Species differences in myocardial beta-adrenergic receptor regulation in response to hyperthyroidism

Takotsubo cardiomyopathy and transient thyrotoxicosis during combination therapy with interferon-alpha and ribavirin for chronic hepatitis C

BACKGROUND

Thyroid dysfunction is a common complication of chronic hepatitis C (CHC) and its therapy. Takotsubo cardiomyopathy (TCM) is a multifactorial, stress related cardiomyopathy, rarely reported in association with thyrotoxicosis. Simultaneous occurrence of TCM and thyrotoxicosis due to hepatitis C and its treatment has never been reported.

CASE PRESENTATION

A 47-year-old woman was admitted for acute chest pain, dyspnea, palpitations and diaphoresis. She had been diagnosed with CHC and had undergone 7 months of IFNα and Ribavirin therapy. At admission electrocardiogram (ECG) showed ST segment elevation, negative T waves and troponin was elevated suggesting ST segment elevation myocardial infarction (STEMI). Echocardiography demonstrated left ventricular apical akinesia and ballooning, with a left ventricular ejection fraction (LVEF) of 35%. Contrast angiography showed normal epicardial coronaries, yet a ventriculogram revealed left ventricular apical ballooning, consistent with TCM. Cardiac MRI showed left ventricle apical ballooning and no late enhancement suggesting the absence of any edema, scar or fibrosis in the left myocardium. She was diagnosed with non-autoimmune destructive thyroiditis: TSH=0.001 mU/L, free T4=2.41 ng/dl, total T3=199 ng/dl and negative thyroid antibodies. The thyroid ultrasonography showed a diffuse small goiter, no nodules and normal vascularization of the parenchyma. Following supportive treatment she experienced a complete recovery after a few weeks and she successfully completed her antiviral treatment, with no thyroid or cardiovascular dysfunction ever since. In patients treated with IFNα for CHC, the prevalence of thyroid dysfunction varies between 2.5-45.3% of cases. TCM is a stress related cardiomyopathy characterized by elevated cardiac enzymes, normal coronary angiography and an acute, transient, left ventricular apical dysfunction that mimics myocardial infarction. Most of the patients survive the initial acute event, typically recover normal ventricular function within one to four weeks and have a favorable outcome, as was the case with our patient. Thyrotoxicosis induced stress cardiomyopathy is rare and has been mostly reported in association with Graves' disease, thyroid storm, thyrotoxicosis factitia or following radioiodine therapy for toxic multinodular goiter.

CONCLUSION

Routine thyroid screening should be done in patients receiving IFN-alpha and Ribavirin for CHC and thyrotoxicosis should be considered as a possible and treatable underlying cause of TCM.

Takotsubo cardiomyopathy associated with thyrotoxicosis: a case report and review of the literature

BACKGROUND

Takotsubo or stress-induced cardiomyopathy is a form of reversible cardiomyopathy commonly associated with emotional or physical stress. Thyrotoxicosis has been identified as a rare cause of Takotsubo cardiomyopathy, with only 12 cases reported in the literature. Here, we report a case of thyroid storm presenting with Takotsubo cardiomyopathy in the setting of Graves' disease.

PATIENT FINDINGS

A 71-year-old woman presented with abdominal pain, vomiting, confusion, and history of weight loss. She was initially diagnosed and treated for diabetic ketoacidosis at another hospital and was transferred to our hospital one day after initial presentation because of concern for acute coronary syndrome. A diagnosis of Takotsubo cardiomyopathy was made on the basis of cardiac catheterization. At that time, she was diagnosed and treated for thyroid storm. Follow-up 7 weeks later revealed improvement of her cardiac function and near-normalization of thyroid hormone levels.

SUMMARY

In this patient, who presented with symptoms of heart failure, acute coronary syndrome was initially considered, but the diagnosis of Takotsubo cardiomyopathy associated with thyroid storm was ultimately made based on cardiac catheterization and laboratory investigation.

CONCLUSIONS

Thyrotoxicosis is associated with adverse disturbances in the cardiovascular system. Takotsubo cardiomyopathy could be a presenting manifestation of thyroid storm, perhaps related to excess catecholamine levels or sensitivity.

Myocardial Rac1 exhibits partial involvement in thyroxin-induced cardiomyocyte hypertrophy and its inhibition is not sufficient to improve cardiac dysfunction or contractile abnormalities in mouse papillary muscles

: Development of cardiac hypertrophy after thyroxin (T4) treatment is well recognized. Recently, we observed that T4-induced cardiac hypertrophy is associated with increased cardiac Rac1 expression and activity. Whether this Rac1 increase has a role in inducing this cardiac phenotype is, however, still unknown. Here, we showed that T4 treatment (500 µg/kg/d) for 2 weeks resulted in increased myocardial Rac1 activity with subsequent hypertension, cardiac hypertrophy, and left ventricular systolic dysfunction in vivo. Isolated right ventricular papillary muscles of T4-treated mice maintained their peak isometric active developed tension but exhibited significant decreases in their corresponding time to peak and in relaxation times. Positive inotropic responses to increasing pacing rate and β-adrenergic stimulation were also depressed in these muscles. Pravastatin (10 mg/kg/d), a Rac1 inhibitor, significantly decreased myocardial Rac1 activity, hypertension, and cardiomyocyte size in T4-treated mice but could not attenuate gross heart weight or functional cardiac changes in these mice. Our data showed that T4 could activate different signaling pathways with distinct cardiovascular outcomes. We also provide the first mechanistic evidence for the partial involvement of Rac1 activation in T4-induced cardiomyocyte hypertrophy and reveal a putative role for Rac1 in the development of T4-induced hypertension.

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.

To scale or not to scale: the principles of dose extrapolation

The principles of inter-species dose extrapolation are poorly understood and applied. We provide an overview of the principles underlying dose scaling for size and dose adjustment for size-independent differences. Scaling of a dose is required in three main situations: the anticipation of first-in-human doses for clinical trials, dose extrapolation in veterinary practice and dose extrapolation for experimental purposes. Each of these situations is discussed. Allometric scaling of drug doses is commonly used for practical reasons, but can be more accurate when one takes into account species differences in pharmacokinetic parameters (clearance, volume of distribution). Simple scaling of drug doses can be misleading for some drugs; correction for protein binding, physicochemical properties of the drug or species differences in physiological time can improve scaling. However, differences in drug transport and metabolism, and in the dose-response relationship, can override the effect of size alone. For this reason, a range of modelling approaches have been developed, which combine in silico simulations with data obtained in vitro and/or in vivo. Drugs that are unlikely to be amenable to simple allometric scaling of their clearance or dose include drugs that are highly protein-bound, drugs that undergo extensive metabolism and active transport, drugs that undergo significant biliary excretion (MW > 500, ampiphilic, conjugated), drugs whose targets are subject to inter-species differences in expression, affinity and distribution and drugs that undergo extensive renal secretion. In addition to inter-species dose extrapolation, we provide an overview of dose extrapolation within species, discussing drug dosing in paediatrics and in the elderly.

Thyroid-adrenergic interactions: physiological and clinical implications

The sympathoadrenal system, including the sympathetic nervous system and the adrenal medulla, interacts with thyroid hormone (TH) at various levels. Both systems are evolutionary old and regulate independent functions, playing probably independent roles in poikilothermic species. With the advent of homeothermy, TH acquired a new role, which is to stimulate thermogenic mechanisms and synergize with the sympathoadrenal system to produce heat and maintain body temperature. An important part of this new function is mediated through coordinated and, most of the time, synergistic interactions with the sympathoadrenal system. Catecholamines can in turn activate TH in a tissue-specific manner, most notably in brown adipose tissue. Such interactions are of great adaptive value in cold adaptation and in states needing high-energy output. Conversely, in states of emergency where energy demand should be reduced, such as disease and starvation, both systems are turned down. In pathological states, where one of the systems is fixed at a high or a low level, coordination is lost with disruption of the physiology and development of symptoms. Exaggerated responses to catecholamines dominate the manifestations of thyrotoxicosis, while hypothyroidism is characterized by a narrowing of adaptive responses (e.g., thermogenic, cardiovascular, and lipolytic). Finally, emerging results suggest the possibility that disrupted interactions between the two systems contribute to explain metabolic variability, for example, fuel efficiency, energy expenditure, and lipolytic responses.

T4-induced cardiac hypertrophy disrupts cyclic GMP mediated responses to brain natriuretic peptide in rabbit myocardium

Brain natriuretic peptide (BNP) affects the regulation of myocardial metabolism through the production of cGMP and these effects may be altered by cardiac hypertrophy. We tested the hypothesis that BNP would cause decreased metabolism and function in the heart and cardiac myocytes by increasing cGMP and that these effects would be disrupted after thyroxine-induced cardiac hypertrophy (T4). Open-chest control and T4 rabbits were instrumented to determine local effects of epicardial BNP (10(-3) M). Function of isolated cardiac myocytes was examined with BNP (10(-8)-10(-7) M) with or without KT5823 (10(-6) M, cGMP protein kinase inhibitor). Cyclic GMP levels were measured in myocytes. In open-chest controls, O2 consumption was reduced in the BNP area of the subepicardium (6.6+/-1.3 ml O2/min/100 g versus 8.9+/-1.4 ml O2/min/100 g) and subendocardium (9.4+/-1.3 versus 11.3+/-0.99). In T4 animals, functional and metabolic rates were higher than controls, but there was no difference between BNP-treated and untreated areas. In isolated control myocytes, BNP (10(-7) M) reduced percent shortening (PSH) from 6.5+/-0.6 to 4.3+/-0.4%. With KT5823 there was no effect of BNP on PSH. In T4 myocytes, BNP had no effect on PSH. In control myocytes, BNP caused cGMP levels to rise from 279+/-8 to 584+/-14 fmol/10(5) cells. In T4 myocytes, baseline cGMP levels were lower (117+/-2 l) and were not significantly increased by BNP. Thus, BNP caused decreased metabolism and function while increasing cGMP in control. These effects were lost after T4 due to lack of cGMP production. These data indicated that the effects of BNP on heart function operated through a cGMP-dependent mechanism, and that this mechanism was disrupted in T4-induced cardiac hypertrophy.

Molecular defects in cardiac myofibrillar proteins due to thyroid hormone imbalance and diabetesThis paper is a part of a series in the Journal's "Made in Canada" section. The paper has undergone peer review

The heart very often becomes a victim of endocrine abnormalities such as thyroid hormone imbalance and insulin deficiency, which are manifested in a broad spectrum of cardiac dysfunction from mildly compromised function to severe heart failure. These functional changes in the heart are largely independent of alterations in the coronary arteries and instead reside at the level of cardiomyocytes. The status of cardiac function reflects the net of underlying subcellular modifications induced by an increase or decrease in thyroid hormone and insulin plasma levels. Changes in the contractile and regulatory proteins constitute molecular and structural alterations in myofibrillar assembly, called myofibrillar remodeling. These alterations may be adaptive or maladaptive with respect to the functional and metabolic demands on the heart as a consequence of the altered endocrine status in the body. There is a substantial body of information to indicate alterations in myofibrillar proteins including actin, myosin, tropomyosin, troponin, titin, desmin, and myosin-binding protein C in conditions such as hyperthyroidism, hypothyroidism, and diabetes. The present article is focussed on discussion how myofibrillar proteins are altered in response to thyroid hormone imbalance and lack of insulin or its responsiveness, and how their structural and functional changes explain the contractile defects in the heart.

Cyclic GMP protein kinase activity is reduced in thyroxine-induced hypertrophic cardiac myocytes

1. We tested the hypothesis that the cGMP-dependent protein kinase has major negative functional effects in cardiac myocytes and that the importance of this pathway is reduced in thyroxine (T4; 0.5 mg/kg per day for 16 days) hypertrophic myocytes. 2. Using isolated ventricular myocytes from control (n = 7) and T4-treated (n = 9) rabbit hypertrophic hearts, myocyte shortening was studied with a video edge detector. Oxygen consumption was measured using O2 electrodes. Protein phosphorylation was measured autoradiographically. 3. Data were collected following treatment with: (i) 8-(4-chlorophenylthio)guanosine-3',5'-monophosphate (PCPT; 10-7 or 10-5 mol/L); (ii) 8-bromo-cAMP (10-5 mol/L) followed by PCPT; (iii) beta-phenyl-1,N2-etheno-8-bromoguanosine-3',5'-monophosphorothioate, SP-isomer (SP; 10-7 or 10-5 mol/L); or (iv) 8-bromo-cAMP (10-5 mol/L) followed by SP. 4. There were no significant differences between groups in baseline percentage shortening (Pcs; 4.9 +/- 0.2 vs 5.6 +/- 0.4% for control and T4 groups, respectively) and maximal rate of shortening (Rs; 64.8 +/- 5.9 vs 79.9 +/- 7.1 micro m/ s for control and T4 groups, respectively). Both SP and PCPT decreased Pcs (-43 vs-21% for control and T4 groups, respectively) and Rs (-36 vs-22% for control and T4 groups, respectively), but the effect was significantly reduced in T4 myocytes. 8-Bromo-cAMP similarly increased Pcs (28 vs 23% for control and T4 groups, respectively) and Rs (20 vs 19% for control and T4 groups, respectively). After 8-bromo-cAMP, SP and PCPT decreased Pcs (-34%) and Rs (-29%) less in the control group. However, the effects of these drugs were not altered in T4 myocytes (Pcs -24%; Rs -22%). Both PCPT and cAMP phosphorylated the same five protein bands. In T4 myocytes, these five bands were enhanced less. 5. We conclude that, in control ventricular myocytes, the cGMP-dependent protein kinase exerted major negative functional effects but, in T4-induced hypertrophic myocytes, the importance of this pathway was reduced and the interaction between cAMP and the cGMP protein kinase was diminished.

The metabolic and cardiovascular effects of hyperthyroidism are largely independent of beta-adrenergic stimulation

Hyperthyroidism and states of adrenergic hyperactivity have many common clinical features, suggesting similar pathogenic mechanisms of action. The widespread use of beta-adrenergic receptor (betaAR) antagonists (beta-blockers) to treat hyperthyroidism has led to the belief that the physiological consequences of thyroid hormone (TH) excess are mediated in part via catecholamine signaling through betaARs. To test this hypothesis, we compared the response to TH excess in mice lacking the three known betaARs (beta-less) vs. wild-type (WT) mice. Although beta-less mice had a lower heart rate at baseline in comparison to WT mice, the metabolic and cardiovascular responses to hyperthyroidism were equivalent in both WT and beta-less mice. These data indicate that the metabolic and cardiovascular effects of TH excess are largely independent of betaARs. These findings suggest that the efficacy of clinical treatment of hyperthyroidism with beta-blockers is due to antagonism of sympathetic signaling, and that this process functions independently of TH action.

Severe reversible dilated cardiomyopathy and hyperthyroidism: case report and review of the literature

OBJECTIVE

To describe a case of a 46-year-old woman with Graves' disease and reversible low-output congestive heart failure and present a comparative analysis of 23 similar cases reported in the literature.

METHODS

A detailed case report is presented. In addition, a review of the pertinent literature published between 1960 and 2002 was performed to identify similar cases of dilated cardiomyopathy and thyrotoxicosis and to assess the findings in these patients.

RESULTS

A 46-year-old woman without primary heart disease was admitted to the hospital with Graves' thyrotoxicosis and severe low-output congestive heart failure. Her left ventricular ejection fraction (LVEF) at the time of initial assessment was less than 20%, and her condition was categorized as New York Heart Association (NYHA) functional class III. Nineteen months after she was treated for hyperthyroidism, her LVEF was 49% and her status was NYHA class I. A severe hypotensive episode occurred when b-adrenergic blockade therapy was initiated. The group of 23 similar cases from the literature plus our currently described patient had a mean age of 45 years, a male-to-female ratio of 1:1.2, Graves' disease as the principal cause, and LVEF improvement from 29% to 58%.

CONCLUSION

Dilated cardiomyopathy is an unusual manifestation of hyperthyroidism with unclear cause. Clinicians should be aware of this entity because it is treatable and hypotension can occur if b-adrenergic blockade treatment is initiated.

Negative functional effects of cGMP mediated by cGMP protein kinase are reduced in T4 cardiac myocytes

We tested the hypothesis that in isolated rabbit cardiac myocytes, the negative functional effects of cyclic GMP are partly mediated by cyclic GMP-dependent protein kinase activity, and that these effects are altered in thyroxine (T4, 0.5 mg/kg/day for 16 days)-induced hypertrophic myocytes. Using isolated ventricular myocytes from control (N=8) and T4 (N=8) hypertrophic hearts, data for percent cell shortening (%) and maximum rate of contraction (microm/s) were collected using a video edge detector at baseline, after the addition of 10(-6) M 8-bromo-cyclic GMP (8-Br-cGMP), 10(-5) M 8-Br-cGMP, and 10(-6) M KT5823 (10-methoxy-10-methoxycarbonyl-9, 10, 11, 12-tetrahydro-9, 12-epoxy-(1H)-diinidolo [1, 2, 3, f-g: 3', 2', 1'-k-j]-pyrrolidino-[3,4-i] [1,6]-benzodiazocin-2-methyl-1-one, cyclic GMP protein kinase inhibitor). Protein phosphorylation was determined autoradiographically after gel electrophoresis. In both control and T(4) myocytes, 8-Br-cGMP caused a significant decrease in percent shortening (5.56+/-0.49% to 3.02+/-0.47% in control and 4.34+/-0.33% to 3.13+/-0.17% in T4 myocytes) and maximal rate of contraction 57.35+/-6.05 to 36.82+/-3.17 microm/s in control and 58.49+/-3.28 to 42.88+/-2.29 microm/s in T4 myocytes). KT5823 significantly increased percent shortening to 3.77+/-0.28% and rate to 48.68+/-4.71 microm/s after 8-Br-cGMP only in control myocytes. In T4 myocytes, the changes in percent shortening and rate after KT5823 were not significant. Protein phosphorylation was increased by 8-Br-cGMP in control and to a lesser extent in T4 myocytes, but the increment was reduced by KT-5823 in control only. These data demonstrated that cyclic GMP had negative functional effects partially mediated by cyclic GMP protein kinase in control myocytes. Cyclic GMP also exerted negative functional effects in thyroxine-induced hypertrophic myocytes, but cyclic GMP protein kinase activity was not an important regulator of these effects in T4 ventricular myocytes.

Molecular characterization of myocardial fibrosis during hypothyroidism: evidence for negative regulation of the pro-alpha1(I) collagen gene expression by thyroid hormone receptor

The purpose of this study was to gain insights into the underlying mechanism of myocardial fibrosis during hypothyroidism. Treatment of cardiac fibroblasts with a medium lacking thyroid hormone led to a 47% increase in [3H]thymidine incorporation into the cell nuclei compared with that in untreated cells. Northern blot analysis of RNA from cardiac fibroblasts grown in a thyroid hormone depleted medium resulted in a 38% increase in the abundance of mRNA for pro-alpha1(I) collagen. At the protein level, the amount of type I collagen, as determined by immunoprecipitation, was increased either in the cell lysate (46%) of cardiac fibroblasts grown in a thyroid hormone depleted medium or in the medium (44%). The chimeric plasmid, ColCAT 3.6, contains the 5'-flanking region of the rat pro-alphal(I) collagen gene (from bases -3520 to +115) fused to the chloramphenicol acetyltransferase (CAT) gene. The plasmid was cotransfected with thyroid hormone receptor (TR) expression plasmid into rat cardiac fibroblasts and COS-l cells (monkey mesangial cells). Cells transfected with the ColCAT plasmid in the presence of thyroid hormone (100 nM T(3)) had a significant decrease (39% in fibroblasts, P<0.01; 52% in COS-1 cells, P<0.001) in CAT activity when compared to cells not exposed to thyroid hormone. Transient co-transfection of TR with various pro-alphal(I) collagen/CAT deletion constructs showed that T(3)-dependent repression was preserved with the deletion from 3520 bp of the flanking sequence to a 5' end point at position -224, indicating that a thyroid hormone-response element (TRE) was localized at the region -224 to +115. The TR-DNA binding assays demonstrated binding of the human TRbeta1 to a fragment containing a proposed TRE located between position -35 and +115 in the 5'-flanking region of the rat pro-alphal(I) collagen gene.

Changes in adenylyl cyclase isoforms as a mechanism for thyroid hormone modulation of cardiac beta-adrenergic receptor responsiveness

Although thyroid hormones are known to modulate cardiac beta-adrenergic receptor expression, the physiologic implications of these changes in the cardiac manifestations of altered thyroid hormone metabolism have been disputed. This study examined whether thyroid hormone modulates signaling via the cyclic adenosine monophosphate (cAMP) pathway by regulating cardiac adenylyl cyclase (AC) isoform expression. Northern blot analyses and AC enzyme assays were performed on preparations from hypothyroid, euthyroid, and hyperthyroid rat ventricles. Steady-state levels of cardiac AC mRNA types V and VI in hypothyroid ventricles were 173% +/- 8% and 149% +/- 12%, respectively, of the values in euthyroid ventricles (P < .01). This increase in AC mRNA isoforms was accompanied by a 1.5-fold increase (P < .05) in the activation of catalytic AC by forskolin and Mn. In contrast, the relative abundance of transcripts for types V and VI AC was similar in hyperthyroid and euthyroid ventricles, but catalytic AC activation by forskolin and Mn was significantly reduced by 35% in membranes obtained from hyperthyroid ventricles. AC activation through beta-adrenergic receptor stimulation by isoproterenol was not altered by thyroid hormone status. Thus, the effect of thyroid hormone to repress AC catalytic activity would be anticipated to offset the increase in beta-adrenergic receptor expression in hyperthyroidism. These studies identify cardiac AC enzymes as important targets for thyroid hormone-dependent regulation of signaling via the cAMP pathway, and support the finding that cardiac adrenergic responsiveness is unaltered in thyroid disease states.

Molecular mechanisms of myocardial remodeling

"Remodeling" implies changes that result in rearrangement of normally existing structures. This review focuses only on permanent modifications in relation to clinical dysfunction in cardiac remodeling (CR) secondary to myocardial infarction (MI) and/or arterial hypertension and includes a special section on the senescent heart, since CR is mainly a disease of the elderly. From a biological point of view, CR is determined by 1 ) the general process of adaptation which allows both the myocyte and the collagen network to adapt to new working conditions; 2) ventricular fibrosis, i.e., increased collagen concentration, which is multifactorial and caused by senescence, ischemia, various hormones, and/or inflammatory processes; 3) cell death, a parameter linked to fibrosis, which is usually due to necrosis and apoptosis and occurs in nearly all models of CR. The process of adaptation is associated with various changes in genetic expression, including a general activation that causes hypertrophy, isogenic shifts which result in the appearance of a slow isomyosin, and a new Na+-K+-ATPase with a low affinity for sodium, reactivation of genes encoding for atrial natriuretic factor and the renin-angiotensin system, and a diminished concentration of sarcoplasmic reticulum Ca2+-ATPase, beta-adrenergic receptors, and the potassium channel responsible for transient outward current. From a clinical point of view, fibrosis is for the moment a major marker for cardiac failure and a crucial determinant of myocardial heterogeneity, increasing diastolic stiffness, and the propensity for reentry arrhythmias. In addition, systolic dysfunction is facilitated by slowing of the calcium transient and the downregulation of the entire adrenergic system. Modifications of intracellular calcium movements are the main determinants of the triggered activity and automaticity that cause arrhythmias and alterations in relaxation.

Contribution of the autonomic nervous system to blood pressure and heart rate variability changes in early experimental hyperthyroidism

A great deal of uncertainty persists regarding the exact nature of the interaction between autonomic nervous system activity and thyroid hormones in the control of heart rate and blood pressure. We now report on thyrotoxicosis produced by daily intraperitoneal (i.p.) injection of L-thyroxine (0.5 mg/kg body wt. in 1 ml of 5 mM NaOH for 5 days). Control rats received i.p. daily injections of the thyroxine solvent. In order to estimate the degree of autonomic activation in hyperthyroidism, specific blockers were administered intravenously: atropine (0.5 mg/kg), prazosin (1 mg/kg), atenolol (1 mg/kg) or the combination of atenolol and atropine. A jet of air was administered in other animals to induce sympathoactivation. Eight animals were studied in each group. The dose and duration of L-thyroxine treatment was sufficient to induce a significant degree of hyperthyroidism with accompanying tachycardia, systolic blood pressure elevation, increased pulse pressure, cardiac hypertrophy, weight loss, tachypnea and hyperthermia. In addition, the intrinsic heart period observed after double blockade (atenolol + atropine) was markedly decreased after treatment with L-thyroxine (121.5+/-3.6 ms vs. 141.2+/-3.7 ms, P < 0.01). Of the autonomic indices, vagal tone (difference between heart period obtained after atenolol and intrinsic heart period) was negatively linearly related to intrinsic heart period (r = 0.71, P < 0.05). Atenolol modified neither the heart period nor blood pressure variability in rats with hyperthyroidism and in these rats the jet of air did not significantly affect the heart period level. The thyrotoxicosis was associated with a reduction of the 0.4 Hz component of blood pressure variability (analyses on 102.4 s segments, modulus 1.10+/-0.07 vs. 1.41+/-0.06 mm Hg, P < 0.01) and prazosin was without effect on this 0.4 Hz component in these animals. These data show a functional diminution of the vascular and cardiac sympathetic tone in early experimental hyperthyroidism. The marked rise in the intrinsic heart rate could be the main determinant of tachycardia. The blood pressure elevation may reflexly induce vagal activation and sympathetic (vascular and cardiac) inhibition.

Thyroid status affects the rat cardiac beta-adrenoceptor system transiently and time-dependently

1. The aim of this study was to investigate the time-dependency of the influence of dysthyroid states on the beta-adrenoceptor system in rat heart left ventricle. Therefore, the influence of acute and chronic hyper- and hypothyroidism on beta-adrenoceptor-induced left ventricular responses, beta-adrenoceptor density, cardiac noradrenaline tissue concentrations, Gs alpha-proteins, and basal and stimulated adenylate cyclase activities was determined. 2. Hyperthyroid rats were obtained by feeding with thyroxine (T4)-containing rat-chow for 1, 4 and 8 weeks. Hypothyroidism was induced by adding 0.05% propylthiouracil (PTU) to the drinking water. Rats of varying ages were used in order to compensate for the differences in the duration of the treatments. Rats were aged 3 and 5 months at the end of the experiments. 3. Thyroxine treatment for 4 and 8 weeks increased the cardiac sensitivity to isoprenaline, but maximal induced inotropic responses were decreased. Cardiac ventricular beta-adrenoceptor density was increased only in rats treated with T4 for 1 week. This transient effect of hyperthyroidism on cardiac beta-adrenoceptor density was not observed in older rats. The PTU treatment resulted in a stable decrease of cardiac beta-adrenoceptor density. 4. Left ventricular tissue noradrenaline concentrations were unaffected by hyperthyroidism, where a decrease was observed in hypothyroid rats. Density of Gs alpha proteins was increased in hearts from chronic hyperthyroid rats. 5. These results indicate that the increased sensitivity to beta-adrenoceptor-mediated stimulation in chronic hyperthyroidism cannot be attributed to changes in cardiac beta-adrenoceptor density, but is probably caused by an enhanced content of Gs alpha. Accordingly, in hyperthyroidism, the beta-adrenoceptor system is influenced time-dependently, whereas hypothyroidism affects the beta-adrenoceptor system independent of time.

Basal muscarinic activity does not impede beta-adrenergic activation in rabbit hearts in controls or thyroxine-induced cardiac hypertrophy

We tested the hypothesis that basal myocardial muscarinic receptor activity acts as a "brake" on beta-adrenergic activation and that this effect would be greater in hearts subjected to thyroxine (T4)-induced (0.5 mg/kg for 16 days) hypertrophy due to an increase in muscarinic receptor density. Twenty control and 20 T4-treated open-chest anesthetized New Zealand white rabbits were given isoproterenol (0.5 microg/kg/min, 10 min i.v.) and/or atropine (3 mg/kg bolus). Coronary blood flow (radioactive microspheres), aortic and left ventricular (LV) pressure, and wall thickening of the LV free wall were recorded. Hearts were quickly excised and stored in liquid nitrogen. Cyclic guanosine monophosphate (GMP) and cyclic adenosine monophosphate (AMP) were determined by radioimmunoassay. T4 increased heart weight/body weight ratio, blood pressures, and the first derivative of the maximal rate of increase of LV systolic pressure (dP/dt[max]). Isoproterenol increased heart rate in both groups. Atropine had no effects on hemodynamic parameters either alone or after stimulation with isoproterenol. At this dose, atropine completely blocked the depressant effects of acetylcholine (10 microg/kg). Isoproterenol increased the maximal time derivative of wall thickening (dWT/dt[max]) in control (from 11.0 +/- 1.0 to 16.4 +/- 1.5 mm/s) but not in T4 animals. T4 increased subepicardial (EPI) and subendocardial (ENDO) coronary blood flow. Isoproterenol increased coronary flow (control: EPI, from 173 +/- 11 to 346 +/- 28 ml/min/100 g; ENDO, from 197 +/- 15 to 364 +/- 30 ml/min/100 g; T4: EPI, from 314 +/- 45 to 459 +/- 43 ml/min/100 g; ENDO, from 339 +/- 48 to 458 +/- 43 ml/min/100 g). Cyclic AMP levels were higher in T4 animals. Isoproterenol increased cyclic AMP (control: EPI, from 540 +/- 82 pmol/g to 1,096 +/- 110; ENDO, 596 +/- 58 to 1,050 +/- 145 pmol/g; T4: EPI, from 882 +/- 107 pmol/g to 1,319 +/- 222; ENDO, from 954 +/- 134 to 1 ,409 +/- 261 pmol/g). Atropine, alone or after stimulation with isoproterenol, had no effect on coronary flow or cyclic AMP in either group. Cyclic GMP levels were unaffected by T4-induced hypertrophy or by any of the treatments in either group. Thus it appears that basal muscarinic activity does not significantly influence function or signal transduction either at baseline or during beta-adrenergic stimulation in controls or in T4-induced hypertrophy.

Medium-term effectiveness of L-thyroxine treatment in idiopathic dilated cardiomyopathy

BACKGROUND

In dilated cardiomyopathy, short-term administration of L-thyroxine (100 micrograms/ day) improves cardiac and exercise performance without changing the heart's adrenergic sensitivity. The aim of this study was to test the medium-term (3 months) efficacy of L-thyroxine (10 patients) compared with placebo (10 patients) and to find out whether later effects are obtainable.

METHODS

Echocardiographic parameters in the control state and during acute changes of left ventricular afterload, cardiopulmonary exercise test, and hemodynamic parameters, including cardiac beta 1 responses to dobutamine, were obtained before and at the end of treatment.

RESULTS

Significant (P < 0.05) changes were observed only with the active drug. After L-thyroxine, patients did not show evidence of chemical hyperthyroidism, despite the increase in thyroxine and the reduction in thyroid-stimulating hormone plasma levels. Cardiac performance improved, as shown by the increase in the left ventricular ejection fraction and rightward shift of the slope of the relation left ventricular ejection fraction/end-systolic stress. Resting cardiac output increased, and the left ventricular diastolic dimensions and systemic vascular resistances decreased. The responses of cardiac output and heart rate to dobutamine infusion were also enhanced. Functional capacity markedly improved, together with an increase in peak exercise cardiac output.

CONCLUSION

L-thyroxine does not lose its beneficial effects on cardiac and exercise performance on medium-term administration and does not induce adverse effects. In addition to the short-term study, the left ventricular diastolic dimensions were decreased. An upregulation of beta 1 receptors might explain the cardiac response to dobutamine.

Myocardial metabolic and functional responses to acetylcholine are altered in thyroxine-induced cardiac hypertrophy

We tested the hypothesis that acetylcholine would reduce myocardial O2 consumption and function, and that thyroxine (T4, 0.5 mg/kg for 16 days) induced cardiac hypertrophy would change this relationship. Anesthetized open-chest New Zealand white rabbits were divided into four groups: control-vehicle (CV, n = 8), control-acetylcholine (CA, n = 10), T4-vehicle (T4V, n = 9), and T4-acetylcholine (T4A, n = 10). Either vehicle or acetylcholine (10(-3) M) was topically applied to the left ventricular surface. Coronary blood flow (radioactive microspheres) and O2 extraction (microspectrophotometry) were used to determine O2 consumption, and muscarinic receptor density and affinity were also determined. T4 increased the heart weight/body weight ratio from 2.6 +/- 0.1 to 3.4 +/- 0.1. T4-treated animals had higher heart rates, blood pressures, and left ventricular dP/dtmax than control rabbits. Topical acetylcholine depressed hemodynamic parameters with a greater decrement in pressures and cardiac output in the T4A group (CA, -25%, T4A, -40%). Myocardial O2 consumption and coronary blood flow were higher in the T4-treated hearts. Myocardial O2 consumption significantly declined in both groups during acetylcholine, but the reduction was greater in the T4-treated hearts (CV 7.9 +/- 0.4 to CA 5.8 +/- 0.6 and T4V 18.8 +/- 3.0 to T4A 7.3 +/- 1.0 mL O2.min-1.100 g-1). Muscarinic receptor density (Bmax) was elevated by 41% in the T4-treated hearts, but affinity (Kd) was not altered. Thus, the T4-treated hearts responded to acetylcholine to a greater extent than control hearts in terms of functional and O2 consumption decrements.(ABSTRACT TRUNCATED AT 250 WORDS)

Beta adrenergic and muscarinic receptors in compensatory cardiac hypertrophy of the adult rat

The beta adrenergic (beta AR) and muscarinic (MR) receptors have been quantitated in parallel, using 125I-pindolol and 3H-quinuclidinylbenzilate, in a model of compensatory left ventricular (LV) hypertrophy (LVH), which developed in rats 4-6 weeks after an abdominal aortic stenosis. Since aortic banding resulted in a pronounced LVH of 62%, the results were expressed both in terms of density (fmol/mg protein) and quantity (fmol per LV). In addition, competition curves using either a specific beta 1-antagonist or isoproterenol or carbachol allowed the determination of the two beta AR subtypes and of the low and high affinity sites (defined by the inhibitory constant Ki) for both beta 1AR and MR. In LVH, receptor density decreased for each of total beta AR, beta 1AR subtype, high affinity (Ki 6-8 nM) beta 1AR sites (from 26 +/- 2 to 19 +/- 3 fmol/mg protein, P < 0.05), total MR and high affinity (Ki 12 nM) MR sites (from 63 +/- 6 to 40 +/- 4 fmol/mg protein, P < 0.001). The beta AR and MR densities dropped in parallel so that the MR/beta AR ratio remained unchanged. In sharp contrast (because the LVs were bigger) the quantities of total beta AR, beta 1AR subtype, beta 1AR high affinity sites, total MR and MR high affinity sites per LV were unmodified.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiac beta-adrenoceptor changes in experimental hyperthyroidism in dogs

1. Triiodothyronine (T3; 1.0 mg/kg per day subcutaneously) was administered to 10 dogs for 14 days; 10 saline-treated dogs served as controls. T3-treated dogs showed the expected physiological responses of hyperthyroidism; further, chronotropic responses to isoprenaline in vivo were significantly increased in T3-treated dogs. 2. Beta-adrenoceptor subtype density was measured in membrane preparations by displacement of 125I-iodocyanopindolol binding by the selective beta 2-adrenoceptor antagonist, ICI 118, 551. T3 treatment led to a 93% increase in right atrial beta 1-adrenoceptor density and a 141% increase in left ventricular beta 1-adrenoceptor density; beta 2-adrenoceptor densities in right atrial, left ventricular and lung membranes were unchanged. 3. T3-treatment did not change basal or maximally stimulated adenylate cyclase activities in left ventricular membranes. 4. Thus, the cardiovascular changes in experimental hyperthyroidism in dogs were accompanied by an increased chronotropic response in vivo to isoprenaline and an increased beta 1-adrenoceptor density in atrial and ventricular membranes. However, there was no corresponding change in basal or maximal responsiveness of adenylate cyclase in ventricular membranes.