of endurance run training on outward K+ currents with rapidly inactivating (I(to)) and sustained or slowly inactivating (I(sus)) characteristics was examined in left ventricular (LV) cardiocytes isolated from sedentary (Sed) and treadmill-trained (Tr) female Sprague-Dawley rats. Isolated LV cardiocytes were used in whole cell patch-clamp studies to characterize whole cell I(to) and I(sus). Peak I(to) was greatest in cells isolated from the Tr group. When I(to) was corrected for cell capacitance to yield a current density, most, but not all, of the Sed vs. Tr differences in I(to) magnitude were eliminated. Regardless of how I(to) was expressed (e.g., I(to) or I(to) density), the time required to achieve a peak value was markedly shortened in the cardiocytes isolated from the Tr group. Training elicited a reduction in I(sus) density. Action potential characteristics were determined in Sed and Tr cardiocytes in primary culture. Training did not affect resting membrane potential, whereas peak membrane potential was reduced and time to peak membrane potential was prolonged in the Tr group. In addition, time to 50% repolarization was significantly increased in cells from the Tr group. Collectively, these data indicate that I(to) and I(sus) characteristics are altered by training in isolated LV cardiocytes. These alterations in I(to) and I(sus) may be responsible, at least in part, for the training-induced alterations in action potential configuration in cardiocytes in primary culture.

Regional alterations of repolarizing K+ currents among the left ventricular free wall of rats with ascending aortic stenosis

The effect of cardiac hypertrophy on electrocardiogram (ECG), action potential duration (APD) and repolarizing K+ currents was investigated in epicardial, midmyocardial and endocardial myocytes isolated from the rat left ventricular free wall. Cardiac hypertrophy was induced by stenosis of the ascending aorta (AS), which led to an increased pressure load (+85 +/- 10 u1v1vZ mm11Z Hg) of the left ventricle; sham-operated animals served as controls. In ECG recordings from AS rats, the QTc interval was prolonged and the main vectors of the QRS complex and the T-wave pointed in opposite directions, indicating an abnormal sequence of repolarization. APD and K+ currents were recorded using the whole-cell patch-clamp technique. In the AS group, APD90 (90 % repolarization) was significantly prolonged in epicardial and midmyocardial, but not endocardial myocytes. Corresponding to the increase in APD, the magnitude of the transient outward K+ current (Ito1) was significantly smaller (-30 %) in epicardial and midmyocardial, but not endocardial myocytes. Inactivation and steady-state inactivation of Ito1 were not affected by hypertrophy. Recovery from inactivation was slightly prolonged in endocardial myocytes from AS rats. No differences in delayed rectifier currents (IK) or inwardly rectifying K+ currents (IK1) were detected between myocytes of the three regions of sham-operated or AS animals. However, both currents were reduced by AS. The present data show that cardiac hypertrophy caused by pressure overload leads to an increase in APD and a decrease in Ito1 primarily in epicardial and midmyocardial myocytes, which implies a major role of alterations in Ito1 for the reduced gradient in APD. The effects of AS on IK1 and IK may slightly counteract the decrease in APD gradient. The observed changes in APD and underlying ionic currents could well explain the alterations in repolarization observed in the ECG induced by cardiac hypertrophy.

Protein kinase C regulation of K+ currents in rat ventricular myocytes and its modification by hormonal status

1. The effects of protein kinase C (PKC) activation on cardiac K+ currents were studied in rat ventricular myocytes, using whole-cell voltage clamp methods. Control rats were compared to hypothyroid or diabetic rats, in which PKC expression and activity were enhanced. 2. In control myocytes, two calcium-independent outward K+ currents, the transient It and the sustained Iss, were attenuated by 18.9 +/- 2.0 and 16.8 +/- 3.5 %, respectively (mean +/- s.e.m.), following addition of a synthetic analogue of diacylglycerol, DiC8 (20 microM). In myocytes from hypothyroid or diabetic rats, It and Iss were not affected by DiC8. 3. The effects of DiC8 were restored in myocytes from thyroidectomized rats by injection of physiological doses of tri-iodothyronine (T3; 10 microg kg-1 for 6-8 days). Incubating cells from diabetic rats with 100 nM insulin for 5-9 h also restored the ability of DiC8 to attenuate It and Iss. 4. The attenuation of K+ currents by DiC8 in control cells was absent in the presence of a peptide known to inhibit the translocation of the isoform PKCepsilon (EAVSKPLT, 24 microM introduced through the recording pipette). A scrambled peptide (LSETKPAV) was without effect. 5. Under hypothyroid conditions the inhibitory peptide restored the effects of DiC8 on It and Iss. These currents were attenuated by 11.9 +/- 1. 5 and 9.8 +/- 1.5 %, respectively, which was significantly (P < 0. 001) more than without the peptide or with the scrambled peptide. 6. These results show that the PKC-mediated suppression of cardiac K+ currents is normally mediated by PKCepsilon translocation. This effect is absent under hypothyroid and diabetic conditions, presumably due to prior PKC activation and translocation. A PKCepsilon translocation inhibitor restores the ability of DiC8 to attenuate K+ currents under hypothyroid conditions. This presumably reflects a (partial) reversal of a chronic translocation and a shift in the balance between PKC and its anchoring proteins.