Biochemical and morphological study of cardiac hypertrophy. Effects of thyroxine on enzyme activities in the rat myocardium

Distinct metabolomic signatures are associated with longevity in humans

Alterations in metabolism influence lifespan in experimental models, but data in humans are lacking. Here we use liquid chromatography/mass spectrometry to quantify 217 plasma metabolites and examine their relation to longevity in a large cohort of men and women followed for up to 20 years. We find that, higher concentrations of the citric acid cycle intermediate, isocitrate, and the bile acid, taurocholate, are associated with lower odds of longevity, defined as attaining 80 years of age. Higher concentrations of isocitrate, but not taurocholate, are also associated with worse cardiovascular health at baseline, as well as risk of future cardiovascular disease and death. None of the metabolites identified are associated with cancer risk. Our findings suggest that some, but not all, metabolic pathways related to human longevity are linked to the risk of common causes of death.

Extrinsic and intrinsic regulation of DOR/TP53INP2 expression in mice: effects of dietary fat content, tissue type and sex in adipose and muscle tissues

Background

DOR/TP53INP2 acts both at the chromosomal level as a nuclear co-factor e.g. for the thyroid hormone receptor and at the extrachromosomal level as an organizing factor of the autophagosome. In a previous study, DOR was shown to be down-regulated in skeletal muscle of obese diabetic Zucker fa/fa rats.

Methods

To identify sites of differential DOR expression in metabolically active tissues, we measured differences in DOR expression in white adipose tissue (WAT), brown adipose tissue (BAT), skeletal muscle (SM) and heart muscle (HM) by qPCR. To assess whether DOR expression is influenced in the short term by nutritional factors, NMRI mice were fed different fat rich diets (fat diet, FD: 18% or high fat diet, HFD: 80% fat) for one week and DOR expression was compared to NMRI mice fed a control diet (normal diet, ND: 3.3% fat). Additionally, DOR expression was measured in young (45 days old) and adult (100 days old) genetically obese (DU6/DU6i) mice and compared to control (DUKs/DUKsi) animals.

Results

ANOVA results demonstrate a significant influence of diet, tissue type and sex on DOR expression in adipose and muscle tissues of FD and HFD mice. In SM, DOR expression was higher in HFD than in FD male mice. In WAT, DOR expression was increased compared to BAT in male FD and HFD mice. In contrast, expression levels in female mice were higher in BAT for both dietary conditions.

DOR expression levels in all tissues of 100 days old genetically obese animals were mainly influenced by sex. In HM, DOR expression was higher in male than female animals.

Conclusions

DOR expression varies under the influence of dietary fat content, tissue type and sex. We identified target tissues for further studies to analyze the specific function of DOR in obesity. DOR might be part of a defense mechanism against fat storage in high fat diets or obesity.

Thyroid hormone and myocardial mitochondrial biogenesis

Mitochondria have been central in the development of some of the most important ideas in modern biology. Since the discovery that mitochondria have its own DNA and specific mutations and deletions were found in association with neuromuscular and heart diseases, as well as in aging, an extraordinary number of publications have followed, and the term mitochondrial medicine was coined. Recently, it has been found that thyroid hormone (TH) stimulates cardiac mitochondrial biogenesis increasing myocardial mitochondrial mass, mitochondrial respiration, oxidative phosphorylation (OXPHOS), enzyme activities, mitochondrial protein synthesis (by stimulation in a T3-dependent manner), cytochrome, phospholipid and mtDNA content. Also, TH therapy may modulate cardiac mitochondrial protein-import apparatus. To identify the sequence of events, molecules and signaling pathways that is activated by TH affecting mitochondrial structure, biogenesis and function further research is warranted.

Mitochondrial gene expression is regulated at multiple levels and differentially in the heart and liver by thyroid hormones

Biogenesis of the oxidative phosphorylation system (OXPHOS) requires the coordinated expression of the nuclear and the mitochondrial genomes. Thyroid hormones play an important role in cell growth and differentiation and are one of the main effectors in mitochondrial biogenesis. To determine how mtDNA expression is regulated, we have investigated the response of two different tissues, the heart and liver, to the thyroid hormone status in vivo and in vitro. We show here that mtDNA expression is a tightly regulated process and that several levels of control can take place simultaneously. In addition, we show that the mechanisms operating in the control of mtDNA expression and their relevance differ between the two tissues, being gene dosage important only in heart while transcription rate and translation efficiency have more weight in liver cells. Another interesting difference is the lack of a direct effect of thyroid hormones on heart mitochondrial transcription.

Association of calcineurin with mitochondrial proteins

Using mouse hearts from Swiss Webster mice, calcineurin was immunoprecipitated using commercially available anti-calcineurin antibody and the resulting complex analyzed by using sodium dodecyl sulfate-gel electrophoresis with silver staining. Distinct proteins were observed and subjected to in situ trypsin digestion followed by extraction of the resulting peptides. Peptides from each protein band were loaded onto a target for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and analyzed. The resulting peptide mass spectrum was compared with the Mascot and Protein Prospector databases and resulted in the specific identification of heart mitochondrial proteins, specifically Mn-superoxide dismutase (SOD), aconitase (ACN), and malate dehydrogenase (MDH). Each of the three mitochondrial enzymes was identified with approximately 15-25% sequence coverage and all with statistical significance (P < 0.05) according to the Mascot database search engine. Tandem mass spectrometry analysis of the peptide fragmentation spectra confirmed the identification of these protein partners and also yielded the identification of mitochondrial isocitrate dehydrogenase (ICDH) as another protein in the immunoprecipitated complex. Using antibody preparations against Mn-SOD, ACN, and ICDH showed the presence of calcineurin and each of the three proteins in the immunoprecipitated complex by Western slot blotting. The activity of ACN, but not MDH or ICDH, was enhanced after incubation with calcineurin indicating one possible regulatory function for the complex. The mitochondrial forms of Mn-SOD, ACN, MDH, and ICDH were identified as partner proteins of calcineurin with all the proteins present in a single multiprotein complex.

Bioenergetic remodeling of heart mitochondria by thyroid hormone

Changes in thyroid status are associated with profound alterations in biochemical and physiological functioning of cardiac muscle impacting metabolic rate, contractility and structural hypertrophy. Using an in vivo model of chronic treatment with thyroid hormone (T4, 0.3 mg/kg/day), we evaluated how mitochondria are regulated in response to T4, and assessed the relationship of T4-induced mitochondrial biogenesis and bioenergetics to overall cardiac hypertrophy. The role of thyroid hormone in cardiac bioenergetic remodeling was addressed in rats treated with T4 for 5, 10 and 15 days. Over that time, myocardial oxygen consumption substantially increased as did cardiac hypertrophy. Myocardial levels of mitochondrial enzyme activities, mitochondrial DNA (mtDNA), specific proteins and transcript were assessed. Activity levels of respiratory complexes I-V and citrate synthase significantly increased with 15 but not with 5 or 10-day T4 treatment. Myocardial levels of mtDNA, mitochondrial proteins (e.g. cytochrome c, cytochrome b, ATPase subunits, MnSOD) and the global transcription factor PPARalpha were significantly elevated with 15-day T4. Transcript analysis revealed increased expression of transcription factors and cofactors involved in mitochondrial biogenesis including PPARalpha, mtTFA, ErbAalpha and PGC-1alpha. Our findings indicate parallel increases in myocardial mitochondrial bioenergetic capacity, oxygen consumption and markers of mitochondrial biogenesis with 15-day T4; these changes were not present with 10-day T4 even with significant cardiac hypertrophy. The marked, parallel increases in PPARalpha levels suggest its potential involvement in mediating myocardial-specific remodeling of mitochondria in response to T4.

Heart mitochondria signaling pathways: appraisal of an emerging field

The contribution that mitochondria make to cardiac function extends well beyond their critical bioenergetic role as a supplier of ATP. The organelle plays an integral part in the regulatory and signaling events that occur in response to physiological stresses, including but not limited to myocardial ischemia and reperfusion, hypoxia, oxidative stress, and hormonal and cytokine stimuli. Research on both intact cardiac muscle tissue and cultured cardiomyocytes has just begun to probe the nature and the extent of mitochondrial involvement in interorganelle communication, hypertropic growth, and cell death. This review covers particular aspects of the newly emerging field of mitochondrial medicine offering a critical guide in the assessment of mitochondrial participation at the molecular and biochemical levels in the multiple and interrelated signaling pathways, gauging the effect that mitochondria have as a receiver, integrator, and transmitter of signals on cardiac phenotype. We also discuss future directions that may impact on the treatment of cardiac diseases.

Functional and metabolic adaptation of the heart to prolonged thyroid hormone treatment

In heart failure, thyroid hormone (TH) treatment improves cardiac performance. The long-term effects of TH on cardiac function and metabolism, however, are incompletely known. To investigate the effects of up to 28 days of TH treatment, male Wistar rats received 3,3',5-triiodo-l-thyronine (200 microg/kg sc per day) leading to a 2.5-fold rise in plasma fatty acid (FA) level and progressive cardiac hypertrophy (+47% after 28 days) (P < 0.001). Ejection fraction (echocardiography) was increased (+12%; P < 0.05) between 7 and 14 days and declined thereafter. Neither cardiac FA oxidation, glycolytic capacity (homogenates) per unit muscle mass, nor mRNA levels of proteins involved in FA and glucose uptake and metabolism (Northern blots and microarray) were altered. After 28 days of treatment, mRNA levels of uncoupling proteins (UCP) 2 and 3 and atrial natriuretic factor were increased (P < 0.05). This indicates that TH-induced hypertrophy is associated with an initial increase in cardiac performance, followed by a decline in cardiac function and increased expression of UCPs and atrial natriuretic factor, suggesting that detrimental effects eventually prevail.

T(3) increases mitochondrial ATP production in oxidative muscle despite increased expression of UCP2 and -3

Triiodothyronine (T(3)) increases O(2) and nutrient flux through mitochondria (Mito) of many tissues, but it is unclear whether ATP synthesis is increased, particularly in different types of skeletal muscle, because variable changes in uncoupling proteins (UCP) and enzymes have been reported. Thus Mito ATP production was measured in oxidative and glycolytic muscles, as well as in liver and heart, in rats administered T(3) for 14 days. Relative to saline-treated controls, T(3) rats had 80, 168, and 62% higher ATP production in soleus muscle, liver, and heart, respectively, as well as higher activities of citrate synthase (CS; 63, 90, 25%) and cytochrome c oxidase (COX; 119, 225, 52%) in the same tissues (all P < 0.01). In plantaris muscle of T(3) rats, CS was only slightly higher (17%, P < 0.05) than in controls, and ATP production and COX were unaffected. mRNA levels of COX I and III were 33 and 47% higher in soleus of T(3) rats (P < 0.01), but there were no differences in plantaris. In contrast, UCP2 and -3 mRNAs were 2.5- to 14-fold higher, and protein levels were 3- to 10-fold higher in both plantaris and soleus of the T(3) group. We conclude that T(3) increases oxidative enzymes and Mito ATP production and Mito-encoded transcripts in oxidative but not glycolytic rodent tissues. Despite large increases in UCP expression, ATP production was enhanced in oxidative tissues and maintained in glycolytic muscle of hyperthyroid rats.

Energy metabolism response to calcium activation in isolated rat hearts during development and regression of T3-induced hypertrophy

The effect of calcium activation on energy production was investigated in isolated perfused hearts from rats treated with triiodothyronine (T3) during 15 days (0.2 mg/kg/day) and in hearts of rats allowed to recover after T3-treatment during 15 days. Changes in phosphorylated compound concentrations were followed in the isolated hearts perfused with a glucose-pyruvate medium by 31P-NMR spectroscopy, when the external calcium concentration was increased from 0.5-1, 1.5 and 2 mM. As expected, T3-treatment resulted in the hypertrophy of the heart (50% increase in HW/BW) that was nearly reversible 15 days after discontinuation of the treatment. When compared to controls, creatine, phosphocreatine (PCr) and glycogen contents were lower (58, 24 and 17% decrease respectively) in the hypertrophied hearts and higher (10, 14 and 18% respectively) after regression of hypertrophy. Intracellular pH, ATP, inorganic phosphate concentrations and the phosphorylation potential were not altered under T3-treatment and after regression of hypertrophy, while calculated free ADP concentration was lower in hypertrophied hearts (control: 40 +/- 2 microM, T3-treatment: 21 +/- 1 microM, regression: 37 +/- 1 microM). Increasing the calcium concentration induced a similar increase in left ventricular developed pressure in the three groups of hearts, with inorganic phosphate concentration increasing with cardiac work. The PCr concentration slightly decreased while the ATP concentration did not change. In spite of different initial PCr concentrations, the evolutions of PCr and Pi concentrations for each stepwise increase in external calcium were similar in the three groups. It is concluded that, in spite of the well-known decrease in efficiency induced by the drug, the mechanisms of PCr (ATP) production remain able to respond to an acute moderate increase in energy demand provoked by a physiological stimulus. This adaptation also persists after the treatment when the energy metabolism balance is apparently improved.

Response of the rat heart to catecholamines and thyroid hormones

Catecholamines and thyroid hormones have a similar influence on heart function and metabolism, but this may occur in a differential manner and to a different extent. In this study, the effects of norepinephrine (NE) and of triiodothyronine (T3) were studied in regard to the function of the left (LV) and right ventricle (RV) and to the oxidative pentose phosphate pathway (PPP). NE was applied in rats as continuous i.v. infusion (0.2 mg/kg/h) for three days. T3 was given as daily s.c. injections (0.2 mg/kg) for the same period of time. LV and RV function was measured in the closed-chest trapanal-anesthetized animals using special Millar ultraminature catheter pressure transducers. NE induced an increase in heart rate, in mean arterial pressure, and in total peripheral resistance (TPR). The cardiac RNA/DNA and the left ventricular weight/body weight ratios were increased by about 40%. These effects were prevented by simultaneous alpha- and beta-receptor blockade with prazosin and metoprolol, respectively, but not by verapamil which abolished the hemodynamic effects. RVSP was significantly elevated by NE in a dose-dependent manner. The functional effects of T3 on the LV were not as pronounced as those induced by NE. Heart rate and LV dp/dtmax were increased by T3, and this increase was prevented by concomitant beta-receptor blockade with metoprolol. In contrast to NE, T3 induced an increase in cardiac output and a concomitant decrease in TPR. The RNA/DNA ratio was elevated and cardiac hypertrophy had developed after treatment for three days with T3. These changes were not affected by beta-receptor blockade with metoprolol. RVSP was increased by T3 to a lesser extent than with NE. In metabolic terms it turned out that only NE, but not T3 had a stimulating effect on the cardiac PPP. NE increased the mRNA and activity of glucose-6-phosphate dehydrogenase (G-6-PD), the first and regulating enzyme of this pathway. However, there was no effect of T3 on G-6-PD activity nor on 6-phosphogluconate dehydrogenase activity, one of the following enzymes in the pathway within the first 5 days of T3 treatment. These results demonstrate that the functional effects of T3 were not as pronounced as or even different from those of NE, and that T3 lacked a stimulating effect on the cardiac PPP.

Thyroid hormones and the creatine kinase system in cardiac cells

The paper reviews the current evidence on the role of thyroid hormones in regulating the creatine kinase energy transfer system at multiple structures in cardiac cells. 1) Thyroid hormones modulate the overall synthesis of phosphocreatine (PCr) by increasing the rate of mitochondrial oxidative phosphorylation. 2) Thyroid hormones regulate the total activity of creatine kinase and its isoenzyme distribution. In comparison with normal thyroid state (euthyroidism), hypothyroidism is characterized by decreased total creatine kinase activity owing to diminished fraction of creatine kinase. On the other hand, hyperthyroidism, while causing no change in total creatine kinase activity, leads to increased fractions of neonatal isoforms of creatine kinase, and, in case of prolonged hyperthyroidism, to decreased fraction of mitochondrial creatine kinase. The latter change is associated with partial uncoupling between mitochondrial creatine kinase and adenine nucleotide translocase reflected by decreased PCr/O ratio. 3) Hyperthyroidism leads to increased passive sarcolemmal permeability due to which the leakage of creatine along its concentration gradient occurs. As a result of (i) increased sarcolemmal permeability for creatine, (ii) uncoupling of mitochondrial PCr synthesis, and (iii) increased energy utilization rate the steady state intracellular PCr content decreases under hyperthyroidism which, in turn, increases the myocardial susceptibility to hypoxic damage. Thyroid state also modulates the protective effects of exogenous PCr on energetically depleted myocardium.

Hyperthyroidism results in increased glycolytic capacity in the rat heart. A 31P-NMR study

We have investigated the metabolic adaptations that occur in the thyroxine-treated rat heart. Rats were made hyperthyroid by daily intra-peritoneal injections of thyroxine (35 micrograms/100 g body weight) over seven days. 31P-NMR investigations of isolated glucose-perfused isometric hearts showed that thyroxine treatment caused an increase in Pi (from 4.9 mumols.(g dry wt.)-1 in control hearts to 11.7 mumols.(g dry wt.)-1 in hyperthyroid hearts), a decrease in phosphocreatine (from 36.5 mumols.(g dry wt.)-1 to 21.8 mumols.(g dry wt.)-1) with no change in ATP or ADP concentrations under the same conditions of cardiac work. The unidirectional exchange flux Pi----ATP was measured by saturation transfer NMR in hyperthyroid rat hearts. This exchange (which has been shown to contain a significant glycolytic component) increased by 2.2-fold in thyroxine-treated hearts in comparison to control hearts (to 3.6 mumols.(g dry wt.)-1.s-1, from 1.6 mumols.(g dry wt.)-1.s-1). In parallel experiments, NMR analysis of extracts from hyperthyroid rat hearts showed significantly elevated levels of glucose 6-phosphate, and fructose 6-phosphate. Measurements of enzyme activities isolated from hyperthyroid and control tissue showed a 40% increase in phosphofructokinase activity. These data together with the increased concentration of Pi show that both glycolytic and glycogenolytic fluxes are increased in the hyperthyroid rat heart. This metabolic adaptation may be necessary to cope with the increased number and activity of Na+/K(+)-ATPase pumps that occur in response to thyroxine treatment.

Insulin stimulates mitochondrial protein synthesis and respiration in isolated perfused rat heart

The rates of synthesis of mitochondrial proteins by both the cytoplasmic and mitochondrial protein synthetic systems, as well as parameters of respiration, were measured and compared in mitochondria isolated from fresh, control perfused, and insulin-perfused rat hearts. The respiratory control ratio (RCR) in mitochondria from fresh hearts was 8.1 +/- 0.4 and decreased to 6.0 +/- 0.2 (P less than 0.001 vs. fresh) in mitochondria from control perfused hearts and to 6.7 +/- 0.2 (P less than 0.005 vs. fresh and P less than 0.02 vs. control perfused) for mitochondria from hearts perfused in the presence of insulin. A positive correlation between the RCR and the rate of mitochondrial translation was demonstrated in mitochondria from fresh hearts. In mitochondria isolated from control perfused hearts, the rate of protein synthesis decreased to 84 +/- 3% of the fresh rate after 30 min of perfusion and fell further to 64 +/- 3% after 3 h of perfusion. The inclusion of insulin in the perfusion buffer stimulated mitochondrial protein synthesis 1.2-fold by 1 h (P less than 0.005) and 1.34-fold by 3 h of perfusion (P less than 0.001). The addition of insulin to 1-h control perfused hearts shifted the rate of mitochondrial protein synthesis from the control level to the insulin-perfused level within 30 min of additional perfusion, whereas 1 h was required to shift the RCR values of these mitochondria from control levels to insulin-perfused levels. Thus, whereas RCR was a useful predictor of mitochondrial translation rates, it did not account for the effects of insulin on mitochondrial translation.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin, thyroid hormone, and heart function of diabetic spontaneously hypertensive rat

Diabetes impairs cardiac performance more extensively in hypertensive rats than it does in nonhypertensive strains. A "low thyroid state" may contribute to the adverse cardiovascular effects of diabetes in spontaneously hypertensive rats (SHR). We tested this hypothesis by comparing the effects of thyroid hormone with those of insulin treatment on cardiac performance of diabetic SHR. Diabetes was induced with streptozotocin (45 mg/kg). Subsets of diabetic rats were treated with either insulin (10-20 units/kg/day) or triiodothyronine (8-10 micrograms/kg/day). Heart rate and systolic arterial pressure were obtained at weekly intervals. After 8 weeks, cardiac function was assessed using an isolated working heart preparation. Diabetes reduced arterial pressure and heart rate in vivo and markedly depressed cardiac performance under volume and pressure loading conditions ex vivo, confirming previous observations. As expected, insulin treatment prevented the bradycardia and depressor effect in vivo and the impairment of cardiac performance ex vivo caused by diabetes. The triiodothyronine treatment duplicated the effects of insulin on the hemodynamic measurements in vivo, and corrected nearly all depressed indexes of performance of diabetic SHR hearts ex vivo. Both treatment regimens successfully reduced 8-week mortality when compared with the untreated diabetic group. The results support the hypothesis that a low thyroid state may contribute to the cardiovascular dysfunction in diabetic SHR. Left ventricular hypertrophy may be an important factor in this phenomenon.

Mitochondrial protein synthesis during thyroxine-induced cardiac hypertrophy

The goal of this paper was to determine the effects of 3,5,3'-triiodothyronine (T3)-thyroxine-induced cardiac hypertrophy on the rates of synthesis of mitochondrial proteins by both the cytoplasmic and mitochondrial protein synthesis systems and to compare the results with total protein synthesis and cardiac enlargement. Daily injections of T3-thyroxine in the rat resulted in a 25% increase in the growth of the ventricle compared with controls. The cytoplasmic synthesis of both mitochondrial and total proteins as measured in the isolated perfused heart was stimulated by T3-thyroxine injection to a peak of 155 and 146%, respectively, of vehicle-injected controls after 3 days of hormone treatment. This peak was followed by a gradual decline in stimulation in total protein synthesis to 132% of control by 9 days of injection, whereas the decline in stimulation of cytoplasmic synthesis of mitochondrial proteins was significantly steeper, falling to 119% of vehicle control. The rate of protein synthesis within the mitochondrial compartment was also measured during the time course of T3-thyroxine-induced hypertrophy. These rates were measured in an isolated intact heart mitochondrial protein synthesis system described and characterized in the companion papers [E. E. McKee, B. L. Grier, G. S. Thompson, and J. D. McCourt. Am. J. Physiol. 258 (Endocrinol. Metab. 21): E492-E502, 1990; and E. E. McKee, B. L. Grier, G. S. Thompson, A. C. F. Leung, and J. D. McCourt. Am. J. Physiol. 258 (Endocrinol. Metab. 21): E503-E510, 1990]. Rates of mitochondrial protein synthesis were dramatically stimulated by T3-thyroxine injection.(ABSTRACT TRUNCATED AT 250 WORDS)

Triiodothyronine-induced changes in function, metabolism and weight of the rat heart: effects of alpha- and beta-adrenergic blockade

The involvement of alpha- and beta-adrenergic receptors in the triiodothyronine (T3)-induced hemodynamic and metabolic alterations and in the development of cardiac hypertrophy was analyzed in time-course studies. Female Sprague-Dawley rats received daily injections of T3 (200 micrograms/kg s.c.) and a continuous i.v. infusion of 0.9% NaCl or alpha- or beta-receptor blocking agents. NaCl-injected animals served as controls. Eighteen hours after T3 administration, heart rate and LV dP/dtmax were considerably elevated. Cardiac output (CO) was not significantly changed. These alterations were abolished by simultaneous infusion of the beta-adrenergic blocker metoprolol. After 48 hours, CO as well as the cardiac RNA concentration were markedly elevated. The rise in LV dP/dtmax and heart rate was similar to the 18-h-value and was prevented by metoprolol. However, metoprolol did not influence the increase in CO and RNA concentration. Likewise, after 72 hours, metoprolol antagonized the T3-induced increase in heart rate and LV dP/dtmax, but had no effect on the elevation of CO and RNA concentration, the enhancement of adenine nucleotide synthesis and cardiac hypertrophy. Like metoprolol, the alpha-adrenergic blocker prazosin did not influence the T3-evoked cardiac hypertrophy. Thus, the development of cardiac hypertrophy in this model can occur independently of alpha- and beta-adrenergic stimulation.

Hepatic indices of thyroid status in rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin

The functional thyroid status of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated rats is unknown. Therefore, activities of certain thyroid-responsive enzymes were examined in the livers of adult male Sprague-Dawley rats 1 week after treatment with TCDD (6.25, 25 or 100 micrograms/kg). Activity of the thyroid-responsive flavin L-glycerol-3-phosphate dehydrogenase (per mg mitochondrial protein) was decreased slightly in livers of TCDD-treated rats, while that of succinate dehydrogenase remained unchanged. In contrast, activities (per mg supernatant protein) of three thyroid-responsive NADP-dependent cytosolic enzymes, malic enzyme, glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, were increased by TCDD treatment in a dose-dependent manner. Lactate dehydrogenase (activity per mg supernatant protein) was also augmented slightly 1 week after TCDD administration. Liver mass was increased by TCDD treatment in a dose-dependent manner, but DNA content per liver was similar at all doses examined. Total hepatic protein, expressed per liver or mg hepatic DNA, was increased in TCDD-treated rats when compared to their pair-fed counterparts. The decreased activity of the mitochondrial L-glycerol-3-phosphate dehydrogenase, in contrast to the increased activities of the supernatant enzymes, malic enzyme, glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, is not consistent with a shift in functional thyroid status following TCDD treatment.