Triiodothyronine, beta-adrenergic receptors, agonist responses, and exercise capacity

Thyroxine therapy in euthyroid patients does not affect body composition or muscular function

OBJECTIVE

The main objective of the study was to evaluate the effects of small increments in thyroxine (T4) levels following levothyroxine (L-T4) administration on the body composition of women patients. The secondary objective was to assess the effect of the therapy on energy expenditure and muscular function.

METHODS

The prospective, randomized study consisted of a 12-month follow-up of 37 women with thyroid nodules. The patients were divided into two groups for comparison, one treated with L-T4 (20 women) and the other untreated (17 women). L-T4 dose was individually adjusted to obtain a serum thyroid-stimulating hormone in the lower portion of the normal range. Multiple tests, including bioelectrical impedance analysis, dual-energy X-ray absorptiometry, air displacement plethysmography, measurement of waist circumference, and skinfold anthropometry, were used to investigate the muscular, fat, and water compartments; energy expenditure and muscular function were assessed by cycle ergometry.

RESULTS

There were no significant differences in body composition, heart rate, energy metabolism, or muscular function between the group of women treated with L-T4 and the untreated group.

CONCLUSION

The controlled increase of circulating T4 does not appear to modify the body composition or muscular function in women patients.

Increased thermogenic responsiveness to intravenous beta-adrenergic stimulation in habitually exercising humans is not related to skeletal muscle beta2-adrenergic receptor density

Habitually exercising adults demonstrate greater thermogenic responsiveness to beta-adrenergic receptor (beta-AR) stimulation compared with their sedentary peers, but the molecular mechanisms involved are unknown. To determine the possible role of increased beta-AR density, we studied 32 healthy adults: 17 habitual aerobic exercisers (age 45 +/- 5 years, 11 males) and 15 sedentary (49 +/- 5 years, 7 males). Maximal oxygen uptake (43.7 +/- 2.5 versus 31.6 +/- 2.9 ml kg(-1) min(-1), P = 0.002, mean +/- S.E.M.) and vastus lateralis muscle maximal citrate synthase activity (1.70 +/- 0.36 versus 0.58 +/- 0.11 micromol min(-1) g(-1), P = 0.008) were higher in the habitually exercising subjects. Resting energy expenditure (EE) adjusted for fat-free mass (FFM) was similar in the habitually exercising (5903 +/- 280 kJ day(-1)) and sedentary adults (6054 +/- 289 kJ day(-1), P = 0.43). The percentage increase in EE (DeltaEE%; indirect calorimetry, ventilated hood) above resting EE in response to beta-AR stimulation (intravenous isoproterenol at 6, 12 and 24 ng (kg FFM)(-1) min(-1)) was greater (7.1 +/- 1.2, 13.7 +/- 1.0, 20.7 +/- 1.3 versus 5.9 +/- 0.9, 9.9 +/- 1.4, 15.9 +/- 1.70%, respectively, P = 0.04), and the dose of isoproterenol required to increase EE by 10% above resting EE was lower (8.2 +/- 1.5 versus 17.1 +/- 4.1 ng (kg FFM)(-1) min(-1), P = 0.03) in the habitually exercising adults. In contrast, vastus lateralis muscle beta(2)-AR density was similar in the habitually exercising and sedentary subjects (7.46 +/- 0.29 versus 7.44 +/- 0.60 fmol (mg dry weight muscle)(-1), P = 0.98), and was not related to DeltaEE% (r = 0.02, P = 0.94) or to the isoproterenol dose required to increase EE by 10% above resting EE (r = -0.06, P = 0.76). These findings indicate that increased beta(2)-AR density is not a mechanism contributing to the greater thermogenic responsiveness to beta-AR stimulation in adult humans who regularly perform aerobic exercise.

Molecular basis of skeletal muscle plasticity--from gene to form and function

Skeletal muscle shows an enormous plasticity to adapt to stimuli such as contractile activity (endurance exercise, electrical stimulation, denervation), loading conditions (resistance training, microgravity), substrate supply (nutritional interventions) or environmental factors (hypoxia). The presented data show that adaptive structural events occur in both muscle fibres (myofibrils, mitochondria) and associated structures (motoneurons and capillaries). Functional adaptations appear to involve alterations in regulatory mechanisms (neuronal, endocrine and intracellular signalling), contractile properties and metabolic capacities. With the appropriate molecular techniques it has been demonstrated over the past 10 years that rapid changes in skeletal muscle mRNA expression occur with exercise in human and rodent species. Recently, gene expression profiling analysis has demonstrated that transcriptional adaptations in skeletal muscle due to changes in loading involve a broad range of genes and that mRNA changes often run parallel for genes in the same functional categories. These changes can be matched to the structural/functional adaptations known to occur with corresponding stimuli. Several signalling pathways involving cytoplasmic protein kinases and nuclear-encoded transcription factors are recognized as potential master regulators that transduce physiological stress into transcriptional adaptations of batteries of metabolic and contractile genes. Nuclear reprogramming is recognized as an important event in muscle plasticity and may be related to the adaptations in the myosin type, protein turnover, and the cytoplasma-to-myonucleus ratio. The accessibility of muscle tissue to biopsies in conjunction with the advent of high-throughput gene expression analysis technology points to skeletal muscle plasticity as a particularly useful paradigm for studying gene regulatory phenomena in humans.

Smaller muscle ATP reduction in women than in men by repeated bouts of sprint exercise

It was hypothesized that the reduction of high-energy phosphates in muscle after repeated sprints is smaller in women than in men. Fifteen healthy and physically active women and men with an average age of 25 yr (range of 19-42 yr) performed three 30-s cycle sprints (Wingate test) with 20 min of rest between sprints. Repeated blood and muscle samples were obtained. Freeze-dried pooled muscle fibers of types I and II were analyzed for high-energy phosphates and their breakdown products and for glycogen. Accumulation of plasma ATP breakdown products, plasma catecholamines, and blood lactate, as well as glycogen reduction in type I fibers, was all lower in women than in men during sprint exercise. Repeated sprints induced smaller reduction of ATP and smaller accumulation of IMP and inosine in women than in men in type II muscle fibers, with no gender differences in changes of ATP and its breakdown products during the bouts of exercise themselves. This indicates that the smaller ATP reduction in women than in men during repeated sprints was created during recovery periods between the sprint exercises and that women possess a faster recovery of ATP via reamination of IMP during these recovery periods.

Metabolic response in type I and type II muscle fibers during a 30-s cycle sprint in men and women

The acute metabolic response to sprint exercise was studied in 20 male and 19 female students. We hypothesized that the reduction of muscle glycogen content during sprint exercise would be smaller in women than in men and that a possible gender difference in glycogen reduction would be higher in type II than in type I fibers. The exercise-induced increase in blood lactate concentration was 22% smaller in women than in men. A considerable reduction of ATP (50%), phosphocreatine (83%), and glycogen (35%) was found in type II muscle fibers, and it did not differ between the genders. A smaller reduction of ATP (17%) and phosphocreatine (78%) was found in type I fibers, and it did not differ between the genders. However, the exercise-induced reduction in glycogen content in type I fibers was 50% smaller in women than in men. The hypothesis was indeed partly confirmed: the exercise-induced glycogen reduction was attenuated in women compared with men, but the gender difference was in type I rather than in type II fibers. Fiber-type-specific and gender-related differences in the metabolic response to sprint exercise might have implications for the design of training programs for men and women.

[Cardiopulmonary parameters in hyperthyroidism]

BACKGROUND

Hyperthyroid patients often suffer from impaired exercise capacity with dyspnoea. Two well established, non-invasive methods were used to evaluate the influence of hyperthyroidism on cardiopulmonary function.

PATIENTS AND METHODS

In 42 patients with hyperthyroidism we performed spirometry and cardiopulmonary exercise testing before and after 7 days of propranolol therapy as well as in euthyroidism.

RESULTS

In hyperthyroidism reduced vital capacity and 1-second capacity were observed (95.5 +/- 2.4% vs 102.6 +/- 1.5%; p = 0.0087; 89.4 +/- 2.3% vs 95.2 +/- 2.2%; p = 0.0179). No changes showed during beta-blockade. At the anaerobic threshold reduced tidal volume and enhanced respiratory frequency were noted (1119.8 +/- 48.9 ml vs 1289.3 +/- 62.7 ml; p = 0.0227; 28.3 +/- 0.8 vs 25.4 +/- 0.9; p = 0.0012). A significant tachycardia could be shown. Impaired response to exercise in pulse and respiratory frequency were observed. Work at the anaerobic threshold was impaired in hyperthyroidism (70 +/- 5 watts vs 86.9 +/- 5.7 watts; p = 0.016) and did not change during propranolol therapy. Oxygen pulse at the anaeorbic threshold was reduced in hyperthyroidism (7.7 +/- 0.4 ml O2/beat vs 9.1 +/- 0.4 ml O2/beat; p = 0.0012) and increased with propranolol (8.9 +/- 0.4 ml O2/beat; p = 0.0001).

CONCLUSION

In hyperthyroidism significant changes in cardiopulmonary function were noted at rest and exercise. High resting function and impaired response to exercise suggest a cardiopulmonary work with low efficiency. Propranolol leads to economization and lowers patients complaints.

Reduced metabolic efficiency of skeletal muscle energetics in hyperthyroid patients evidenced quantitatively by in vivo phosphorus-31 magnetic resonance spectroscopy

Skeletal muscle energetics of seven hyperthyroid patients were investigated throughout a rest-exercise-recovery protocol using phosphorus-31 magnetic resonance spectroscopy (31P MRS) to quantitatively document in vivo the metabolic bases of impaired muscle performance in hyperthyroidism. The contributions of the main pathways of adenosine triphosphate (ATP) synthesis to energy production and proton efflux were measured and compared with results from normal muscle. At rest, a reduced concentration of phosphocreatine (PCr) was calculated for hyperthyroid patients when compared with controls, whereas pH and concentrations of inorganic phosphate (Pi) and phosphomonoesters (PME) were not different from controls. During exercise, the analysis of changes in pH and PCr concentration demonstrated that (1) at the onset of exercise, the magnitude of glycolysis activation is significantly larger for patients, resulting in a marked pH decrease; (2) the energy cost of exercise is higher for patients as compared with controls performing the same amount of work; and (3) both anaerobic and aerobic pathways are significantly more activated in the hyperthyroid group throughout the 3 minutes of exercise. During recovery, the rates of proton efflux and PCr resynthesis were similar in both groups, excluding any alteration in oxidative function and proton handling as a cause of initial glycolytic hyperactivation. The increased energy cost measured for patients during exercise evidences an increased need for energy, which is (1) probably linked to the existence of additional ATP-consuming mechanism(s), and (2) supported by hyperactivation of both aerobic and anaerobic pathways. These findings imply that, all things equal, a hyperthyroid muscle requires more energy to function than normal, and as a result is potentially more fatiguable.

Effect of exercise on erythrocyte beta-adrenergic receptors and plasma concentrations of catecholamines and thyroid hormones in Thoroughbred horses

The effects of exercise stress on erythrocyte beta-adrenergic receptor characteristics and plasma concentrations of adrenaline, noradrenaline and thyroid hormones were studied in Thoroughbred racehorses during rest and after exercise. Five minutes after a maximal speed race of 1200 +/- 200 m (mean +/- s.d.), both plasma adrenaline and noradrenaline concentrations increased with respect to basal values (from 2.48 +/- 0.15 to 3.83 +/- 0.27 and from 2.13 +/- 0.11 to 3.53 +/- 0.27 nmol/l respectively). The increment of adrenaline was greater in high performance (HP) as compared to low performance (LP) horses (76.9 vs. 43.5%), in accordance with the contribution of the adrenal medulla in the sympathoadrenal response to exercise. Triiodothyronine (T3), but not thyroxine (T4) levels increased 5 min after exercise (from 55.6 +/- 2.9 to 81 +/- 3.7 ng/dl and from 0.67 +/- 0.04 to 0.70 +/- 0.05 micrograms/dl respectively). No differences were observed in basal values of thyroid hormones or in the percentage of T3 increment, when comparing HP vs. LP horses. Erythrocyte membranes obtained 5 min after racing showed decreased concentrations of beta-adrenergic receptors (beta-AR) and dissociation constant as compared to basal values (50.1 +/- 7.0 vs. 95.7 +/- 12.0 fmol/mg protein and 0.97 +/- 0.24 vs. 2.04 +/- 0.3 nmol/l respectively). This temporal pattern suggest that the observed changes in beta-AR characteristics could be mediated by catecholamines, but not by thyroid hormones, in this model. This down regulation of beta-AR may act as a protecting mechanism preventing the erythrocytes from the decrease in membrane fluidity known to be provoked by adrenergic agonists. The accomplished study showed that, in the Thoroughbred horse, there is a homeostatic response to race stress, characterised by a sudden increase in plasma catecholamines and T3 and a parallel decrease in beta-AR concentration on the erythrocyte membrane. In this way the racing horse could rapidly adjust its metabolism to the exercise stress, but at the same time override one possible undesirable side-effect caused by these hormonal changes. Further studies will be required to establish performance-related differences occurring in endocrine changes.