Thyroid status and response to endothelin-1 in rat arterial vessels

Regulation of nitric oxide production in hypothyroidism

Hypothyroidism is a common endocrine disorder that predominantly occurs in females. It is associated with an increased risk of cardiovascular diseases (CVD), but the molecular mechanism is not known. Disturbance in lipid metabolism, the regulation of oxidative stress, and inflammation characterize the progression of subclinical hypothyroidism. The initiation and progression of endothelial dysfunction also exhibit these changes, which is the initial step in developing CVD. Animal and human studies highlight the critical role of nitric oxide (NO) as a reliable biomarker for cardiovascular risk in subclinical and clinical hypothyroidism. In this review, we summarize the recent literature findings associated with NO production by the thyroid hormones in both physiological and pathophysiological conditions. We also discuss the levothyroxine treatment effect on serum NO levels in hypothyroid patients.

Thyroid hormone receptor-α deletion decreases heart function and exercise performance in apolipoprotein E-deficient mice

The deletion of thyroid hormone receptor-α (TRα) in atherosclerosis-prone apolipoprotein E-deficient (ApoE(-/-)) mice (ApoE(-/-)TRα(0/0)) accelerates the formation of atherosclerotic plaques without aggravation of hypercholesterolemia. To evaluate other predisposition risk factors to atherosclerosis in this model, we studied blood pressure (BP) and cardiac and vascular functions, as well as exercise tolerance in young adult ApoE(-/-)TRα(0/0) mice before the development of atherosclerotic plaques. Telemetric BP recorded for 4 consecutive days showed that the spontaneous systolic BP was slightly decreased in ApoE(-/-)TRα(0/0) compared with ApoE(-/-) mice associated with a reduced locomotor activity. The percentage of animals that completed endurance (57% vs. 89%) and maximal running (0% vs. 89% at 46 cm/s speed in ApoE(-/-)TRα(0/0) and ApoE(-/-) mice, respectively) tests was lower in ApoE(-/-)TRα(0/0) mice. Moreover, during the maximal running test, both maximal running speed and running distance were significantly reduced in ApoE(-/-)TRα(0/0) mice, associated with a blunted BP response to exercise. Transthoracic echocardiography revealed a decreased interventricular septum thickness and an increased end-systolic left ventricular volume in ApoE(-/-)TRα(0/0) mice. Accordingly, left ventricular fractional shortening, ejection fraction, and stroke volume were all significantly decreased in ApoE(-/-)TRα(0/0) mice with a concomitant blunted cardiac output. No interstrain difference was observed in vascular reactivity, except that ApoE(-/-)TRα(0/0) mice exhibited an enhanced acetylcholine-induced relaxation in mesenteric and distal femoral arteries. In conclusion, the deletion of TRα in ApoE(-/-) mice alters cardiac structure and contractility; both could contribute to blunted BP response to physical exercise and impaired exercise performance.

The incidence and severity of pulmonary hypertension in obstructive sleep apnea with hypothyroidism

BACKGROUND

Hypothyroidism and obstructive sleep apnea (OSA) are both common health problems and can be seen together. Each of these 2 diseases can cause pulmonary hypertension (PH). We aimed to determine whether hypothyroidism with OSA has a significant effect on the frequency and severity of PH.

MATERIAL AND METHODS

A total of 236 patients were included in the study. Patients were divided into 3 groups: Group I, Obstructive Sleep Apnea (n=149); Group II, Hypothyroidism (n=56); and Group III, Obstructive Sleep Apnea-Hypothyroidism (n=31). All patients underwent polysomnography and echocardiography and serum levels of thyroid-stimulating hormone (TSH) and free thyroxine 4 (FT4) were analyzed.

RESULTS

There were 167 male and 69 female participants, and the mean age was 47.8 ± 11.5 (Group I: 81.9% male, 18.1% female; Group II: 44.6% male, 55.4% female; Group III: 64.6% male, 35.4% female). Distribution of mean pulmonary arterial pressure on echocardiography was statistically different among the 3 groups (x(2)=14.99, p=0.006). When adjusted according to the apnea-hypopnea index (AHI), age, and body mass index (BMI), a significant relation with PH was determined (p=0.002).

CONCLUSIONS

The combination of hypothyroidism with OSA is associated with an increased frequency and severity of PH. When PH is found out of line with the severity of OSA, thyroid dysfunction should be investigated.

Effect of acute hyperthyroidism on blood flow, muscle oxygenation, and sympathetic nerve activity during dynamic handgrip

Hyperthyroidism induces marked changes in hemodynamics. Although considerable research has been done to study the effect of hyperthyroidism on the cardiovascular system, few studies have isolated the short-term, nongenomic effects of thyroid hormone on cardiovascular responses to exercise. We used near-infrared spectroscopy to measure muscle oxygenation, Doppler ultrasound to measure skeletal muscle blood flow, and microneurography to measure muscle sympathetic nerve activity (MSNA) during fatiguing dynamic handgrip in twelve healthy males (26 ± 1 years). Subjects were measured separately in both the euthyroid state, and acute hyperthyroid state (approximately ten times the normal levels of T₃), induced by oral dosage of 300 μg of triiodothyronine (T₃). Forearm blood flow was increased as a function of exercise time in the euthyroid and hyperthyroid state (Δ161.8 ± 45.0 mL/min and Δ140.7 ± 16.3 mL/min, respectively) but there was no significant difference between trials. Forearm vascular conductance (FVC) also increased as a function of exercise time with no significant difference between treatments at submaximal exercise but was significantly less with T₃ treatment. MSNA was not different at rest or during submaximal exercise; however, MSNA was significantly greater at fatigue during the hyperthyroid state. Muscle oxyhemoglobin concentration was decreased during exercise in both euthyroid and hyperthyroid states (Δ19.7 ± 10.8% and Δ14.8 ± 9.6%, respectively); whereas deoxyhemoglobin concentration was increased (Δ50.0 ± 4.1% and Δ50.0 ± 6.2%, respectively). These results indicate that T₃ had no direct effect on skeletal muscle oxygenation or blood flow during dynamic exercise, but elicited greater MSNA and lower FVC during fatiguing exercise.

Thyroid hormone stimulates NO production via activation of the PI3K/Akt pathway in vascular myocytes

AIMS

Thyroid hormone (TH) rapidly relaxes vascular smooth muscle cells (VSMCs). However, the mechanisms involved in this effect remain unclear. We hypothesize that TH-induced rapid vascular relaxation is mediated by VSMC-derived nitric oxide (NO) production and is associated with the phosphatidylinositol 3-kinase/protein kinase B (PI3K/Akt) signalling pathway.

METHODS AND RESULTS

NO levels were determined using a NO-specific fluorescent dye (DAF-2) and nitrite (NO2-) levels. Expression of NO synthase (NOS) isoforms and proteins of the PI3K/Akt pathway was determined by both western blotting and immunocytochemistry. Myosin light chain (MLC) phosphorylation levels were also investigated by western blotting. Exposure of cultured VSMCs from rat thoracic aortas to triiodothyronine (T3) resulted in a significant decrease of MLC phosphorylation levels. T3 also induced a rapid increase in Akt phosphorylation and increased NO production in a dose-dependent manner (0.001-1 microM). VSMCs stimulated with T3 for 30 min showed an increase in the expression of all three NOS isoforms and augmented NO production, effects that were prevented by inhibitors of PI3K. Vascular reactivity studies showed that vessels treated with T3 displayed a decreased response to phenylephrine, which was reversed by NOS inhibition. These data suggest that T3 treatment induces greater generation of NO both in aorta and VSMCs and that this phenomenon is endothelium independent. In addition, these findings show for the first time that the PI3K/Akt signalling pathway is involved in T3-induced NO production by VSMCs, which occurs with expressive participation of inducible and neuronal NOS.

CONCLUSION

Our data strongly indicate that T3 causes NO-dependent rapid relaxation of VSMC and that this effect is mediated by the PI3K/Akt signalling pathway.

Inducible nitric oxide synthase is involved in endothelial dysfunction of mesenteric small arteries from hypothyroid rats

The time-dependent effects of mild hypothyroidism on endothelial function were assessed in rat mesenteric arteries. Male Wistar rats were treated with methimazole (MMI; 0.003%) or placebo up to 16 wk. Endothelial function of mesenteric small arteries was assessed by pressurized myograph. MMI-treated animals displayed a decrease in serum thyroid hormones, an increment of plasma TSH and inflammatory cytokines, and a blunted vascular relaxation to acetylcholine, as compared with controls. Endothelial dysfunction resulted from a reduced nitric oxide (NO) availability caused by oxidative excess. Vascular-inducible NO synthase (iNOS) expression was up-regulated. S-methylisothiourea (an iNOS inhibitor) normalized endothelium-dependent relaxations and restored NO availability in arteries from 8-wk MMI-animals and partly ameliorated these alterations in 16-wk MMI rats. Similar results were obtained when MMI-induced hypothyroidism was prevented by T(4) replacement. Among controls, an impaired NO availability, secondary to oxidative excess, occurred at 16 wk, and it was less pronounced than in age-matched MMI animals. Both endothelial dysfunction and oxidant excess secondary to aging were prevented by apocynin (nicotinamide adenine dinucleotide phosphate oxidase inhibitor). Mesenteric superoxide production was reduced by S-methylisothiourea and T(4) replacement in MMI animals and abolished by apocynin in controls (dihydroethidium staining). MMI-induced mild hypothyroidism is associated with endothelial dysfunction caused by a reduced NO availability, secondary to oxidative excess. It is suggested that in this animal model, characterized by TSH elevation and low-grade inflammation, an increased expression and function of iNOS, resulting in superoxide generation, accounts for an impaired NO availability.

Enhancement of endothelium-dependent flow-mediated vasodilation in hyperthyroidism

OBJECTIVE

Vascular responsiveness changes in hyperthyroid patients remains controversial. This study attempts to determine whether the vasomotor activity can be influenced by hyperthyroid conditions, and, if so, whether changes induced by hyperthyroidism may be restored to normal during the euthyroid state after treatment.

DESIGN

A case-control clinical study.

PATIENTS AND MEASUREMENTS

Forty-five pretreated hyperthyroid patients (mean age 36.62 +/- 10.12 years, 36 female) were compared with 45 gender- and age-matched control subjects (mean age 38.98 +/- 11.17 years, 40 female). Brachial artery endothelium-dependent flow-mediated vasodilation (FMD) and endothelium-independent nitroglycerin-mediated vasodilation (NMD) responses were assessed noninvasively by high-resolution ultrasound imaging. Among the 45 hyperthyroid patients, 27 patients underwent the same procedures prospectively in the post-treatment euthyroid state.

RESULTS

The FMD values were significantly increased in hyperthyroid patients vs. those of controls (8.94 +/- 5.65%vs. 3.77 +/- 3.42%, P < 0.001), whereas NMD levels were not significantly different (18.17 +/- 7.76%vs. 17.28 +/- 6.63%, P = 0.560). Multiple regression analysis revealed that the presence of hyperthyroidism was the only significant factor associated with FMD. In the follow-up study of 27 hyperthyroid patients, the FMD values were significantly decreased in the post-treatment euthyroid state compared with those in the pretreated hyperthyroid state (6.40 +/- 4.27%vs. 8.83 +/- 4.61%, P = 0.021), although these values were still higher than those of controls.

CONCLUSIONS

This study demonstrated that endothelium-dependent FMD was increased in the hyperthyroid patients, and could be partially restored by treatment with antithyroid agents.

Thyroid disease and vascular function

Altered cardiac function in thyroid disease is well recognized and has been extensively investigated, vascular function has however been less well studied in those with thyroid dysfunction. Thyroid hormones, thyroxine (T(4)) and triiodothyronine (T(3)) are important regulators of cardiac function and cardiovascular hemodynamics. The cardiovascular system responds to minimal but persistent changes in circulating thyroid hormone levels producing changes in vascular reactivity and endothelial function. The detection of endothelial dysfunction and/or arterial stiffness allows early identification of individuals at risk as these occur in both patients with risk factors for coronary artery disease and in those with established disease. This may allow treatment to be targeted at high risk individuals with the aim of slowing the progression of vascular disease. The various methods used to assess arterial function are reviewed and the changes demonstrated in human and animal models of thyroid dysfunction.

Thyroid status and endothelium-dependent vasodilation in skeletal muscle

Cardiovascular dysfunction is characteristic of both hypo- and hyperthyroidism. Endothelium-dependent dilation of conductance vessels is impaired in hypothyroidism but augmented in hyperthyroidism. We hypothesized that these alterations in dilation extend into the resistance vasculature of skeletal muscle. To test this hypothesis, rats were made hypothyroid with propylthiouracil (Hypo; n = 13) or hyperthyroid with triiodothyronine (Hyper; n = 9) over 3-4 mo. Compared with euthyroid controls (Eut; n = 14), Hypo rats were characterized by reduced skeletal muscle oxidative capacity and blunted growth; Hyper rats exhibited increased muscle oxidative capacity and left ventricular hypertrophy (P < 0.05 for all effects). Vasodilation to the endothelium-dependent agent acetylcholine ( approximately 2 x 10(-4) M) in skeletal muscle was determined in situ. Conductance in certain muscles increased from control [e.g., soleus: 0.98 +/- 0.15 (Eut), 0.79 +/- 0.14 (Hypo), and 1.06 +/- 0.24 ml.min(-1).100 g(-1).mmHg(-1) (Hyper); not significant among groups] to acetylcholine [1.91 +/- 0.21 (Eut), 2.28 +/- 0.26 (Hypo), and 2.15 +/- 0.33 ml.min(-1).100 g(-1).mmHg(-1) (Hyper); P < 0.05 vs. control values for all groups] but did not differ among groups. Expression of mRNA for the endothelial isoform of nitric oxide synthase in resistance vessels isolated from various muscles was similarly unchanged with alterations in thyroid status [e.g., soleus 1A arterioles: 33.15 +/- 0.58 (Eut), 32.73 +/- 0.27 (Hypo), and 32.80 +/- 0.54 (Hyper) cycles at threshold; not significant]. These data suggest that endothelium-dependent dilation of resistance vasculature in skeletal muscle is unchanged in both hypo- and hyperthyroidism. These data also emphasize the importance of examining resistance vasculature to improve understanding of effects of chronic disease on integrated cardiovascular function.