Interaction of thyroid status and diet on muscle protein breakdown in the rat, as measured by N tau-methylhistidine excretion

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.

Effects of long-term growth hormone (GH) and triiodothyronine (T3) administration on functional hepatic nitrogen clearance in normal man

BACKGROUND/AIMS

A decline in urea excretion is seen following long-term growth hormone administration, reflecting overall protein anabolism. Conversely, hyperthyroidism is characterized by increased urea synthesis and negative nitrogen metabolism. These seemingly opposite effects are presumed to reflect different actions on peripheral protein metabolism. The extent to which these hormonal systems have different direct effects on hepatic urea genesis has not been fully characterized.

METHODS

We measured urea nitrogen synthesis rates and blood alanine levels concomitantly before, during, and after a 4-h constant intravenous infusion of alanine (2 mmol.kg bw-1.h-1). Urea nitrogen synthesis rate was estimated hourly as urinary excretion corrected for gut hydrolysis and accumulation in body water. The slope of the linear relationship between urea nitrogen synthesis rate and alanine concentration represents the liver function as to conversion of amino-N, and is denoted the functional hepatic nitrogen clearance. Eight normal male subjects (age 21-27 years; body mass index 22.4-27.0 kg/m2) were randomly studied four times: 1) after 10 days of subcutaneous saline injections, 2) after 10 days of subcutaneous growth hormone injections (0.1 IU/kg per day), 3) after 10 days of triiodothyronine administration (40 micrograms on even dates, 20 micrograms on uneven dates) and 4) after 10 days given 2)+3). All injections were given at 20 00 h.

RESULTS

Growth hormone decreased functional hepatic nitrogen clearance (l/h) by 30% (from 33.8 +/- 3.2 l/h (control) to 23.8 +/- 1.5 l/h (10 days growth hormone) (mean +/- SE) (ANOVA; p < 0.01)). Triiodothyronine did not change functional hepatic nitrogen clearance (36.7 +/- 3.2 l/h), but triiodothyronine given together with growth hormone abolished the effect of growth hormone functional hepatic nitrogen clearance (38.8 +/- 4.8 l/h).

CONCLUSIONS

The results show that long-term growth hormone administration acts on liver by decreasing functional hepatic nitrogen clearance, thereby retaining amino-N in the body. Triiodothyronine has no effect on functional hepatic nitrogen clearance, but given together with growth hormone, it abolishes the effect of growth hormone on functional hepatic nitrogen clearance. A possible mechanism is the known effect of thyroid hormones in reducing the bioavailability of insulin-like growth factor-I. Thus, the effects of growth hormone and triiodothyronine on amino-N homeostasis are interdependent and to some extent exerted via interplay in their regulation of liver function as to amino-N conversion.

Hepatic conversion of amino nitrogen to urea nitrogen in hypothyroid patients and upon L-thyroxine therapy

Conflicting studies have been reported regarding the influence of thyroid hormones on hepatic nitrogen metabolism and liver metabolic activity. We studied urea N synthesis rate (UNSR), functional hepatic N clearance (FHNC), galactose elimination capacity, and antipyrine clearance in six hypothyroid female patients before and after achievement of a stable euthyroid status. In both conditions, UNSR measured at intervals in response to constant alanine infusion was linearly related to the average alpha-amino N concentrations. In the hypothyroid state, peak UNSR was decreased by 31% in comparison with values measured in euthyroidism, which were in the normal range. FHNC (ie, the slope of the linear relation between UNSR and blood alpha-amino N concentration) is a measure of the kinetics of the process of hepatic amino N to urea N conversion; it was 19.8 +/- 4.0 L.h-1 in hypothyroid patients and increased to normal values after L-thyroxine replacement (30.4 +/- 3.3 L.h-1, P < .01; normal values > 25 L.h-1). Hepatic microsomal and cytosolic activities (antipyrine clearance and galactose elimination) were normal in hypothyroid patients and did not change significantly after therapy. Our data show a specific defect in hepatic handling of amino acids in hypothyroid patients, leading to reduced alpha-amino N to urea N conversion, in the absence of any detectable impairment in different hepatic metabolic activities.

Thyroid function during isoflurane anesthesia and valvular heart surgery

Proper thyroid function is essential for maintaining cardiovascular integrity during normal and stressful situations. In this study, the effects of isoflurane-O2 anesthesia and surgical stress on serum TSH, T4, free T4, T3, rT3, and cortisol were investigated in nine patients before, during, and after valve surgery. Compared with preoperative control values, serum TSH decreased in the postoperative period. Both T4 and free T4 had similar decreases after cardiopulmonary bypass (CPB) and remained depressed postoperatively. Both T3 and rT3 decreased at the start of cardiopulmonary bypass; T3 remained low in the postoperative period, while rT3 increased. Cortisol decreased during anesthesia and surgery in the prebypass period, but increased during cardiopulmonary bypass and in the postoperative period. The results suggest that isoflurane-O2 anesthesia during valve surgery produces a rapid decrease in T3, resulting in the low T3 syndrome postoperatively. Isoflurane, in the dose studied, similar to fentanyl, can suppress the cortisol response to anesthesia and surgery in the prebypass period, but not during and after CPB.

Influence of clofibrate on thyroid hormone and muscle protein turnover

Clofibrate, a hypolipidemic agent, has been shown to increase muscle protein degradation. The possible role of thyroid hormones in this phenomena was examined. Clofibrate treatment of rats for 2 weeks resulted in a significant decrease in total thyroxine and triiodothyronine levels in serum. Reverse T3 and resin uptake values remained unchanged. When exogenous thyroxine was co-administered with clofibrate, serum TSH levels were suppressed, but the increased muscle protein degradation was not reversed. Equilibrium dialysis and Scatchard analysis of the binding of 125I-thyroxine to serum proteins indicated that clofibrate competitively inhibits the binding of thyroid hormone to serum proteins by decreasing its apparent binding affinity. In the presence of lower total thyroid hormone concentrations and an elevated free thyroxine fraction, the total free hormone levels are estimated to be in the normal range in the serum of clofibrate treated rats. Clofibrate seems to act like thyroid hormone since it binds to and displaces T4 from plasma proteins. Because free thyroid hormone levels are in the normal range, the thyroid hormone-like effects of clofibrate on the cell may be additive to the T4 effects, and are probably responsible for the hypermetabolic state seen in the muscle of clofibrate-treated animals. Our data suggest that the effects of clofibrate in muscle are complex. In addition to competitively altering the binding of thyroxine to serum proteins, this substance may also exert a hitherto unrecognized thyroid-hormone-like subcellular effect resulting in increased muscle protein degradation, and in augmented ouabain-sensitive ATPase activities.

Interactive effects of insulin and corticosterone on myofibrillar protein turnover in rats as determined by N tau-methylhistidine excretion

The effects of graded doses of insulin and corticosterone on myofibrillar protein turnover were investigated in growing diabetic rats in order to assess their counteractive roles in the control of protein accretion. N tau-Methylhistidine excretion and carcass protein accretion were measured over 6 days in streptozotocin-diabetic rats receiving either a constant catabolic dose of corticosterone accompanied by graded doses of insulin or a constant dose of insulin accompanied by graded doses of corticosterone. The high corticosterone dose decreased the rate of protein accretion by both increasing the rate of degradation and decreasing the rate of synthesis. Increasing insulin dosage counteracted these effects, but could not restore positive accretion rates. Direct measurement of protein-synthesis rates gave results comparable with those obtained from use of N tau-methylhistidine excretion. At constant insulin dosage, increased corticosterone to 45 mg/kg body wt. per day caused a dose-related linear decrease in protein accretion rates from +4.5 to -3.2% per day. Growth ceased at 28 mg of corticosterone/kg body wt. per day, largely owing to a fall in synthesis rates (-3.5%/day) rather than the increase in degradation rates (+1.0%/day). However, at steroid doses greater than 30 mg/kg body wt. per day the degradation rate increased markedly and accounted for most of the additional fall in accretion. These results show that insulin antagonizes the action of glucocorticoids on both the synthesis and degradative pathways of myofibrillar protein turnover. The changes in fractional degradation rates appear relatively more attenuated by insulin than are those of synthesis.

Modification of glucocorticoid-induced changes in myofibrillar protein turnover in rats by protein and energy deficiency as assessed by urinary excretion of Ntau-methylhistidine

The effects of differing degrees of experimental protein-energy malnutrition on the response of myofibrillar protein turnover rates to administration of corticosteroid has been studied in two experiments on rats. The basal control diet, offered ad lib. in each case, contained 40 g protein/kg, and other groups received diets containing 62 X 5, 95 or 220 g protein/kg at 0 X 67, 1 or 1 X 5 times the level of the control energy intake. Daily administration of 25 or 30 mg corticosterone/kg body-weight after 18 d pre-feeding caused an increase in plasma protein, glucose and insulin concentrations, but a decrease in the corticosterone: insulin values. Liver size and protein content increased, as did the fractional excretion of dietary nitrogen as urea-N in all treated groups. However, whereas a fall in food intake and body-weight occurred in one experiment the reverse occurred in the other. Ntau -Methylhistidine excretion was 12% lower for rats receiving 40 v. 220 g protein/kg diet and excretion was increased by only 57 v. 90% respectively, when the two groups of rats were given 30 mg corticosterone/kg per d. Rats which received 25 mg corticosterone/kg per d and up to 95 g protein/kg diet increased excretion of Ntau -methylhistidine by an average 35%. The fractional degradation rate of myofibrillar protein (kd) was reduced by about 10% by the low-protein diet from 3 X 1 to 2 X 8%/d. During corticosterone treatment the increment in kd for rats on this diet was only 60% of that for rats receiving the 220 g protein/kg diet, i.e. an increase of 1 X 8 v. 3 X 0%/d. Energy restriction further reduced kd during low-protein intake but did not affect the response to the corticosterone. Variations in dietary protein from 40 to 95 g/kg had little effect on the increase in kd during steroid treatment. The effect of corticosterone on calculated synthesis rates (ks) differed markedly between experiments. While ks fell by 50-65% in rats which lost weight on treatment, it rose by up to 60% in rats where carcass non-collagen-protein accretion remained unchanged or increased, despite an increase in kd. Protein deficiency decreases the catabolic response to glucocorticoid, but the net metabolic response appears crucially dependent on changes in food intake or the stage of growth of the rat or both. A net anabolic response with increased fractional rates of myofibrillar protein breakdown, synthesis and accretion was observed in growing rats fed on relatively-low-protein diets and given 25 mg corticosterone/kg per d.(ABSTRACT TRUNCATED AT 400 WORDS)

Thyroid hormone action on intermediary metabolism. Part III. Protein metabolism in hyper- and hypothyroidism

In their physiological concentrations, thyroid hormones stimulate the synthesis as well as the degradation of proteins, whereas in supraphysiological doses protein catabolism predominates. In hyperthyroidism skeletal muscle protein stores suffer depletion which is reflected by an increased urinary N- and methylhistidine -excretion. Due to the enhanced skeletal muscle amino acid release, the plasma concentration of glucoplastic amino acids are often enhanced, contributing by means of an elevated substrate supply to the increased hepatic gluconeogenesis. Thyroid hormone excess induces cardiac hypertrophy which is in direct contrast to the hypotroph skeletal muscle in hyperthyroid patients. Thyroid hormones stimulate a series of intracellular and secretory proteins in the liver, although in hyperthyroid liver alcohol dehydrogenase and the enzymes of histidine and tryptophan metabolism show reduced activities. The stimulatory effect is due to thyroid hormone-induced increase in the protein synthesis at a pretranslational level and is supported experimentally for malic enzyme, alpha 2u-globulin and albumin by the measurement of their specific messenger RNA activities. Thyroid hormone action at the cellular level is reflected by a generalized increase in total cellular RNA with a selective increase or decrease in a small population of specific mRNA. The activities of protein catabolizing lysosomal enzymes are stimulated by thyroid hormones; up to now effects of T3 on the degradation of specific enzymes have not been reported. Serum total protein concentration is slightly reduced or even unchanged in hyperthyroidism. The thyroid hormone-induced increase in the turnover of total body protein is part of the hypermetabolism observed in hyperthyroidism.

Effect of a protein-free diet on muscle protein turnover and nitrogen conservation in euthyroid and hyperthyroid rats

Although protein turnover in skeletal muscle is increased in hyperthyroidism and decreased in hypothyroidism, a deficient protein intake tends to increase serum T3 (tri-iodothyronine) while decreasing muscle protein turnover. To determine whether this diet-induced decrease in protein turnover can occur independent of thyroid status, we have examined muscle protein turnover and nitrogen conservation in hyperthyroid rats fed on a protein-free diet. After inducing hyperthyroidism by giving 20 micrograms of T3/100g body wt. daily for 7 days, groups of euthyroid and hyperthyroid animals were divided into subgroups fed on basal and protein-free diets. Muscle protein turnover was measured by N tau-methylhistidine excretion and [14C]tyrosine infusion. Urinary nitrogen output of euthyroid and hyperthyroid animals fed on the protein-free diet was also measured. Although hyperthyroidism increased the baseline rates of muscle protein synthesis and degradation, it did not prevent a decrease in these values in response to protein depletion. Furthermore, hyperthyroid rats showed greatly decreased nitrogen excretion in response to the protein-free diet, although not to values for euthyroid rats. These findings suggest that protein depletion made the experimental animals less responsive to the protein-catabolic effects of T3.

Thyroid status and muscle protein breakdown as assessed by urinary 3-methylhistidine excretion: study in thyrotoxic patients before and after treatment

Urinary 3-methylhistidine (3MH) excretion was studied in nine thyrotoxic patients before and after treatment. Urinary creatinine (Cr) output was also measured and was low in the thyrotoxic subjects before treatment. Thus, although urinary output of 3MH was not greater than among the control population when expressed per subject, it was significantly elevated when expressed as the ratio of 3MH to Cr; this ratio fell significantly, reaching normal control values after a euthyroid state was obtained. In one patient who became hypothyroid, the 3MH/Cr ratio fell under the control value. There was a significant linear correlation between the 3MH/Cr ratio and the hormonal variables (T3, T4, FT4l); moreover, variations in the 3MH/Cr ratio and variations in the T3 level were closely correlated. 3-Methylhistidine appears to be a reliable index of muscular breakdown in thyrotoxicosis. From our results, it can be concluded, first of all, that hyperthyroidism is accompanied by an increased muscular catabolism, and, second, that the return to a euthyroid state results in an immediate normalization of muscular breakdown.

Direct anabolic effects of thyroid hormone on isolated mouse heart

The direct effects of L-and D-triiodothyronine (T3) on cardiac protein metabolism were investigated using fetal mouse hearts in organ culture. This model allowed the production of "thyrotoxicosis" in isolated hearts in vitro in the absence of the usual systemic metabolic and hemodynamic effects of thyroid hormones. Hearts were studied during the first 24 h of T3 exposure in culture, before changes in beating rate due to T3 occurred. Phenylalanine release was decreased by 26 +/- 2.3% (P less than 0.001) by the optimal concentrations of T3 (10(-7) to 10(-6) M). Changes were similar in the presence or absence of insulin. D-T3 was also anabolic, decreasing phenylalanine release by 24 +/- 2.5% (P less than 0.001) at concentrations of 10(-6) to 10(-5) M. The L-isomer increased protein synthesis by 23 +/- 6.8% (P less than 0.05) and decreased protein degradation, as measured by phenylalanine release in the presence of cycloheximide, by 5 +/- 1.6% (P less than 0.01). The D-isomer also increased protein synthesis but had no measurable effect on protein degradation. We conclude that thyroid hormones can exert direct anabolic effects on heart in the absence of systemic hemodynamic and metabolic changes. These effects are mediated primarily through an acceleration of the rate of protein synthesis; in the case of L-T3, a small inhibition of proteolysis may also occur.

Dose response of protein turnover in rat skeletal muscle to triiodothyronine treatment

Skeletal muscle protein turnover has been examined in thyroidectomized rats treated with 0, 0.3, 0.75, 2, 20 and 100 micrograms triidothyronine/day for 7 days by implanted osmotic minipump. Protein synthesis in gastrocnemius, plantaris and soleus muscle were measured in vivo by the constant infusion method and protein degradation estimated as the difference between gross and net rates of synthesis. Serum levels of triidothyronine (T3) and insulin were also measured in addition to oxygen consumption rates in some cases. Compared with untreated intact rats muscle growth rates were unchanged at 0.3, 0.75 and 2 micrograms T3/day and, judging by plasma T3 levels, 0.75 microgram T3/day was a replacement dose. Slowing of growth was evident in the untreated thyroidectomized rats mid-way through the 7 day experimental period (6-7 days after throidectomy). High doses of T3 (20 and 100 micrograms/day) promptly supressed growth but there was subsequent recovery. Protein synthesis and degradation were generally lower in the hypothyroid state and normal or elevated in the hyperthyroid state. The changes in protein synthesis were mediated by changes in both RNA concentration and RNA activity (protein synthesis per unit RNA). Gastrocnemius and plantaris muscles were most responsive in the hypothyroid range. Since protein synthesis is particularly depressed in these muscles in malnutrition, the fall in protein degradation induced by the lowered thyroid status in this condition will be an important adaptive response to conserve protein. The increased protein turnover in the hyperthyroid rats was most marked in the soleus muscle and it is argued that this is necessary to allow the changes in protein composition and metabolic character which occur in response to hyperthyroidism in this muscle.

Effect of corticosterone on myofibrillar protein turnover in diabetic rats as assessed by Ntau-methylhistidine excretion

The effect of corticosterone on myofibrillar protein breakdown in diabetic rats was investigated in order to assess the possible counteracting effects of the secondary rise in plasma insulin concentrations which normally accompanies such treatment. N(tau)-Methylhistidine excretion, an index of myofibrillar protein breakdown, was compared before and after corticosterone treatment (4.0 mg/100 g body wt. per day) of normal control, adrenalectomized, 10-day-streptozotocin-diabetic and adrenalectomized diabetic rats. Diabetic rats received 1.5 units of insulin/100 g body wt. per day throughout the experiment and showed marked hyperglycaemia and glucosuria during corticosterone treatment, whereas non-diabetic rats had only mild hyperglycaemia but elevated insulin concentrations. Corticosterone treatment increased the average rate of myofibrillar protein breakdown by 68% and 95% respectively in non-diabetic and diabetic rats. Net loss of muscle non-collagen protein for the same 7-day period was greater in diabetic than in non-diabetic animals (4.15 versus 2.84% per day), and the calculated average synthesis rates were lowest in diabetic rats. Adrenalectomy had little effect except to decrease slightly the rate of muscle protein breakdown. These results show that the rise in plasma insulin concentrations that accompanies exogenous corticosterone administration to non-diabetic rats diminishes the catabolic effect of this glucocorticoid on muscle. Insulin appears to antagonize the effects of the glucocorticoid by attenuating the increased rates of myofibrillar protein breakdown and, to a lesser extent, by limiting the decrease in synthesis rates.