Regulation of cardiac sarcolemmal Ca2+ channels and Ca2+ transporters by thyroid hormone

Factors predicting the progression from paroxysmal to persistent atrial fibrillation despite an index catheter ablation

INTRODUCTION

Despite undergoing an index ablation, some patients progress from paroxysmal atrial fibrillation (PAF) to persistent AF (PersAF), and the mechanism behind this is unclear. The aim of this study was to investigate the predictors of progression to PersAF after catheter ablation in patients with PAF.

METHODS

This study included 400 PAF patients who underwent an index ablation between 2015 and 2019. The patients were classified into three groups based on their outcomes: Group 1 (PAF to sinus rhythm, n = 226), Group 2 (PAF to PAF, n = 146), and Group 3 (PAF to PersAF, n = 28). Baseline and procedural characteristics were collected, and predictors for AF recurrence and progression were evaluated.

RESULTS

The mean age of the patients was 58.4 ± 11.1 years, with 272 males. After 3 years of follow-up, 7% of the PAF cases recurred and progressed to PersAF despite undergoing an index catheter ablation. In the multivariable analysis, a larger left atrial (LA) diameter and the presence of non-pulmonary vein (PV) triggers during the index procedure independently predicted recurrence. Moreover, a larger LA diameter, the presence of non-PV triggers, and a history of thyroid disease independently predicted AF progression.

CONCLUSION

The progression from PAF to PersAF after catheter ablation is associated with a larger LA diameter, history of thyroid disease, and the presence of non-PV triggers. Meticulous preprocedural evaluation, patient selection, and comprehensive provocation tests during catheter ablation are recommended.

Both hypothyroidism and hyperthyroidism increase atrial fibrillation inducibility in rats

BACKGROUND

Evidence indicates that cardiac hypothyroidism may contribute to heart failure progression. It is also known that heart failure is associated with an increased risk of atrial fibrillation (AF). Although it is established that hyperthyroidism increases AF incidence, the effect of hypothyroidism on AF is unclear. This study investigated the effects of different thyroid hormone levels, ranging from hypothyroidism to hyperthyroidism on AF inducibility in thyroidectomized rats.

METHODS AND RESULTS

Thyroidectomized rats with serum-confirmed hypothyroidism 1 month after surgery were randomized into hypothyroid (N=9), euthyroid (N=9), and hyperthyroid (N=9) groups. Rats received placebo, 3.3-mg l-thyroxine (T4), or 20-mg T4 pellets (60-day release form) for 2 months, respectively. At the end of treatment, hypothyroid, euthyroid, and hyperthyroid status was confirmed. Hypothyroid animals showed cardiac atrophy and reduced cardiac systolic and diastolic functions, whereas hyperthyroid rats exhibited cardiac hypertrophy and increased cardiac function. Hypothyroidism and hyperthyroidism produced opposite electrophysiological changes in heart rates and atrial effective refractory period, but both significantly increased AF susceptibility. AF incidence was 78% in hypothyroid, 67% in hyperthyroid, and the duration of induced AF was also longer, compared with 11% in the euthyroid group (all P<0.05). Hypothyroidism increased atrial interstitial fibrosis, but connexin 43 was not affected.

CONCLUSIONS

Both hypothyroidism and hyperthyroidism lead to increased AF vulnerability in a rat thyroidectomy model. Our results stress that normal thyroid hormone levels are required to maintain normal cardiac electrophysiology and to prevent cardiac arrhythmias and AF.

Heart rate, body temperature and physical activity are variously affected during insulin treatment in alloxan-induced type 1 diabetic rat

Diabetes mellitus is associated with a variety of cardiovascular complications including impaired cardiac muscle function. The effects of insulin treatment on heart rate, body temperature and physical activity in the alloxan (ALX)-induced diabetic rat were investigated using in vivo biotelemetry techniques. The electrocardiogram, physical activity and body temperature were recorded in vivo with a biotelemetry system for 10 days before ALX treatment, for 20 days following administration of ALX (120 mg/kg) and thereafter, for 15 days whilst rats received daily insulin. Heart rate declined rapidly after administration of ALX. Pre-ALX heart rate was 321+/-9 beats per minute, falling to 285+/-12 beats per minute 15-20 days after ALX and recovering to 331+/-10 beats per minute 5-10 days after commencement of insulin. Heart rate variability declined and PQ, QRS and QT intervals were prolonged after administration of ALX. Physical activity and body temperature declined after administration of ALX. Pre-ALX body temperature was 37.6+/-0.1 °C, falling to 37.3+/-0.1 °C 15-20 days after ALX and recovering to 37.8+/-0.1 °C 5-10 days after commencement insulin. ALX-induced diabetes is associated with disturbances in heart rhythm, physical activity and body temperature that are variously affected during insulin treatment.

Thyrotoxic hypokalemic periodic paralysis: An overlooked pathology in western countries

Thyrotoxic hypokalemic periodic paralysis (THPP) is a complication of hyperthyroidism that is mostly diagnosed in Asian populations; consequently, it can be difficult to recognize in western populations. THPP represents an endocrine emergency that can result in respiratory insufficiency, cardiac arrhythmias, and death. Its differential diagnosis from the other more common forms of hypokalemic paralysis is important to avoid inappropriate therapy. Here, we discuss the main pathogenetic hypotheses, clinical features, and therapies of this disease. We also report an example of THPP management in our primary care unit.

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.

Modest actomyosin energy conservation increases myocardial postischemic function

We have proposed that pharmacological preconditioning, leading to PKC-epsilon activation, in hearts improves postischemic functional recovery through a decrease in actomyosin ATPase activity and subsequent ATP conservation. The purpose of the present study was to determine whether moderate PKC-independent decreases in actomyosin ATPase are sufficient to improve myocardial postischemic function. Rats were given propylthiouracil (PTU) for 8 days to induce a 25% increase in beta-myosin heavy chain with a 28% reduction in actomyosin ATPase activity. Recovery of postischemic left ventricular developed pressure (LVDP) was significantly higher in PTU-treated rat hearts subjected to 30 min of global ischemia than in control hearts: 57.9 +/- 6.2 vs. 32.6 +/- 5.1% of preischemic values. In addition, PTU-treated hearts exhibited a delayed onset of rigor contracture during ischemia and a higher global ATP content after ischemia. In the second part of our study, we demonstrated a lower maximal actomyosin ATPase and a higher global ATP content after ischemia in human troponin T (TnT) transgenic mouse hearts. In mouse hearts with and without a point mutation at F110I of human TnT, recovery of postischemic LVDP was 55.4 +/- 5.5 and 62.5 +/- 14.5% compared with 20.0 +/- 2.9% in nontransgenic mouse hearts after 35 min of global ischemia. These results are consistent with the hypothesis that moderate decreases in actomyosin ATPase activity result in net ATP conservation that is sufficient to improve postischemic contractile function.

Effect of thyroid hormones on the gene expression of calcium transport systems in rat muscles

It has been previously shown that modification of thyroid hormone levels have a profound impact on cardiac function, predominantly through a direct regulation of the sarcoplasmic reticulum protein levels. Nevertheless, little is known about the regulation of calcium transport systems in skeletal muscle due to the altered concentration of thyroid hormones. Thus, the goal of our study was to find out whether altered thyroid status could change the gene expression of the Na(+)/Ca(2+) exchanger (NCX), the inositol 1,4,5-trisphosphate (IP(3)) receptors and ryanodine receptors (RyRs) in slow and fast skeletal muscles of rats. A hyperthyroid state was maintained in rats by triiodothyronine (T(3)) administration, while methimazole was employed for inducing hypothyroidism. After a period of 2-10 months of T(3) treatment we observed a significant increase in mRNA levels of the NCX, RyRs and IP(3) receptors. This increase was more pronounced in the slow soleus than in the fast extensor digitorum longus (EDL) muscle. It is tempting to speculate that thyroid hormones also alter calcium concentration and thus influence the process of excitation-contraction coupling in the skeletal muscle.

Thyroid hormone regulation of calcium cycling proteins

Alterations in thyroid hormone levels have a profound impact on myocardial contractility, speed of relaxation, cardiac output, and heart rate. The mechanisms for these changes include altered expression of several key proteins, involved in the regulation of intracellular calcium homeostasis. Most notably, increases in thyroid hormone and the coordinated increases in cardiac contractile parameters are marked by increases in the levels of the sarcoplasmic reticulum (SR) Ca2+-adenosine triphosphatase (ATPase) and decreases in its inhibitor, phospholamban. These changes at the protein level result in enhanced SR calcium transport and myocyte calcium cycling, leading to increases in the force and rates of contraction as well as relaxation rates at the organ level. However, decreases in thyroid hormone levels are associated with opposite alterations in these two proteins, leading to reduced myocyte calcium handling capacity and lower cardiac contractility. Furthermore, changes in the relative ratio of phospholamban/Ca2+-ATPase correlate with changes in the affinity of the SR Ca2+-transport system and relaxation rates in beating hearts. These findings suggest that thyroid hormone directly regulates SR protein levels and thus, cardiac function.

Mutations linked to familial hypokalaemic periodic paralysis in the calcium channel alpha1 subunit gene (Cav1.1) are not associated with thyrotoxic hypokalaemic periodic paralysis

OBJECTIVE

To investigate whether patients with thyrotoxic hypokalaemic periodic paralysis (THPP) have the same molecular defect in the calcium channel gene described in familial hypokalaemic periodic paralysis (FHPP), as the symptoms of both diseases are comparable, we analysed, in patients with THPP, the presence of mutations R528H, R1239H and R1239G on the S4 voltage-sensing transmembrane segment of the alpha1 subunit of the calcium channel gene (Cav1.1).

DESIGN AND PATIENTS

Genomic DNA was extracted from peripheral blood from 14 patients with THPP, 13 sporadic cases and one with a family history. An FHPP family was selected as a positive control. The exons bearing the described mutations were amplified by PCR, screened by single-strand conformation polymorphism (SSCP), and further sequenced.

MEASUREMENTS

THPP was diagnosed both clinically and through laboratory tests, all patients having elevated levels of thyroid hormones (T4, T3 or free T4), suppressed TSH and plasma potassium below 3 small middle dot5 mmol/l.

RESULTS

No evidence of the described mutations was found in patients with THPP. Furthermore, we did not detect any mutations in any of the four full S4 voltage-sensing transmembrane segments of Cav1 small middle dot1 (DIS4, DIIS4, DIIIS4 and DIVS4) by direct sequencing. However, close to the R528H mutation, we identified two single nucleotide polymorphisms at nucleotides 1551 and 1564 in both familial and sporadic cases with THPP. In addition, we were able to detect the R528H mutation in the DIIS4 transmembrane segment in all members of the FHPP family.

CONCLUSION

Mutations linked to familial hypokalaemic periodic paralysis in the calcium channel alpha1 subunit gene (Cav1.1) are not associated with thyrotoxic hypokalaemic periodic paralysis. However, polymorphisms in nucleotides 1551 and 1564 in the exon 11 were found in patients with familial hypokalaemic periodic paralysis and thyrotoxic hypokalaemic periodic paralysis in higher frequency than in controls. The polymorphisms identified within the Cav1.1 gene are associated with thyrotoxic hypokalaemic periodic paralysis and represent a novel finding.

Inhibitory effects of calcium channel blockers on thyroid hormone uptake in neonatal rat cardiomyocytes

The effects of the Ca2+ channel blockers verapamil, nifedipine, and diltiazem on triiodothyronine (T3) and thyroxine (T4) uptake were tested in cultured cardiomyocytes from 2-day-old rats. Experiments were performed at 37 degrees C in medium with 0.5% BSA for [125I]T3 (100 pM) or 0.1% BSA for [125I]T4 (350 pM). The 15-min uptake of [125I]T3 was 0.124 +/- 0.013 fmol/pM free T3 (n = 6); [125I]T4 uptake was 0.032 +/- 0.003 fmol/pM free T4 (n = 12). Neither T3 nor T4 uptake was affected by 1% DMSO (diluent for nifedipine and verapamil). Uptake of [125I]T3 but not of [125I]T4 was dose dependently reduced by incubation with 1-100 microM verapamil (49-87%, P < 0.05) or nifedipine (53-81%, P < 0.05). The relative decline in [125I]T3 uptake after 4 h of incubation with 10 microM verapamil or nifedipine was less than after 15 min or 1 h, indicating that the major inhibitory effect of the Ca2+ channel blockers occurred at the level of the plasma membrane. The reduction of nuclear [125I]T3 binding by 10 microM verapamil or nifedipine was proportional to the reduction of cellular [125I]T3 uptake. Diltiazem (1-100 microM) had no dose-dependent effect on [125I]T3 uptake but reduced [125I]T4 uptake by 45% (P < 0.05) at each concentration tested. Neither the presence of 20 mM K+ nor the presence of low Ca2+ in the medium affected [125I]T3 uptake. In conclusion, the inhibitory effects of Ca2+ channel blockers on T3 uptake in cardiomyocytes are not secondary to their effects on Ca2+ influx but, rather, reflect interference with the putative T3 carrier in the plasma membrane.

Thyroid hormone-induced overexpression of functional ryanodine receptors in the rabbit heart

Modifications in the Ca(2+)-uptake and -release functions of the sarcoplasmic reticulum (SR) may be a major component of the mechanisms underlying thyroid state-dependent alterations in heart rate, myocardial contractility, and metabolism. We investigated the influence of hyperthyroid state on the expression and functional properties of the ryanodine receptor (RyR), a major protein in the junctional SR (JSR), which mediates Ca(2+) release to trigger muscle contraction. Experiments were performed using homogenates and JSR vesicles derived from ventricular myocardium of euthyroid and hyperthyroid rabbits. Hyperthyroidism, with attendant cardiac hypertrophy, was induced by the injection of L-thyroxine (200 microg/kg body wt) daily for 7 days. Western blotting analysis using cardiac RyR-specific antibody revealed a significant increase (>50%) in the relative amount of RyR in the hyperthyroid compared with euthyroid rabbits. Ca(2+)-dependent, high-affinity [(3)H]ryanodine binding was also significantly greater ( approximately 40%) in JSR from hyperthyroid rabbits. The Ca(2+ )sensitivity of [(3)H]ryanodine binding and the dissociation constant for [(3)H]ryanodine did not differ significantly between euthyroid and hyperthyroid hearts. Measurement of Ca(2+)-release rates from passively Ca(2+)-preloaded JSR vesicles and assessment of the effect of RyR-Ca(2+)-release channel (CRC) blockade on active Ca(2+)-uptake rates revealed significantly enhanced (>2-fold) CRC activity in the hyperthyroid, compared with euthyroid, JSR. These results demonstrate overexpression of functional RyR in thyroid hormone-induced cardiac hypertrophy. Relative abundance of RyR may be responsible, in part, for the changes in SR Ca(2+) release, cytosolic Ca(2+) transient, and cardiac systolic function associated with thyroid hormone-induced cardiac hypertrophy.

The effect of isoproterenol on phospholamban-deficient mouse hearts with altered thyroid conditions

The aim of the present study was to determine the effects of beta -adrenergic stimulation in wild-type and phospholamban-deficient mouse hearts with altered thyroid conditions. Hypothyroidism was associated with significant decreases in heart/body weight ratio in wild-type and phospholamban-deficient mice, whereas hyperthyroidism was associated with significant increases in heart/body weight ratio in both groups. Hypothyroid hearts of wild-type and phospholamban-deficient mice exhibited similar increases in beta -myosin heavy chain protein levels and decreases in alpha -myosin heavy chain protein levels. In hyperthyroidism, there were increases in the alpha -myosin heavy chain protein levels and these were similar in wild-type and phospholamban-deficient hearts. There were no detectable levels of beta -myosin heavy chain protein in the hyperthyroid hearts. The relative tissue level of phospholamban in wild-type hearts was increased (133%, P<0.01) in hypothyroidism, and decreased (69%, P<0.01) in hyperthyroidism, when compared to euthyroid controls (100%). Similar increases and decreases in SR Ca(2+)-ATPase protein levels were observed between phospholamban-deficient and wild-type hearts in hyperthyroidism and hypothyroidism, respectively. The basal contractile state of wild-type and phospholamban-deficient hearts was significantly depressed in hypothyroidism. On the other hand, the basal contractile state of wild-type and phospholamban-deficient hearts was significantly increased in hyperthyroidism. During beta -agonist stimulation of wild-type hearts, the responses in the rates of contraction and relaxation were highest in the hypothyroid group, followed by the euthyroid, and lastly by the hyperthyroid groups. There was a close linear correlation between the magnitude of the contractile parameter responses and the phospholamban/SERCA2 ratios in these hearts. However, the phospholamban-deficient hypothyroid, euthyroid, and hyperthyroid hearts did not exhibit any responses to isoproterenol, indicating that the alterations in the thyroid states of these hearts do not influence the effects of isoproterenol on cardiac function. These findings suggest that phospholamban is an important regulator of the heart's responses to beta -adrenergic stimulation under various thyroid states.

Heat acclimation induces changes in cardiac mechanical performance: the role of thyroid hormone

The involvement of reduced thyroxine level in the emergence of heat acclimation-induced negative lusitropic effect was examined. Experiments were carried out on 1) control rat hearts maintained at 24 +/- 1 degreesC (C); 2) rat hearts acclimated at 34 degreesC for 1 mo (AC); 3) AC-euthyroid rat hearts, via administration of thyroxine in the drinking water (AT); and 4) hypothyroid rat hearts, maintained at 24 +/- 1 degreesC, via administration of thiouracil in the drinking water (CP). Systolic pressure and velocities of contraction (dP/dt. P) and relaxation (-dP/dt. P) were measured using the Langendorff perfusion system. The steady-state levels of Ca2+-ATPase and phospholamban mRNAs and the expression of the encoded proteins Ca2+-ATPase (SERCA) and phospholamban (PLB) were measured, using semi-quantitative RT-PCR and Western immunoblotting, respectively. Rat thyroxine levels were measured using RIA. Heat acclimation, which brought about a reduced thyroxine level, led to downregulation of Ca2+-ATPase mRNA expression and translation and upregulation of phospholamban mRNA and PLB. Consequently, the PLB-to-SERCA ratio (PLB/SERCA) of the AC hearts showed a significant increase. These changes, as well as the greater pressure generation and the reduced dP/dt. P and -dP/dt. P observed in AC hearts were blunted in the AT hearts. Our data suggest that sustained heat acclimation-induced low thyroxine level has a decisive effect on the contractile machinery of the AC heart. Elevated PLB/SERCA apparently explains the negative lusitropic effect observed in these hearts.

Antioxidant-sensitive shortening of ventricular action potential in hyperthyroid rats is independent of lipid peroxidation

The effects of substances able to reduce peroxidative processes on thyroid hormone-induced electrophysiological changes in ventricular muscle fibres were examined. For this study, 60 day old euthyroid and hyperthyroid rats were used. One group of hyperthyroid rats was untreated and the others were treated with vitamin E, N-acetylcysteine, and cholesterol, respectively. Hyperthyroidism was elicited by 10 day treatment with daily i.p. injections of triiodothyronine (10 microg/100 g body weight). Vitamin E and N-acetylcysteine were administered for 10 days by daily i.m. injections (20 mg/100 g body weight) and daily i.p. injections (100 mg/100 g body weight), respectively. Cholesterol was administered by cholesterol-supplemented diet (4%) from day 30. Hyperthyroidism induced a decrease in the whole antioxidant capacity and an increase in both lipid peroxidation and susceptibility to oxidative stress. Vitamin E and N-acetylcysteine administration to hyperthyroid rats led to reduction in lipid peroxidation and susceptibility to oxidative stress and to increase in antioxidant level, while the diet addition of cholesterol decreased lipid peroxidation but did not modify the other parameters. The hyperthyroid state was also associated with a decrease in the duration of the ventricular action potential recorded in vitro. The vitamin E and N-acetylcysteine administration attenuated the thyroid hormone-induced changes in action potential duration, which was however, significantly different from that of the euthyroid rats. In contrast, cholesterol supplementation did not modify the electrical activity of hyperthyroid heart. These results demonstrate that the triiodothyronine effects on ventricular electrophysiological properties are mediated, at least in part, through a membrane modification involving a free radical mechanism. Moreover, they indicate that the antioxidant-sensitive shortening of action potential duration induced by thyroid hormone is likely independent of enhanced peroxidative processes in sarcolemmal membrane.

Ca(2+)-ATPases in the cochlear duct

Differing levels of the Ca(2+)-ATPase enzymes that reside on the plasma membrane (PM) and on the endoplasmic reticulum (ER) were identified in individual rat cochlear tissues by the use of a semi-quantitative enzyme-linked immunosorbent assay (ELISA). Unlike other studies, a specific antibody to PM Ca(2+)-ATPase was used to detect significantly greater levels (about 2x) of PM Ca(2+)-ATPase in the stria vascularis (SV) than that in the spiral ligament (SL) and organ of Corti (OC) tissues. Similarly, levels of ER Ca(2+)-ATPase were also significantly higher in the SV than in the SL and OC tissues. The presence of ER Ca(2+)-ATPase in the tissues of the SV has not been demonstrated previously. Given the importance of Ca2+ homeostasis in the inner ear, the statistically significantly higher densities of both PM and ER Ca(2+)-ATPase measured in the SV relative to the SL and OC regions would indicate tissue-specific responses to fluctuations in systemic and local Ca2+ concentrations.

Action potential configuration in heart papillary muscles from female rats in different thyroid states

We have studied the effects of thyroidectomy and the in vivo administration of different triiodothyronine (T3) doses in thyroidectomized female rats on electrophysiological properties, measured in vitro, of the anterior and posterior papillary muscle fibers from the right ventricle. In each thyroid state, the action potential duration (APD) measured by stimulating at 1 Hz was shorter for the posterior papillary muscle. APD from both preparations was found significantly lengthened in thyroidectomized animals in comparison to euthyroid controls. APD was shortened owing to treatment of thyroidectomized rats with T3 doses up to 10 micrograms/100 g body weight every second day. Treatment with larger doses of T3 tended to restore the values of APD found for ventricular fibres from both controls and thyroidectomized animals treated with substitutive T3 doses (5 micrograms/100 g body weight every second day). As the stimulation rate was increased from 1 to 5 Hz, APD increased in both preparations of all groups. The changes were of different amounts but the APD difference between the rat groups, which were significant at 1 Hz, remained significant at 5 Hz, while the differences between anterior and posterior preparations were cancelled in animals treated with 50 micrograms of T3 and reversed in those treated with 100 micrograms.