Thyroid thermogenesis

Molecular functions of thyroid hormones and their clinical significance in liver-related diseases

Thyroid hormones (THs) are potent mediators of several physiological processes, including embryonic development, cellular differentiation, metabolism, and cell growth. Triiodothyronine (T3) is the most biologically active TH form. Thyroid hormone receptors (TRs) belong to the nuclear receptor superfamily and mediate the biological functions of T3 via transcriptional regulation. TRs generally form heterodimers with the retinoid X receptor (RXR) and regulate target genes upon T3 stimulation. Research over the past few decades has revealed that disruption of cellular TH signaling triggers chronic liver diseases, including alcoholic or nonalcoholic fatty liver disease and hepatocellular carcinoma (HCC). Animal model experiments and epidemiologic studies to date imply close associations between high TH levels and prevention of liver disease. Moreover, several investigations spanning four decades have reported the therapeutic potential of T3 analogs in lowering lipids, preventing chronic liver disease, and as anticancer agents. Thus, elucidating downstream genes/signaling pathways and molecular mechanisms of TH actions is critical for the treatment of significant public health issues. Here, we have reviewed recent studies focusing on the roles of THs and TRs in several disorders, in particular, liver diseases. We also discuss the potential therapeutic applications of THs and underlying molecular mechanisms.

Renal intercalated cells are rather energized by a proton than a sodium pump

The Na(+) concentration of the intracellular milieu is very low compared with the extracellular medium. Transport of Na(+) along this gradient is used to fuel secondary transport of many solutes, and thus plays a major role for most cell functions including the control of cell volume and resting membrane potential. Because of a continuous leak, Na(+) has to be permanently removed from the intracellular milieu, a process that is thought to be exclusively mediated by the Na(+)/K(+)-ATPase in animal cells. Here, we show that intercalated cells of the mouse kidney are an exception to this general rule. By an approach combining two-photon imaging of isolated renal tubules, physiological studies, and genetically engineered animals, we demonstrate that inhibition of the H(+) vacuolar-type ATPase (V-ATPase) caused drastic cell swelling and depolarization, and also inhibited the NaCl absorption pathway that we recently discovered in intercalated cells. In contrast, pharmacological blockade of the Na(+)/K(+)-ATPase had no effects. Basolateral NaCl exit from β-intercalated cells was independent of the Na(+)/K(+)-ATPase but critically relied on the presence of the basolateral ion transporter anion exchanger 4. We conclude that not all animal cells critically rely on the sodium pump as the unique bioenergizer, but can be replaced by the H(+) V-ATPase in renal intercalated cells. This concept is likely to apply to other animal cell types characterized by plasma membrane expression of the H(+) V-ATPase.

TSH and Thyrotropic Agonists: Key Actors in Thyroid Homeostasis

This paper provides the reader with an overview of our current knowledge of hypothalamic-pituitary-thyroid feedback from a cybernetic standpoint. Over the past decades we have gained a plethora of information from biochemical, clinical, and epidemiological investigation, especially on the role of TSH and other thyrotropic agonists as critical components of this complex relationship. Integrating these data into a systems perspective delivers new insights into static and dynamic behaviour of thyroid homeostasis. Explicit usage of this information with mathematical methods promises to deliver a better understanding of thyrotropic feedback control and new options for personalised diagnosis of thyroid dysfunction and targeted therapy, also by permitting a new perspective on the conundrum of the TSH reference range.

Neutrophil function in hypothyroid patients

We investigated whether some in vitro polymorphonuclear (PMN) granulocyte functions were impaired in patients with hypothyroidism, since this disease has previously been associated with an increased susceptibility to infectious agents and decreased leukocyte heat production. PMNs from 9 of 17 hypothyroid patients exhibited a decreased ability to kill Staph. aureus in vitro compared with euthyroid controls (p less than 0.02). This was normalized in all patients tested after therapy with levothyroxine. In 3 patients, studied repeatedly during the initial phase of therapy, PMN bactericidal capacity was gradually normalized. In addition, PMN adherence to nylon fibres showed a transient decrease approximately 6 weeks after initiation of therapy. PMN chemiluminescence during phagocytosis of Staph. aureus and stimulated and spontaneous migration under agarose were normal and did not change during therapy. Thus, the decreased bactericidal capacity found in half of these hypothyroid patients might confer an increased susceptibility to cerain infectious agents.

Potential therapeutic applications of thyroid hormone analogs

Thyroid hormone (T3 and T4) has many beneficial effects including enhancing cardiac function, promoting weight loss and reducing serum cholesterol. Excess thyroid hormone is, however, associated with unwanted effects on the heart, bone and skeletal muscle. We therefore need analogs that harness the beneficial effects of thyroid hormone without the untoward effects. Such work is largely based on understanding the cellular mechanisms of thyroid hormone action, specifically the crystal structure of the nuclear receptor proteins. In clinical studies, use of naturally occurring thyroid hormone analogs can suppress TSH levels in patients with thyroid cancer without producing tachycardia. Many thyromimetic compounds have been tested in animal models and shown to increase total body oxygen consumption, and to lower weight and serum cholesterol and triglyceride levels while having minor effects on heart rate. Alternatively, analogs that specifically enhance both systolic and diastolic function are potentially useful in the treatment of chronic congestive heart failure. In addition to analogs that are thyroid hormone receptor agonists, several compounds that are thyroid hormone receptor antagonists have been identified and tested. This Review discusses the potential application of thyroid hormone analogs (both agonists and antagonists) in a variety of human disease states.

Thermogenic mechanisms and their hormonal regulation

Increased heat generation from biological processes is inherent to homeothermy. Homeothermic species produce more heat from sustaining a more active metabolism as well as from reducing fuel efficiency. This article reviews the mechanisms used by homeothermic species to generate more heat and their regulation largely by thyroid hormone (TH) and the sympathetic nervous system (SNS). Thermogenic mechanisms antecede homeothermy, but in homeothermic species they are activated and regulated. Some of these mechanisms increase ATP utilization (same amount of heat per ATP), whereas others increase the heat resulting from aerobic ATP synthesis (more heat per ATP). Among the former, ATP utilization in the maintenance of ionic gradient through membranes seems quantitatively more important, particularly in birds. Regulated reduction of the proton-motive force to produce heat, originally believed specific to brown adipose tissue, is indeed an ancient thermogenic mechanism. A regulated proton leak has been described in the mitochondria of several tissues, but its precise mechanism remains undefined. This leak is more active in homeothermic species and is regulated by TH, explaining a significant fraction of its thermogenic effect. Homeothermic species generate additional heat, in a facultative manner, when obligatory thermogenesis and heat-saving mechanisms become limiting. Facultative thermogenesis is activated by the SNS but is modulated by TH. The type II iodothyronine deiodinase plays a critical role in modulating the amount of the active TH, T(3), in BAT, thereby modulating the responses to SNS. Other hormones affect thermogenesis in an indirect or permissive manner, providing fuel and modulating thermogenesis depending on food availability, but they do not seem to have a primary role in temperature homeostasis. Thermogenesis has a very high energy cost. Cold adaptation and food availability may have been conflicting selection pressures accounting for the variability of thermogenesis in humans.

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.

Thyroid hormone control of thermogenesis and energy balance

The mechanisms whereby thyroid hormone increases heat production have been analyzed with emphasis in more recent developments. Thyroid hormone increases obligatory thermogenesis as a result of the stimulation of numerous metabolic pathways involved in development, remodeling, and delivery of energy to the tissues. In addition, thyroid hormone may specifically stimulate some thermogenic mechanisms selected during evolution of homeotherms (e.g., Na/K-ATPase, Ca2+ cycling in muscle). Thyroid hormone also plays an essential role in facultative thermogenesis interacting with the sympathetic nervous system (SNS) at various levels. Peripherally, thyroid hormone potentiates the effects of the SNS at the level of the adrenergic receptor and adenylyl cyclase complex as well as distal from this point. Synergistic interactions between T3 and cAMP on the regulation of gene expression have been described. Brown adipose tissue (BAT) T4-5'-deiodinase plays a central role in controlling heat production. When this enzyme is stimulated by norepinephrine in the euthyroid and hypothyroid condition, it provides high concentrations of T3 to BAT; inhibition by T4 in hyperthyroidism may limit brown fat thermogenic responses. Also, thyrotoxicosis uniquely reduces the expression of beta 3-adrenergic receptors in brown adipose tissue, and the increased obligatory thermogenesis of this condition, via afferent neural pathways, may reduce the hypothalamic stimulation of brown fat, providing additional mechanisms to limit brown adipose tissue thermogenesis in hyperthyroidism.

Impaired action of thyroid hormone associated with smoking in women with hypothyroidism

BACKGROUND

The effect of smoking on thyroid function is controversial, and its effect on thyroid hormone action is unknown. We investigated the effects of cigarette smoking in women with various grades of hypothyroidism and in normal women.

METHODS

We studied 138 normal women and 135 women with primary hypothyroidism, of whom 84 had subclinical hypothyroidism and 51 overt hypothyroidism. Sixty of the women with hypothyroidism were reevaluated during thyroxine therapy. The women were categorized as smokers or nonsmokers according to their responses to a questionnaire. Thyroid function was evaluated by measurements of serum thyrotropin, free thyroxine, and triiodothyronine. Peripheral thyroid hormone action was assessed by a clinical score and measurements of ankle-reflex time and serum lipids and creatine kinase.

RESULTS

Among the women with subclinical hypothyroidism, the smokers had a higher mean (+/- SD) serum thyrotropin concentration (21.3 +/- 16.6 vs. 12.7 +/- 7.2 mU per liter, P = 0.004) and a higher ratio of serum triiodothyronine to serum free thyroxine (by 30 percent, P = 0.003) than the nonsmokers. Their serum concentrations of total cholesterol and low-density lipoprotein (LDL) cholesterol were higher (by 16 percent, P = 0.013; and 28 percent, P = 0.003, respectively). Among the women with overt hypothyroidism, the serum concentrations of thyrotropin, free thyroxine, and triiodothyronine were similar in the smokers and nonsmokers. As compared with the nonsmokers, the smokers had a clinical score indicating a greater degree of hypothyroidism (P < 0.001), higher serum concentrations of total and LDL cholesterol (by 25 percent, P < 0.001; and 24 percent, P = 0.002, respectively), longer ankle-reflex time (by 25 percent, P < 0.001), and higher serum concentrations of creatine kinase (by 236 percent, P < 0.001). There were dose-response relations between smoking and serum concentrations of total and LDL cholesterol, serum creatine kinase concentrations, and ankle-reflex time in the women with overt hypothyroidism, and between smoking and serum concentrations of total and LDL cholesterol in the women with subclinical hypothyroidism.

CONCLUSIONS

Smoking increases the metabolic effects of hypothyroidism in a dose-dependent way. This may be explained by alteration of both thyroid function and hormone action.

Inhibition of Na(+)-K(+)-ATPase by digoxin and its relation with energy expenditure and nutrient oxidation rate

This study investigates the effects of digoxin, an inhibitor of the Na+ pump (Na(+)-K(+)-ATPase), on resting metabolic rate (RMR), respiratory quotient (RQ), and nutrient oxidation rate. Twelve healthy male subjects followed a double-blind protocol design and received either 1 mg/day digoxin or a placebo 2 days before indirect calorimetry measurements. Digoxin induced a 0.22 +/- 0.07 kJ/min or 3.8 +/- 1.5% (mean +/- SE, P = 0.01) decrease in RMR and a 0.40 +/- 0.13 kJ/min (P = 0.01) decrease in fat oxidation rate, whereas carbohydrate and protein oxidation rates did not change significantly. A dose-response relationship between serum digoxin and RQ was observed. These results suggest that digoxin reduces not only RMR but also fat oxidation rate by mechanisms that remain to be elucidated. Because a linkage and an association between genes coding the Na(+)-K(+)-ATPase and the RQ have been previously observed, the present demonstration of an effect of Na(+)-K(+)-ATPase inhibition on fat oxidation rate strengthens the concept that the activity of this enzyme may play a role in body weight regulation.

Cardiolipins and biomembrane function

Evidence is discussed for roles of cardiolipins in oxidative phosphorylation mechanisms that regulate State 4 respiration by returning ejected protons across and over bacterial and mitochondrial membrane phospholipids, and that regulate State 3 respiration through the relative contributions of proteins that transport protons, electrons and/or metabolites. The barrier properties of phospholipid bilayers support and regulate the slow proton leak that is the basis for State 4 respiration. Proton permeability is in the range 10(-3)-10(-4) cm s-1 in mitochondria and in protein-free membranes formed from extracted mitochondrial phospholipids or from stable synthetic phosphatidylcholines or phosphatidylethanolamines. The roles of cardiolipins in proton conductance in model phospholipid membrane systems need to be assessed in view of new findings by Hübner et al. [313]: saturated cardiolipins form bilayers whilst natural highly unsaturated cardiolipins form nonlamellar phases. Mitochondrial cardiolipins apparently participate in bilayers formed by phosphatidylcholines and phosphatidylethanolamines. It is not yet clear if cardiolipins themselves conduct protons back across the membrane according to their degree of fatty acyl saturation, and/or modulate proton conductance by phosphatidylcholines and phosphatidylethanolamines. Mitochondrial cardiolipins, especially those with high 18:2 acyl contents, strongly bind many carrier and enzyme proteins that are involved in oxidative phosphorylation, some of which contribute to regulation of State 3 respiration. The role of cardiolipins in biomembrane protein function has been examined by measuring retained phospholipids and phospholipid binding in purified proteins, and by reconstituting delipidated proteins. The reconstitution criterion for the significance of cardiolipin-protein interactions has been catalytical activity; proton-pumping and multiprotein interactions have yet to be correlated. Some proteins, e.g., cytochrome c oxidase are catalytically active when dimyristoylphosphatidylcholine replaces retained cardiolipins. Cardiolipin-protein interactions orient membrane proteins, matrix proteins, and on the outerface receptors, enzymes, and some leader peptides for import; activate enzymes or keep them inactive unless the inner membrane is disrupted; and modulate formation of nonbilayer HII-phases. The capacity of the proton-exchanging uncoupling protein to accelerate thermogenic respiration in brown adipose tissue mitochondria of cold-adapted animals is not apparently affected by the increased cardiolipin unsaturation; this protein seems to take over the protonophoric role of cardiolipins in other mitochondria. Many in vivo influences that affect proton leakage and carrier rates selectively alter cardiolipins in amount per mitochondrial phospholipids, in fatty acyl composition and perhaps in sidedness; other mitochondrial membrane phospholipids respond less or not at all.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium transport in chicken leukocytes and erythrocytes

1. In the present study, Ca2+ uptake and Ca2(+)-ATPase activity of two different chicken leukocyte populations and erythrocytes isolated from 1- to 6-week-old chickens were determined. 2. The Ca2(+)-ATPase activity of the two leukocyte populations significantly increased at 3 weeks of age. Erythrocyte Ca2(+)-ATPase activity significantly increased at 2 weeks of age. 3. Calcium transport activities into the two leukocyte populations did not differ significantly with age.

3H-ouabain binding sites in porcine skeletal muscle as influenced by environmental temperature and energy intake

The influence of environmental temperature and energy intake on 3H-ouabain binding sites in skeletal muscle has been investigated in young growing pigs at 8 weeks of age. Animals lived for several weeks at 35 or 10 degrees C on a high (H) or low (L) level of energy intake. The four treatment groups were thus: 35H, 35L, 10H and 10L. The total number of 3H-ouabain binding sites (Bmax) in longissimus dorsi muscle (mean values +/- SEM) were 221 +/- 66, 214 +/- 61, 350 +/- 76 and 486 +/- 114 pmol/g wet weight for the 35H, 35L, 10H and 10L groups respectively. Bmax was significantly greater in those living in the cold than the warm (P less than 0.001). Moreover, at 10 degrees C energy intake had a significant effect, with Bmax being greater in the 10L than the 10H group (P less than 0.005). Level of energy intake had no influence on Bmax at 35 degrees C. The apparent dissociation constant was not affected by either temperature or intake. The elevated Bmax and hence the increase in number of Na+,K+-pumping sites in the cold is probably related to increased muscular activity associated with shivering. However, thyroid status also influences the number of Na+,K+-pumping sites and this may have been a contributory factor in the present study. In addition, the elevated Bmax suggests a greater potential for non-shivering thermogenesis associated with increased Na+,K+-ATPase concentration in the cold. Differences in relative stage of development between the four groups may help to explain the results for Bmax in relation to level of energy intake.

Red cell sodium and ionic fluxes in patients with hyper- and hypothyroidism

To investigate the status of the Na+ concentrations [Na+]i, K+ concentrations [K+]i and ionic fluxes in red cells of human subjects with abnormal thyroid function, we measured the Na(+)-K+ pump activity as well as Na(+)-K+ cotransport (CoT), Na(+)-Li+ countertransport (CTT) and Na+ passive permeability in erythrocytes of 37 normal subjects, 19 untreated hyperthyroid patients, 12 treated hyperthyroid patients and 9 hypothyroid patients with T4 replacement. The mean [Na+]i value in the untreated hyperthyroidism group was significantly higher than that in the normal subjects (p less than .05), but not significantly different from that in the treated hyperthyroidism group. The mean [Na+]i value in the hypothyroidism with T4 replacement group, however, was significantly lower than that in the normal group (p less than .01). We did not find any significant difference of [K+]i in comparing each group. It was found that the Na(+)-K+ pump activity in erythrocytes was significantly increased in untreated hyperthyroidism (mean; 23.4% above control, p less than 10(-5], but there was no significant difference in treated hyperthyroidism and hypothyroid patients with T4 replacement. The rate constant for ouabain-sensitive Na+ efflux in the hypothyroidism with T4 replacement group was markedly higher than that in normal subjects (p less than .01), but not significantly different in the untreated hyperthyroidism group. We observed a significant increase of the Na+ CoT value in the patients with untreated hyperthyroidism as compared with that of the normal subjects (p less than .05), but there was no significant difference in the patients treated for hyperthyroidism and the hypothyroidism with T4 replacement group. However, the rate constant for Na(+)-CoT in the patients with hypothyroidism with T4 replacement was significantly higher than that in normal subjects (p less than .05). We observed a marked decrease of Na(+)-Li+CTT value in the patients with untreated hyperthyroidism versus that in the normal group (p less than .01). Passive Na+ permeability in the patients with untreated hyperthyroidism was markedly increased (p less than .05), and was markedly decreased in the patients with hypothyroidism with T4 replacement compared to normal subjects (p less than .01). It can be concluded from these studies that an increase in Na(+)-K+ pump activity in the patients untreated for hyperthyroidism might then be regarded as a secondary adaptive cellular response to higher [Na+]i values due to enhanced passive Na+ permeability, rather than a direct effect of the thyroid hormone.

The haematology of hyperthyroidism: abnormalities of erythrocytes, leucocytes, thrombocytes and haemostasis

The abnormalities of erythrocytes, leucocytes, thrombocytes and coagulation that have been reported, particularly in more recent years, to be associated with hyperthyroidism are surveyed. Several areas are highlighted where further investigations could lead to clinically useful insights, improved information about the haematological processes involved or to a better understanding of thyroid hormone action.

Graves' disease presenting as pyrexia of unknown origin

Fever is a common clinical manifestation of inflammatory processes of the thyroid and thyroid crisis. On the other hand, fever alone as a presenting symptom of thyrotoxicosis, without other manifestations, is extremely rare. A female patient is described in whom fever persisted for two months prior to hospitalization, but without clinical symptoms or signs to lead to suspicion of thyroid disease. After exhaustive investigation it was found that the patient was suffering from hyperthyroidism. Fever disappeared gradually on antithyroid therapy, recurred when the drugs were withdrawn for a rechallenge trial, and cleared up again after renewal. Four other cases of persistent fever as a presenting symptom of hyperthyroidism were found on a review of previous publications. Thyrotoxicosis should, therefore, be included in the differential diagnosis of pyrexia of unknown origin.

Effects of hypothyroidism and hypoparathyroidism on rat myocardium: mechanical and electrical alterations

The mechanical and electrical effects of hypoparathyroidism (Px), hypothyroidism (Tx), and hypothyroidism combined with hypoparathyroidism (TPx) were investigated by comparing simultaneously recorded transmembrane action potentials and isometric and isotonic contractions recorded from the myocardium. Left ventricular papillary muscles from male Wistar rats were studied electrically and mechanically in a muscle bath at 30 degree C, stimulated at 0.1 Hz and external calcium = 2.4 mM. No significant difference was found between control (C), Px, Tx, and TPx preparations with regard to resting tension and developed tension. However, time to peak tension, time to one half relaxation and time to peak shortening were significantly increased in preparations from animals that were Px, Tx, and TPx as compared with age-matched controls. Maximum velocities of shortening (Vs) and relengthening (Vr) at all relative loads studied were significantly depressed in Px preparations when compared with those of C muscles. A greater depression was found in the Tx muscle and still greater depression of these indices was noted in TPx muscles. No significant difference was found between C, Px, Tx, and TPx action potential with regard to resting membrane potential (RMP), action potential amplitude (AMP), overshoot (OS), or maximum rate of rise of the upstroke (Vmax).(ABSTRACT TRUNCATED AT 250 WORDS)

Altered erythrocyte Na-K pump in anorectic patients

The status of the erythrocyte sodium pump was evaluated in a group of patients suffering from anorexia nervosa and a group of healthy female control subjects. Anorectic patients showed significantly higher mean values of digoxin-binding sites/cell (ie, the number of Na-K-ATPase units) with respect to control subjects while no differences were found in the specific 86Rb uptake (which reflects the Na-K-ATPase activity) between the two groups. A significant correlation was found between relative weight and the number of Na-K-ATPase pump units (r = -0.66; P less than 0.0001). Anorectic patients showed lower serum T3 concentrations (71.3 +/- 53 ng/dL) with respect to control subjects (100.8 +/- 4.7 ng/dL; P less than 0.0005) and a significant negative correlation between T3 levels and the number of pump units (r = -0.52; P less than 0.003) was found. Our study therefore shows that the erythrocyte Na-K pump may be altered in several anorectic patients. We suggest that this feature could be interrelated with the degree of underweight and/or malnutrition.

The plasma membrane sodium-hydrogen exchanger and its role in physiological and pathophysiological processes

The plasma membranes of most if not all vertebrate cells contain a transport system that mediates the transmembrane exchange of sodium for hydrogen. The kinetic properties of this transport system include a 1:1 stoichiometry, affinity for lithium and ammonium ion in addition to sodium and hydrogen, the ability to function in multiple 1:1 exchange modes involving these four cations, sensitivity to inhibition by amiloride and its analogues, and allosteric regulation by intracellular protons. The plasma membrane sodium-hydrogen exchanger plays a physiological role in the regulation of intracellular pH, the control of cell growth and proliferation, stimulus-response coupling in white cells and platelets, the metabolic response to hormones such as insulin and glucocorticoids, the regulation of cell volume, and the transepithelial absorption and secretion of sodium, hydrogen, bicarbonate and chloride ions, and organic anions. Preliminary evidence raises the possibility that the sodium-hydrogen exchanger may play a pathophysiological role in such diverse conditions as renal acid-base disorders, essential hypertension, cancer, and tissue or organ hypertrophy. Thus, future research on cellular acid-base homeostasis in general, and on plasma membrane sodium-hydrogen exchange in particular, will enhance our understanding of a great variety of physiological and pathophysiological processes.

Hyperthyroid heart disease

The importance of cardiovascular system involvement in hyperthyroidism has been recognized for many years. In the middle-aged and elderly patient, often with mild but prolonged elevation of plasma thyroid hormones, symptoms and signs of heart failure and complicating atrial fibrillation may dominate the clinical picture and mask the more classical endocrine manifestations of the disease. Pitfalls in diagnosis and the importance of early recognition and treatment are discussed. Despite experimental evidence for a short-term inotropic action of thyroid hormone excess, clinical data support the existence of a reversible cardiomyopathy in hyperthyroidism with impaired contractile reserve. Enhanced myocardial performance at rest primarily reflects the peripheral actions of thyroid hormone excess. Most, if not all, of the cardiac abnormalities return to normal once a euthyroid state has been achieved, although atrial fibrillation may persist in a minority. Optimum treatment requires rapid and definitive antithyroid therapy, usually using a large dose of radio-iodine, and rapid control of heart failure. Systemic anticoagulation is indicated in the presence of atrial fibrillation and should be continued until sinus rhythm has been present for at least three months, either spontaneously or after cardioversion.

Regulation of volume reabsorption by thyroid hormones in the proximal tubule of rat: minor role of luminal sodium permeability

In order to investigate whether changes in luminal membrane sodium permeability can explain the increase in isotonic fluid reabsorption (Jv) found in proximal tubules of thyroidectomized rats (TX) treated with tri-iodothyronine (T3), experiments were carried out on TX rats and TX rats treated for 3 days (TX + T3) with physiological doses (10 micrograms/kg body wt) of T3. Two sets of experiments were performed: 1) in vivo, using the micropuncture technique for the measurements of Jv; 2) in vitro, using isolated brush border membrane vesicles for the direct measurement of Na+ permeability. In micropuncture studies a 65% increase in Jv of TX rats was observed after treatment with T3. Luminal perfusion of proximal tubules of TX rats with Amphotericin B (10 micrograms/ml), to increase luminal sodium permeability, enhanced Jv only by 15%. Brush border membrane vesicles isolated from TX and TX + T3 rats showed the same sodium permeability in uptake or efflux experiments. These results were confirmed by the fact that sodium gradient dependent histidine transport into brush border membrane vesicles did not change after T3 treatment. Finally, measuring the amiloride sensitive sodium uptake, it was also found that Na+-H+ exchange was also only slightly affected by T3. These micropuncture and vesicle data indicate that the large effect of T3 on the trans-cellular sodium transport and volume reabsorption in the proximal tubule, cannot be explained by an action of T3 on the sodium entry step across the brush border membrane.

The use of micropuncture, isolated tubule, and vesicle technique in the study of the action of thyroid hormones on the proximal tubule function

In hypothyroid rats (TX), the isotonic fluid reabsorption (Jv), that is closely linked to the transepithelial sodium transport (JNa), is impaired. The administration of physiological doses (10 micrograms/kg body weight per day) of tri-iodothyronine (T3) doubles Jv in three days (TX+T3). This phenomenon could be explained by several mechanisms: a direct stimulation of Na-K-ATPase, an increase in the Na+ entry step, changes in the permeability properties of the luminal and/or basal lateral membranes. Using a kinetic microassay, Na-K-ATPase activity was measured in early (S1) and late (S2) proximal tubules segments isolated from control, TX, and TX+3T3 animals. In TX rats the enzyme activity was lower (70%) in both segments versus control rats, it remained unchanged after 3 days, and it increased after 7 days of T3 substitution. The Na+ permeability of brush border membrane (BBM) vesicles isolated from TX and TX+T3 rats was identical. However the valuation of the K+ membrane permeability by in vivo perfusion of the lumen and peritubular space of proximal tubules of TX rats, with perfusate containing the K+ ionophore valinomycin (1 microgram/ml), induced a significant increase in Jv that accounted for 40% of that elicited by T3. Taken together, the in vivo and in vitro experiments suggest that the early effect on Jv of physiological doses of T3 cannot be explained by a direct action of T3 either on the Na+ entry step across the BBM or on the Na+ exit step (i.e., the Na-K-ATPase), but rather by an increase in K+ permeability of proximal tubular cell membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of thyroid hormone on sodium pump sites, sodium content, and contractile responses to cardiac glycosides in cultured chick ventricular cells

Sensitivity of cardiac muscle to digitalis glycosides depends on the thyroid state. The mechanism of this interaction was investigated at the cellular level using spontaneously beating monolayers of cultured chick embryo ventricular cells. Cells were grown for 48 h in serum-free medium containing concentrations of triiodothyronine (T3) from zero to 10(-7) M, and the total number of sodium pump sites, sodium content, and contractile amplitude in the presence and absence of various concentrations of ouabain were determined. T3 caused a concentration-dependent increase in the number of specific ouabain binding sites; the maximal increase to 160% of control was observed in response to 10(-8) M T3. T3 lowered steady-state cellular sodium content in a concentration-dependent manner, also. Ouabain (1 microM) exposure elevated cellular sodium content in all cells, but the increase was greatest in cells grown in T3-free medium and least in cells grown in 10(-8) M T3. The positive inotropic and toxic effects of ouabain in cells grown in 10(-8) M T3 were diminished at any given ouabain concentration, and thus, the dose-response curve was shifted to the right. These results indicate that T3 causes induction of additional sodium pump sites that are functional. The increased tolerance of hyperthyroid cells and reduced tolerance of hypothyroid cells to cardiac glycosides can be explained by these changes in the number of sodium pump sites and cellular sodium content, and consequently, calcium influx via sodium-calcium exchange.

Stimulation of rat renal Na+, K+-ATPase activity by thyroid hormones

The effect of thyroid hormones (T4, T3 and reverse T3) on rat renal Na+,K+-ATPase activity was investigated by a cytochemical technique. T3 caused stimulation of Na+,K+-ATPase activity in the renal medulla but not in the renal cortex. There was a peak in enzyme activity after cultured renal segments had been exposed to T3 for 11 min and this time of maximal stimulation did not vary with the concentration of T3. A rectilinear response in Na+,K+-ATPase activity was observed over T3 concentration range 10 pmol l-1 to 100 nmol l-1; at higher T3 concentrations, Na+,K+-ATPase activity was inhibited. The enzyme response was totally blocked by specific T3 antiserum. Addition of T4 and reverse T3 (100 fmol l-1 -1 mmol l-1) failed to stimulate Na+,K+-ATPase activity in any part of the kidney. Plasma (neat and diluted 1:10) stimulated the enzyme in parallel with the dose response curve and the stimulatory effect was abolished by prior addition of specific T3 antiserum.

Cold stress and congenital adrenal hyperplasia heterozygotes

Congenital adrenal hyperplasia due to 21-hydroxylase deficiency is common in certain regions of the world characterised by cold winters. The persistence of this potentially lethal recessively inherited disease suggests that an evolutionary advantage is conferred upon the partially affected heterozygotes. Profound hypothermia following acute cold exposure in normal subjects carries a considerable mortality especially from cardiac arrhythmias and dehydration. A hypothesis is proposed to suggest that the incomplete block at the 21-hydroxylase step of steroid biosynthesis decreases stress-induced steroid responses, especially in the younger heterozygote and clinically non-salt-losing homozygote; glucocorticoid and mineralocorticoid insufficiency reduces the severity of cold-induced water diuresis; prevention of severe dehydration contributes towards the ability to survive profound hypothermia. Studies into the salt and water metabolism of the congenital adrenal hyperplasia heterozygote at various ages as well as examination of antidiuretic hormone and steroid hormone interactions upon the renal tubule in cold-exposed normal individuals are merited.

Inverse relationship between ouabain sites on human erythrocytes and body mass index in normal healthy subjects

Binding of [3H]ouabain by erythrocytes and levels of (Na+ + K+) ATPase, cell sodium and potassium, and plasma triiodothyronine (T3) were studied in 16 normal, apparently healthy men. The subjects were between 28 and 50 years of age. Based on their economic status, the subjects were divided into two groups of eight each. Group A consisted of subjects with a higher economic status; these subjects had a mean body mass index (weight/height2) of 24.1 +/- 1.2 (mean +/- SEM). Group B comprised subjects with a lower economic status; these subjects had a mean body mass index of 18.8 +/- 1.0. The number of ouabain binding sites per red cell in group B was higher (466 +/- 29) compared with that in group A (348 +/- 21; P less than 0.01). The dissociation constant (Kd) between the two groups was not different. The (Na+ + K+) ATPase activity per 10(10) cells in group B (0.50 +/- 0.07) was higher compared with that in group A (0.29 +/- 0.03). The increased number of pump sites and increased enzyme activity in group B were associated with a lower [Na]-to-[K] ratio in the cell (P less than 0.01). The plasma T3 level in group A (202 +/- 16.7 ng/dL) was not statistically different from that of group B (163 +/- 17.6 ng/dL). The number of sodium pump sites showed an inverse relationship with body mass index (r = -0.55; P less than 0.05) and with the cell [Na]-to-[K] ratio (r = -0.74; P less than 0.01). The number of pump sites in group A showed a positive correlation (r = 0.60) with plasma T3 levels. Such a relationship was, however, weak in group B (r = 0.36). The results lead to the conclusion that there was increased utilization of available cell energy by sodium pump activity in the subjects in group B. This may be a physiologic adaptation for efficient utilization of ingested nutrients via the sodium pump in response to marginal nutrient deficits in these subjects. Dissociation of increased pump sites from plasma T3 levels may mean that the adaptive phenomenon does not represent a wasteful loss of energy.

Status of the red cell Na,K-pump in hyper- and hypothyroidism

To investigate the status of the sodium-potassium pump in cells of human subjects with abnormal thyroid function, we measured the number of pump units as well as the cation transport activity of the pump in erythrocytes from 23 hyperthyroid and 7 hypothyroid patients. It was found that the number of Na+-K+-ATPase units in erythrocytes (as measured by ouabain binding) was significantly reduced in hyperthyroidism (mean 36% below controls, p less than 0.001). The rate of rubidium uptake by the same cells was also reduced, but to a smaller extent (mean 9%, p less than 0.02). These changes were reversible with control of the hyperthyroidism. Hypothyroid individuals showed changes in erythrocyte Na+-K+ pump which were in the opposite direction to those seen in hyperthyroidism. It is concluded that thyroid hormone exerts a marked negative influence on the number of Na+-K+ pump units in one easily available human cell type. The direction of the effect suggests a complex relationship between thyroid hormone and the level of the Na,K-ATPase in any one tissue. Whatever the cellular mechanism responsible for the effects observed in the red cell, these changes should provide a measure of thyroid hormone action at a cellular level and this may prove useful in the study of thyroid hormone physiology in man.

Age-dependence of the stimulatory effect of triiodothyronine on the DNA synthesis of rat hepatocytes

The effect of a single subcutaneous injection of triiodothyronine (T3) on DNA synthesis rate and liver growth was studied in young and old female Wistar rats. The hormonal treatment was effective in stimulating DNA synthesis, resulting in an increase of the specific liver volume as well as volume and numerical density of nuclei/100 g body wt. in young animals. In the old animals T3 injection induced a reduced and delayed synthesis of DNA, which did not affect the morphometric parameters of liver/100 g body wt.

Lack of stimulation of renal (Na+ +K+)-ATPase by thyroid hormones in the rabbit

The effect of 1-3,5,3'-triiodothyronine (T3) and thyroxine (T4) on (Na+ +K+)-ATPase activities was examined in rabbit kidneys because in this tissue almost 80% of the metabolism is connected to active sodium transport. T3-receptor concentrations were estimated as 0.62 and 0.80 pmol/mg per DNA in the cortex and outer medulla, respectively. A dose of 0.5 mg T3/kg body weight for 3 days increased basal metabolic rate by almost 60%, and the mitochondrial 1-alpha-glycerophosphate dehydrogenase activity was increased by 50% in both the cortex and medulla. (Na+ +K+)-ATPase activity in the liver was raised by almost 50%. However, no changes in (Na+ +K+)-ATPase activities or binding sites for [3H]ouabain in either the kidney cortex or medulla could be observed. T4 at 16 mg/kg daily for 14 days was also without effect on renal (Na+ +K+)-ATPase activities. Furthermore, the response to T3 was absent at high sodium excretion rates induced by unilateral nephrectomy and extracellular volume expansion. Thus, despite stimulation of basal metabolic rate and renal 1-alpha-glycerophosphate dehydrogenase activity by T3 and T4, the (Na+ +K+)-ATPase activity in the rabbit kidney is identical in euthyroid and hyperthyroid states. However, thyroid hormones prevent the normal natriuretic response to extracellular volume expansion.

An expanded concept of "insurance" supplementation--broad-spectrum protection from cardiovascular disease

The preventive merits of "nutritional insurance" supplementation can be considerably broadened if meaningful doses of nutrients such as mitochondrial "metavitamins" (coenzyme Q, lipoic acid, carnitine), lipotropes, and key essential fatty acids, are included in insurance supplements. From the standpoint of cardiovascular protection, these nutrients, as well as magnesium, selenium, and GTF-chromium, appear to have particular value. Sophisticated insurance supplementation would likely have a favorable impact on many parameters which govern cardiovascular risk--serum lipid profiles, blood pressure, platelet stability, glucose tolerance, bioenergetics, action potential regulation--and as a life-long preventive health strategy might confer substantial benefit.

The structure and function of brown adipose tissue in the neonate

Presented is a discussion of brown adipose cell structure as compared to that of the white adipose cell, the role which brown adipose tissue (BAT) plays in thermoregulation, the physiologic mechanisms which control BAT heat generation, and the proposed biochemical processes which are responsible for heat production in the cell. Based on the physiology of BAT thermogenesis, specific nursing measures to minimize cold stimuli are recommended.

Hypothyroid myopathy. A clinical and pathologaical study

Ten patients with varying degrees of hypothroid myopathy were studied clinically and by serial percutaneous needle muscle biopsies before and during treatment with L-thyroxine. The biochemical evidence of hypothyroidism was related to the severity of the myopathic and signs before treatment. The severity of myopathic symptoms before and during treatment correlated with the biochemical evidence of hypothyrodism, a type II fibre atrophy and increased central nuclear counts. Likewise, the clinical evidence of a myopathy before and during treatment was correlated with both a type II fibre atrophy and loss and increased central nuclear counts but was not related to the biochemical parameters of hypothyroidism, except the level of thyroid stimulating hormone. In the muscle, before and during treatment, of the two most severely affected patients, intracellular glycogen inclusions were seen in scattered muscle fibres. On light microscopy and on electronmicroscopy, numerous mitochondria were seen responding to L-thyroxine with accumulations of subsarcolemmal honey-combing. Vesicular abnormalities, an electron dense matrix or occasional crystalline deposits were seen in muscle mitochondria from less severely azffected patients. Severely myopathic muscle contained excessive glycogen, membrane bound glycogen and excess lipid in a mainly perinuclear distribution. Occasional myelin and membranous bodies were seen and satellite cells during the recovery phase. A group of patients with hypothyroid myopathy who are likely to have a delayed recovery of full muscle strength on L-thyroxine may be recognised by the presence of severe proximal muscle weakness and characteristic changes on histochemical and electronmicroscopic examination of muscle. The spectrum of histochemical and electronmicroscopic abnormalities of muscle revealed with increasing degree of hypothyrodism, suggests that a generally reversible acquired glycogen storage and mictochondrial disorder is an important feature in the pathogenesis of this condition.

The relation between thyroid function and human regulatory nonshivering thermogenesis

By a standardized exposure to cold air under simultaneous vasodilation, it was possible to elucidate the chemical factors responsible for heat production independent of muscular activity in seven patients with thyrotoxicosis and in nine patients with hypothyroidism. The results showed that hyperthryroid patients have a thermoregulatory reserve equal to normal persons, as they were able to increase their oxygen consumption significantly by an average of 0.035 mmol . kg-1 . min-1 (16%) (range: 0.014--0.064). The plasma concentration of non-esterified fatty acids (NEFA) increased significantly by an average of 0.675 mmol/l (64%) (range: 0.015--2.220). The hypothyroid patients also increased their oxygen consumption significantly by an average of 0.032 mmol . kg-1 . min-1 (30) (range: -0.016--0.066), but in this group the normal fat mobilization was inhibited as no rise in plasma NEFA was seen. Administration of exogenous thyroxine restored thermoregulation to the level and pattern seen in normothyroid persons, as the increase in plasma NEFA after treatment was on average 0.483 nnol/l (192%) (range: 0.342--0.733).