Endothelial dysfunction in a murine model of thyroid hormone resistance

Aortic effects of thyroid hormone in male mice

It is well established that thyroid hormones are required for cardiovascular functions; however, the molecular mechanisms remain incompletely understood, especially the individual contributions of genomic and non-genomic signalling pathways. In this study, we dissected how thyroid hormones modulate aortic contractility. To test the immediate effects of thyroid hormones on vasocontractility, we used a wire myograph to record the contractile response of dissected mouse aortas to the adrenergic agonist phenylephrine in the presence of different doses of T3 (3,3',5-triiodothyronine). Interestingly, we observed reduced vasoconstriction under low and high T3 concentrations, indicating an inversed U-shaped curve with maximal constrictive capacity at euthyroid conditions. We then tested for possible genomic actions of thyroid hormones on vasocontractility by treating mice for 4 days with 1 mg/L thyroxine in drinking water. The study revealed that in contrast to the non-genomic actions the aortas of these animals were hyperresponsive to the contractile stimulus, an effect not observed in endogenously hyperthyroid TRβ knockout mice. To identify targets of genomic thyroid hormone action, we analysed aortic gene expression by microarray, revealing several altered genes including the well-known thyroid hormone target gene hairless. Taken together, the findings demonstrate that thyroid hormones regulate aortic tone through genomic and non-genomic actions, although genomic actions seem to prevail in vivo. Moreover, we identified several novel thyroid hormone target genes that could provide a better understanding of the molecular changes occurring in the hyperthyroid aorta.

Augmentation index in resistance to thyroid hormone (RTH)

OBJECTIVE

Resistance to thyroid hormone (RTH) is associated with a varied clinical presentation. The cardiac effects of RTH have been described but vascular function has yet to be fully evaluated in this condition. We have measured the arterial function of those with RTH to assess any vascular changes.

DESIGN

An observational study.

PATIENTS

Twelve RTH patients were recruited from the thyroid clinic (mean value +/- SD), age 40.8 +/- 18.7 years; BMI 27.2 +/- 4.2 kg/m(2) and compared with 12 healthy, euthyroid, age-matched controls (age 41.4 +/- 19.3; BMI 24.8 +/- 4.4 kg/m(2)) with no history of cardiovascular disease. No interventional measures were instituted.

MEASUREMENTS

Arterial stiffness was measured using pulse wave analysis at the radial artery. Thyroid function, fasting lipids and glucose were also measured on the same occasion in both patients and controls. Results The corrected augmentation index, a surrogate marker of arterial stiffness was significantly higher in patients compared with controls (21.0% +/- 14.1%vs. 5.4% +/- 18.2%, P < 0.03). Low density lipoprotein cholesterol (LDL-cholesterol) levels were also significantly elevated in patients compared with controls (3.0 +/- 0.6 vs. 2.1 +/- 0.5 mmol/l; P < 0.002).

CONCLUSION

RTH patients show evidence in this study of increased augmentation index consistent with an increase in arterial stiffness compared with euthyroid controls. They also demonstrate elevated LDL-cholesterol levels. Both these measures may lead to increased cardiovascular risk.

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.

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.