Effects of thyroid hormone on the calcium current and isoprenaline-induced background current in rabbit ventricular myocytes

New aspects of endocrine control of atrial fibrillation and possibilities for clinical translation

Hormones are potent endo-, para-, and autocrine endogenous regulators of the function of multiple organs, including the heart. Endocrine dysfunction promotes a number of cardiovascular diseases, including atrial fibrillation (AF). While the heart is a target for endocrine regulation, it is also an active endocrine organ itself, secreting a number of important bioactive hormones that convey significant endocrine effects, but also through para-/autocrine actions, actively participate in cardiac self-regulation. The hormones regulating heart-function work in concert to support myocardial performance. AF is a serious clinical problem associated with increased morbidity and mortality, mainly due to stroke and heart failure. Current therapies for AF remain inadequate. AF is characterized by altered atrial function and structure, including electrical and profibrotic remodelling in the atria and ventricles, which facilitates AF progression and hampers its treatment. Although features of this remodelling are well-established and its mechanisms are partly understood, important pathways pertinent to AF arrhythmogenesis are still unidentified. The discovery of these missing pathways has the potential to lead to therapeutic breakthroughs. Endocrine dysfunction is well-recognized to lead to AF. In this review, we discuss endocrine and cardiocrine signalling systems that directly, or as a consequence of an underlying cardiac pathology, contribute to AF pathogenesis. More specifically, we consider the roles of products from the hypothalamic-pituitary axis, the adrenal glands, adipose tissue, the renin–angiotensin system, atrial cardiomyocytes, and the thyroid gland in controlling atrial electrical and structural properties. The influence of endocrine/paracrine dysfunction on AF risk and mechanisms is evaluated and discussed. We focus on the most recent findings and reflect on the potential of translating them into clinical application.

Protein kinase A regulates C-terminally truncated CaV 1.2 in Xenopus oocytes: roles of N- and C-termini of the α1C subunit

KEY POINTS

β-Adrenergic stimulation enhances Ca²⁺ entry via L-type Ca_V 1.2 channels, causing stronger contraction of cardiac muscle cells. The signalling pathway involves activation of protein kinase A (PKA), but the molecular details of PKA regulation of Ca_V 1.2 remain controversial despite extensive research. We show that PKA regulation of Ca_V 1.2 can be reconstituted in Xenopus oocytes when the distal C-terminus (dCT) of the main subunit, α_1C , is truncated. The PKA upregulation of Ca_V 1.2 does not require key factors previously implicated in this mechanism: the clipped dCT, the A kinase-anchoring protein 15 (AKAP15), the phosphorylation sites S1700, T1704 and S1928, or the β subunit of Ca_V 1.2. The gating element within the initial segment of the N-terminus of the cardiac isoform of α_1C is essential for the PKA effect. We propose that the regulation described here is one of two or several mechanisms that jointly mediate the PKA regulation of Ca_V 1.2 in the heart.

ABSTRACT

β-Adrenergic stimulation enhances Ca²⁺ currents via L-type, voltage-gated Ca_V 1.2 channels, strengthening cardiac contraction. The signalling via β-adrenergic receptors (β-ARs) involves elevation of cyclic AMP (cAMP) levels and activation of protein kinase A (PKA). However, how PKA affects the channel remains controversial. Recent studies in heterologous systems and genetically engineered mice stress the importance of the post-translational proteolytic truncation of the distal C-terminus (dCT) of the main (α_1C ) subunit. Here, we successfully reconstituted the cAMP/PKA regulation of the dCT-truncated Ca_V 1.2 in Xenopus oocytes, which previously failed with the non-truncated α_1C . cAMP and the purified catalytic subunit of PKA, PKA-CS, injected into intact oocytes, enhanced Ca_V 1.2 currents by ∼40% (rabbit α_1C ) to ∼130% (mouse α_1C ). PKA blockers were used to confirm specificity and the need for dissociation of the PKA holoenzyme. The regulation persisted in the absence of the clipped dCT (as a separate protein), the A kinase-anchoring protein AKAP15, and the phosphorylation sites S1700 and T1704, previously proposed as essential for the PKA effect. The Ca_V β_2b subunit was not involved, as suggested by extensive mutagenesis. Using deletion/chimeric mutagenesis, we have identified the initial segment of the cardiac long-N-terminal isoform of α_1C as a previously unrecognized essential element involved in PKA regulation. We propose that the observed regulation, that exclusively involves the α_1C subunit, is one of several mechanisms underlying the overall PKA action on Ca_V 1.2 in the heart. We hypothesize that PKA is acting on Ca_V 1.2, in part, by affecting a structural 'scaffold' comprising the interacting cytosolic N- and C-termini of α_1C .

Evaluation of P-Wave Dispersion, Diastolic Function, and Atrial Electromechanical Conduction in Pediatric Patients with Subclinical Hypothyroidism

AIM

This study aimed to evaluate ventricular diastolic dysfunction, inter- and intraatrial conduction delay, and P-wave dispersion in pediatric patients with subclinical hypothyroidism.

METHODS

The study comprised a total of 30 pediatric patients with subclinical hypothyroidism (SH) (mean age 7.8 ± 3.2 years) and 30 healthy children (mean age 8.4 ± 3.6 years) as the control group. A SH diagnosis was made in the event of increased serum thyroid-stimulating hormone (TSH) and decreased serum free triiodothyronine (T3 ) and free thyroxine (T4 ) concentrations.

RESULTS

Conventional Doppler imaging (TDI) showed low mitral early diastolic E-wave velocity and E/A ratio (P < 0.001) and significantly higher mitral late diastolic A-wave velocity (P = 0.001) in hypothyroidism patients. Moreover, patients with hypothyroidism had significantly lower left ventricular (LV) septal Em velocity and Em /Am ratios compared with the control group (P < 0.001), whereas Am velocity was higher in hypothyroidism patients (P = 0.018). LV lateral Em velocity and Em /Am ratio were significantly lower in patients with hypothyroidism compared with the control group (P < 0.001). With regard to atrial electromechanical conduction, atrial electromechanical delay (PA) lateral, PA septum, PA tricuspid, and each of interatrial and intraatrial conduction delay were significantly prolonged in hypothyroidism patients as compared with the control group (P < 0.001, P < 0.001, P = 0.023, P = 0.002, and P = 0.003, respectively). P-wave dispersion was significantly different in the pediatric patients with hypothyroidism (P < 0.001).

CONCLUSION

This study demonstrated atrial electromechanical conduction delay, abnormal P-wave dispersion, and ventricle diastolic dysfunction in pediatric patients with hypothyroidism.

Thyroid-induced worsening of parkinsonian tremor resistant to drugs and subthalamic nucleus deep brain stimulation

Introduction. Symptoms of both hypothyroidism and thyrotoxicosis can be easily overlooked in patients with Parkinson's disease (PD). We report on a patient whose parkinsonian tremor worsened and proved refractory not only to common treatment, but also to deep brain stimulation (DBS). Case Presentation. A 61-year-old woman with advanced PD underwent bilateral subthalamic DBS, with an excellent outcome. Twenty-one months after the surgery, however, patient's resting/postural tremor markedly worsened. There was a slight improvement for 1 month after repeated adjustments of DBS parameters, but then the tremor worsened again. Since even a minimal increase of the dose of dopaminergic drugs caused extremely severe dyskinesias, an anticholinergic drug biperiden and benzodiazepine clonazepam were introduced, what helped for another month. With the onset of severe diarrhoea, a laboratory workup was performed. Thyrotoxicosis was detected. During treatment with the antithyroid agent carbimazole, the parkinsonian tremor clearly improved within two weeks. Conclusion. A hyperthyroid state can markedly exaggerate all forms of tremor, as well as other types of movement disorders. This condition can be overlooked or masked by other symptoms. Therefore, if the tremor in a patient with PD gradually worsens and proves resistant to the usual treatment, examine the thyroid gland.

Thyroid hormones and cardiac arrhythmias

Thyroid hormone plays an important role in cardiac electrophysiology and Ca2+ handling through both genomic and nongenomic mechanisms of action, while both actions can interfere. Chronic changes in the amount of circulating thyroid hormone due to thyroid dysfunction or systemic disease result in structural, electrophysiological and Ca2+ handling remodeling, while acute changes may affect basal activity of cardiac cells membrane systems. Consequently, long-term or rapid modulation of sarcolemmal ion channels, Ca2+ cycling proteins and intercellular communicating channels by thyroid hormone may affect heart function as well as susceptibility of the heart to arrhythmias. This aspect including pro- and anti-arrhythmic potential of thyroid hormone is highlighted in this review.

Acute, nongenomic effect of thyroid hormones in preventing calcium overload in newborn rat cardiocytes

In this study, we examined the acute effects of thyroid hormones (TH) T(3) and T(4), leading to improvement of myocardial function through activation of Ca(2+) extrusion mechanisms and, consequently, prevention of intracellular calcium overload. Extracellular calcium elevation from 1.8 to 3.8 mM caused immediate increase in intracellular calcium level ([Ca(2+)](i)) in newborn cardiomyocyte cultures. Administration of 10 or 100 nM T(3) or T(4) rapidly (within 10 sec) decreased [Ca(2+)](i) to its control level. Similar results were obtained when [Ca(2+)](i) was elevated by decreasing extracellular Na(+) concentration, causing backward influx of Ca(2+) through Na(+)/Ca(2+) exchanger, or by administration of caffeine, releasing Ca(2+) from the sarcoplasmic reticulum (SR). Under these conditions, T(3) or T(4) decreased [Ca(2+)](i). T(3) and T(4) also exhibited protective effects during ischemia. T(3) or T(4) presence during hypoxia for 120 min in culture medium restricted the increase of [Ca(2+)](i) and prevented the pathological effects of its overload. An inhibitor of SR Ca(2+)-ATPase (SERCA2a), thapsigargin, increases [Ca(2+)](i) and in its presence neither T(3) nor T(4) had any effect on the [Ca(2+)](i) level. The reduction of [Ca(2+)](i) level by T(3) and T(4) was also blocked in the presence of H-89 (a PKA inhibitor), and by calmodulin inhibitors. The effect of TH on the reduction of [Ca(2+)](i) was prevented by propranolol, indicating that the hormones exert their effect through interaction with adrenergic receptors. These results support our hypothesis that TH prevent calcium overload in newborn rat cardiomyocytes, most likely by a direct, acute, and nongenomic effect on Ca(2+) transport into the SR.

Simple electrocardiographic markers for the prediction of paroxysmal atrial fibrillation in hyperthyroidism

BACKGROUND

Hyperthyroidism is a major cause of paroxysmal atrial fibrillation (AF). The purpose of this study was to evaluate the predictors of AF in the patients with clinical and subclinical hyperthyroidism.

METHODS AND RESULTS

The study population consisted of four groups: group I (57 euthyroid healthy persons), group II (33 patients with subclinical hyperthyroidism), group III (69 patients with overt hyperthyroidism) and group IV (31 patients with overt hyperthyroidism and documented paroxysmal AF). The maximum P wave duration (P maximum) in group IV (114 +/- 8 ms) was significantly higher than group I (102 +/- 7 ms, p < 0.001), group II (106 +/- 7 ms, p < 0.001) and group III (108 +/- 9 ms, p0.005). The P wave dispersion (PWD) was measured as 46 +/- 9 ms in group IV and this was significantly higher than group I (29 +/- 8 ms, p < 0.001), group II (36 +/- 9 ms, p < 0.001) and grup III (38 +/- 8 ms, p = 0.001). The P maximum and PWD were higher in the patients with subclinical hyperthyroidism compared to healthy individuals. Univariate regression analysis revealed that age, P maximum and PWD, multivariate analysis showed that P maximum and PWD were significant predictors of paroxysmal AF. A PWD value of 37.5 ms separated group IV from others with a sensitivity of 90%, specificity of 85%, and positive predictive accuracy of 77%.

CONCLUSION

Simply measuring P maximum and PWD values, we could identify the patients with high risk for the development of AF and these simple ECG parameters may help in clinical judgement to determine the requirement for treatment in the patients with subclinical hyperthyroidism.

Genomic and non-genomic regulation of L-type calcium channels in rat ventricle by thyroid hormone

Hyperthyroidism is associated with low exercise tolerance despite high cardiac output and sometimes with the development of heart failure. L-type calcium channels may play a role in the mechanism, but this has not been fully understood. We examined the effects of thyroid hormone on gene expression and function of L-type calcium channels in rat ventricles by the ribonuclease protection assay and whole-cell patch-clamp technique, respectively. The effects of bisoprolol, beta-blocking agent, on the regulation of calcium channel by thyroid hormone was also studied. In hyperthyroid animals, the mRNA of the calcium channel alpha1c subunit was reduced on day 4, compared with that in euthyroid animals, and remained low on day 8. Bisoprolol did not affect the thyroid hormone mediated decrease in alpha1c subunit mRNA. While L-type calcium current was greater in hyperthyroid than euthyroid myocytes on day 4, it was smaller on day 8. In addition, the isoproterenol-induced increase in calcium current in euthyroid rats was attenuated in hyperthyroid rats. Acetylcholine decreased calcium current in hyperthyroid myocytes, but not in euthyroid myocytes. In conclusion, L-type calcium current was increased by thyroid hormone in rat ventricular myocytes by the activation of the adenylate cyclase cascade, despite a decreased calcium channel gene expression. These genomic and non-genomic modifications may play an important role in the association of high cardiac output with low exercise tolerance, and in the development of heart failure in hyperthyroidism.

G protein modulates thyroid hormone-induced Na(+) channel activation in ventricular myocytes

To evaluate the effects of liothyronine (3,5,3'-triiodo-L-thyronine, T(3)) on Na(+) channel current (I(Na)) properties, I(Na) was recorded in adult guinea pig ventricular myocytes. T(3) (1 nM) acutely increased whole cell I(Na) and shifted the steady-state I(Na) inactivation curve dose dependently. When the pipette solution contained 100 microM GTP or GTPgammaS, the effect of T(3) on the whole cell I(Na) was increased two- to threefold. This effect was almost completely abolished by pertussis toxin preincubation. In the cell-attached patch, T(3) increased the open probability of single I(Na) by reducing the null probability. In the inside-out patch, T(3) effect was 10 times faster than that in whole cell and cell-attached patches while GTPgammaS was present and could be completely washed out. T(3) alone slightly increased the channel open probability by increasing the closed state to open state rate constant (k(CO)) and reducing the null probability. GTPgammaS exposure only increased the number of functional channels. T(3) and GTPgammaS synergistically enhanced the channel open probability 5.8 +/- 0.5-fold by increasing k(CO), decreasing the open state to absorbing inactivated state rate constant, and greatly reducing the null probability. These results demonstrate that T(3) acts on the cytosolic side of the membrane and acutely activates I(Na). Pertussis toxin-sensitive G protein modulation greatly magnifies the T(3) effects on the channel kinetics and null probability, thereby increasing the channel open probability.

Matrix is the site of indirect effects of propranolol on the ion channelopathies in cardiac remodeling by L-thyroxine

Cardiac remodeling by chronic L-thyroxine medication causes exaggerated cardiac arrhythmias in relation to ion channelopathies that involve multichannels. The matrix of lipid membrane is likely the key site where channel lesions, possibly will develop and be benefitted by drug intervention. Cardiac remodeling in rats and guinea pigs was developed by L-thyroxine 0.5 mg/kg SC for 10 days. Propranolol was instituted on days 8-10. Whole cell holding was applied to measure ion currents. An increase in HR, dispersion of QTc, mitochondrial Na+/K+ ATPase, Ca2+/Mg2+ ATPase, and LPO production were found in the model. T3 and T4 levels in plasma were high. Propranolol was effective in regressing cardiac remodeling, together with lowering all the parameters and the enhanced I(Ca.L),I(KS), and I(KR) currents, but T3 and T4 remained basically unchanged. The changes in ion channels are likely the consequence of the cardiac remodeling that is formed by oxidative stress and increased energy consumption provoked by L-thyroxine. The benefit of propranolol on the disordered ion channels is mediated by its ability to ameliorate lesions of the matrix.

Nongenomic actions of thyroid hormone on the heart

Extranuclear or nongenomic actions of thyroid hormone do not require formation of a nuclear complex between the hormone and its traditional 3,5,3'-triiodo-L-thyronine (T3) receptor (TR). Among nongenomic actions of iodothyronines that are relevant to the heart are those on membrane ion channels or pumps. These include stimulation of the sarcolemmal Na+ channel, inward-rectifying K+ channel, voltage-activated potassium channels, and calcium pump (Ca2+-adenosine triphosphatases [ATPases]) and have been shown in intact cells or isolated membranes. Because circulating levels of thyroid hormone are relatively stable, actions on channels or pumps may contribute to setting of basal activity of these transport functions. The mechanism of certain of these membrane effects may involve actions of the hormone on signal transducing protein kinases that modulate levels of activity of plasma membrane channels. Thyroid hormone nongenomically enhances myocardial contractility in isolated myocardial cells, in the isolated perfused rat heart and in human subjects. Iodothyronines also decrease vasomotor tone in a variety of models and in man by a mechanism independent of cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP), or nitric oxide generation. Acutely increased myocardial mitochondrial respiration has been demonstrated in isolated organelles exposed to thyroid hormone. Genomic and nongenomic actions of thyroid hormone can interface, e.g., at the level of sarcoplasmic reticulum Ca2+-ATPase, where gene expression is regulated by the TR-T3 complex and activity of the enzyme can be modulated nongenomically. The relevance of nongenomic actions of thyroid hormone on the heart has been demonstrated in acute effects of the hormone on cardiac output and systemic vascular resistance in human subjects.

Effects of thyroid hormone on the arrhythmogenic activity of pulmonary vein cardiomyocytes

OBJECTIVES

This study was conducted to investigate the effects of thyroid hormone on the electrophysiological characteristics of pulmonary vein (PV) cardiomyocytes.

BACKGROUND

Hyperthyroidism is an important etiology of paroxysmal atrial fibrillation (AF). Pulmonary veins are known to initiate paroxysmal AF.

METHODS

The action potential and ionic currents were investigated in single rabbit PV and atrial cardiomyocytes with (hyperthyroid) and without (control) incubation of L-triiodothyronine using the whole-cell clamp technique.

RESULTS

Compared with the control cardiomyocytes, hyperthyroid PV and atrial cardiomyocytes had shorter action potential duration. Hyperthyroid PV cardiomyocytes had faster beating rates (1.82 +/- 0.13 Hz vs. 1.03 +/- 0.15 Hz, p < 0.005) and a higher incidence of delayed after depolarization (beating: 92% vs. 6%, p < 0.0001; non-beating: 45% vs. 3%, p < 0.005). However, only hyperthyroid PV beating cardiomyocytes had a higher incidence of early after depolarization (46% vs. 0%, p < 0.0001). The ionic current experiments showed that hyperthyroid PV beating cardiomyocytes had larger densities of overall slow inward (2.72 +/- 0.21 pA/pF vs. 2.07 +/- 0.19 pA/pF, p < 0.05), overall transient outward (1.39 +/- 0.21 pA/pF vs. 0.48 +/- 0.08 pA/pF, p < 0.001) and steady state outward currents (0.78 +/- 0.06 pA/pF vs. 0.58 +/- 0.04 pA/pF, p < 0.05) on depolarization and larger transient inward (0.021 +/- 0.004 pA/pF vs. 0.005 +/- 0.001 pA/pF, p < 0.001) on repolarization. By contrast, the hyperthyroid PV non-beating cardiomyocytes had larger densities of overall transient outward (1.01 +/- 0.14 pA/pF vs. 0.37 +/- 0.07 pA/pF, p < 0.001), steady state outward (0.61 +/- 0.06 pA/pF vs. 0.44 +/- 0.04 pA/pF, p < 0.05) and transient inward currents (0.011 +/- 0.002 pA/pF vs. 0.003 +/- 0.001 pA/pF, p < 0.05).

CONCLUSIONS

Thyroid hormone changes the electrophysiological activity of the PV cardiomyocytes. Increased automaticity and enhanced triggered activity may increase the arrhythmogenic activity of PVs in hyperthyroidism.

Electrophysiological abnormalities of the atrial muscle in patients with paroxysmal atrial fibrillation associated with hyperthyroidism

OBJECTIVE

Atrial fibrillation (AF) is common in patients with hyperthyroidism. Although the choice of an antiarrhythmic agent should be based on its electrophysiological effects and the electrophysiological properties of the arrhythmia in question, the atrial electrophysiological features of AF associated with hyperthyroidism are unknown. The purposes of this study are to clarify the atrial electrophysiological abnormalities of AF with hyperthyroidism, and to propose effective therapies for AF in patients with hyperthyroidism.

SUBJECTS AND DESIGN

This study included 117 patients who underwent electrophysiological study and were evaluated for thyroid function: 29 patients without AF or hyperthyroidism (Group I), 78 patients with lone paroxysmal AF (Group II), and 10 patients with paroxysmal AF and hyperthyroidism (Group III). The following electrophysiological parameters were assessed and measured quantitatively: (1) the incidence of abnormal right atrial electrograms during sinus rhythm, indicating areas of altered anatomy and conduction where AF is likely to develop; (2) the atrial effective refractory period (ERP); and (3) the atrial conduction delay (CD), which is induced by early atrial premature beats close to the atrial ERP and is thought to facilitate the occurrence of AF.

RESULTS

The incidence of abnormal right atrial electrograms during sinus rhythm was significantly higher in Group II (67.1%) than in Group I (20.0%, P < 0.001) and Group III (22.2%, P = 0.009). The atrial ERP was significantly shorter in Group III (187 +/- 7 ms) than in Group I (215 +/- 36 ms, P = 0.019) and Group II (208 +/- 28 ms, P = 0.022). The atrial CD was observed in Group III as well as in Group II.

CONCLUSIONS

Our data indicate that the electrophysiological features of paroxysmal AF associated with hyperthyroidism are essentially different from those of lone paroxysmal AF. In patients with paroxysmal AF and hyperthyroidism, a shortening of the refractory period in association with a facilitation of the atrial CD could be expected to increase the propensity for AF, and a pre-existent arrhythmogenic substrate might not be essential to the genesis of AF. These findings suggest that the agents that prolong the atrial ERP are effective against AF in patients with hyperthyroidism.

Single-channel activity and expression of atrial L-type Ca(2+) channels in patients with latent hyperthyroidism

Patients with "latent hyperthyroidism" (suppressed thyroid-stimulating hormone and normal circulating thyroid hormones) are at risk to develop atrial fibrillation. In animal models, hyperthyroidism is associated with increased cardiac L-type Ca(2+) current. Therefore, we assessed L-type channel function and expression in right atria from patients undergoing cardiac surgery. Single L-type channels were studied in the cell-attached condition. Voltage dependence of gating was similar in patients with and without latent hyperthyroidism. With use of a pulse protocol leading to maximum channel availability, single-channel activity was further analyzed. Average peak current was significantly enhanced in latent hyperthyroidism, mainly because of an increased channel availability (P < 0.05). Protein expression was analyzed by Western blot. In latent hyperthyroidism, expression of Ca(2+) channel alpha(1)-subunits was increased more than threefold (P < 0.01). In contrast, sarco(endo)plasmic reticulum Ca(2+)-ATPase and phospholamban levels were not significantly changed. We only observed a trend toward increased sarco(endo)plasmic reticulum Ca(2+)-ATPase expression (P = 0.085). Function and expression of human atrial L-type Ca(2+) channels are increased in latent hyperthyroidism. These endocrine effects on the heart may be clinically relevant.

Anion transport in heart

Anion transport proteins in mammalian cells participate in a wide variety of cell and intracellular organelle functions, including regulation of electrical activity, pH, volume, and the transport of osmolites and metabolites, and may even play a role in the control of immunological responses, cell migration, cell proliferation, and differentiation. Although significant progress over the past decade has been achieved in understanding electrogenic and electroneutral anion transport proteins in sarcolemmal and intracellular membranes, information on the molecular nature and physiological significance of many of these proteins, especially in the heart, is incomplete. Functional and molecular studies presently suggest that four primary types of sarcolemmal anion channels are expressed in cardiac cells: channels regulated by protein kinase A (PKA), protein kinase C, and purinergic receptors (I(Cl.PKA)); channels regulated by changes in cell volume (I(Cl.vol)); channels activated by intracellular Ca(2+) (I(Cl.Ca)); and inwardly rectifying anion channels (I(Cl.ir)). In most animal species, I(Cl.PKA) is due to expression of a cardiac isoform of the epithelial cystic fibrosis transmembrane conductance regulator Cl(-) channel. New molecular candidates responsible for I(Cl.vol), I(Cl.Ca), and I(Cl.ir) (ClC-3, CLCA1, and ClC-2, respectively) have recently been identified and are presently being evaluated. Two isoforms of the band 3 anion exchange protein, originally characterized in erythrocytes, are responsible for Cl(-)/HCO(3)(-) exchange, and at least two members of a large vertebrate family of electroneutral cotransporters (ENCC1 and ENCC3) are responsible for Na(+)-dependent Cl(-) cotransport in heart. A 223-amino acid protein in the outer mitochondrial membrane of most eukaryotic cells comprises a voltage-dependent anion channel. The molecular entities responsible for other types of electroneutral anion exchange or Cl(-) conductances in intracellular membranes of the sarcoplasmic reticulum or nucleus are unknown. Evidence of cardiac expression of up to five additional members of the ClC gene family suggest a rich new variety of molecular candidates that may underlie existing or novel Cl(-) channel subtypes in sarcolemmal and intracellular membranes. The application of modern molecular biological and genetic approaches to the study of anion transport proteins during the next decade holds exciting promise for eventually revealing the actual physiological, pathophysiological, and clinical significance of these unique transport processes in cardiac and other mammalian cells.

Cardiovascular phenotype and temperature control in mice lacking thyroid hormone receptor-beta or both alpha1 and beta

We have used a telemetry system to record heart rate, body temperature, electrocardiogram (ECG), and locomotor activity in awake, freely moving mice lacking thyroid hormone receptor (TR)-beta or TR-alpha1 and -beta (TR-alpha1/beta). The TR-alpha1/beta-deficient mice had a reduced heart rate compared with wild-type controls. The TR-beta-deficient mice showed an elevated heart rate, which, however, was unresponsive to thyroid hormone treatment regardless of hormonal serum levels. ECG revealed that the TR-beta-deficient mice had a shortened Q-Tend time in contrast to the TR-alpha1/beta-deficient mice, which exhibited prolonged P-Q and Q-Tend times. Mental or pharmacological stimulation of the sympathetic nervous system resulted in a parallel increase in heart rate in all animals. A single injection of a nonselective beta-adrenergic-receptor blocker resulted in a parallel decrease in all mice. The TR-alpha1/beta-deficient mice also had a 0.4 degrees C lower body temperature than controls, whereas no difference was observed in locomotor activity between the different strains of mice. Our present and previous results support the hypothesis that TR-alpha1 has a major role in determining heart rate under baseline conditions and body temperature and that TR-beta mediates a hormone-induced increase in heart rate.

Evidence that decreased heart rate in thyroid hormone receptor-alpha1-deficient mice is an intrinsic defect

Using a telemetry system with implantable transmitters, we recorded heart rate, electrocardiogram (ECG), body temperature, and locomotor activity continuously in awake, freely moving mice deficient in the thyroid hormone receptor-alpha1 (TRalpha1). We have previously reported that the TRalpha1-deficient mice have a 20% lower mean heart rate and a 0.5 degrees C lower body temperature compared with wild-type control animals. In this study we found that when 3,5, 3'-triiodothyronine (T3) was given once a day, there was a parallel increase in heart rate (occurring 1 day later in the TRalpha1-deficient mice than in controls) and body temperature. Analysis of single-lead ECG revealed a prolonged QRS and Q-Tend time in the TRalpha1-deficient mice, which was shortened after T3 treatment. Monophasic action potential durations, measured in hearts from anesthetized mice at 90% of repolarization, were significantly prolonged in TRalpha1-deficient mice. Air-jet stress and a single injection of an anticholinergic agent induced a parallel increase, and a beta-adrenergic receptor blocker induced a decrease in heart rate in both groups. There was no difference in beta-adrenergic receptor density. The results indicate that the TRalpha1-deficient mice have a specific defect in intrinsic heart rate regulation.

Nongenomic actions of thyroid hormone

Nongenomic actions of thyroid hormone are by definition independent of nuclear receptors for the hormone and have been described at the plasma membrane, various cell organelles, the cytoskeleton, and in cytoplasm. The actions include alterations in solute transport (Ca2+, Na+, glucose), changes in activities of several kinases, including protein kinase C, cAMP-dependent protein kinase and pyruvate kinase M2 (PKM2), effects on efficiency of specific mRNA translation and mRNA t1/2, modulation of mitochondrial respiration, and regulation of actin polymerization (promotion of formation of F-actin). Iodothyronines also can regulate nongenomically the state of contractile elements in vascular smooth muscle cells (VSMC). The physiologic significance at the cellular level of certain of these actions has been demonstrated, for example, in the cases of myocardiocyte Na+ current, red cell Ca2+ content, and the control by hormone-induced alterations in actin solubility of cell surface activity of iodothyronine 5'-monodeiodinase activity and the intracellular distribution of protein disulfide isomerase activity. The physiologic significance of these actions at the organ or system level is less clear, but extranuclear effects of thyroid hormone on myocardial Na+ channel, sarcoplasmic reticulum Ca(2+)-ATPase activity, and contractile state of VSMC may each contribute to acute effects of thyroid hormone on cardiac output that have recently been described clinically. The molecular mechanisms for nongenomic actions are incompletely understood; relevant binding sites and signal transduction pathways have been described for hormone actions on plasma membrane Ca(2+)-ATPase activity, and PKM2 monomer is known to bind T3 and, as a result, prevent activation of the kinase via tetramer formation. Nongenomic actions of thyroid hormone may have different structure-activity relationships of iodothyronines from those effects that depend upon nuclear receptors; they may have different time courses and may invoke complex signal transduction pathways before the action is detected.