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Biomedical subjects

H J Seitz

Publications and source records attributed to H J Seitz.

At least 37 records · Page 2Linked to original sources
PubMed1989

Rapid stimulation of hepatic oxygen consumption by 3,5-di-iodo-L-thyronine.

Tri-iodothyronine (T3) and thyroxine (T4) as well as 3,5-di-iodothyronine (T2) stimulated O2 consumption by isolated perfused livers from hypothyroid rats at a concentration as low as 1 pM by about 30% within 90 min. Application of T2 resulted in a faster stimulation than with application of T3 or T4. Inhibition of iodothyronine monodeiodinase by propylthiouracil, thereby blocking the degradation of T4 to T3 and of T3 to T2, demonstrated that only T2 is the active hormone for the rapid stimulation of hepatic O2 consumption: T3 and T4 lost all of their stimulative activity, whereas T2 was as potent as in the absence of propylthiouracil. Perfusion experiments with thyroid-hormone analogues confirmed the specificity of the T2 effect. The nucleus is unlikely to contribute to the rapid T2 effect, as can be deduced from perfusion experiments with cycloheximide and lack of induction of malic enzyme by T2. In conclusion, a new scheme of regulation of mitochondrial activity is proposed: T2 acts rapidly and directly via a mitochondrial pathway, whereas T3 exerts its long-term action indirectly by induction of specific enzymes.

Animals↗
PubMed1989

Glucoregulatory function of thyroid hormones: evidence for an insulin-independent effect.

The effect of hyperglycemia on glucose kinetics was investigated in normal and T4-treated miniature pigs. 1. T4-treatment increased basal plasma glucose (G) (+ 20%) and glucose turnover (+36%). 2. Glucose infusion (2 mg/kg x min) in controls increased insulin and glucose utilization (Rd), but decreased glucagon and hepatic glucose production (Ra). After T4-treatment glucose increased insulin, decreased glucagon and Ra but only a slight effect on G and Rd were observed. 3. Infusing glucose + somatostatin resulted in hyperglycemia in both groups due to an initial fall in Ra and Rd followed by an increase in Rd, where total Ra (endogenous + exogenous exceeded Rd. Glucose intolerance was more pronounced in controls, due to a T4-induced increase in Rd. During this non-steady state the increment in Rd per increment in G was calculated and showed 1.5 in controls and 2.5 after T4-treatment. These data give evidence that thyroid hormones increase glucose utilization during hypoinsulinemia.

Animals↗
PubMed1988

Glucoregulatory function of glucagon in hypo-, eu- and hyperthyroid miniature pigs.

The glucoregulatory function of glucagon was investigated in hypo-, eu- and hyperthyroid miniature pigs. Infusion glucagon, (3 ng x kg body weight-1.min-1) transiently increased blood glucose (p less than 0.01) and hepatic glucose production (p less than 0.01) in euthyroidism, but was without effect in hyperthyroidism. Infusing glucagon plus somatostatin (2 ng x kg body weight-1.min-1 and 0.2 microgram x kg body weight-1.min-1) transiently increased blood glucose (delta 3.0 to 4.3 mmol/l) and hepatic glucose production (delta 3.3 to 7.7 mumol x kg body weight-1.min-1) in all thyroid states, the effect was less pronounced in hyperthyroid pigs. By contrast, hypoglucagonaemia (74 to 107 pg/ml) at basal insulin (28 to 35 microU/ml) provoked hypoglycaemia (1.4 to 2.2 mmol/l) and a fall in glucose production (delta 4.7 to 8.3 mumol x kg body weight-1.min-1), which was independent of the thyroid state; the effect was most pronounced in hyperthyroidism (p less than 0.01). Hepatic glycogen content, arterial gluconeogenic precursor concentrations as well as the glycaemic response (delta 0.60 mmol/l) to alanine infusion (23 mumol x kg body weight-1.min-1) were all unaffected by hyperthyroidism. We conclude that moderate experimental hyperthyroidism reduces glucagon action due to reduced glycogen mobilisation. This may in part result from increased insulin sensitivity.

Animals↗
PubMed1988

Regulation of hepatic glucose output by glucose in vivo.

The effect of infusing glucose at a rate estimated to equal hepatic glucose production (2 mg/kg BW X min) was investigated in postabsorptive unrestrained miniature pigs. Glucose turnover was estimated by the use of 3-H-glucose before and during glucose infusion where the contribution of the glucose-induced hormonal changes were estimated by infusing glucose plus somatostatin with or without insulin and/or glucagon substitution. Our data give evidence for an inhibitory effect of glucose on endogenous glucose production in the postabsorptive state. This effect seems to be independent of the hormonal changes induced by glucose infusion.

Animals↗
PubMed1988

Thyroid hormone effect on gene expression of the adenine nucleotide translocase in different rat tissues.

The ADP/ATP transport across the mitochondrial membrane is achieved by the adenine nucleotide translocase (ANT), an integral inner mitochondrial membrane protein. As deduced from experiments in rat liver in vivo and in isolated rat liver mitochondria this ADP/ATP transport is accelerated by thyroid hormone application, thus explaining, at least to a considerable extent, the thyroid hormone mediated increase in mitochondrial metabolic activity. The present study investigates the effect of T3 on rat liver, heart, and kidney ANT gene expression. As shown by Northern blot analysis, a cDNA for beef heart ANT-mRNA showed cross-hybridization with the ANT-mRNA from rat heart, liver, and kidney. Hypo- and hyperthyroid rats showed no differences in size nor in amounts of heart, liver, and kidney ANT-mRNA. Measurement of heart ANT-protein level revealed no major differences among the various thyroid states. Thus, the long-term action of thyroid hormones on increasing the carrier-mediated ADP/ATP translocation cannot be ascribed to an effect of T3 on ANT gene expression. The mechanism by which T3 activates this transporter system remains to be identified but some possibilities are suggested.

Animals↗
PubMed1987

Interrelation between thyroid state and the effect of glucagon on gluconeogenesis in perfused rat livers.

The effects of different thyroid states on glucagon/dBcAMP-induced gluconeogenesis from alanine or lactate were investigated in the isolated perfused liver from 24-hr starved rats. Gluconeogenesis from alanine varied with the thyroid state, being increased in hyperthyroidism and decreased in hypothyroidism. Both glucagon and dBcAMP increased glucose production from alanine in euthyroid and even less pronounced in hypothyroid livers, the effect was dose dependent; concomitantly alanine and [14C] alpha-amino-isobutyric acid uptake increased. In hyperthyroid liver, both glucagon and dBcAMP stimulated neither hepatic uptake of alanine and [14C] alpha-amino-isobutyric acid nor gluconeogenesis from alanine. Lactate uptake as well as glucose production from lactate varied with the thyroid state, being increased in the hyper- and decreased in the hypothyroid state. Both glucagon and dBcAMP increased lactate uptake as well as gluconeogenesis from lactate: the effect was even more pronounced in hyperthyroid and reduced in hypothyroid liver. We conclude that the glucogenic effect of glucagon/dBcAMP is reduced in the hypo- and--at unlimited substrate supply--stimulated in the hyperthyroid liver.

1-Methyl-3-isobutylxanthine↗
PubMed1987

Regulation of hepatic glucokinase gene expression. Role of carbohydrates, and glucocorticoid and thyroid hormones.

The present study investigates the effect of thyroid and glucocorticoid hormones on the induction of hepatic glucokinase mRNA activity, enzyme synthesis and activity in starved/refed adrenalectomized, thyroidectomized and intact rats. In intact rats glucose refeeding resulted within 2 h in a more than tenfold increase in the functional messenger, followed by a corresponding increase in glucokinase synthesis and, a little later, in enzyme activity. Glucokinase mRNA and synthesis remained elevated at this level for about further 6 h. Then the mRNA activity and enzyme synthesis declined considerably to a new steady state (a factor of about 4 above the starvation level) within a further 8 h, while enzyme activity remained constantly elevated. The half-life of glucokinase mRNA, as determined after administration of cordycepin, was identical during the different refeeding periods. Thus the overshoot phenomenon, provoked by carbohydrate refeeding, in glucokinase mRNA is not explained by alteration of the glucokinase mRNA decay rates. In thyroidectomized or adrenalectomized rats, glucose refeeding resulted in only a small increase in glucokinase mRNA, synthesis and activity. Application of thyroid hormones in thyroidectomized rats, refed a carbohydrate-rich diet, enhanced the specific mRNA considerably within 8-10 h, while it took 20-24 h to enhance glucokinase mRNA by glucocorticoids in adrenalectomized rats refed a carbohydrate-rich diet. The decay in translatable glucokinase mRNA, as determined after administration of cordycepin, was identical in the hypothyroid and euthyroid fed state, while adrenalectomy resulted in a significant decrease in the specific mRNA half-life. We conclude that refeeding a carbohydrate-rich diet rapidly stimulates glucokinase mRNA regeneration showing overshoot kinetics. 3,3',5-Triiodothyronine in its physiological concentration significantly enhances the response in glucokinase mRNA at the nuclear level, while glucocorticoids in their physiological concentration predominantly stabilize the translatable glucokinase mRNA.

Adrenalectomy↗
PubMed1987

Hexokinase isoenzymes of RIN-m5F insulinoma cells. Expression of glucokinase gene in insulin-producing cells.

We have analysed the pattern of expression of the hexokinase isoenzyme group in RIN-m5F insulinoma cells. Three hexokinase forms were resolved by DEAE-cellulose chromatography. The most abundant isoenzyme co-eluted with hexokinase type II from rat adipose tissue and displayed a Km for glucose of 0.15 mM, similar to the adipose-tissue enzyme. Hexokinase type II was in large part associated with a particulate subcellular fraction in RIN-m5F cells. The two other hexokinases separated by ion-exchange chromatography were an enzyme similar to hexokinase type I from brain and glucokinase (or hexokinase type IV). The latter isoenzyme was identified as the liver-type glucokinase by the following properties: co-elution with hepatic glucokinase from DEAE-cellulose and DEAE-Sephadex; sigmoid saturation kinetics with glucose with half-maximal velocity at 5.6 mM and Hill coefficient (h) of 1.54; suppression of enzyme activity by antibodies raised against rat liver glucokinase; apparent Mr of 56,500 and pI of 5.6, as shown by immunoblotting after one- and two-dimensional gel electrophoresis; peptide map identical with that of hepatic glucokinase after proteolysis with chymotrypsin and papain. These data indicate that the gene coding for hepatic glucokinase is expressed in RIN-m5F cells, a finding consistent with indirect evidence for the presence of glucokinase in the beta-cell of the islet of Langerhans. On the other hand, the overall pattern of hexokinases is distinctly different in RIN-m5F cells and islets of Langerhans, since hexokinase type II appears to be lacking in islets. Alteration in hexokinase expression after tumoral transformation has been reported in other systems.

Adenoma, Islet Cell↗
PubMed1986

Cooperative effect of thyroid and glucocorticoid hormones on the induction of hepatic phosphoenolpyruvate carboxykinase in vivo and in cultured hepatocytes.

The present study investigates the effect and interaction of glucocorticoid and thyroid hormones on the induction of phosphoenolpyruvate carboxykinase (PEPck) mRNA and enzyme protein under in vivo conditions and in serum-free cultured hepatocytes from hypothyroid rats. In hypothyroid/adrenalectomized rats T3 significantly enhanced the cAMP induced PEPck mRNA activity within 3-6 h. This effect was further enhanced by the presence of glucocorticoids. The half-life of PEPck mRNA, as determined after administration of cordycepin, was not affected by hypothyroidism or hyperthyroidism (t 1/2 approximately equal to 45 min), but considerably prolonged by the absence of glucocorticoid hormones (t 1/2 less than 80 min). In hepatocytes in culture Bt2cAMP (0.2 mM) provoked an increase in translatable PEPck mRNA within 2 h incubation time. Preincubation with either T3 (0.1 microM) or dexamethasone (0.1 microM) for 4 h significantly enhanced the cAMP response on PEPck mRNA. Addition of both, T3 plus dexamethasone further enhanced this Bt2cAMP-mediated effect. By measurement of PEPck synthesis corresponding findings were observed. It is concluded that glucocorticoid and thyroid hormones predominantly enhance the cAMP-provoked induction of hepatic PEPck mRNA and, consequently, of PEPck synthesis. Their effect is rapid, significant and additive, indicating an independent action. While glucocorticoids, in addition, accelerate PEPck mRNA degradation, the PEPck mRNA decay rate is similar in the presence and absence of thyroid hormones.

Animals↗
PubMed1986

Experimental hyperthyroidism does not induce hepatic insulin resistance in the miniature pig.

The effect of hypo- and hyper-thyroidism on insulin-mediated alterations in tracer-determined glucose kinetics and the arterial concentration of gluconeogenic precursors were investigated in 24 h-starved conscious unrestrained miniature pigs. Hyperinsulinaemia (about 40 microunits/ml) decreased blood glucose and, transiently, glucose output at unaltered glucose utilization in all thyroid states: this effect was pronounced in hyperthyroid (-50%) and less in hypothyroid pigs (-25%) compared with euthyroid controls (-35%). We conclude that moderate experimental hyperthyroidism does not induce hepatic insulin resistance, whereas hypothyroidism slightly impairs insulin action with respect to the regulation of glucose output.

Animals↗
PubMed1986

Tissue-specific expression of glucokinase: identification of the gene product in liver and pancreatic islets.

The tissue distribution of glucokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) was examined by protein blotting analysis. Antibodies raised against rat liver glucokinase recognized a single protein subunit with an apparent Mr of 56,500 on nitrocellulose blots of cytosol protein from liver, separated by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. A protein of identical electrophoretic mobility was detected by immunoblotting of cytosol protein from pancreatic islets. Hepatic glucokinase and the immunoreactive islet product bound to and were eluted from DEAE-cellulose at the same ionic strength. Glucokinase was displayed as a set of two spots with apparent pI values of 5.54 and 5.64 by immunoblotting after two-dimensional gel electrophoresis. The two isoforms appeared equally abundant in liver extract, whereas the component with a pI of 5.64 was predominant in islets. By quantitative immunoblotting, glucokinase was estimated to represent 0.1% of total cytosol protein in liver and 1/20th as much in islets. The glucokinase activity of both liver and islet cytosols was suppressed by the antibodies to hepatic glucokinase. Immunoblotting of cytosol protein from intestinal mucosa, exocrine pancreas, epididymal adipose tissue, kidney, brain, and spleen failed to reveal the glucokinase protein. Thus, significant expression of the glucokinase gene appears restricted to the liver and pancreatic islets.

Animals↗
PubMed1985

Rapid thyroid-hormone effect on mitochondrial and cytosolic ATP/ADP ratios in the intact liver cell.

The effect of thyroid-hormone application on cytosolic and mitochondrial ATP/ADP ratio was investigated in rat liver in vivo and in the isolated perfused organ. In vivo the ATP/ADP ratio in livers from hypothyroid rats was 0.84 +/- 0.08 in the mitochondrial matrix and 5.6 +/- 0.9 in the cytosol, as was observed in euthyroid controls. In contrast, hyperthyroidism was followed by a significant decrease in the mitochondrial and by an increase in the cytosolic ATP/ADP ratio (to 0.34 +/- 0.06 and 11.3 +/- 2.8 respectively). In the perfused liver from hypothyroid animals, addition of L-3,3',5-tri-iodothyronine in the perfusate also provoked, within 2 h, a significant decrease in the mitochondrial ATP/ADP ratio, whereas the cytosolic ratio was unaffected. From these and previous data in the isolated perfused liver and in isolated mitochondria from hypothyroid and tri-iodothyronine-treated rats it is concluded that thyroid hormones increase mitochondrial respiration and ATP regeneration, which is associated with an acceleration of mitochondrial adenine nucleotide transport and significant alterations in the mitochondrial and cytosolic ATP/ADP ratios.

Adenosine Diphosphate↗
PubMed1985

Effect of thyroid state on cyclic AMP-mediated induction of hepatic phosphoenolpyruvate carboxykinase.

Hepatic phosphoenolpyruvate carboxykinase (PEPCK) is significantly increased in the hyperthyroid starved rat, and moderately decreased in the hypothyroid starved rat. As tri-iodothyronine by itself has only a small and sustained effect on the induction of this enzyme, as was previously shown in the isolated perfused organ, the effect of hypo- and hyper-thyroidism on the increase in cytosolic PEPCK provoked by dibutyryl cyclic AMP (Bt2cAMP) was investigated in vivo and in the isolated perfused liver. Compared with euthyroid fed controls, in hypothyroid fed rats Bt2cAMP provoked in 2 h only a small increase in translatable mRNA coding for PEPCK. In contrast, in hyperthyroid animals PEPCK mRNA as measured by translation in vitro was already increased in the fed state, and further enhanced by Bt2cAMP injection to values as in euthyroid controls. Under all thyroid states a close correlation between PEPCK mRNA activity and PEPCK synthesis was observed. In the isolated perfused liver from the hyperthyroid fed rat, the increase in PEPCK provoked by Bt2cAMP or Bt2cAMP + isobutylmethylxanthine was considerably enhanced compared with those obtained in livers of hypothyroid rats. Also, adrenaline provoked a stimulated induction of PEPCK in hyperthyroid rats compared with hypothyroid rats. To summarize, our data indicate that the primary action of thyroid hormones on the synthesis of hepatic cytosolic PEPCK is to accelerate the cyclic AMP- or adrenaline-induction of the enzyme, acting primarily at a pretranslational level.

Animals↗
PubMed1984

Permissive action of thyroid hormones in the cAMP-mediated induction of phosphoenolpyruvate carboxykinase in hepatocytes in culture.

Thyroid hormones are known to stimulate in vivo the synthesis of several liver proteins. In order to determine their role in the regulation of important hepatic proteins at the cell level, the effect of T3 on the induction of the glucoregulatory enzyme phosphoenolpyruvate carboxykinase (PEPck) was investigated in cultured hepatocytes. The cells were isolated from adult hypothyroid rats and exposed to a chemically defined medium, devoid of serum supplement and hormone-free over a period of 48 h. Addition of T3 to the medium had only a minor effect on PEPck induction. Dibutyryladenosine 3',5'-monophosphate (Bt2cAMP) or epinephrine provoked significant increase in PEPck synthesis within 4-6 h. Simultaneous addition of T3 was found significantly to promote the Bt2cAMP-mediated enzyme induction. Physiological concentrations of T3 (10 pM to 1 nM) were found to be effective in enhancing this cAMP-mediated signal. Exposure of the cells to T3 for only 90 min, 16 h prior to addition of Bt2cAMP was nearly as effective as continuous exposure to T3 in enhancing the cAMP-effect on PEPck synthesis. In contrast, no effect of T3 on the Bt2cAMP-induced tyrosine-aminotransferase activity could be detected. It is concluded that the role of T3 on the cAMP-elicited PEPck synthesis is selective, rapid, primarily permissive and significant within the physiological range of the hormone. In addition, these data indicate that such hepatocyte cultures prepared from hypothyroid rats are ideally suited to the analysis of the regulatory role of thyroid hormones on specific gene expression.

Animals↗
PubMed1984

Effect of long-chain fatty acyl-CoA on mitochondrial and cytosolic ATP/ADP ratios in the intact liver cell.

The effect of long-chain acyl-CoA on subcellular adenine nucleotide systems was studied in the intact liver cell. Long-chain acyl-CoA content was varied by varying the nutritional state (fed and starved states) or by addition of oleate. Starvation led to an increase in the mitochondrial and a decrease in the cytosolic ATP/ADP ratio in liver both in vivo and in the isolated perfused organ as compared with the fed state. The changes were reversed on re-feeding glucose in liver in vivo or on infusion of substrates (glucose, glycerol) in the perfused liver, respectively. Similar changes in mitochondrial and cytosolic ATP/ADP ratios occurred on addition of oleate, but, importantly, not with a short-chain fatty acid such as octanoate. It is concluded that long-chain acyl-CoA exerts an inhibitory effect on mitochondrial adenine nucleotide translocation in the intact cell, as was previously postulated in the literature from data obtained with isolated mitochondria. The physiological relevance with respect to pyruvate metabolism, i.e. regulation of pyruvate carboxylase and pyruvate dehydrogenase by the mitochondrial ATP/ADP ratio, is discussed.

Acyl Coenzyme A↗
PubMed1984

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.

Amino Acids↗