Exercise capacity and cardiac function in trained and untrained thyroid-deficient rats

Effect of thyroid hormone on uncoupling protein-3 mRNA expression in rat heart and skeletal muscle

Thyroid hormones (triiodothyronine [T3] and thyroxine [T4]) stimulate UCP-3 expression in skeletal muscle. We examined whether thyroid hormone-induced changes in uncoupling protein (UCP)-3 mRNA expression are related to directs effects of T3 or reflect secondary effects of the hormone through stimulation of renin-angiotensin or beta-adrenergic systems. Hyperthyroidism was produced by three injections of 100 microg T3/100 g body weight on alternate days with or without concomitant treatment with either captopril (an angiotensin-converting enzyme [ACE] inhibitor), propranolol (a beta-blocker) or clenbuterol (a beta2-agonist). The relative abundance of UCP-3 mRNA was measured in ventricular myocardium and skeletal muscle (gastrocnemius and soleus). T3 resulted in a significant increase in the relative abundance of UCP-3 in heart and skeletal muscle (p < 0.05), and the effect was not altered by captopril or propanolol; the inhibitors alone had no effect of UCP-3 mRNA content. There was no synergistic or additive effect of T3 and clenbuterol on UCP-3 mRNA expression in skeletal muscle. Increased UCP-3 mRNA levels were associated with increased UCP-3 protein expression in skeletal muscle. We conclude that the effect of T3 on UCP-3 expression in cardiac and skeletal muscle is not dependent on either angiotensin II or the beta-adrenergic system and probably reflects a direct action of the hormone on UCP-3 gene expression.

Cardiovascular hemodynamics and exercise tolerance in thyroid disease

The heart is an organ sensitive to the action of thyroid hormone, and measurable changes in cardiovascular performance are detected with small variations in thyroid hormone serum concentrations. Most patients with thyroid disease experience cardiovascular manifestations, and the most serious complications of thyroid dysfunction occur as a result of cardiac involvement. The increased metabolic state and oxygen consumption that occur in hyperthyroid patients require an increased supply of oxygen and removal of metabolic products from the periphery. This is accomplished by increasing the cardiac output to meet the needs of the periphery. Circulation time is decreased in hyperthyroid patients, and a lowered arterial resistance and increased venous resistance promote the return of blood to the heart. Thyroid hormones may significantly decrease the strength of respiratory and skeletal muscles and affect regulatory mechanisms of adaptation to incremental effort. In hyperthyroidism, cardiovascular exercise testing and analysis of respiratory gas exchange demonstrate low efficiency of cardiopulmonary function as well as impaired chronotropic, contractile, and vasodilatatory reserves, which are reversible when euthyroidism is restored. During exercise, the increment (delta) of minute ventilation (respiratory rate x tidal volume), and oxygen pulse (oxygen uptake per heart beat) are significantly lower in dysthyroidism versus euthyroidism. Especially in older patients with thyroid dysfunction, markedly reduced workload, delta ejection fraction, and delta heart rate, both at the anaerobic threshold as well as at maximal exercise, are observed. In thyrotoxicosis, mitochondria oxidative dysfunction during exercise mostly causes intracellular acidosis, whereas in hypothyroidism, inadequate cardiovascular support appears to be one of the principal factors involved. These abnormalities partly explain why subjects with dysthyroidism are intolerant to exertion. Thus, in thyroid disease, both cardiac structures and function may remain normal at rest, however impaired cardiovascular and respiratory adaptation to effort becomes unmasked during exercise.

Plasma free fatty acids in neonates with congenital hypothyroidism

Since the introduction of neonatal mass screening for congenital hypothyroidism (CH), numerous cases have been detected. It is of interest that even severely hypothyroid neonates rarely exhibit bradycardia, hypothermia, or inactivity, which have been recognized as typical signs of CH. Regarding neonates and young infants, few reported data are available on the effects of thyroid hormones on energy expenditure. Plasma free fatty acids (FFAs), markers for lipolysis, play essential roles in maintaining physiologic homeostasis. To study fuel utilization in CH neonates, we measured heart rates, plasma FFA, and thyroid hormones before and after levothryoxine (LT4) replacement therapy. Fifty-five screen-detected CH neonates and 29 age-matched normal neonates for controls were enrolled. The CH neonates were divided into two groups according to serum thyroid hormone levels: a mildly hypothyroid group (n = 37), serum thyrotropin (TSH) less than 100 microIU/mL and free thyroxine (FT4) 0.6 ng/dL or more; and a severely hypothyroid group (n = 18), TSH 100 microIU/mL or more and FT4 less than 0.6 ng/dL. Twenty-four of the 55 patients had their heart rates measured by electrocardiography. Fasting blood samples were taken from the subjects during physical movements. Serum levels of TSH, FT4, FFA, and other blood chemicals, measured on an autoanalyzer system in our hospital, were compared before and after LT4 substitution therapy. The following results were obtained. The mean plasma FFA values before LT4 replacement were 208.5 +/- 89.4 microEq/L in the mildly hypothyroid group, 228.5 +/- 114.7 microEq/L in the severely hypothyroid group, and 213.9 +/- 97.7 microEq/L in controls. No statistical differences were noted among the three values. Two months after LT4 replacement therapy, at the age of 3 months, plasma FFA concentrations significantly increased in both groups compared with those before the therapy. Control infants also showed a significant increase in plasma FFA concentrations from 1 to 3 months of age. There were no significant differences in plasma FFA concentrations among the three groups at the age of 3 months. No significant correlations were found between plasma FFA and serum thyroid hormones. From these results it is suggested that in neonates and young infants, thyroid hormones do not play major roles in mobilization of fats through the adrenergic regulation of lipolysis for energy supply. This may be one of the reasons for the unexpectedly mild signs and symptoms in the screen-detected hypothyroid neonates.

Effects of exercise training on cardiac function, gene expression, and apoptosis in rats

This study determined the effects of exercise training on cardiac function, gene expression, and apoptosis. Rats exposed to a regimen of treadmill exercise for 13 wk had a significant increase in cardiac index and stroke volume index and a concomitant decrease in systemic vascular resistance compared with both age-matched and body weight-matched sedentary controls in the conscious state at rest. In exercise-trained animals, there was no change in the expression of several marker genes known to be associated with pathological cardiac adaptation, including atrial natriuretic factor, beta-myosin heavy chain, alpha-skeletal and smooth muscle actins, and collagens I and III. Exercise training, however, produced a significant induction of alpha-myosin heavy chain, which was not observed in rats with myocardial infarction. No histological features of cardiac apoptosis were observed in the treadmill-trained rats. In contrast, apoptotic myocytes were detected in animals with myocardial infarction. In summary, exercise training improves cardiac function without evidence of cardiac apoptosis and produces a pattern of cardiac gene expression distinct from pathological cardiac adaptation.

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.

31P-NMR study of different hypothyroid states in rat leg muscle

Using phosphorus nuclear magnetic resonance spectroscopy, this study was undertaken to determine the effects of experimental hypothyroidism on muscle bioenergetics. The peaks of phosphocreatine (PCr), Pi, phosphodiesters (PDE), sugar phosphomonoesters, and ATP were obtained at rest, during a 2-Hz hindleg muscle stimulation, and during a subsequent recovery period from four groups of anesthetized rats as follows: one control and three hypothyroid (HT) groups treated by propylthyouracil during 2, 4, and 6 wk, respectively. Resting spectra showed a significant rise in Pi by 30% and decreased intracellular pH and PCr/Pi in all three HT groups. PDE progressively increased to 200% of its initial value with hypothyroidism duration. Muscle stimulation did not lead to significant differences in PCr depletion. The percentage of PCr recovery is less in HT muscle than in control muscle. An abnormal H+ metabolism is obvious in all three HT groups. These results indicate abnormal bioenergetics in HT muscle and suggest an impairment of mitochondrial metabolism and of the H+ efflux. They also evoke a high sensitivity of cellular energetics to thyroid deficiency.

Control of myosin heavy chain expression: interaction of hypothyroidism and hindlimb suspension

The aim of this study was to contrast competing influences, hypothyroidism and hindlimb suspension, on myosin heavy chain (MHC) expression studied at the protein level and mRNA level. Female Sprague-Dawley rats were assigned to either normal control (NC), normal suspended (NS), or hypothyroid (thyroidectomized) control (TC) and suspended (TS) groups. NS and TS animals were suspended for 14 days following which myofibrils and total RNA were purified from the hindlimb muscles. In the soleus and vastus intermedius (VI), there was an increase in type I MHC and a decrease in type IIa MHC in both the TC and TS groups and a decrease in type I and increase in type IIa MHC in the NS group. At the mRNA level, similar shifts were observed with the exception that 1) the increased type IIa MHC seen in the soleus and VI of the NS animals was not accompanied by an increase in IIa mRNA and 2) type IIb mRNA was increased in the NS soleus without concomitant changes in IIb protein levels. These data suggest the following: 1) a hypothyroid state predominates over mechanical unweighting factors in the control of MHC distribution in slow muscles; and 2) translational or posttranslational factors may be important in the regulation of type IIa and IIb MHC expression during hindlimb suspension.

Effect of increased carbohydrate utilization potential on cardiac isomyosin in thyroidectomized rats

Previous studies have shown that dietary carbohydrate (CHO) can impact on cardiac isomyosin expression in hormonally deficient animals. The primary objective of this study was to determine whether a high-CHO diet alters cardiac isomyosin expression in severe thyroid-deficient rats. Also the effects of targeting the heart with episodes of biasing cardiac metabolism toward CHO were studied. Female Sprague-Dawley rats were assigned to one of two major groups: 1) normal control and 2) thyroidectomized (TX) propylthiouracil treated. The TX rats were allocated into four experimental subgroups as follows: 1) mixed diet; 2) high-CHO diet; 3) high-CHO diet and treated with oxfenicine, a fatty acid oxidation inhibitor; and 4) high-CHO diet, treated with oxfenicine, and trained. Results show that, at the end of 12 wk of thyroidectomy, there was a marked shift in cardiac isomyosin distribution to predominance of the V3 isoform. However, 6 wk of experimental manipulation failed to redirect cardiac isomyosin expression in TX rats. It is concluded that increased CHO utilization does not influence cardiac isoenzyme expression in markedly hypothyroid female rats. Dietary effects of CHO on cardiac isomyosin require some critical level of thyroid hormone for mediating the response.

Functional and metabolic consequences of skeletal muscle remodeling in hypothyroidism

Functional and metabolic responses of hypothyroid skeletal muscle were evaluated during steady-state isometric contraction conditions, using an isolated perfused rat hindlimb preparation. Treating rats with propylthiouracil (PTU) for 4-5 mo resulted in a 55% decrease (P less than 0.001) in citrate synthase activity in plantaris muscle and phenotypic remodeling of the plantaris, evident by a threefold increase in type I fiber area and a 13% decrease in type II fiber area. Perfusion of PTU (n = 9) and control (n = 9) rat hindlimbs of similar size, with similar inflow (approximately 10 ml/min) and oxygen content (approximately 20 g/100 ml), resulted in similar oxygen deliveries to the contracting muscles (PTU 11.4 +/- 0.58, control 9.54 +/- 0.75 mumol.min-1.g-1; P greater than 0.05). Ten-minute tetanic contraction (100 ms at 100 Hz) periods at 4, 8, 15, 30, and 45 tetani/min were elicited in consecutive ascending order. Oxygen consumption (VO2) was lower in the PTU group at all contraction frequencies (P less than 0.005), with a decrease in peak VO2 of 44% (PTU 3.01 +/- 0.29, control 5.35 +/- 0.42 mumol.min-1.g-1; P less than 0.001). Oxygen extraction by the PTU muscle was only approximately 25% of that delivered. Developed tension was initially less (15%; P less than 0.05) in the PTU group but declined in a similar manner, as a percent of initial, to that of the control group. The slightly lower absolute tension development of the PTU muscle could not account for the large reduction in VO2.(ABSTRACT TRUNCATED AT 250 WORDS)

Oxidative muscular injury and its relevance to hyperthyroidism

In experimental hyperthyroidism, acceleration of lipid peroxidation occurs in heart and slow-oxidative muscles, suggesting the contribution of reactive oxygen species to the muscular injury caused by thyroid hormones. This article reviews various models of oxidative muscular injury and considers the relevance of the accompanying metabolic derangements to thyrotoxic myopathy and cardiomyopathy, which are the major complications of hyperthyroidism. The muscular injury models in which reactive oxygen species are supposed to play a role are ischemia/reperfusion syndrome, exercise-induced myopathy, heart and skeletal muscle diseases related to the nutritional deficiency of selenium and vitamin E and related disorders, and genetic muscular dystrophies. These models provide evidence that mitochondrial function and the glutathione-dependent antioxidant system are important for the maintenance of the structural and functional integrity of muscular tissues. Thyroid hormones have a profound effect on mitochondrial oxidative activity, synthesis and degradation of proteins and vitamin E, the sensitivity of the tissues to catecholamine, the differentiation of muscle fibers, and the levels of antioxidant enzymes. The large volume of circumstantial evidence presented here indicates that hyperthyroid muscular tissues undergo several biochemical changes that predispose them to free radical-mediated injury.

Effects of thyroid hormones on skeletal muscle bioenergetics. In vivo phosphorus-31 magnetic resonance spectroscopy study of humans and rats

The pathophysiology of the myopathy in dysthyroid states is poorly understood. We therefore tested the effects of thyroid hormones on muscle bioenergetics in humans and rats, using in vivo 31P NMR. Two hypothyroid patients had: low phosphocreatine to inorganic phosphate ratio (PCr/Pi) at rest, increased PCr depletion during exercise and delayed postexercise recovery of PCr/Pi. Eight thyroidectomized rats did not show abnormalities at rest, but muscle work induced by nerve stimulation resulted in a significantly (P less than 0.0001) lower PCr/Pi (35-45% of control) at each of the three stimulation frequencies tested (0.25, 0.5, and 1.0 Hz). Recovery rate was markedly slowed to one-third of normal values. Thyroxine therapy reversed these abnormalities in both human and rat muscle. Five patients and six rats with hyperthyroidism did not differ from normal controls during rest and exercise but had an unusually rapid recovery after exercise. The bioenergetic abnormalities in hypothyroid muscle suggest the existence of a hormone-dependent, reversible mitochondrial impairment in this disorder. The exercise intolerance and fatigue experienced in hypothyroid muscle may be due to such a bioenergetic impairment. The changes in energy metabolism in hyperthyroid muscle probably do not cause the muscular disease in this disorder.