Thyroid hormone modulates inotropic responses, alpha-adrenoceptor density and catecholamine concentrations in the rat heart

Protective Effects of Thyroid Hormone Deprivation on Progression of Maladaptive Cardiac Hypertrophy and Heart Failure

Purpose: Thyroid hormones (TH) play a central role for cardiac function. TH influence heart rate and cardiac contractility, and altered thyroid function is associated with increased cardiovascular morbidity and mortality. The precise role of TH in onset and progression of heart failure still requires clarification. Methods: Chronic left ventricular pressure overload was induced in mouse hearts by transverse aortic constriction (TAC). One week after TAC, alteration of TH status was induced and the impact on cardiac disease progression was studied longitudinally over 4 weeks in mice with hypo- or hyperthyroidism and was compared to euthyroid TAC controls. Serial assessment was performed for heart function (2D M-mode echocardiography), heart morphology (weight, fibrosis, and cardiomyocyte cross-sectional area), and molecular changes in heart tissues (TH target gene expression, apoptosis, and mTOR activation) at 2 and 4 weeks. Results: In diseased heart, subsequent TH restriction stopped progression of maladaptive cardiac hypertrophy and improved cardiac function. In contrast and compared to euthyroid TAC controls, increased TH availability after TAC propelled maladaptive cardiac growth and development of heart failure. This was accompanied by a rise in cardiomyocyte apoptosis and mTOR pathway activation. Conclusion: This study shows, for the first time, a protective effect of TH deprivation against progression of pathological cardiac hypertrophy and development of congestive heart failure in mice with left ventricular pressure overload. Whether this also applies to the human situation needs to be determined in clinical studies and would infer a critical re-thinking of management of TH status in patients with hypertensive heart disease.

Cardiovascular Responses to 5-hydroxytryptamine in Methimazole-induced Hypothyroid Pithed Rats

BACKGROUND

Patients suffering from hypothyroidism tend to develop diastolic hypertension. 5-Hydroxytryptamine (5-HT) is an amine that contributes to the maintenance of the blood pressure through central and peripheral 5-HT receptors. Curiously, the hypothyroidism alters the density of the 5-HT receptors in rodent brains.

AIM OF THE STUDY

Analyze the effect of the methimazole-induced hypothyroidism on the peripheral cardiovascular responses elicited by 5-HT.

METHODS

The vasopressor and tachycardic responses to 5-HT (3-300 μg/kg), and the vasodepressor responses to 5-HT, 5-carboxamidotryptamine (5-CT, 0.001-0.1 μg/kg), isoprenaline (0.03-1 μg/kg) and acetylcholine (ACh, 0.03-3 μg/kg), during an infusion of methoxamine, were determined in pithed hypothyroid rats.

RESULTS

The tachycardic and vasopressor responses to 5-HT and the vasodepressor responses to 5-CT and ACh remained unaffected, the vasodepressor response to 5-HT reduced, and the vasodepressor response to isoprenaline enhanced and reduced at the lowest and highest dose, respectively.

CONCLUSION

These results suggest that hypothyroidism impairs the vasodepressor response to 5-HT, which could contribute to hypothyroidism-induced hypertension.

Impact of experimental hypothyroidism on monoamines level in discrete brain regions and other peripheral tissues of young and adult male rats

The levels of dopamine (DA), norepinephrine (NE) and serotonin (5-HT) in different brain regions as well as in blood plasma, cardiac muscle and adrenal gland of young and adult male albino rats were measured following experimentally induced hypothyroidism. Hypothyroidism induced by daily oral administration of propylthiouracil (PTU, 5mg/kg body wt) caused a significant reduction in DA levels in most of the tissues examined of both young and adult rats after 21 and 28 days, in NE levels after all the time intervals studied in young rats, and after 21 and 28 days in adult rats. 5-HT exhibited a significant reduction in the selected brain regions and blood plasma after 21 and 28 days and in cardiac muscle after all the time intervals in the two age groups of animals. It may be suggested that the changes in monoamine levels induced by hypothyroidism may be due to disturbance in the synthesis and release of these amines through the neurons impairment or may be due to an alteration pattern of their synthesizing and/or degradative enzymes.

Serum thyroid hormone levels may not accurately reflect thyroid tissue levels and cardiac function in mild hypothyroidism

The link between thyroid dysfunction and cardiovascular diseases has been recognized for more than 100 years. Although overt hypothyroidism leads to impaired cardiac function and possibly heart failure, the cardiovascular consequences of borderline low thyroid function are not clear. Establishment of a suitable animal model would be helpful. In this study, we characterized a rat model to study the relationship between cardiovascular function and graded levels of thyroid activity. We used rats with surgical thyroidectomy and subcutaneous implantation of slow release pellets with three different T4 doses for 3 wk. In terminal experiments, cardiac function was evaluated by echocardiograms and hemodynamics. Myocardial arteriolar density was also quantified morphometrically. Thyroid hormone levels in serum and heart tissue were determined by RIA assays. Thyroidectomy alone led to cardiac atrophy, severe cardiac dysfunction, and a dramatic loss of arterioles. The low T4 dose normalized serum T3 and T4 levels, but cardiac tissue T3 and T4 remained below normal. Low-dose T4 failed to prevent cardiac atrophy or restore cardiac function and arteriolar density to normal values. All cardiac function parameters and myocardial arteriolar density were normalized with the middle dose of T4, whereas the high dose produced hyperthyroidism. Our results show that thyroid hormones are important regulators of cardiac function and myocardial arteriolar density. This animal model will be useful in studying the pathophysiological consequences of mild thyroid dysfunction. Results also suggest that cardiac function may provide valuable supplemental information in proper diagnosis of mild thyroid conditions.

Upregulation of the α1-adrenoceptor-induced phosphoinositide and inotropic response in hypothyroid rat heart

In this study, we examined changes in the biochemical and inotropic events of the alpha(1)-adrenoceptor signaling pathway in hypothyroid rat hearts. Hypothyroidism was induced by treating experimental animals with 0.05% 6-n-propyl-2-thiouracil (PTU) in drinking water for 7 weeks. A significant decrease of beta- and an increase in alpha(1)-adrenoceptor density as well as an increase in the basal activity of the phosphoinositide (4,5) bisphosphate hydrolyzing phospholipase C was observed in sarcolemmal membranes purified from hypothyroid hearts as compared to age-matched euthyroid controls. Following stimulation with 10 microM phenylephrine (in the presence of 10 microM atenolol), the increase of contractile parameters over baseline values was significantly higher in hypo- than euthyroid hearts, while the opposite occurred under beta-stimulation with 0.1 microM isoproterenol. Interestingly, the increase in phenylephrine-mediated positive inotropy was accompanied by a significant increase in the sarcolemmal phospholipase C activity and in the inositol 1,4,5-trisphosphate content in hypothyroid as compared to euthyroid controls. Our results suggest that cardiac alpha(1)-adrenoceptor and its associated phosphoinositide signaling pathway may act as a reserve for catecholamine inotropic response in hypothyroidism, where the beta-adrenoceptors are compromised.

Effects of triiodothyronine pretreatment on beta-adrenergic responses in stunned cardiac myocytes

OBJECTIVE

To investigate whether triiodothyronine pretreatment enhanced beta-adrenergic responses in stunned myocardium and whether this acute effect of triiodothyronine was mediated through the cyclic adenosine 3',5'-monophosphate (AMP) system.

DESIGN

A prospective study.

SETTING

University laboratory.

PARTICIPANTS

Rabbits.

INTERVENTIONS

Rabbit ventricular myocytes were isolated and placed in a medium equilibrated with either air (control) or with 95% N(2) and 5% CO(2) (stunned) for 15 minutes at 37 degrees C. The stunned myocytes were reoxygenated with air for 30 minutes. Triiodothyronine (10 nmol/L) and/or isoproterenol (0.1 nmol/L) was added to the myocytes. Myocyte shortening was measured by using a video-edge detector.

MEASUREMENTS AND MAIN RESULTS

In electrically stimulated cells, the basal values of the percent shortening (22%-30%) and the maximum rate of shortening (22%-25%) were significantly reduced in the stunned myocytes. Isoproterenol (5 minutes) alone significantly increased the percent shortening in the control (from 3.70 +/- 0.36 to 4.14 +/- 0.37) but not in the stunned myocytes (from 2.60 +/- 0.30 to 3.15 +/- 0.27). Triiodothyronine (5 minutes) alone significantly increased the percent shortening in the control (from 3.75 +/- 0.36 to 4.34 +/- 0.45) and in the stunned myocytes (from 2.91 +/- 0.2 to 3.85 +/- 0.26). After triiodothyronine pretreatment for 5 minutes, isoproterenol caused greater increases in the percent shortening in both the control (37%) and the stunned myocytes (62%) than either agent alone. Isoproterenol or triiodothyronine caused small increases in the maximum rate of shortening in the control (14%-16%) and the stunned myocytes (34%-49%). After triiodothyronine pretreatment, isoproterenol caused greater increases in the maximum rate of shortening in both groups (control: 41%, stunned: 73%) than either agent alone. Isoproterenol caused an increase in the level of cyclic AMP (rho;moles/10(5) myocytes) in the control (from 2.92 +/- 0.47 to 3.77 +/- 0.43) but not in the stunned myocytes (from 2.42 +/- 0.25 to 2.42 +/- 0.20). Triiodothyronine pretreatment did not cause any change in cyclic AMP levels in the control (2.50 +/- 0.29) or in the stunned myocytes (2.60 +/- 0.40). After triiodothyronine pretreatment, isoproterenol caused a small increase in the cyclic AMP level in the control but not in the stunned myocytes.

CONCLUSIONS

The data showed that the myocardial beta-adrenergic responses were more sensitive to ischemic insult than the triiodothyronine responses. Triiodothyronine pretreatment enhanced beta-adrenergic responses in both the control and the stunned myocytes. However, this acute positive inotropic effect of triiodothyronine might not be mediated through the cyclic AMP system.

Role of endothelium-derived relaxing factors in the renal response to vasoactive agents in hypothyroid rats

This study analyzed the role of nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF) in the abnormal renal vascular reactivity of hypothyroid rats. Renal responses to vasoconstrictors [VC: phenylephrine (PHE) and ANG II] and vasodilators [VD: ACh, sodium nitroprusside (SNP), and papaverine (PV)] were studied in kidneys from control and hypothyroid rats under normal conditions and after NO or EDHF blockade. NO was blocked by the administration of Nomega-nitro-l-arginine methyl ester (l-NAME) and EDHF by the administration of tetraethylammonium (TEA) or by an increased extracellular K+. The response to VC was also evaluated after endothelium removal. Hypothyroid kidneys showed reduced responsiveness to PHE and a normal response to ANG II. l-NAME and TEA administration produced an increased sensitivity to PHE and to ANG II in control preparations. l-NAME also increased the response to PHE in hypothyroid kidneys, but the differences between control and hypothyroid kidneys were maintained. TEA administration did not change the response to either VC in hypothyroid preparations. In endothelium-removed preparations, TEA was unable to increase pressor responsiveness to VC. Hypothyroid kidneys showed reduced responsiveness to ACh and SNP and normal response to PV. The differences between hypothyroid and control preparations in the responses to ACh and SNP were maintained after l-NAME or increased K+. In conclusion, this study shows that 1) the attenuated response to PHE in hypothyroidism is not related to an increased production of endothelium-derived relaxing factors NO and EDHF; 2) the response to VC in hypothyroid preparations is insensitive to EDHF blockade; and 3) hypothyroid preparations have a reduced reactivity to the NO donor, and NO-independent vasodilatation remains unaffected.

Arrhythmogenesis in isolated rat hearts with enhanced alpha-adrenoceptor-mediated responsiveness

1. It has been postulated that stimulation of myocardial alpha-adrenoceptors is one of the primary mediators of the dysrhythmias which occur during periods of myocardial ischaemia and reperfusion. This study examines arrhythmogenesis during coronary artery occlusion and reperfusion in isolated perfused rat hearts from control animals and from rats with enhanced myocardial alpha-adrenoceptor responsiveness. 2. Rats were administered propylthiouracil (PTU) in their drinking water for 8 weeks. This treatment resulted in an enhanced responsiveness of isolated left atria to the alpha-adrenoceptor agonist phenylephrine compared with atria from control animals. 3. In Langendorff-perfused isolated hearts, the spontaneous rate of contraction was significantly lower in the PTU-pretreated group than in either age-matched or weight-matched controls. Occlusion of the left anterior descending artery (LAD) for 25 min resulted in ventricular tachycardia (VT) of similar incidence and duration in all groups and ventricular fibrillation (VF) in both control groups but not the PTU-pretreated group. 4. Following the 25-min ischaemic period the myocardium was reperfused for 10 min. The incidence and duration of VT and VF during this period was similar in all groups except that the duration of VF in the PTU-pretreated group was significantly lower than in controls. 5. In perfused hearts paced at 4 Hz, the incidence and duration of dysrhythmias during ischaemia and reperfusion was again similar in all groups, only the duration of VF being affected (reduced) by PTU-pretreatment. 6. In conclusion, this study does not lend support to the hypothesis that myocardial alpha-adrenoceptors have a primary role in arrhythmogenesis, but the data would support a role for these receptors in myocardial protection.

Effects of oral propylthiouracil treatment on nitric oxide production in rat aorta

The effects of oral propylthiouracil (PTU) treatment on vascular nitric oxide (NO) production were studied in the rat aorta. Rats were fed a standard low fat diet with or without 0.1% PTU, for 2 or 4 weeks, or for 2 weeks with additional thyroxine injections. Concentration response curves were then constructed to phenylephrine (PE) in both endothelium-intact and denuded aortic rings from these animals and after incubation with 0.1 mM L-N(G)nitroarginine (L-NOARG). In addition, expression of nitric oxide synthase (NOS) was analysed in sections of aorta from PTU-treated and control rats using rabbit polyclonal antibodies to both inducible NOS (iNOS) and endothelial NOS (eNOS). Oral PTU treatment resulted in a significant reduction in both the maximum response (control, 0.53+/-0.02; 2 week PTU, 0.20+/-0.07; 4 week PTU, 0.07+/-0.02 g mg(-1)) and vessel sensitivity (EC50 values: control, 9.10x10(-8)+/-0.67; 2 week PTU, 7.45x10(-7)+/-1.15; 4 week PTU, 9.73x10(-7)+/-0.45 M) to PE in endothelium-intact vessel rings, as compared to controls (P<0.05). Both endothelial removal and incubation with L-NOARG restored the maximum response after 2, but not 4 weeks, although, in general, vessel sensitivity was not altered by either treatment. Vessels from PTU-treated rats given thyroxine injections showed no significant differences between any of the dose response curve parameters. Immunohistochemical analysis suggested that labelling for eNOS may be increased after PTU treatment as compared to control animals, whereas iNOS antibody immunoreactivity was not different between the two groups. These results suggest that the hyporesponsiveness to PE observed after oral PTU treatment is, in part, due to enhanced nitric oxide (NO) production by the endothelium, and demonstrate for the first time that thyroid hormones may play a role in the regulation of eNOS activity in the rat aorta.

Effects of thyroid hormone on catecholamine and its metabolite concentrations in rat cardiac muscle and cerebral cortex

Clinical and experimental data suggest that thyroid hormone affects the actions of catecholamine (CA). However, the serum or tissue levels of CA during thyroid disorders have not been well defined. Accordingly, we investigated the levels of CA and their metabolites in the cardiac muscle, the cerebral cortex, and the plasma of rats with hyperthyroidism and hypothyroidism versus euthyroid animals. The Neurochem analyzer system (ESA, Inc., Bedford, MA) was used in such determinations. The cardiac muscles of hyperthyroid rats exhibited a 16% decrease in the levels of 1-dopa, 3-methoxytyramine (3-MT) and homovanillic acid (HVA) as compared with those in euthyroid rats. The levels of norepinephrine (NE) in cardiac muscle of these rats increased significantly (5.2-fold) relative to the levels in euthyroid rats. NE was undetectable in the cardiac muscles of the hypothyroid rats. Epinephrine (E) and dopamine (DA) were not detected in the cardiac muscles of the rats with either thyroid disorder. Levels of E and 3,4-dihydroxymandelic acid (DOPEG) were detected only in the cerebral cortex of hyperthyroid rats. The cerebral cortex levels of 3-methyoxytyramine (3-MT), 3,4-dihydroxyphenylacetic acid (DOPAC), metanephrine (MN), and homovanillic acid (HVA) were all significantly increased in the hyperthyroid versus the euthyroid rats. The cerebral cortex levels of DA, NE, normetanephrine (NMN), and VMA in the hyperthyroid rats all showed a significant decrease. Levels of NE, NMN, and DOPAC in the cerebral cortex increased significantly in the hypothyroid rats. The level of VMA was undetectable in cerebral cortex of such animals. Data from studies on cardiac muscle and cerebral cortex indicate that the changes in CA and CA metabolites are responsible in part for the cardiovascular and the central nervous system symptoms observed in hyperthyroidism and hypothyroidism.