Effect of hyperthyroidism on fibre-type composition, fibre area, glycogen content and enzyme activity in human skeletal muscle

Pembrolizumab-induced hypothyroidism caused reversible increased serum creatinine levels: a case report

Background

The advent of immune checkpoint inhibitors (ICIs) has significantly improved the prognosis of patients with advanced malignancies. On the other hand, these drugs might cause immune-related adverse events (irAEs) including endocrinopathies and nephropathies. Thyroid dysfunction is one of the most common irAEs. For ICIs-induced nephropathies, most cases are due to tubulointerstitial nephritis, which might require steroid treatment. Here, we report a patient with non-small cell lung cancer treated with ICI who developed increased serum creatinine (s-Cr) levels due to ICIs-induced hypothyroidism.

Case presentation

A 57-year-old Asian man with refractory non-small cell lung cancer under ICIs therapy (pembrolizumab, an anti-programmed cell death-1 monoclonal antibody) developed increased s-Cr levels 5 months after the pembrolizumab initiation. His laboratory data, renal biopsy, and Gallium-67 scintigraphy findings denied pembrolizumab-induced tubulointerstitial nephritis. His renal function was correlated with thyroid function. Despite the increase of s-Cr levels, serum cystatin C levels were normal, which could be explained by the hypothyroidism. Levothyroxine treatment improved renal function as well as thyroid function. Then pembrolizumab was resumed, and both his thyroid and renal function remained normal level. Ultimately, we concluded that the increased s-Cr levels were caused by pembrolizumab-induced hypothyroidism.

Conclusion

All clinicians involved in ICI treatment need to recognize the possible increase in s-Cr levels caused by ICIs-induced hypothyroidism, and we propose monitoring serum cystatin C levels to differentiate ICIs-induced hypothyroidism from tubulointerstitial nephritis before invasive renal biopsies or steroid treatment, which are recommended by the prescribing information for pembrolizumab, are performed.

Differential diagnosis of idiopathic inflammatory myopathies in adults - the first step when approaching a patient with muscle weakness

Despite its misleading adjective, the most commonly used diagnostic criteria of idiopathic inflammatory myopathies (IIM) are applicable only after all other non-autoimmune muscle diseases have been excluded. It makes differential diagnosis the first step when approaching a patient with muscle weakness. This article is designed to list the most common conditions from which to differentiate in rheumatological care. In fact, many patients with the diseases described here have been initially misdiagnosed with IIM. For the purpose of this article, only the most commonly found and important conditions according to the authors are listed with the essence of information; other autoimmune muscle diseases, such as sarcoidosis and eosinophilic myositis, are not portrayed. The attached bibliography may serve as a source, when further exploration of a specific subject is needed.

The effect of TSH change per year on the risk of incident chronic kidney disease in euthyroid subjects

The objective of this study is to evaluate the predictive values of baseline thyroid-stimulating hormone and the rate of thyroid-stimulating hormone change within the euthyroid state on the development of chronic kidney disease. We conducted a longitudinal study in 17,067 Korean adults with normal thyroid function and no history of thyroid disease. Incident chronic kidney disease was defined as an estimated glomerular filtration rate <60 ml/min/1.73 m². The rate of thyroid-stimulating hormone change was determined by subtracting the baseline thyroid-stimulating hormone level from the thyroid-stimulating hormone level measured at the last visit prior to the diagnosis of chronic kidney disease or at the final visit in subjects without chronic kidney disease, divided by the observation period (years). Subjects were stratified into quintiles according to rates of thyroid-stimulating hormone change. During 86,583 person-years of follow-up (median follow-up 5.2 years), there were 561 incident cases of chronic kidney disease. The risk of incident chronic kidney disease was significantly higher in subjects with rapid increases (quintile 5) or decreases (quintile 1) in thyroid-stimulating hormone levels compared to the reference group (quintile 3). In fully adjusted models, the hazard ratios of quintiles 1 and 5 were 3.15 (95 % confidence interval 2.34 to 4.24; p < 0.001) and 3.37 (95 % confidence interval 2.52 to 4.51; p < 0.001), respectively. However, there was no significant association between baseline thyroid-stimulating hormone and risk of incident chronic kidney disease. The development of chronic kidney disease is associated with the rate of changes in thyroid-stimulating hormone level rather than with baseline thyroid-stimulating hormone levels.

Interactions between thyroid disorders and kidney disease

There are several interactions between thyroid and kidney functions in each other organ's disease states. Thyroid hormones affect renal development and physiology. Thyroid hormones have pre-renal and intrinsic renal effects by which they increase the renal blood flow and the glomerular filtration rate (GFR). Hypothyroidism is associated with reduced GFR and hyperthyroidism results in increased GFR as well as increased renin - angiotensin - aldosterone activation. Chronic kidney disease (CKD) is characterized by a low T3 syndrome which is now considered a part of an atypical nonthyroidal illness. CKD patients also have increased incidence of primary hypothyroidism and subclinical hypothyroidism. The physiological benefits of a hypothyroid state in CKD, and the risk of CKD progression with hyperthyroidism emphasize on a conservative approach in the treatment of thyroid hormone abnormalities in CKD. Thyroid dysfunction is also associated with glomerulonephritis often by a common autoimmune etiology. Several drugs could affect both thyroid and kidney functions. There are few described interactions between thyroid and renal malignancies. A detailed knowledge of all these interactions is important for both the nephrologists and endocrinologists for optimal management of the patient.

Why can insulin resistance be a natural consequence of thyroid dysfunction?

Evidence for a relationship between T4 and T3 and glucose metabolism appeared over 100 years ago when the influence of thyroid hormone excess in the deterioration of glucose metabolism was first noticed. Since then, it has been known that hyperthyroidism is associated with insulin resistance. More recently, hypothyroidism has also been linked to decreased insulin sensitivity. The explanation to this apparent paradox may lie in the differential effects of thyroid hormones at the liver and peripheral tissues level. The purpose of this paper is to explore the effects of thyroid hormones in glucose metabolism and analyze the mechanisms whereby alterations of thyroid hormones lead to insulin resistance.

Higher proportion of fast-twitch (type II) muscle fibres in idiopathic inflammatory myopathies - evident in chronic but not in untreated newly diagnosed patients

OBJECTIVE

Polymyositis and dermatomyositis are idiopathic, inflammatory myopathies characterized by proximal muscle fatigue. Conventional immunosuppressive treatment gives a variable response. Biopsies from chronic patients display a low proportion type I and a high proportion of type II muscle fibres. This raised a suspicion that the low proportion of type I fibres might play a role in the muscle fatigue.

AIM

To investigate whether the muscle fibre attributes evident in chronic myositis are characteristic for the polymyositis and dermatomyosistis diseases themselves.

METHODS

Muscle biopsies were obtained from thigh muscle from untreated patients (n = 18), treated responders (n = 14) and non-responders (n = 6) and from healthy controls (n = 11), respectively. For clinical evaluations, creatine kinase, functional index of myositis and cumulative dose of cortisone were established.

RESULTS

  Chronic patients had a lower proportion of type I fibres and a higher proportion of type II fibres compared to untreated myositis patients and healthy controls. Fibre cross-sectional area (CSA) did not differ between patients and healthy individuals but all women had a 20% smaller type II fibre CSA compared to men.

CONCLUSIONS

Untreated polymyositis and dermatomyositis patients and healthy controls have a different fibre type composition than chronic polymyositis and dermatomyositis patients. Fibre CSA did not differ between healthy controls or any of the patient groups. A low proportion of oxidative muscle fibres can therefore be excluded as a contributing factor causing muscle fatigue at disease onset and the gender difference should be taken into consideration when evaluating fibre CSA in myositis.

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.

Interactions between thyroid and kidney function in pathological conditions of these organ systems: a review

Thyroidal status affects kidney function already in the embryonic stage. Thyroid hormones influence general tissue growth as well as tubular functions, electrolyte handling and neural input. Hyper- and hypo-functioning of the thyroid influences mature kidney function indirectly by affecting the cardiovascular system and the renal blood flow, and directly by affecting glomerular filtration, electrolyte pumps, the secretory and absorptive capacity of the tubuli, and the structure of the kidney. Hyperthyroidism accelerates several physiologic processes, a fact which is reflected in the decreased systemic vascular resistance, increased cardiac output (CO), increased renal blood flow (RBF), hypertrophic and hyperplastic tubuli, and increased glomerular filtration rate (GFR). Renal failure can progress due to glomerulosclerosis, proteinuria and oxidative stress. Hypothyroidism has a more negative influence on kidney function. Peripheral vascular resistance is increased with intrarenal vasoconstriction, and CO is decreased, causing decreased RBF. The influence on the different tubular functions is modest, although the transport capacity is below normal. The GFR is decreased up to 40% in hypothyroid humans. Despite the negative influences on glomerular and tubular kidney function, a hypothyroid state has been described as beneficial in kidney disease. Kidney disease is associated with decreased thyroid hormone concentrations caused by central effects and by changes in peripheral hormone metabolism and thyroid hormone binding proteins. Geriatric cats form an animal model of disease because both hyperthyroidism and chronic kidney disease (CKD) have high prevalence among them, and the link between thyroid and kidney affects the evaluation of clinical wellbeing and the possible treatment options.

Neuromuscular complications of thyrotoxicosis

Thyroid hormones exert multiple effects on the neuromuscular system and the brain, with the most important being their role in stimulating the development and differentiation of the neuromuscular system and brain in foetal and neonatal life. In the presence of hyperthyroidism, muscular and neurological symptoms may be the presenting clinical features of the disease. The frequency and severity of neuromuscular complications vary considerably and are probably related to the degree of hyperthyroidism, although in some patients the neuromuscular dysfunction is caused by associated disorders rather than by hyperthyroidism per se. This update focuses on the most common neurological and muscular disorders that occur in patients with thyrotoxicosis. It is beyond the scope of this paper to discuss thyroid eye disease and cardiac complications, in themselves separate complications of specific myocytes.

Thigh muscle stiffness assessed with magnetic resonance elastography in hyperthyroid patients before and after medical treatment

PURPOSE

To measure the stiffness of the vastus medialis (VM) in hyperthyroid patients before and after treatment.

MATERIALS AND METHODS

A total of five healthy euthyroid patients and five hyperthyroid patients were tested using magnetic resonance elastography (MRE), which involves the induction of shear waves in the thigh muscles using a pneumatic driver at 90 Hz.

RESULTS

Among the pretreatment hyperthyroid cohort a lower stiffness was found when the muscle was relaxed (2.11 +/- 0.61 kPa) compared the stiffness following treatment of hyperthyroidism (5.52 +/- 1.52 kPa), which was accompanied by an improvement in the contractile function of the VM. Pretreatment muscle stiffness was also less than that of age matched healthy volunteers (4.56 +/- 0.40 kPa). The behavior of the waves was sensitive to the stage of this myopathy and to the amount of free thyroxine (FT4).

CONCLUSION

The MRE technique provides a new tool to gain new insights into pathophysiology of thyroid associated and other muscle diseases and their response to treatment.

Thyroid hormone replacement normalizes renal renin and angiotensin receptor expression in thyroidectomized fetal sheep

Previous studies have suggested that thyroid hormone influences maturation of the renin-angiotensin system (RAS) and cardiovascular function in the late-gestation fetal sheep. To further examine the importance of thyroid hormone in this regard, we used the technique of thyroidectomy (TX) to remove endogenous thyroid hormone from the circulation and then replaced it with physiological amounts of exogenous thyroxine. We hypothesized that the previously observed changes in RAS activity and cardiovascular function associated with TX would be normalized. TX was performed at 120 days of gestational age (dGA), and control fetuses were sham operated. After 3 days of recovery, TX fetuses were continuously intravenously infused with thyroxine until delivery by cesarean section close to term (around 138 dGA). Immediately before necropsy, fetuses were infused with isoproterenol, and the hemodynamic responses were noted. Thyroid hormone replacement normalized not only plasma triiodothyronine (T3) and thyroxine (T4) levels but also the TX-induced decreases in renal renin mRNA and renal renin content. Renal ANG II subtype receptor expression levels were also normalized for both mRNA and protein. Decreased basal heat rate and systolic blood pressure associated with TX returned to normal following replacement; however, changes in mean blood pressure and isoproterenol-induced changes in mean blood pressure were not altered. These findings demonstrate that replacement of thyroid hormone in hypothyroid sheep fetuses can restore renal ANG II receptor and renin expression and secretion to normal.

Resistance Training Improves Muscle Function and Body Composition in Patients With Hyperthyroidism

OBJECTIVE

To evaluate the effect of resistance training on skeletal muscle performance and body composition in patients with medically treated hyperthyroidism.

DESIGN

Nonrandomized controlled trial.

SETTING

Large public tertiary hospital.

PARTICIPANTS

Sixteen sedentary patients with recent clinical diagnosis and laboratory confirmation of hyperthyroidism (7 men, 9 women; age, 38+/-11 y; weight, 58.4+/-2.6 kg; height, 1.6+/-0.3m) were assigned to the control group (medical therapy; n=9) or training group (medical therapy associated with resistance training; n=7). An age- and sex-matched healthy group served as controls (3 men, 5 women; age, 40+/-3 y; weight, 68.4+/-4.3 kg; height, 1.6+/-0.3m).

INTERVENTION

Resistance training twice a week for 16 weeks.

MAIN OUTCOME MEASURES

Peak muscular strength (by dynamometry and 1 repetition maximum method) and endurance (30% of peak force) for 7 movements and anthropometric measurements.

RESULTS

The hyperthyroid patients as a group had lower baseline overall strength values when compared with healthy subjects (200.3+/-16.0 kg vs 274.9+/-21.8 kg, respectively; P=.006). Overall absolute increases in strength (49 kg vs 91 kg, P<.05) and endurance (78.5x10(2)kg/s vs 176.9x10(2)kg/s, P<.05) were higher in the training group compared with the control group. Body weight increased in both groups, but the sum of muscular circumference increased only in the training group (training group, 92.6+/-3.3 cm vs 97.1+/-3.8 cm; control group, 94.6+/-2.2 cm vs 94.4+/-2.1cm; P<.05), with no change in the sum of skinfolds.

CONCLUSIONS

Resistance training accelerates the recovery of skeletal muscle function and promotes weight gain based on muscle mass improvement in patients with medically treated hyperthyroidism.

Disfunção muscular esquelética e composição corporal no hipertireoidismo

O objetivo deste artigo é revisar os aspectos da disfunção muscular esquelética e composição corporal no hipertireoidismo. O hipertireoidismo está associado a uma fraqueza muscular generalizada que é parte da manifestação clínica inicial de cerca de 80% dos pacientes, comprometendo a realização de tarefas cotidianas e a qualidade de vida. Um fator que contribui para a redução da força é a atrofia muscular, que tende a afetar mais comumente os grupos musculares proximais. Além disso, o hipertireoidismo é acompanhado de perda ponderal associada à depleção de massa muscular e tecido adiposo. Estudos demonstram que o tratamento medicamentoso é capaz de recuperar a força e mais lentamente a resistência, definida como a capacidade de sustentar cargas submáximas por períodos prolongados, e que o treinamento contra resistência associado ao tratamento medicamentoso, é capaz de promover um maior ganho de força e de resistência muscular nestes pacientes. Embora não tenha sido estabelecido um padrão de composição corporal na recuperação do peso após o tratamento da doença, sabe-se que pacientes submetidos ao treinamento de força apresentam recuperação de peso acompanhado principalmente de ganho de massa muscular.

Testosterone-induced increase in muscle size in healthy young men is associated with muscle fiber hypertrophy

Administration of replacement doses of testosterone to healthy hypogonadal men and supraphysiological doses to eugonadal men increases muscle size. To determine whether testosterone-induced increase in muscle size is due to muscle fiber hypertrophy, 61 healthy men, 18-35 yr of age, received monthly injections of a long-acting gonadotropin-releasing hormone (GnRH) agonist to suppress endogenous testosterone secretion and weekly injections of 25, 50, 125, 300, or 600 mg testosterone enanthate (TE) for 20 wk. Thigh muscle volume was measured by magnetic resonance imaging (MRI) scan, and muscle biopsies were obtained from vastus lateralis muscle in 39 men before and after 20 wk of combined treatment with GnRH agonist and testosterone. Administration of GnRH agonist plus TE resulted in mean nadir testosterone concentrations of 234, 289, 695, 1,344, and 2,435 ng/dl at the 25-, 50-, 125-, 300-, and 600-mg doses, respectively. Graded doses of testosterone administration were associated with testosterone dose and concentration-dependent increase in muscle volume measured by MRI (changes in vastus lateralis volume, -4, +7, +15, +32, and +48 ml at 25-, 50-, 125-, 300-, and 600-mg doses, respectively). Changes in cross-sectional areas of both type I and II fibers were dependent on testosterone dose and significantly correlated with total (r = 0.35, and 0.44, P < 0.0001 for type I and II fibers, respectively) and free (r = 0.34 and 0.35, P < 0.005) testosterone concentrations during treatment. The men receiving 300 and 600 mg of TE weekly experienced significant increases from baseline in areas of type I (baseline vs. 20 wk, 3,176 +/- 186 vs. 4,201 +/- 252 microm(2), P < 0.05 at 300-mg dose, and 3,347 +/- 253 vs. 4,984 +/- 374 microm(2), P = 0.006 at 600-mg dose) muscle fibers; the men in the 600-mg group also had significant increments in cross-sectional area of type II (4,060 +/- 401 vs. 5,526 +/- 544 microm(2), P = 0.03) fibers. The relative proportions of type I and type II fibers did not change significantly after treatment in any group. The myonuclear number per fiber increased significantly in men receiving the 300- and 600-mg doses of TE and was significantly correlated with testosterone concentration and muscle fiber cross-sectional area. In conclusion, the increases in muscle volume in healthy eugonadal men treated with graded doses of testosterone are associated with concentration-dependent increases in cross-sectional areas of both type I and type II muscle fibers and myonuclear number. We conclude that the testosterone induced increase in muscle volume is due to muscle fiber hypertrophy.

Neuromuscular manifestations of endocrine disorders

This article reviews the neuromuscular disorders associated with many endocrine disturbances. The severity of neuromuscular disorders varies. Some of these disturbances are mild, and others are severe and life threatening.

Impact of hyperthyroidism and its correction on vascular reactivity in humans

BACKGROUND

Although thyroid hormone (TH) exerts relevant effects on the cardiovascular system, it is unknown whether TH also regulates vascular reactivity in humans. Methods and Results- We studied 8 patients with hyperthyroidism, basally (H) and 6 months after euthyroidism was restored by methimazole (EU). Thirteen healthy subjects served as control subjects (C). We measured forearm blood flow (FBF) by strain-gauge plethysmography during intrabrachial graded infusion of acetylcholine, sodium nitroprusside (SNP), norepinephrine, and L-NMMA (inhibitor of NO synthesis). Basal FBF (in mL. dL(-1). min(-1)) was markedly higher in H than in C (5.8+/-1.2 and 1.9+/-0.1, respectively; P<0.001) and was close to normal in EU (2.6+/-0.3, P<0.01 versus H). During acetylcholine infusion, FBF increased much more in H (+33+/-5) than in C (+14+/-3, P<0.01 versus H) and in EU (+20+/-5, P=0.01 versus H and P=NS versus C). In contrast, the response to SNP infusion was comparable in the patients and control subjects. During norepinephrine infusion, the fall in FBF was much more pronounced in H (-6+/-1) than in C (-0.7+/-0.3, P<0.005 versus H) and in EU (-1.5+/-0.3, P<0.01 versus H). Finally, inhibition of NO synthesis by L-NMMA decreased FBF by 2.8+/-0.6, 0.61+/-0.7, and 1.4+/-0.3 in H, C, and EU, respectively (H versus C and EU, P<0.05).

CONCLUSIONS

In hyperthyroidism, (1) the marked basal vasodilation is largely accounted for by excessive endothelial NO production, (2) vascular reactivity is exaggerated because of enhanced sensitivity of the endothelial component, (3) the vasoconstrictory response to norepinephrine is potentiated, and (4) this abnormal vascular profile is corrected when euthyroidism is restored by medical therapy. The data demonstrate that vascular endothelium is a specific target of TH.

Effects of hyperthyroidism on vascular contractile and relaxation responses

Previous research has shown that skeletal muscle blood flow, at rest and during muscular contractions, is elevated in the hyperthyroid state. We hypothesized that reduced vascular contractile and enhanced endothelium-dependent relaxation responses contribute to these observations. To test these hypotheses, male rats were administered triiodothyronine (Hyper, n = 27; 300 micrograms/kg) for 6-12 wk. Compared with euthyroid control rats (Eut, n = 27), Hyper exhibited left ventricular hypertrophy (Eut, 2.01 +/- 0.04 mg/g body wt; Hyper, 2.70 +/- 0.06; P < 0.0005) and greater oxidative enzyme activity in several skeletal muscles (all P < 0.0005). Vascular rings, 2-3 mm in axial length, were prepared from abdominal aortas, and responses to vasoactive agents were determined in vitro. Compared with Eut, vascular rings with intact endothelium from Hyper exhibited reductions in contractile responses to norepinephrine (NE) across a range of NE concentrations (P < 0.05). Maximal tension developed in response to NE was reduced approximately 30% in hyperthyroidism (Eut, 3.8 +/- 0.2 g; Hyper, 2.6 +/- 0.4; P < 0.01). Contractile responses to NE were not different between Eut and Hyper in rings denuded of endothelium. Maximal vasorelaxation responses to acetylcholine (ACh), after precontraction with NE (10(-7) M), were enhanced in the hyperthyroid state (Eut, 65.1 +/- 4.8%; Hyper, 84.0 +/- 7.1; P < 0.05). Enhanced vasorelaxation to ACh was also observed when precontraction was induced by prostaglandin F2 alpha. These findings indicate that vascular contractile and relaxation responses are altered in male hyperthyroid rats.

Endocrine neuromyopathies

The myopathies associated with endocrine disorders range in clinical presentation from the relatively nonspecific pattern of proximal muscle weakness of glucocorticoid excess states to specific presentations of contractions produced in tetany. All endocrine neuromyopathies emphasize the role of skeletal muscle in protein, carbohydrate, and electrolyte metabolism. Hormonal abnormalities tend to compromise muscle force generation by indirect effects on muscle function. The recognition and effective treatment of all these disorders require the identification of the underlying hormonal imbalances and awareness of general medical problems produced by the endocrine disorders.

Mammalian skeletal muscle fiber type transitions

Mammalian skeletal muscle is an extremely heterogeneous tissue, composed of a large variety of fiber types. These fibers, however, are not fixed units but represent highly versatile entities capable of responding to altered functional demands and a variety of signals by changing their phenotypic profiles. This adaptive responsiveness is the basis of fiber type transitions. The fiber population of a given muscle is in a dynamic state, constantly adjusting to the current conditions. The full range of adaptive ability spans fast to slow characteristics. However, it is now clear that fiber type transitions do not proceed in immediate jumps from one extreme to the other, but occur in a graded and orderly sequential manner. At the molecular level, the best examples of these stepwise transitions are myofibrillar protein isoform exchanges. For the myosin heavy chain, this entails a sequence going from the fastest (MHCIIb) to the slowest (MHCI) isoform, and vice-versa. Depending on the basal protein isoform profile and hence the position within the fast-slow spectrum, the adaptive ranges of different fibers vary. A simple transition scheme has emerged from the multitude of data collected on fiber type conversions under a variety of conditions.

Low efficiency of oxygen utilization during exercise in hyperthyroidism

STUDY OBJECTIVE

The mechanism of exercise intolerance in hyperthyroidism has not been fully elucidated. This study was undertaken to determine if hyperthyroidism reduced the efficiency of sub-maximal exercise.

STUDY DESIGN

We measured cardiorespiratory variables up to the anaerobic threshold (AT) during ramp-loading cycle ergometry in 12 patients (New York Heart Association functional class II or III). Studies were performed in the hyperthyroid state and repeated in the euthyroid state after 10 months of medical treatment. In 10-W steps from rest to the AT, we measured oxygen uptake (VO2) as a measure of total body work rate, and pressure rate product (PRP) as a measure of cardiac work rate. Loading watts at AT divided by the increment of Vo2 from rest to the AT (delta Watt/delta VO2) was calculated as an index of work efficiency (where delta means the increment of each value from rest to the AT).

RESULTS

VO2 and PRP at the AT were not significantly different between hyperthyroid and euthyroid states (VO2, 16.6 +/- 3.0 vs 17.5 +/- 2.3 mL/min/kg; PRP, 229 +/- 41 vs 218 +/- 28 x 10(2) mm Hg/min). However, loading watts at the AT were significantly lower in the hyperthyroid than the euthyroid state (28 +/- 22 vs 60 +/- 14 W: p < 0.01). VO2 and PRP while hyperthyroid were significantly higher than when euthyroid at every 10-W step during ramp-loading exercise. Furthermore, delta Watt/delta VO2 was significantly lower in hyperthyroid than euthyroid states (p < 0.001). There was a significant inverse correlation-ship between triiodothyronine and delta Watt/delta Vo2 (r = -0.654, p < 0.001).

CONCLUSION

Hyperthyroidism causes low work efficiency, which may limit exercise tolerance.

Early changes in plasma glucagon and growth hormone response to oral glucose in experimental hyperthyroidism

The mechanisms underlying deterioration of glucose tolerance associated with hyperthyroidism are not completely understood. Increases in glucagon and growth hormone (GH) secretion have been previously found in hyperthyroid subjects, and could play a crucial role in this phenomenon. However, studies have not yet established the time sequence of changes in plasma glucose on the one hand and glucagon and GH on the other. To assess the early effects of thyroid hormone excess on glucose tolerance and plasma concentrations of the main glucoregulatory hormones, 12 nondiabetic euthyroid subjects underwent an oral glucose tolerance test (OGTT) before and after triiodothyronine ([T3] 120 micrograms/d) was administered for 10 days. Plasma levels of glucose, insulin, glucagon, and GH were determined at fasting and after the glucose load. T3 administration caused a marked increase in serum T3 (8.8 +/- 0.6 v 2.0 +/- 0.1 nmol/L), with clinical and biochemical signs of thyrotoxicosis. During the treatment, plasma glucose significantly increased both at fasting and after the glucose load (basal, 5.3 +/- 0.1 v 4.9 +/- 0.2 mmol/L, P < .05; area under the curve [AUC] for OGTT, 7.7 +/- 0.3 v 6.7 +/- 0.4 mmol/L min, P < .01) without any change in plasma insulin levels. After T3 administration, plasma glucagon levels were lower than at baseline (basal, 92 +/- 7 v 148 +/- 35 ng/L; AUC, 74 +/- 6 v 98 +/- 16 ng/L.min, P < .05), showing an appropriate reduction by the increased glucose levels. Conversely, plasma GH showed impaired suppression by hyperglycemia (AUC, 1.2 +/- 0.3 v 0.7 +/- 0.2 microgram/L.min, P < .05). In conclusion, thyroid hormone excess rapidly impairs glucose tolerance. Altered secretion of GH is an early event in thyrotoxicosis accompanying the onset of hyperglycemia, whereas plasma glucagon is appropriately suppressed by the increased plasma glucose levels. Thus, GH but not glucagon may contribute to the early hyperglycemic effect of thyrotoxicosis.

Skeletal muscle metabolism during exercise and recovery in patients with respiratory failure

BACKGROUND

Patients with respiratory failure have early fatiguability which may be due to limitation of oxygen supply for oxidative (mitochondrial) ATP synthesis. Skeletal muscle in exercise and recovery was studied to examine the effect of chronic hypoxia on mitochondrial activity in vivo.

METHODS

The skeletal muscle of five patients with respiratory failure (PaO2 < 9 kPa) was studied by phosphorus-31 magnetic resonance spectroscopy and compared with 10 age and sex matched controls. Patients lay in a 1.9 Tesla superconducting magnet with the gastrocnemius muscle overlying a six cm surface coil. Spectra were acquired at rest, during plantar flexion exercise, and during recovery from exercise. Relative concentrations of inorganic phosphate (Pi), phosphocreatine (PCr) and ATP were measured from peak areas, and pH and free ADP concentration were calculated. For the start of exercise, the rates of PCr depletion and estimated lactic acid production were calculated. For the post exercise recovery period, the initial rate of PCr recovery (a quantitative measure of mitochondrial ATP synthesis), the apparent Vmax for mitochondrial ATP synthesis (calculated from initial PCr resynthesis and the end exercise ADP concentration which drives this process), and the recovery half times of PCr, Pi, and ADP (also measures of mitochondrial function) were determined.

RESULTS

Considerably greater and faster PCr depletion and intracellular acidosis were found during exercise. This is consistent with limitation of oxygen supply to the muscle and might explain the early fatiguability of these patients. There was no abnormality in recovery from exercise, however, suggesting that mitochondria function normally after exercise.

CONCLUSIONS

These results are consistent with one or more of the following: (a) decreased level of activity of these patients; (b) changes in the fibre type of the muscle; (c) decreased oxygen supply to the muscle during exercise but not during recovery. They are not consistent with an intrinsic defect of mitochondrial ATP synthesis in skeletal muscle in respiratory failure.

Changes in muscle fibre type from adolescence to adulthood in women and men

Age-related changes in muscle fibre characteristics have been presented in cross-sectional studies previously. The aim of the present study was to investigate longitudinally whether the muscle fibre type composition and muscle fibre area change from adolescence to adulthood. Fifty-five men and 28 women were studied at the age of 16 and again at the age of 27. Biopsies were taken from the vastus lateralis muscle and analysed for fibre types (I, IIA, IIB, IIC) and fibre areas. Different development of fibre type composition with increased age were seen in women and men: the type I percentage tended to increase in the women (51 +/- 9 to 55 +/- 12) and decrease significantly in the men (55 +/- 12 to 48 +/- 13). The fibre areas remained unchanged in both sexes. It is suggested that there is a sex-related fibre adaptation to increased age.

Does chronic hypoxaemia induce transformations of fibre types?

The study comprised nine patients with chronic obstructive lung disease. Quadriceps muscle biopsies were studied with respect to fibre type composition before and after haemodilution that brought haemoglobin (Hb) to within normal limits. Ten days elapsed between the two biopsy occasions. The arterial oxygen tension (PaO2) and saturation (SaO2) were depressed to 8.4 +/- 2.0 kPa and 89 +/- 11% in the patients with chronic obstructive lung disease and increased to 9.2 +/- 2.1 and 91 +/- 8% with haemodilation. The type II fibre proportion was 71 +/- 12% before haemodilation and significantly higher than normal (reference group, see Aniansson et al. 1981). Following haemodilation the proportion of type II fibres decreased significantly to 60 +/- 14%. The proportion of type II fibres was directly related to the haemoglobin content before, but not after, haemodilation and was inversely related to PaO2 and SaO2 both before and after haemodilation. In conclusion, hypoxaemia may be a factor underlying the high proportion of type II fibres found in patients with chronic obstructive lung disease.

Muscle strength in hyperthyroid patients before and after medical treatment

Ten hyperthyroid patients were assessed for muscle strength before and after a period of medical treatment that averaged 12 months. The subjects did not change their habitual level of physical activity between the two test occasions. Maximal voluntary isokinetic knee extensor muscle strength was determined during various concentric, eccentric and isometric conditions. Average increases in strength from before to termination of treatment ranged from 25 to 41% for the concentric and isometric tests (P less than 0.01, n = 10), and from 19 to 35% for the eccentric tests (P less than 0.01, n = 6). The present study demonstrates that medical treatment of hyperthyroid patients results in a marked increase in muscle strength.

Mechanisms limiting glycogen storage in muscle during prolonged insulin stimulation

The extent to which muscle glycogen concentrations can be increased during exposure to maximal insulin concentrations and abundant glucose was investigated in the isolated perfused rat hindquarter preparation. Perfusion for 7 h in the presence of 20,000 microU/ml insulin and 11-13 mM glucose increased muscle glycogen concentrations to maximal values 2, 3, and 3.5 times above normal fed levels in fast-twitch white, slow-twitch red, and fast-twitch red fibers, respectively. Glucose uptake decreased (mean +/- SE) from 34.9 +/- 1.2 mumol.g-1.h-1 at 0 h to 7.5 +/- 0.7 after 7 h of perfusion. During the perfusion muscle glycogen synthase activity decreased and free intracellular glucose and glucose 6-phosphate increased indicating that glucose disposal was impaired. However, glucose transport as measured by the uptake of 3-O-[14C]methyl-D-glucose was also markedly decreased after 5 and 7 h of perfusion compared with initial values. Total muscle water concentration decreased during glycogen loading of the muscles. Mechanisms limiting glycogen storage under maximal insulin stimulation include impaired insulin-stimulated membrane transport of glucose as well as impaired intracellular glucose disposal.

Effect of thyroid hormones on oxidative and nonoxidative glucose metabolism in humans

The glucoregulatory function of thyroid hormones was investigated in six healthy subjects before and after 14 day 3,5,3',5'-tetraiodothyronine (T4) treatment (300 micrograms/day) using a sequential clamp protocol for 5 h at euglycemia (0-2 h) and hyperglycemia (165 mg/dl, 2-5 h) and different insulin infusion rates (1.0 for 0-3.5 h and 6.5 mU.kg-1.min-1, for 3.5-5 h). T4 treatment increased basal energy expenditure (+8%), glucose disposal (+31%), and oxidation (+87%) but decreased nonoxidative glucose metabolism (-30%) and was without effect on lipid oxidation. During the euglycemic clamp, T4 treatment enhanced insulin-induced glucose disposal (+16%), glucose oxidation (+34%), and inhibition of lipid oxidation (-66 vs. -40%); nonoxidative glucose metabolism was stimulated to a similar extent before and after T4. During hyperglycemia, 3,5,3'-triiodothyronine (T3) did not affect glucose disposal but increased carbohydrate-induced lipogenesis at both insulin infusion rates. We conclude that T4 treatment promotes glucose disposal and oxidation, T3 decreases noninsulin-mediated glucose storage but does not antagonize insulin action.

Effects of hyperthyroidism on the sensitivity of glycolysis and glycogen synthesis to insulin in the soleus muscle of the rat

1. The effects of hyperthyroidism on the sensitivity and responsiveness of glycolysis and glycogen synthesis to insulin were investigated in the isolated incubated soleus muscle of the rat. 2. Hyperthyroidism, which was induced by administration of tri-iodothyronine (T3) to rats for 2, 5 or 10 days, increased fasting plasma concentrations of glucose, insulin and free fatty acids. 3. Administration of T3 for 2 or 5 days increased the rates of glycolysis at all insulin concentrations studied: this was due to increased rates of both glucose phosphorylation and glycogen breakdown, but there was no effect of T3 on the sensitivity of glycolysis to insulin. However, administration of T3 for 10 days increased the sensitivity of the rate of glycolysis to insulin. 4. The concentration of adenosine in the gastrocnemius muscles of the rats was not different from controls after 5 days, but it was markedly decreased after 10 days of T3 administration. If these changes are indicative of changes in the soleus muscle, the increased sensitivity of glycolysis to insulin found after 10 days' T3 administration could be due to the decrease in the concentration of adenosine. 5. Administration of T3 decreased the sensitivity of glycogen synthesis to insulin and the glycogen content of the soleus muscles. This may explain the decreased rates of non-oxidative glucose disposal found in spontaneous and experimental hyperthyroidism in man. 6. The rates of glucose oxidation did not change after 2 days, but they were increased after 5 and 10 days of T3 administration.