The effects of triiodothyronine (T3) on heart rate, temperature and ECG measured with telemetry in freely moving mice

Environmental heterogeneity affects seasonal variation in thyroid hormone physiology of free-living arctic ground squirrels (Urocitellus parryii)

Thyroid hormones (TH) are key regulators of metabolism that could play an important role in altering physiology and energy allocation across life-history stages. Here, we examine seasonal TH dynamics from 345 plasma samples collected from 134 free-living arctic ground squirrels (Urocitellus parryii (Richardson, 1825)) across three consecutive years (2014–2016). We also examine whether unbound levels of triiodothyronine (free T3) in plasma are correlated with total T3 levels and total thyroxine (T4) levels, and whether fecal T3 metabolite levels correlate with plasma TH levels. We found significant differences in plasma TH levels across stages of the annual cycle, but these differences were highly inconsistent across years. However, within a given time period, pregnant females had lower free T3 levels compared with lactating females. We also found that although free T3 was correlated with both total T3 and total T4 in plasma, fecal T3 was not positively correlated with plasma TH levels. Given the lack of consistent differences across life-history stages, we suggest environmental heterogeneity may be the primary driver of TH dynamics, masking any potential endogenous seasonal rhythms in TH secretion. We urge caution in the use of fecal T3 metabolites as a proxy for circulating levels and encourage further research to understand the observed discrepancy.

American Thyroid Association Guide to investigating thyroid hormone economy and action in rodent and cell models

BACKGROUND

An in-depth understanding of the fundamental principles that regulate thyroid hormone homeostasis is critical for the development of new diagnostic and treatment approaches for patients with thyroid disease.

SUMMARY

Important clinical practices in use today for the treatment of patients with hypothyroidism, hyperthyroidism, or thyroid cancer are the result of laboratory discoveries made by scientists investigating the most basic aspects of thyroid structure and molecular biology. In this document, a panel of experts commissioned by the American Thyroid Association makes a series of recommendations related to the study of thyroid hormone economy and action. These recommendations are intended to promote standardization of study design, which should in turn increase the comparability and reproducibility of experimental findings.

CONCLUSIONS

It is expected that adherence to these recommendations by investigators in the field will facilitate progress towards a better understanding of the thyroid gland and thyroid hormone dependent processes.

Development of autonomic dysfunction with intermittent hypoxia in a lean murine model

Intermittent hypoxia (IH) has been previously shown in a lean murine model to produce sustained hypertension and reverse the diurnal variation of blood glucose (BG). Concomitant glucose infusion attenuated the hypertension but exacerbated the BG fluctuations. In this study, cardiovascular variability analysis was employed to track the development of autonomic dysfunction in mice exposed to room air (IA) or IH, in combination with saline or glucose infusion. Baroreflex sensitivity was found to decrease in all animals, except in the control group. Low-frequency power of pulse interval spectrum, reflecting vagal activity, decreased more rapidly in glucose relative to saline while low-frequency power of blood pressure, reflecting sympathetic activity, decreased more slowly in IH relative to IA. Ultradian (≈ 12 h) rhythmicity was substantially suppressed in IH groups. These findings suggest that IH acted to increase sympathetic activity while glucose infusion led to reduced parasympathetic activity. The combination of IH and hyperglycemia leads to progressively adverse effects on autonomic control independent of obesity.

Enhanced cardiomyocyte Ca(2+) cycling precedes terminal AV-block in mitochondrial cardiomyopathy Mterf3 KO mice

AIMS

Heart disease is commonly associated with altered mitochondrial function and signs of oxidative stress. This study elucidates whether primary cardiac mitochondrial dysfunction causes changes in cardiomyocyte handling of reactive oxygen species (ROS) and Ca(2+). We used a mouse model with a tissue-specific ablation of the recently discovered mtDNA transcription regulator Mterf3 (Mterf3 KO). These mice display a cardiomyopathy with severe respiratory chain dysfunction, cardiac hypertrophy, and shortened lifespan. ROS and Ca(2+) handling were measured using fluorescent indicators and confocal microscopy.

RESULTS

Mterf3 KO hearts displayed no signs of increased ROS production or oxidative stress. Surprisingly, Mterf3 KO cardiomyocytes showed enlarged Ca(2+) transient amplitudes, faster sarcoplasmic reticulum (SR) Ca(2+) reuptake, and increased SR Ca(2+) load, resembling increased adrenergic stimulation. Furthermore, spontaneous releases of Ca(2+) were frequent in Mterf3 KO cardiomyocytes. Electrocardiography (measured with telemetry in freely moving mice) showed a terminal state in Mterf3 KO mice with gradually developing bradycardia and atrioventricular block.

CONCLUSION

In conclusion, mitochondrial dysfunction induced by Mterf3 KO leads to a cardiomyopathy without signs of oxidative stress but with increased cardiomyocyte Ca(2+) cycling and an arrhythmogenic phenotype. These findings highlight the complex interaction between mitochondrial function, cardiomyocyte contractility, and compensatory mechanisms, such as activation of adrenergic signaling.

Early impairment of transmural principal strains in the left ventricular wall after short-term, high-fat feeding of mice predisposed to cardiac steatosis

BACKGROUND

myocardial lipid accumulation precedes some cardiomyopathies, but little is known of concurrent effects on ventricular mechanics. We tested the hypothesis that intramyocardial lipid accumulation during a short-term, high-fat diet (HFD) affects 2-dimensional strains in the heart. We examined the hearts of nontransgenic (NTG) mice and of transgenic mice predisposed to elevated triacylglyceride (TAG) storage linked to low-level overexpression of peroxisome proliferator activated receptor (PPAR-α).

METHODS AND RESULTS

myocardial lipid and transmural principal strains E1 and E2 were determined in vivo with (1)H magnetic resonance spectroscopy/imaging before and after 2 weeks of an HFD in both PPAR-α and NTG littermate mice. Baseline lipid was elevated in PPAR-α compared with NTG mice. An HFD increased mobile lipid by 174% in NTG mice (P<0.05) and by 79% in PPAR-α mice (P<0.05). After an HFD, lipid and TAG were higher in PPAR-α versus NTG mice by 63% and 81%, respectively. However, TAG in PPAR-α mice after an HFD was similar to TAG in PPAR-α mice fed a regular diet, suggesting that the magnetic resonance spectroscopy signal from lipid is not exclusive to TAG. Only at the highest lipid contents, achieved in PPAR-α mice, were strains affected. Endocardial strain was most compromised, with a negative correlation to lipid (P<0.05).

CONCLUSIONS

a short-term HFD elevated myocardial lipid measures as determined by magnetic resonance spectroscopy, which became dissociated from TAG content in hearts predisposed to cardiac steatosis. The increased lipid was associated with concurrent, transmural reductions in E1 and E2 strains across the left ventricular wall. Strains were attenuated at the highest levels of lipid accumulation, suggesting a threshold response. Thus, 2-dimensional strains are impaired early and without left ventricular diastolic dysfunction, owing to cardiac steatosis.

Sex differences in mouse heart rate and body temperature and in their regulation by adenosine A1 receptors

AIM

To examine cardiac function, body temperature and locomotor behaviour in the awake adenosine A(1) receptor knock out mouse of both sexes.

METHODS

Male and female A(1)R (+/+) and (-/-) mice, instrumented with telemetric devices, were recorded during basal conditions and after drug administration.

RESULTS

Female mice had higher heart rate, body temperature and locomotion, both during daytime and during the night. Awake A(1)R (-/-) mice had a slightly elevated heart rate, and this was more clear-cut in males. Heart rate was also higher in Langendorff-perfused denervated A(1)R (-/-) hearts. Body temperature was higher in A(1)R (-/-) males and females; locomotor activity was higher in A(1)R (-/-) females, but not in males. The adenosine receptor agonist R-PIA (0.2 mg kg(-1)) decreased heart rate and body temperature, but less in A(1)R (-/-) animals than in A(1)R (+/+) mice (P < 0.001 in both parameters). The unselective adenosine receptor antagonist caffeine had a minor stimulatory effect on heart rate in lower doses, but depressed it at a dose of 75 mg kg(-1). Body temperature was increased after a low dose (7.5 mg kg(-1)) of caffeine in both sexes and genotypes, and markedly reduced after a high dose (75 mg kg(-1)) of caffeine. An intermediary dose of caffeine 30 mg kg(-1) increased or decreased body temperature depending on genotype and sex. Locomotor responses to caffeine were variable depending both on genotype and sex.

CONCLUSION

Thus, the adenosine A(1) receptor is involved in the regulation of heart rate, body temperature and locomotor activity, but the magnitude of the involvement is different in males and females.

Liver-derived IGF-I regulates exploratory activity in old mice

Growth hormone (GH) replacement in hypopituitary patients improves well-being and initiative. Experimental studies indicate that these psychic effects may be reflected in enhanced locomotor activity in mice. It is unknown whether these phenomena are mediated directly by GH or by circulating IGF-I. IGF-I production in the liver was inactivated at 6-10 wk of age (LI-IGF-I-/- mice), resulting in an 80-85% reduction of circulating IGF-I, and, secondary to this, increased GH secretion. Using activity boxes on three different occasions during 1 wk, 6-mo-old LI-IGF-I-/- mice had similar activity levels, and 14-mo-old mice had a moderate but significant decrease in activity level, compared with control mice. At 20 mo of age, the LI-IGF-I-/- mice displayed a more prominent decrease in activity level with decreased horizontal activity throughout the test period, and at day 1, there were several signs of an altered habituation process with different time patterns of locomotor activity and horizontal activity compared with the control mice. At days 3 and 5, rearing activity was lower in the 20-mo-old LI-IGF-I-/- mice. Anxiety level was unaffected in all age groups, as measured using the Montgomery's elevated plus-maze. In conclusion, old LI-IGF-I-/- mice displayed a decrease in both horizontal and rearing (exploratory) activity level and an altered habituation process. These results indicate that liver-derived IGF-I mediates at least part of the effects of GH on exploratory activity in mice.

Electrocardiographic characterization of stress-induced myocardial infarction in atherosclerotic mice

AIM

We have previously shown that mental and hypoxic stress can trigger the development of myocardial infarction (MI) in atherosclerotic apoE(-/-) x LDLR(-/-) mice. The purpose of the present study was to characterize the interval between stress and MI and determine whether electrophysiological changes precede the precipitation of an infarct by assessing telemetry recordings of the electrocardiogram.

METHODS

Isoflurane anaesthetized apoE(-/-) x LDLR(-/-) (n = 16) and C57BL/6J (n = 8) mice were exposed to systemic hypoxia by reducing the inhaled oxygen concentration to 10% for 10 min. Mental stress was induced in eight conscious apoE(-/-) x LDLR(-/-) and eight C57BL/6J mice by blowing air into the cage. Physiological parameters were recorded every 30 min for 2-6 days by implanted transmitters.

RESULTS

During stress all mice developed transient ischaemic STU-area changes, which returned to normal at the end of stress. During the recovery phase (6 days) 50% (4/8) of the mentally stressed apoE(-/-) x LDLR(-/-) mice developed increased STU-area variability (P < 0.05) followed by dramatic STU-area elevations and spontaneous death at approximately 12-24 h. In hypoxia-exposed apoE(-/-) x LDLR(-/-) mice 56% (9/16) developed MI as determined by elevated serum levels of the infarction marker troponin T which correlated with increased variability in the STU-area (P < 0.05).

CONCLUSION

This is the first mouse model showing that increased STU-area variability is indicative of MI development in atherosclerotic mice following ischaemic stress. Furthermore, our findings suggest a two-phase pathway for the infarction development: an initial phase comprising a transient ischaemic response which triggers a delayed second phase of ischaemia and MI.

Resistance to diet-induced obesity in mice globally overexpressing OGH/GPB5

We identified a glycoprotein hormone beta-subunit (OGH, also called GPB5) that, as a heterodimer with the alpha-subunit GPA2, serves as a second ligand for the thyroid-stimulating hormone receptor. Mice in which the OGH gene is deleted (OGH-/-) are indistinguishable from WT littermates in body weight, response to high-fat diet, metabolic parameters, body composition, and insulin tolerance. Mice engineered to transgenically globally overexpress OGH (OGH-TG) develop approximately 2-fold elevations in their basal thyroid levels and weigh slightly less than WT littermates despite increased food intake because of an increase in their metabolic rates. Moreover, when OGH-TG mice are challenged with a high-fat diet, they gain significantly less weight and body fat than their WT littermates. The OGH-TG mice also have reduced blood glucose, insulin, cholesterol, and triglycerides. In contrast to other approaches in which the thyroid axis is activated, OGH-TG mice exhibit only minor changes in heart rate and blood pressure. Our findings suggest that constitutive low-level activation of the thyroid axis (via OGH or other means) may provide a beneficial therapeutic approach for combating diet-induced obesity.

Reduced sympathetic responsiveness as well as plasma and tissue noradrenaline concentration in growth hormone transgenic mice

AIMS

Acromegaly [overproduction of growth hormone (GH)] and GH deficiency have both been associated with alterations in autonomic nervous system function. The aim of this study was to investigate autonomic nervous system influence on heart rate (HR) in transgenic mice overexpressing bovine GH (bGH).

METHODS

HR and HR variability (HRV) were measured in conscious young (8-13 weeks) and old (5-6 months) female bGH and control mice using telemetry. HR control was studied using antagonists and an agonist of adrenergic and muscarinic receptors. Noradrenaline was measured in plasma, heart and kidney using high performance liquid chromatography.

RESULTS

Average 24 h resting HR did not differ between bGH and control mice. After saline injection and after muscarinic blockade with methylscopolamine HR increase was blunted (in old) or absent (in young) bGH mice compared with control mice (P < 0.05). Phenylephrine caused a baroreflex mediated decrease in HR from around 550 to 300-350 beats min(-1), not different between bGH and control mice. Time- and frequency-domain measures of HRV were reduced in old bGH compared with control mice (P < 0.05). Noradrenaline concentrations were reduced by 25-49% in plasma and tissue of bGH compared with control mice (P < 0.05).

CONCLUSION

The current study suggests reduced autonomic modulation of HR in bGH transgenic mice. Thus, GH appears to have marked effects on autonomic tone, reducing sympathetic nervous system function possibly via reduced noradrenaline stores.

Mouse isolated perfused heart: Characteristics and cautions

1. Owing to the considerable potential for manipulating the murine genome and, as a consequence, the increasing availability of genetically modified models of cardiovascular diseases, the mouse is fast becoming a cornerstone of animal research. However, progress in the use of various murine preparations is hampered by the lack of facilities and skills for the adequate physiological assessment of genetically modified mice. 2. We have attempted to address this problem by refining and characterizing a mouse isolated heart preparation that was originally developed for use with larger hearts. 3. We used the isolated buffer-perfused Langendorff preparation (perfused at constant flow or constant pressure) to characterize: (i) the frequency-response characteristics; (ii) heart isolation conditions; (iii) perfusion chamber conditions; (iv) temperature-function relationships; (v) stability over extended periods of perfusion; (vi) perfusate calcium-function relationships; (vii) pressure-volume relationships; (viii) pressure-rate relationships; and (ix) flow-function relationships.

Age and anesthetic effects on murine electrocardiography

Murine models offer potential insights regarding human cardiac disease, but efficient and reliable methods for phenotype evaluation are necessary. We employed non-invasive electrocardiography (ECG) in mice, investigating statistical reliability of these parameters with respect to anesthetic and animal age. Mice (C57BL/6, 8 or 48 weeks) were anesthetized by ketamine/xylazine (K/X, 80/10 mg/kg ip) or by inhalation anesthetic (halothane, HAL; sevoflurane, SEV) and 6 lead ECGs were recorded. P wave duration and QT interval was significantly prolonged with K/X compared to HAL and SEV, indicating slowed atrial and ventricular conduction. P-R interval (atrio-ventricular conduction) was significantly increased in aged mice under all anesthetics. Heart rate was inversely correlated to QT interval and P wave duration. We also detected significant age effects with respect to optimal approaches for QT interval corrections. Power analysis showed 4-fold higher number of mice/group, were required for K/X, to achieve identical statistical sensitivity. These data demonstrate the importance of anesthetic selection for relevant and reliable ECG analysis in mice and illustrate the selective influences of anesthetics and age on cardiac conductance in this species.

Long-term blood pressure telemetry in AT2 receptor-disrupted mice

OBJECTIVES

The hypertension in AT2 receptor knockout mice is imperfectly defined. Therefore, we investigated the influence of dietary salt loading and deoxycorticosterone (DOCA)-salt treatment on blood pressure and diurnal patterns of blood pressure in these mice by radiotelemetry.

METHODS

We used telemetry in AT2 receptor knockout and wild-type mice to measure blood pressure, heart rate, aortic pressure dp/dt, locomotor activity, and circadian rhythms. Salt-related effects were studied by increasing the salt in chow to 4%, adding 1% saline in drinking water, and by DOCA-salt treatment

RESULTS

Baseline blood pressures were higher in AT2 receptor knockout than in wild-type mice and were not affected by increasing the salt intake. The blood pressure increase was steeper and greater in AT2 receptor knockout than in wild-type mice after DOCA-salt treatment A circadian rhythm of blood pressure and heart rate, with higher values during the night, was seen in wild-type, but not in AT2 receptor knockout mice. In AT2 receptor knockout mice, this rhythm was only significant when daily salt intake was increased or when DOCA-salt hypertension was induced. The acrophase of blood pressure and heart rate was found between 2000 and 2400 h and was in accordance with the maximum physical activity.

CONCLUSION

These data suggest that AT2 knockout mice display slight hypertension which is not salt-sensitive. On the other hand, the susceptibility to develop DOCA-salt hypertension is increased. The study also illustrates the power of telemetry in monitoring long-term cardiovascular changes and circadian blood pressure and heart rate rhythms in genetically engineered mice.

Functions of thyroid hormone receptors in mice

Thyroid hormone receptors (TRs) play a central role in mediating the actions of thyroid hormone in development and homeostasis in vertebrate species. The TRs are nuclear receptors that act as ligand-regulated transcription factors. There are two TR genes (TRalpha and TRbeta), each capable of generating different variant products, suggesting a potentially complex array of TR pathways. Targeted mutagenesis in the mouse has indicated that there are specific individual functions for the TR genes in vivo. The deletion of combinations of TRalpha and TRbeta variants has revealed that additional functions are convergently regulated by both TR genes and indicates that control of an extended range of functions is facilitated by a network of specific and common TR pathways. The TR-deficient mouse models have allowed investigation of the TR pathways underlying many functions of thyroid hormone and provide a unique perspective on receptor-mediated mechanisms of biological control.

Myocardial infarction mediated by endothelin receptor signaling in hypercholesterolemic mice

Myocardial infarction is linked to atherosclerosis, yet the sequence leading from silent coronary atherosclerosis to acute myocardial infarction has remained unclear. Here we show that hypercholesterolemic apolipoprotein E-/- low density lipoprotein receptor-/- mice develop not only coronary atherosclerosis but also myocardial infarction. Exposure of mice to mental stress or hypoxia led to acute ischemia, which, in a large proportion of the mice, was followed by electrocardiographic changes, leakage of troponin T, and loss of dehydrogenase from the myocardium, all indicative of acute myocardial infarction. Apoptotic death of cardiomyocytes was followed by inflammation and fibrosis in the heart. All these pathological changes could be prevented by a blocker of the endothelin type A receptor. Thus, stress elicits myocardial infarction through endothelin receptor signaling in coronary atherosclerosis caused by hypercholesterolemia.

Abnormal heart rate and body temperature in mice lacking thyroid hormone receptor alpha 1

Thyroid hormone, acting through several nuclear hormone receptors, plays important roles in thermogenesis, lipogenesis and maturation of the neonatal brain. The receptor specificity for mediating these effects is largely unknown, and to determine this we developed mice lacking the thyroid hormone receptor TR alpha 1. The mice have an average heart rate 20% lower than that of control animals, both under normal conditions and after thyroid hormone stimulation. Electrocardiograms show that the mice also have prolonged QRS- and QTend-durations. The mice have a body temperature 0.5 degrees C lower than normal and exhibit a mild hypothyroidism, whereas their overall behavior and reproduction are normal. The results identify specific and important roles for TR alpha 1 in regulation of tightly controlled physiological functions, such as cardiac pacemaking, ventricular repolarisation and control of body temperature.