Hypothyroidism provides resistance to reperfusion injury following myocardium ischemia

Changes in the Stoichiometry of Uniplex Decrease Mitochondrial Calcium Overload and Contribute to Tolerance of Cardiac Ischemia/Reperfusion Injury in Hypothyroidism

Background: Thyroid hormone status in hypothyroidism (HT) downregulates key elements in Ca²⁺ handling within the heart, reducing contractility, impairing the basal energetic balance, and increasing the risk of cardiovascular disease. Mitochondrial Ca²⁺ transport is reduced in HT, and tolerance to reperfusion damage has been documented, but the precise mechanism is not well understood. Therefore, we aimed to determine the stoichiometry and activity of the mitochondrial Ca²⁺ uniporter or uniplex in an HT model and the relevance to the opening of the mitochondrial permeability transition pores (mPTP) during ischemia/reperfusion (I/R) injury. Methods: An HT model was established in Wistar rats by treatment with 6-propylthiouracil for 28 days. Uniplex composition and activity were determined in cardiac mitochondria. Hearts were perfused ex vivo to induce I/R injury, and functional parameters related to contractility and tissue viability were evaluated. Results: The cardiac stoichiometry between two subunits of the uniplex (MICU1/MCU) increased by 25% in animals with HT. The intramitochondrial Ca²⁺ content was reduced by 40% and was less prone to the mPTP opening. After I/R injury, ischemic contracture and the onset of ventricular fibrillation were delayed in animals with HT, concomitant with a reduction in oxidative damage and mitochondrial dysfunction. Conclusions: Our results suggest that HT is associated with an increase in the cardiac MICU1/MCU ratio, thereby changing the stoichiometry between these subunits to increase the threshold to cytosolic Ca²⁺ and reduce mitochondrial Ca²⁺ overload. Our results also demonstrate that this HT model can be used to explore the role of mitochondrial Ca²⁺ transport in cardiac diseases due to its induced tolerance to cardiac damage.

Cardioprotective Mechanisms of Hypothyroidism on Ischemia/Reperfusion in Rats and Effects of Carvedilol: Energetic Study

Hypothyroidism is considered a cardiac risk factor, but there is controversial evidence about its effects on coronary disease. The aim of this work was to evaluate the influence of hypothyroidism in rat hearts exposed to 2 degrees of stunning due to ischemia and reperfusion (I/R) as well as the underlying mechanisms. Hypothyroid (HypoT) rats were obtained by drinking 0.02% methimazole during 15 days. Isolated hearts were perfused and introduced in a flow calorimeter to measure contractile performance (P), total heat rate (Ht), and muscle economy (P/Ht). Hearts were exposed to 2 models of I/R, moderate and severe (respectively 20 or 30 minutes I/45 minutes R). Moreover, free cytosolic and mitochondrial calcium changes were measured by confocal fluorometry on cardiomyocytes. Comparison to euthyroid (EuT) hearts was done. Hypothyroidism was cardioprotective, but HypoT hearts were more sensitive than EuT hearts to the preischemic blockade of mitochondrial transporters mNCX and mKATPchannels. Moreover, the postischemic recovery of P and P/Ht in HypoT hearts was strongly reduced by inhibition of the cellular pathways of PI3K/Akt and protein kinase C (PKC), and it was increased by nitric oxide synthase (NOS) inhibition. However, physiological concentrations of adrenaline reduced the cardioprotection of HypoT, but oral treatment with 20 mg/kg/day carvedilol prevented it. Results show that hypothyroidism reduces the mitochondrial Ca2+overload during I/R by mKATPchannel activation and Ca2+extrusion through mNCX, while the PI3K/Akt and PKC pathways are involved in that cardioprotection. Contrarily, NOS activation and adrenaline blunt such cardioprotection, but carvedilol prevented the adrenergic dysfunction. These results would explain why hypothyroidism is a clinical risk factor in angor patients under adrenergic exacerbation but reduced the incidence of acute episodes of coronary syndrome in hospitalized patients. Results suggest that a treatment with carvedilol could be a potential therapeutic agent to prevent cardiac postischemic dysfunction in hypothyroid patients.

Cardiac ischemia/reperfusion injury is inversely affected by thyroid hormones excess or deficiency in male Wistar rats

AIM

Thyroid dysfunctions can increase the risk of myocardial ischemia and infarction. However, the repercussions on cardiac ischemia/reperfusion (IR) injury remain unclear so far. We report here the effects of hypothyroidism and thyrotoxicosis in the susceptibility to IR injury in isolated rat hearts compared to euthyroid condition and the potential role of antioxidant enzymes.

METHODS

Hypothyroidism and thyrotoxicosis were induced by administration of methimazole (MMZ, 300 mg/L) and thyroxine (T4, 12 mg/L), respectively in drinking water for 35 days. Isolated hearts were submitted to IR and evaluated for mechanical dysfunctions and infarct size. Superoxide dismutase types 1 and 2 (SOD1 and SOD2), glutathione peroxidase types 1 and 3 (GPX 1 and GPX3) and catalase mRNA levels were assessed by quantitative RT-PCR to investigate the potential role of antioxidant enzymes.

RESULTS

Thyrotoxicosis elicited cardiac hypertrophy and increased baseline mechanical performance, including increased left ventricle (LV) systolic pressure, LV developed pressure and derivatives of pressure (dP/dt), whereas in hypothyroid hearts exhibited decreased dP/dt. Post-ischemic recovery of LV end-diastolic pressure (LVEDP), LVDP and dP/dt was impaired in thyrotoxic rat hearts, whereas hypothyroid hearts exhibited improved LVEDP and decreased infarct size. Catalase expression was decreased by thyrotoxicosis.

CONCLUSION

Thyrotoxicosis was correlated, at least in part, to cardiac remodeling and increased susceptibility to IR injury possibly due to down-regulation of antioxidant enzymes, whereas hypothyroid hearts were less vulnerable to IR injury.

Involvement of inducible nitric oxide synthase in the loss of cardioprotection by ischemic postconditioning in hypothyroid rats

Cardioprotection by ischemic postconditioning (IPost) is negated in hypothyroidism; the underlying mechanisms however are unknown. This study aimed at determining whether changes in Bax, Bcl-2, eNOS, and iNOS gene expressions are involved in the negating effects of IPost against ischemia-reperfusion (IR) injury in hypothyroidism. The hearts from control and hypothyroid rats were perfused in Langendorff apparatus and exposed to 30 min ischemia, followed by 120 min reperfusion and IPost. In a subgroup of hypothyroid rats, ischemia duration was extended to 40 min. Hemodynamic parameters, infarct size, and gene expressions were measured. Compared to controls, hypothyroid rats with 30 min ischemia had higher recovery of post-ischemic LVDP and ± dp/dt, confirmed by decreased CK and LDH levels (187 ± 16 vs. 485 ± 41 and 191 ± 9 vs. 702 ± 48 U/L, respectively; p<0.05), decreased infarct size (6.7 ± 1.1 vs. 46.1 ± 1.7%; p<0.05), and a reduced DNA laddering pattern. Recovery of post-ischemic LVDP and ± dp/dt decreased and infarct size increased following extension of ischemia period in hypothyroid rats. IPost increased eNOS and Bcl-2 expression by 3.2-fold and 3.7-fold and decreased Bax and iNOS expression by 79% and 38%, respectively; it also reduced IR-induced DNA laddering pattern in controls, whereas no change was observed in hypothyroid rats, regardless of the ischemia period. In conclusion, hearts from hypothyroid rats were resistant to IR injury, partly due to the lower expression of iNOS and subsequent reduction in apoptosis after IR. In hypothyroid rats, IPost was not associated with further reduction in iNOS expression and failed to provide additional cardioprotection against ischemia.

The effect of maternal hypothyroidism on cardiac function and tolerance to ischemia-reperfusion injury in offspring male and female rats

PURPOSE

Accumulating evidence indicates that intrauterine evolution disturbance can contribute to myocardial ischemia reperfusion (IR) injury; in addition, thyroid hormones (THs) have a crucial role in the development of different systems during fetal life. The aim of this study was to determine the effect of TH deficiency during fetal life on tolerance of isolated heart to ischemia during adulthood in both genders.

METHODS

Hypothyroidism was induced in pregnant Wistar rats by administrating 0.025 % 6-propyl-2-thiouracil in drinking water throughout pregnancy. Offspring of rats with maternal hypothyroidism (MH) and control groups were tested in adulthood. Isolated hearts were perfused with Langendorff setup and exposed to 30 min of ischemia, followed by 45 min of reperfusion. Baseline values of the left ventricular end-diastolic pressure (LVEDP), left ventricular developed pressure (LVDP), heart rate (HR), and peak rates of positive and negative changes in left ventricular pressure (±dp/dt) were recorded.

RESULTS

In the MH groups the baseline levels of LVDP (male: 23 %, female: 33 %), HR (male: 31 %, female: 26 %), and ±dp/dt were significantly (p < 0.01) lower, compared to controls. After ischemia, hearts from male rats with MH had less tolerance to IR injury as assessed in terms of reductions in recovery of hemodynamic parameters compared to controls, while in female rats there were no significant differences between MH and controls.

CONCLUSIONS

MH decreases hemodynamic parameters in the heart of both male and female offspring in adulthood; in addition, hearts of male rats with MH show less tolerance to ischemia, compared to those of females.

Thyroid hormone, thyromimetics, and metabolic efficiency

Thyroid hormone (TH) has long been recognized as a major modulator of metabolic efficiency, energy expenditure, and thermogenesis. TH effects in regulating metabolic efficiency are transduced by controlling the coupling of mitochondrial oxidative phosphorylation and the cycling of extramitochondrial substrate/futile cycles. However, despite our present understanding of the genomic and nongenomic modes of action of TH, its control of mitochondrial coupling still remains elusive. This review summarizes historical and up-to-date findings concerned with TH regulation of metabolic energetics, while integrating its genomic and mitochondrial activities. It underscores the role played by TH-induced gating of the mitochondrial permeability transition pore (PTP) in controlling metabolic efficiency. PTP gating may offer a unified target for some TH pleiotropic activities and may serve as a novel target for synthetic functional thyromimetics designed to modulate metabolic efficiency. PTP gating by long-chain fatty acid analogs may serve as a model for such strategy.

Citicoline (CDP-choline) protects myocardium from ischemia/reperfusion injury via inhibiting mitochondrial permeability transition

AIMS

Oxidative stress emerges after reperfusion of an organ following an ischemic period and results in tissue damage. In the heart, an amplified generation of reactive oxygen species and a significant Ca(2+) accumulation cause ventricular arrhythmias and mitochondrial dysfunction. This occurs in consequence of increased non-specific permeability. A number of works have shown that permeability transition is a common substrate that underlies the reperfusion-induced heart injury. The aim of this work was to explore the possibility that CDP-choline may circumvent heart damage and mitochondrial permeability transition.

MAIN METHODS

Rats were injected i.p. with CDP-choline at 20 mg/kg body weight. Heart electric behavior was followed during a closure/opening cycle of the left coronary descendent artery. Heart mitochondria were isolated from rats treated with CDP-choline, and their function was evaluated by analyzing Ca(2+) movements, achievement of a high level of the transmembrane potential, and respiratory control. Oxidative stress was estimated following the activity of the enzymes cis-aconitase and superoxide dismutase, as well as the disruption of mitochondrial DNA.

KEY FINDINGS

This study shows that CDP-choline avoided ventricular arrhythmias and drop of blood pressure. Results also show that mitochondria, isolated from CDP-choline-treated rats, maintained selective permeability, retained accumulated Ca(2+), an elevated value of transmembrane potential, and a high ratio of respiratory control. Furthermore, activity of cis-aconitase enzyme and mDNA structure were preserved.

SIGNIFICANCE

This work introduces CDP-choline as a useful tool to preserve heart function from reperfusion damage by inhibiting mitochondrial permeability transition.

Pleiotropic effects of thyroid hormones: learning from hypothyroidism

Hypothyroidism induces several metabolic changes that allow understanding some physiopathological mechanisms. Under experimental hypothyroid conditions in rats, heart and kidney are protected against oxidative damage induced by ischemia reperfusion. An increased resistance to opening of the permeability transition pore seems to be at the basis of such protection. Moreover, glomerular filtration rate of hypothyroid kidney is low as a result of adenosine receptors-induced renal vasoconstriction. The vascular tone of aorta is also regulated by adenosine in hypothyroid conditions. In other context, thyroid hormones regulate lipoprotein metabolism. High plasma level of LDL cholesterol is a common feature in hypothyroidism, due to a low expression of the hepatic LDL receptor. In contrast, HDL-cholesterol plasma levels are variable in hypothyroidism; several proteins involved in HDL metabolism and structure are expressed at lower levels in experimental hypothyroidism. Based on the positive influence of thyroid hormones on lipoprotein metabolism, thyromimetic drugs are promising for the treatment of dyslipidemias. In summary, hypothyroid status has been useful to understand molecular mechanisms involved in ischemia reperfusion, regulation of vascular function and intravascular metabolism of lipoproteins.

In hyperthyroid rats octylguanidine protects the heart from reperfusion damage

Hyperthyroidism sensitizes the heart for reperfusion injury. As known, mitochondrial permeability transition underlies reperfusion heart damage. This study was undertaken to explore the protective effect of octylguanidine (OG), an inhibitor of permeability transition, on hearts from hyperthyroid rats subjected to ischemia/reperfusion. Hyperthyroidism was induced by a daily injection of 2 mg T3/kg body weight for 5 days. OG was injected at a dose of 5 mg/kg body weight. It was found that the amine protects against reperfusion-induced permeability transition, i.e., mitochondria from hyperthyroid rats, treated with OG, retained accumulated Ca(2+), similarly to control mitochondria. OG maintained post reperfusion cardiac frequency in hyperthyroid rats at 429 +/- 16 in comparison to control and T3 treated rats (70 +/- 12 and 71 +/- 2, respectively). We also found that OG diminished the post reperfusion accumulation of IFNgamma from 34.3 +/- 2.5 to 18.7 +/- 1.35, IL-6 from 38.5 +/- 4.5 to 15.1 +/- 0.12, IL-1 from 16.78 +/- 0.73 to 12.19 +/- 1.54, and TNFalpha from 45.05 +/- 3.14 to 29.85 +/- 4.3 (pg/50 microg myocardial tissue). It is concluded that OG inhibits the hypersensitivity of the hyperthyroid myocardium to undergo reperfusion damage due to its inhibitory action on the permeability transition pore.

Hypothyroidism provides resistance to kidney mitochondria against the injury induced by renal ischemia-reperfusion

Massive Ca(2+) accumulation in mitochondria, plus the stimulating effect of an inducing agent, i.e., oxidative stress, induces the so-called permeability transition, which is characterized by the opening of a nonspecific pore. This work was aimed at studying the influence of thyroid hormone on the opening of such a nonspecific pore in kidney mitochondria, as induced by an oxidative stress. To meet this objective, membrane permeability transition was examined in mitochondria isolated from kidney of euthyroid and hypothyroid rats, after a period of ischemia/reperfusion. It was found that mitochondria from hypothyroid rats were able to retain accumulated Ca(2+) to sustain a transmembrane potential after Ca(2+) addition, as well as to maintain matrix NAD(+) and membrane cytochrome c content. The protective effect of hypothyroidism was clearly opposed to that occurring in ischemic reperfused mitochondria from euthyroid rats. Our findings demonstrate that these mitochondria were unable to preserve selective membrane permeability, except when cyclosporin A was added. It is proposed that the protection is conferred by the low content of cardiolipin found in the inner membrane. This phospholipid is required to switch adenine nucleotide translocase from specific carrier to a non-specific pore.

Ru360, a specific mitochondrial calcium uptake inhibitor, improves cardiac post-ischaemic functional recovery in rats in vivo

BACKGROUND AND PURPOSE

The mitochondrial permeability transition pore (mPTP), an energy-dissipating channel activated by calcium, contributes to reperfusion damage by depolarizing the mitochondrial inner membrane potential. As mitochondrial Ca(2+) overload is a main inductor of mPTP opening, we examined the effect of Ru(360), a selective inhibitor of the mitochondrial calcium uptake system against myocardial damage induced by reperfusion in a rat model.

EXPERIMENTAL APPROACH

Myocardial reperfusion injury was induced by a 5-min occlusion of the left anterior descending coronary artery, followed by a 5-min reperfusion in anaesthetized open-chest rats. We measured reperfusion-induced arrhythmias and functions indicative of unimpaired mitochondrial integrity to evaluate the effect of Ru(360) treatment.

KEY RESULTS

Reperfusion elicited a high incidence of arrhythmias, haemodynamic dysfunction and loss of mitochondrial integrity. A bolus intravenous injection of Ru(360) (15-50 nmol kg(-1)), given 30-min before ischaemia, significantly improved the above mentioned variables in the ischaemic/reperfused myocardium. Calcium uptake in isolated mitochondria from Ru(360)-treated ventricles was partially diminished, suggesting an interaction of this compound with the calcium uniporter.

CONCLUSIONS AND IMPLICATIONS

We showed that Ru(360) treatment abolishes the incidence of arrhythmias and haemodynamic dysfunction elicited by reperfusion in a whole rat model. Ru(360) administration partially inhibits calcium uptake, preventing mitochondria from depolarization by the opening of the mPTP. We conclude that myocardial damage could be a consequence of failure of the mitochondrial network to maintain the membrane potential at reperfusion. Hence, it is plausible that Ru(360) could be used in reperfusion therapy to prevent the occurrence of arrhythmia.

Hypothyroidism attenuates protein tyrosine nitration, oxidative stress and renal damage induced by ischemia and reperfusion: effect unrelated to antioxidant enzymes activities

Background

It has been established that hypothyroidism protects rats against renal ischemia and reperfusion (IR) oxidative damage. However, it is not clear if hypothyroidism is able to prevent protein tyrosine nitration, an index of nitrosative stress, induced by IR or if antioxidant enzymes have involved in this protective effect. In this work it was explored if hypothyroidism is able to prevent the increase in nitrosative and oxidative stress induced by IR. In addition the activity of the antioxidant enzymes catalase, glutathione peroxidase, and superoxide dismutase was studied. Control and thyroidectomized (HTX) rats were studied 24 h of reperfusion after 60 min ischemia.

Methods

Male Wistar rats weighing 380 ± 22 g were subjected to surgical thyroidectomy. Rats were studied 15 days after surgery. Euthyroid sham-operated rats were used as controls (CT). Both groups of rats underwent a right kidney nephrectomy and suffered a 60 min left renal ischemia with 24 h of reperfusion. Rats were divided in four groups: CT, HTX, IR and HTX+IR. Rats were sacrificed and samples of plasma and kidney were obtained. Blood urea nitrogen (BUN) and creatinine were measured in blood plasma. Kidney damage was evaluated by histological analysis. Oxidative stress was measured by immunohistochemical localization of protein carbonyls and 4-hydroxy-2-nonenal modified proteins. The protein carbonyl content was measured using antibodies against dinitrophenol (DNP)-modified proteins. Nitrosative stress was measured by immunohistochemical analysis of 3-nitrotyrosine modified proteins. The activity of the antioxidant enzymes catalase, glutathione peroxidase, and superoxide dismutase was measured by spectrophotometric methods. Multiple comparisons were performed with ANOVA followed by Bonferroni t test.

Results

The histological damage and the rise in plasma creatinine and BUN induced by IR were significantly lower in HTX+IR group. The increase in protein carbonyls and in 3-nitrotyrosine and 4-hydroxy-2-nonenal modified proteins was prevented in HTX+IR group. IR-induced decrease in renal antioxidant enzymes was essentially not prevented by HTX in HTX+IR group.

Conclusion

Hypothyroidism was able to prevent not only oxidative but also nitrosative stress induced by IR. In addition, the antioxidant enzymes catalase, glutathione peroxidase, and superoxide dismutase seem not to play a protective role in this experimental model.

Myocardial protective effect of octylguanidine against the damage induced by ischemia reperfusion in rat heart

This study shows that the hydrophobic cation octylguanidine protects against myocardial damage induced by ischemia-reperfusion. The protective effect of the amine was analyzed after 5 min of coronary occlusion followed by 5 min reperfusion in rat hearts. ECG tracings from rats treated with an i.v., injection of 5 mg/kg of octylguanidine showed a total absence of post-reperfusion arrhythmias, conversely to what was observed in untreated rats. The histological images showed that myocardium fibers from treated rats were in good shape and retained their striae, also there was absence of edema. Furthermore, the accumulation of 201Tl in hearts from these rats indicated that the tissue did not suffer disruption or impairment in membrane functions. The above correlated with the fact that mitochondria isolated from the ventricular free wall from treated rats preserved their ability to synthesize ATP. We propose that the protective effect of octylguanidine might be due to its documented inhibitory action on the opening of mitochondrial non-specific pores, a mechanism which is associated in heart injury as induced by reperfusion.

Thyroid hormone may induce changes in the concentration of the mitochondrial calcium uniporter

We explored the possibility that the hormone 3,3',5-tri-iodothyronine can regulate the biosynthesis of the mitochondrial calcium uniporter. To meet this objective experiments on Ca(2+) transport, and binding of the specific inhibitor Ru(360) were carried out in mitochondria isolated from euthyroid, hyperthyroid and hypothyroid rats. It was found that V(max) for Ca(2+) transport increased from 11.67+/-0.8 in euthyroid to 14.36+/-0.44 in hyperthyroid, and decreased in hypothyroid mitochondria to 8.62+/-0.63 nmol Ca(2+)/mg/s. Furthermore, the K(i) for the specific inhibitor Ru(360), depends on the thyroid status, i.e. 18, 19 and 13 nM for control, hyper- and hypothyroid mitochondria, respectively. In addition, the binding of 103Ru(360) was increased in hyperthyroid and decreased in hypothyroid mitochondria. Scatchard analysis for the binding of 103Ru(360) showed the following values: 28, 40 and 23 pmol/mg for control, hyper- and hypothyroid mitochondria, respectively. The K(d) for 103Ru(360) was found to be 30.39, 37.03 and 35.71 nM for controls, hyper- and hypothyroid groups, respectively. When hypothyroid rats were treated with thyroid hormone, mitochondrial Ca(2+) transport, as well as 103Ru(360) binding, reached similar values to those found for euthyroid mitochondria.