Functional and metabolic consequences of skeletal muscle remodeling in hypothyroidism

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.

Effects of Cyclosporine-A on Rat Soleus Muscle Fiber Size and Phenotype

PURPOSE

Organ transplant patients treated with cyclosporine-A (CsA) often exhibit weight loss and muscle weakness. The cellular target of CsA, calcineurin, has been implicated in maintenance of muscle fiber size and in expression of the type I skeletal muscle phenotype. We hypothesized that CsA treatment would cause fiber atrophy, as well as increase type IIa myosin heavy chain (MHC) content and oxidative enzyme activities in the soleus muscle.

METHODS

Rats were treated with CsA for 21 d (20 mg.kg(-1).d(-1); N = 16) and compared with control rats given olive oil vehicle (Veh; N = 16). Soleus muscles were excised bilaterally. MHC content was determined by gel electrophoresis, oxidative enzyme activities by spectrophotometric methods, and fiber type and size by histochemistry.

RESULTS

Lymphocyte count was depressed in CsA rats (P < 0.05), indicating treatment efficacy. Type IIa MHC content was increased in the soleus muscle with CsA (Veh, 10.4 +/- 1.7%; CsA, 15.1 +/- 2.0; P < 0.05) at the expense of type I MHC. Soleus muscle oxidative enzyme activities were also increased with CsA treatment (P < 0.05). Soleus muscle atrophy occurred, reflected by a 22% decrease in fiber cross-sectional area (Veh, 3255 +/- 105 microm(2); CsA, 2533 +/- 125; P < 0.05).

CONCLUSION

These findings indicate that CsA treatment is associated with changes in skeletal muscle fiber size and phenotype. The former may underlie clinical symptoms of transplant patients treated with CsA.

Effects of hypothyroidism on the skeletal muscle blood flow response to contractions

Hypothyroidism is associated with impaired blood flow to skeletal muscle under whole body exercise conditions. It is unclear whether poor cardiac and/or vascular function account for blunted muscle blood flow. Our experiment isolated a small group of hindlimb muscles and simulated exercise via tetanic contractions. We hypothesized that muscle blood flow would be attenuated in hypothyroid rats (HYPO) compared with euthyroid rats (EUT). Rats were made hypothyroid by mixing propylthiouracil in their drinking water (2.35 x 10-3 mol/l). Treatment efficacy was evidenced by lower serum T3 concentrations and resting heart rates in HYPO (both P<0.05). In the experimental preparation, isometric contractions of the lower right hindlimb muscles at a rate of 30 tetani/min were induced via sciatic nerve stimulation. Regional blood flows were determined by the radiolabelled microsphere method at three time points: rest, 2 min of contractions and 10 min of contractions. Muscle blood flow generally increased from rest ( approximately 5-10 ml/min per 100 g) through contractions for both groups. Further, blood flow during contractions did not differ between groups for any muscle (eg. red section of gastrocnemius muscle; EUT, 59.9 +/- 14.1; HYPO, 61.1 +/- 15.0; NS between groups). These findings indicate that hypothyroidism does not significantly impair skeletal muscle blood flow when only a small muscle mass is contracting. Our findings suggest that impaired blood flow under whole body exercise is accounted for by inadequate cardiac function rather than abnormal vascular function.

Response of mitochondrial function to hypothyroidism in normal and regenerated rat skeletal muscle

Although thyroid hormones induce a well known decrease in muscle oxidative capacity, nothing is known concerning their effects on mitochondrial function and regulation in situ. Similarly, the influence of regeneration process is not completely understood. We investigated the effects of hypothyroidism on mitochondrial function in fast gastrocnemius (GS) and slow soleus (SOL) muscles either intact or having undergone a cycle of degeneration/regeneration (Rg SOL) following a local injection of myotoxin. Thyroid hormone deficiency was induced by thyroidectomy and propylthiouracyl via drinking water. Respiration was measured in muscle fibres permeabilised by saponin in order to assess the oxidative capacity of the muscles and the regulation of mitochondria in situ. Oxidative capacities were 8.9 in SOL, 8.5 in Rg SOL and 5.9 micromol O2/min/g dry weight in GS and decreased by 52, 42 and 39% respectively (P < 0.001) in hypothyroid rats. Moreover, the Km of mitochondrial respiration for the phosphate acceptor ADP exhibited a two-fold decrease in Rg SOL and intact SOL by hypothyroidism (P < 0.01), while mitochondrial creatine kinase activity and sensitivity of mitochondrial respiration to creatine were not altered. The results of this study demonstrate that hypothyroidism markedly altered the sensitivity of mitochondrial respiration to ADP but not to creatine in SOL muscles, suggesting that mitochondrial regulation could be partially controlled by thyroid hormones. On the other hand, mitochondrial function completely recovered following regeneration/degeneration, suggesting that thyroid hormones are not involved in the regeneration process per se.

The alpha(1)- and alpha(2)-isoforms of Na-K-ATPase play different roles in skeletal muscle contractility

The Na-K-ATPase, which maintains the Na(+) and K(+) gradients across the plasma membrane, can play a major role in modulation of skeletal muscle contractility. Although both alpha(1)- and alpha(2)-isoforms of the Na-K-ATPase are expressed in skeletal muscle, the physiological significance of these isoforms in contractility is not known. Evaluation of the contractile parameters of mouse extensor digitorum longus (EDL) was carried out using gene-targeted mice lacking one copy of either the alpha(1)- or alpha(2)-isoform gene of the Na-K-ATPase. The EDL muscles from heterozygous mice contain approximately one-half of the alpha(1)- or alpha(2)-isoform, respectively, which permits differentiation of the functional roles of these isoforms. EDL from the alpha(1)(+/-) mouse shows lower force compared with wild type, whereas that from the alpha(2)(+/-) mouse shows greater force. The different functional roles of these two isoforms are further demonstrated because inhibition of the alpha(2)-isoform with ouabain increases contractility of alpha(1)(+/-) EDL. These results demonstrate that the Na-K-ATPase alpha(1)- and alpha(2)-isoforms may play different roles in skeletal muscle contraction.

Effects of forearm bier block with bretylium on the hemodynamic and metabolic responses to handgrip

We tested the hypothesis that a reduction in sympathetic tone to exercising forearm muscle would increase blood flow, reduce muscle acidosis, and attenuate reflex responses. Subjects performed a progressive, four-stage rhythmic handgrip protocol before and after forearm bier block with bretylium as forearm blood flow (Doppler) and metabolic (venous effluent metabolite concentration and (31)P-NMR indexes) and autonomic reflex responses (heart rate, blood pressure, and sympathetic nerve traffic) were measured. Bretylium inhibits the release of norepinephrine at the neurovascular junction. Bier block increased blood flow as well as oxygen consumption in the exercising forearm (P < 0.03 and P < 0.02, respectively). However, despite this increase in flow, venous K(+) release and H(+) release were both increased during exercise (P < 0.002 for both indexes). Additionally, minimal muscle pH measured during the first minute of recovery with NMR was lower after bier block (6.41 +/- 0.08 vs. 6.20 +/- 0.06; P < 0.036, simple effects). Meanwhile, reflex effects were unaffected by the bretylium bier block. The results support the conclusion that sympathetic stimulation to muscle during exercise not only limits muscle blood flow but also appears to limit anaerobiosis and H(+) release, presumably through a preferential recruitment of oxidative fibers.

Peripheral adaptations in trained aged rats with femoral artery stenosis

The development and functional significance of exercise-induced peripheral adaptations were evaluated in aged animals with peripheral arterial insufficiency. Fisher 344 male rats (21 months old) were subjected to bilateral stenosis of the femoral arteries sufficient to limit active hyperemia but not to impair resting blood flow. Beginning the third day after stenosis, animals were (1) exercised by walking (n = 12) on a treadmill at 20 m/min at 15% inclination, twice a day, 5 days per week, or (2) limited to cage activity (n = 10). Exercise tolerance improved from approximately 5 to approximately 35 minutes (P < .001) over the 8 weeks of the training program but increased only marginally to approximately 8 minutes for the sedentary group. An isolated hind limb preparation perfused at equivalent blood flows (approximately 1 ml.min-1 x g-1 with an arterial blood oxygen content of approximately 20 vol%) was used to assess the functional and metabolic impact of muscle-specific adaptations during sequential contraction periods at 4, 8, 15, 30, 45, 60, 75, and 90 tetani per minute. An initially similar force development of approximately 10 N/g was better maintained (P < .001) by the trained group. The peak oxygen consumption attained by the trained group of 5.68 +/- 0.34 mumol.min-1 x g-1 was greater (P < .01) than that of the sedentary group (4.34 +/- 0.29 mumol.min-1 x g-1). This was due to a greater oxygen extraction, since oxygen delivery was the same (approximately 10 mumol.min-1 x g-1) to muscles of both groups.(ABSTRACT TRUNCATED AT 250 WORDS)