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H L Schwartz

Publications and source records attributed to H L Schwartz.

At least 19 recordsLinked to original sources
PubMed1992

Quantitation of rat tissue thyroid hormone binding receptor isoforms by immunoprecipitation of nuclear triiodothyronine binding capacity.

A panel of anti-thyroid hormone receptor (TR) antisera were generated to allow direct assay of the concentrations of the alpha 1 and beta 1 receptor isoforms in nuclear extracts from adult rat liver, kidney, brain and heart, and fetal brain. An antiserum, immunoglobulin G (IgG)-beta 1, raised against amino acid sequence 62-92 of the rat TR-beta 1 specifically precipitated only TR-beta 1 in vitro translation products. A second antiserum, IgG-alpha 1/beta, generated against a sequence that is identical in the ligand binding region of rat TR-alpha 1 and TR-beta isoforms immunoprecipitated both TR-alpha 1 and -beta 1 translation products. These IgG preparations were used to specifically immunoprecipitate thyroid hormone receptor binding activity from nuclear extracts. IgG-beta 1 cleared almost 80%, and the IgG-alpha 1/beta immunoprecipitated nearly all binding from hepatic nuclear extracts. This distribution of TR protein, 80% beta 1 and 20% alpha 1, is the same as previously reported for their respective mRNAs in liver. In heart, kidney, and brain IgG-beta 1 cleared 45, 43, and 28% of total binding, respectively, and IgG-alpha 1/beta cleared all T3 binding activity from these tissues. In agreement with an earlier study, marked variations in specific protein/mRNA ratios were noted among these tissues. Consistent with our earlier report of the presence of only very low levels of TR-beta 1 mRNA in fetal brain, IgG-beta 1 cleared just 5% of binding in this tissue. Studies using an antiserum (IgG-ch) generated against homologous segments of the hinge region in both TR-alpha 1 and -beta 1 yielded results which contrasted sharply with those of IgG-alpha 1/beta. Whereas IgG-ch could also immunoprecipitate virtually all binding from hepatic extracts it cleared only 40-50% of binding from the other tissues, including fetal brain in which TR-alpha 1 accounts for greater than 90% of binding protein. The data suggest the presence of posttranslational modification of the TR-alpha 1 protein in the hinge region, consistent with the presence in this segment of potential phosphorylation sites.

Amino Acid Sequence↗
PubMed1992

Ontogeny of hepatic nuclear triiodothyronine receptor isoforms in the rat.

We have determined the contribution of the thyroid hormone receptor (TR) isoforms TR alpha 1 and TR beta 1 to the postnatal rise in rat hepatic nuclear T3-binding capacity. In agreement with previous studies, total hepatic nuclear binding capacity rose by about 8-fold from the 19th day of gestation to young adulthood at 2 months of age (0.10 +/- 0.03 to 0.86 +/- 0.17 pmol/mg DNA). The levels of specific TR species were measured by immunoprecipitation of T3-binding activity from hepatic extracts using a panel of antisera directed against specific regions of the TR isoforms. The difference between receptor immunoprecipitated with antibody against TR beta 1 and that precipitated with an antibody against an identical region in both TR beta 1 and TR alpha 1 was tentatively assumed to represent TR alpha 1. TR alpha 1 accounted for virtually all T3-binding activity in fetal liver on gestational day 19 (G19), increased by 2-fold shortly after birth, and remained constant thereafter. TR alpha 1 mRNA, on the other hand, was highest in concentration on G16 and fell by 50-75% in the adult. TR beta 1 was undetectable by immunoprecipitation of hepatic extracts from fetuses on G19. However, Northern analysis showed the presence of TR beta 1 mRNA in the fetal liver, which rose in concentration by 3- to 4-fold in late gestation and then remained constant. The contribution of TR beta 1 to total binding capacity rose to 33% and 40% on postnatal days 15 and 30, respectively, and to 80% in the adult liver. Immunohistochemical analyses of hepatic sections confirmed the presence of very low levels of TR beta 1 in fetal liver as early as G16 and G19, and a sharp rise in TR beta 1 protein concentration in the postnatal period. This indicated that the increase in TR beta 1-binding capacity results from increased TR beta 1 mass. The increase in TR beta 1-binding capacity, thus, is due to increased translational efficiency of the beta 1 mRNA or stabilization of the TR beta 1 protein. The prominence of TR alpha 1 in both rat fetal liver and fetal brain, as previously demonstrated in our laboratory, raises the possibility that this receptor isoform may carry out specialized functions in the fetus and that TR beta 1 subserves still other functions at later stages of development.(ABSTRACT TRUNCATED AT 400 WORDS)

Aging↗
PubMed1991

Immunofluorescence localization of thyroid hormone receptor protein beta 1 and variant alpha 2 in selected tissues: cerebellar Purkinje cells as a model for beta 1 receptor-mediated developmental effects of thyroid hormone in brain.

Rat c-erbA beta 1 mRNA rises in cerebrum during the first 10 days of life, coincident with an increase in tissue triiodothyronine (T3) levels and T3-dependent brain development. These data suggest that the beta 1 receptor may mediate the T3 effect. However, in cerebellum c-erbA beta 1 mRNA levels were very low. Since cerebellar development, including dendritic arborization of Purkinje cells, is a T3-sensitive process, we assessed the levels of the beta 1 receptor protein in cerebellum during development. Antisera to unique peptide regions of beta 1 were raised. Their specificity was demonstrated by specific immunoprecipitation of the in vitro translated product, 85% immunoprecipitation of the T3 binding activity in hepatic nuclear extracts, and Western blot analysis of tissue extracts. Immunohistochemical studies using anti-beta 1 antiserum stained liver nuclei but not testis nuclei, which contain no T3 binding activity or beta 1 mRNA. In cerebellar Purkinje cells, an immunofluorescent signal, localized to the nucleus and more intense than that seen in the liver, was observed. A positive but weaker signal was also present in the granule cells. Thus, we may infer that the cerebellum contains significant concentrations of beta 1 receptor protein despite the low beta 1 mRNA content. Both the intensity of staining in Purkinje cell nuclei and immunoprecipitable beta 1 receptor binding capacity rose in the neonatal period. Antiserum to the non-T3 binding alpha 2 variant protein was also prepared and a distinctive pattern of fluorescence was observed. Strong fluorescence was seen in the nuclei of granule cells, but none was seen in Purkinje cells. The alpha 2 fluorescence in testis was high, consistent with the high levels of alpha 2 mRNA in this tissue. The fluorescent signal appeared to originate primarily in dividing spermatogonia. Our findings support the concept that the beta 1 receptor plays a central role in T3-induced brain development and strongly suggest that the Purkinje cell is a direct target for T3.

Amino Acid Sequence↗
PubMed1991

Functional relationship of thyroid hormone-induced lipogenesis, lipolysis, and thermogenesis in the rat.

Metabolic balance studies were carried out to determine the interrelationships of thyroid hormone-induced lipogenesis, lipolysis, and energy balance in the free-living rat. Intraperitoneal doses of 15 micrograms triiodothyronine (T3)/100 g body wt per d caused an increase in caloric intake from 26.5 +/- 1.7 (mean +/- SEM) kcal/100 g per d to 38.1 +/- 1.5 kcal/100 g per d. Food intake, however, rose only after 4-6 d of treatment and was maximal by the 8th day. In contrast, total body basal oxygen consumption rose by 24 h and reached a maximum by 4 d. Since total urinary nitrogen excretion and hepatic phosphoenolpyruvate carboxykinase mRNA did not rise, gluconeogenesis from protein sources did not supply the needed substrate for the early increase in calorigenesis. Total body fat stores fell approximately 50% by the 6th day of treatment and could account for the entire increase in caloric expenditure during the initial period of T3 treatment. Total body lipogenesis increased within 1 d and reached a plateau 4-5 d after the start of T3 treatment. 15-19% of the increased caloric intake was channeled through lipogenesis, assuming glucose to be the sole substrate for lipogenesis. The metabolic cost of the increased lipogenesis, however, accounted for only 3-4% of the T3-induced increase in calorigenesis. These results suggest that fatty acids derived from adipose tissue are the primary source of substrate for thyroid hormone-induced calorigenesis and that the early increase in lipogenesis serves simply to maintain fat stores. Since the mRNAs coding for lipogenic enzymes rise many hours before oxygen consumption and lipolysis, these results suggest that T3 acts at least in part by an early coordinate induction of the genes responsible for these processes.

Animals↗
PubMed1991

Prolonged fasting reduces rat hepatic beta 1 thyroid hormone receptor protein without changing the level of its messenger ribonucleic acid.

The level of hepatic nuclear T3-binding capacity falls in rats subjected to fasting. To define the mechanism underlying these changes, we have assayed in liver the concentration of the mRNA coding for the beta 1-receptor (beta 1-TR) isoform, the total nuclear T3-binding capacity, and the fraction of the total binding capacity that can be specifically immunoprecipitated with an anti-beta 1-TR immunoglobulin G preparation. Although no changes in beta 1-TR mRNA concentration were noted, we observed a 60% fall in total binding capacity. beta 1-TR mRNA levels were preserved despite a 50% fall in total poly(A)+ RNA. The fall in beta 1-TR protein, however, was consistent with a generalized decrease in total hepatic protein content. This study provides yet another instance in which measurement of receptor mRNA is not consonant with the behavior of the nuclear T3 receptor protein.

Animals↗
PubMed1990

Relationship of c-erbA mRNA content to tissue triiodothyronine nuclear binding capacity and function in developing and adult rats.

We have quantitated in adult and developing rat tissues the molar concentrations of c-erbA alpha 1- and beta 1-mRNAs, which code for nuclear T3-binding proteins, and c-erbA alpha 2-mRNA, which is generated by alternate splicing of the alpha gene transcript and codes for a receptor variant that does not bind T3. Comparison of the concentrations of c-erbA alpha 1-mRNA, beta 1-mRNA, or their sum to the T3 nuclear binding capacity per mg of DNA in adult liver, kidney, heart, cerebrum, and cerebellum and during the ontogeny of liver and brain shows that the T3 binding capacity/c-erbA mRNA ratio is tissue-specific and related to developmental state. Administration of T3 resulted in a 40-50% fall in the alpha 1 signal of adult liver, kidney, and heart without changing either the beta 1 signal or T3 binding capacity. A 40-fold increase in rat brain beta 1-mRNA occurred in the transition between the 19-day gestational fetus and the 10-day-old neonate. This corresponds to the period during which the T3 content rises in brain and during which T3 is known to influence central nervous system development. Our findings indicate that important translational or post-translational factors influence nuclear binding capacity and raise the possibility that c-erbA beta 1 may play a primary role in mediating T3 effects in developing and adult animals.

Aging↗
PubMed1990

Binding of 3,5,3'-triiodothyronine (T3) and its analogs to the in vitro translational products of c-erbA protooncogenes: differences in the affinity of the alpha- and beta-forms for the acetic acid analog and failure of the human testis and kidney alpha-2 products to bind T3.

We have compared the affinities for T3 and the T3 analog binding characteristics of the in vitro translational products of seven c-erbA cDNAs (chicken c-erbA alpha; human placental c-erbA beta; rat c-erbA beta-1; rat c-erbA alpha-1; rat c-erbA alpha-2; human testis c-erbA alpha-2; and human kidney c-erbA alpha-2). Four of these (chicken c-erbA alpha, human placental c-erbA beta, rat c-erbA beta-1, rat c-erbA alpha-1) bound T3 with high affinity as previously described. When compared under identical conditions of synthesis and [125I]T3 binding, there was no significant difference between the affinity of the chicken c-erb A alpha-1 and the human c-erbA beta but in a more limited series the affinity of rat c-erbA beta-1 for T3 was 4.6-fold higher than that of the rat c-erbA alpha-1. In vitro translational products of the beta-probes showed a characteristic 2.2-fold higher triiodothyroacetic acid/T3 ratio than did the products of the alpha-probes, regardless of the species of origin of the probe. As previously established, the rat c-erbA alpha-2 product did not bind T3. However, in contrast to two published reports, the human testis and kidney alpha-2 probe products also failed to bind T3. These findings indicate that highly conserved C-terminal 37-40 residues are important for high affinity T3 binding by proteins encoded by the c-erb A family of genes.

Amino Acid Sequence↗
PubMed1989

Thyroid hormone and circadian regulation of the binding activity of a liver-specific protein associated with the 5'-flanking region of the S14 gene.

Recent studies have described a DNase I hypersensitive site in the 5'-flanking region of the rat hepatic S14 gene that is closely associated with its expression. A 111-base pair subfragment (-389 to -279) of this region interacts specifically in a gel shift assay with a protein present in hepatic nuclear protein extracts. This protein, designated P1, was not present in extracts of other tissues, even those in which the gene is expressed and hormonally regulated. The binding activity of P1 is exceedingly low in extracts from hypothyroid rats and is markedly increased by administration of thyroid hormone. However, the slow accumulation of P1 after thyroid hormone administration indicates that increased levels of P1 are not necessary for the acute hormonal induction of S14 gene expression. The level of P1 binding activity increases in the evening, synchronous with circadian variation of hepatic mRNA S14. Since neither P1 binding activity nor circadian variation in mRNA-S14 levels are observed in the other tissues expressing the S14 gene, P1 may function to modulate the circadian rhythm observed in hepatic S14 gene expression. DNase I footprinting analysis revealed that P1 binds to a defined nucleotide sequence, 5'-AAAAGAGCTATTGATTGCCTGCA-3', located between -310 and -288 in the S14 gene.

Animals↗
PubMed1989

Time course of hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase activity and messenger ribonucleic acid, biliary lipid secretion, and hepatic cholesterol content in methimazole-treated hypothyroid and hypophysectomized rats after triiodothyronine administration: possible linkage of cholesterol synthesis to biliary secretion.

In an effort to define the mechanism by which thyroid hormone increases the synthesis of hepatic cholesterol, we have investigated both in hypophysectomized and methimazole-treated hypothyroid rats the time course of T3 effects on plasma cholesterol concentration, total hepatic cholesterol, the rate of biliary secretion of cholesterol, bile acids, and phospholipids, and the activity and mRNA levels of 3-hydroxy-3-methylglutaryl coenzyme-A (HMG-CoA) reductase, the rate-limiting enzyme in the hepatic synthesis of cholesterol. A single dose of 200 micrograms T3 was estimated to maintain at least 90% nuclear occupancy for the ensuing 54 h of the experiment. In both preparations the relative rise in biliary secretion of cholesterol exceeded that of other biliary constituents and preceded by 12 h an increase in HMG-CoA reductase enzyme activity and its mRNA. The level of total hepatic cholesterol remained constant throughout the experiment. We interpret these findings to suggest that T3-stimulated cholesterol synthesis is mediated by an antecedent T3-induced rise in biliary cholesterol secretion. We postulate that biliary cholesterol secretion is augmented by an intrahepatic shift of cholesterol and depletion of the hepatic sampling center responsible for the feedback regulation of cholesterol synthesis. The level of HMG CoA reductase mRNA appeared to govern enzyme activity in both preparations, but the ratio of mRNA to hepatic enzyme activity was substantially greater in the methimazole-treated compared with the hyphophysectomized animals.

Animals↗
PubMed1989

The regulation of lipogenesis by thyroid hormone and its contribution to thermogenesis.

We have used the tritiated water method to quantitate the effects of thyroid hormone on lipogenesis in the rat and then determined the contribution of this process to thyroid hormone-induced thermogenesis. After thyroid hormone administration to hypothyroid animals, fatty acid synthesis rose after a lag time of 12-16 h and reached a plateau after 4-5 days. This is consistent with the kinetics of an increase in oxygen consumption measured by others in similar animals. A diurnal variation was maintained in all thyroid states, with the peak value in the middle of the dark period being 3-fold higher than the nadir. Fatty acid synthesis in the livers of hyperthyroid animals was 3- to 4-fold higher than that in euthyroid rats, which, in turn, was 3- to 5-fold higher than the rate observed in hypothyroid rats. Slightly smaller but similar fold increases were measured in epididymal fat. A stimulation of fatty acid synthesis by thyroid hormone was also measured in the rest of the carcass, with hyperthyroid rates being twice those in hypothyroid animals. The contribution of the liver was much greater in hyperthyroid rats (34% of total fatty acid synthesis) than in hypothyroid animals (5%). The energy costs of this synthesis were calculated and compared to published values for total oxygen consumption in different thyroid states. Thus, 6-10% of the total increment in oxygen consumption between hyperthyroid and hypothyroid animals could be attributed to lipogenesis, depending on which published figures were used. About 3% of this increment was due to the liver alone.

Adenosine Triphosphate↗
PubMed1989

Thyroid hormone-, carbohydrate, and age-dependent regulation of a methylation site in the hepatic S14 gene.

The rat hepatic S14 gene has served as a model of thyroid hormone regulation of gene expression. Earlier studies of the S14-containing chromatin region demonstrated that a cytosine residue at position 625 (C-625) in the 3' untranslated exon was hypermethylated in hepatic DNA derived from hypothyroid animals. This observation was consistent with the markedly reduced level of expression of the S14 gene in these rats. The current studies have extended these observations to groups of rats in various thyroidal states. By using the restriction enzyme Hhal, the percent demethylation of this site was quantitated (hypothyroid, 9.3%; euthyroid, 19.2%; hyperthyroid, 66.6%). Moreover, the level of methylation was shown to be reversible as the thyroidal state was altered. Our data also indicate that these changes are probably independent of de novo DNA synthesis. Kinetic studies of the demethylation of this cytosine residue after T3 administration showed no change for at least 1 day and maximal change after about 4 days. This contrasts with the significant rise in S14 mRNA evident within 30 min and suggests that demethylation plays no role in the acute induction of this gene by T3. Carbohydrate feeding, another stimulus of S14 expression, similarly caused the demethylation of this cytosine residue. Earlier studies had demonstrated that mRNA S14 expression was not detectable in rat pups before about 20 days of age and continued to rise through the first year of life. Consistent with those findings, S-14 C-625 was fully methylated up to 15 days of age. Progressive demethylation then occurred up to 12 months of age. These results indicate that increased demethylation of a specific site in the 3' untranslated region of the S14 gene, possibly resulting from augmented excision repair processes, is correlated with increased expression of the gene.

Aging↗
PubMed1988

Triiodothyronine rapidly reverses inhibition of S14 gene transcription by glucagon.

The rapid response of hepatic mRNA-S14 to T3 has made this sequence an important model for studying the mechanism of hormonal induction of gene expression. In previous studies we showed, in the intact rat, that glucagon administration during the peak of the mRNA S14 diurnal rhythm causes a monoexponential fall in the level of mRNA-S14, and that T3 reverses this effect. We have now defined more precisely the mechanism governing this interaction. Measurement of in vitro nuclear transcriptional rates shows that T3 can reverse the glucagon-induced reduction of mRNA-S14 transcription. Reversal can be demonstrated within 5 min after the iv injection of T3. Further, the reversal appears to be related to the occupation of specific nuclear receptors, as inferred from the calculated nuclear occupancy and the effects of various iodothyronine analogs of T3. These results suggest that the effects of T3 are mediated by varying rates of production of the nuclear precursor and not by its stabilization, as previously proposed. Ancillary evidence supporting this conclusion came from the demonstration that the apparent t1/2 of the 4.5-kilobase precursor was not prolonged by T3.

Animals↗
PubMed1988

Triiodothyronine nuclear binding capacity in rat tissues correlates with a 6.0 kilobase (kb) and not a 2.6 kb messenger ribonucleic acid hybridization signal generated by a human c-erbA probe.

Recent studies have raised the possibility of multiple structurally distinctive tissue-specific nuclear T3 receptors, all exhibiting homology with the v-erbA oncogene and represented by mRNAs of various sizes. We have assayed the level of mRNAs recognized by a 32P-labeled cRNA derived from human plancetal c-erbA-A beta cDNA by solution hybridization and by Northern transfer in different rat tissues, as well as human liver and placenta. Two related mRNAs were demonstrated in the rat tissues analyzed, one measuring 6.0 and the other 2.6 kilobases (kb). In human liver and placenta a 6.0 kb mRNA was seen, but not a 2.6 kb mRNA. Only the 6.0 kb sequence correlated with the receptor concentration determined by 125I-T3 displacement analysis.

Animals↗
PubMed1987

Replacement dose, metabolism, and bioavailability of levothyroxine in the treatment of hypothyroidism. Role of triiodothyronine in pituitary feedback in humans.

A change in the formulation of the levothyroxine preparation Synthroid (Flint) in 1982 prompted us to reevaluate the replacement dose of this drug in 19 patients with hypothyroidism. The dose was titrated monthly until thyrotropin levels became normal. The mean replacement dose (+/- SD) was 112 +/- 19 micrograms per day, significantly less (P less than 0.001) than the dose of an earlier formulation--169 +/- 66 micrograms per day--used in a similar study (Stock JM, et al. N Engl J Med 1974; 290:529-33). The fractional gastrointestinal absorption of a tablet of the current formulation is 81 percent, considerably higher than the earlier estimate of 48 percent. Using high-performance liquid chromatographic analysis, we found that the current tablet contains the amount of thyroxine stated by the manufacturer. By measuring the bioavailability of the earlier type of tablet in five patients, we inferred that the strength of the previous tablet had been overestimated. In the present study, the thyrotropin levels of patients on replacement therapy returned to normal when serum triiodothyronine concentrations were not significantly different from those of controls (122 vs. 115 ng per deciliter [1.87 vs. 1.77 nmol per liter]), but when serum thyroxine levels were significantly above those of controls (11.3 vs. 8.7 micrograms per deciliter [145 vs. 112 nmol per liter], P less than 0.001). These findings suggest the possibility that in humans, serum triiodothyronine may play a more important part than serum thyroxine in regulating the serum thyrotropin concentration.

Biological Availability↗
PubMed1987

Rat hepatic mRNA-S14 and lipogenic enzymes during weaning: role of S14 in lipogenesis.

The rapid and marked response of hepatic mRNA-S14 sequence to both triiodothyronine and carbohydrate intake has made this sequence an attractive model for studying the action of hormonal and dietary factors. Because it is highly expressed and regulated only in lipogenic tissues, we have suggested that it plays a role in some aspect of lipid synthesis, transport, or metabolism. To provide more precise information regarding the function of S14 we have measured lipogenesis, lipogenic enzymes, beta-oxidation, and mRNA-S14 levels in spontaneously weaning neonatal rats and in rats prematurely weaned to a laboratory diet on postnatal day 17. After birth, the levels of lipogenesis, mRNA-S14, and the lipogenic enzymes malic enzyme (ME) and fatty acid synthase (FAS) were almost undetectable but increased with the onset of spontaneous weaning. Coincident with these changes, beta-oxidation decreased. Premature weaning beginning on day 17 resulted in an earlier and even more marked increase in lipogenesis, ME, FAS, and mRNA-S14. On day 19, ME and FAS activities were 6- to 19-fold more than activities in control suckling pups, whereas mRNA-S14 levels had risen to greater than 100 times the control values. Thus directional shifts in mRNA-S14 corresponded with indices of lipogenesis and were opposite to indices reflecting beta-oxidation. The response of mRNA-S14 therefore suggests that it may be related to the synthesis of fatty acids. On the other hand, the level of lipogenesis in the fetus was high despite the fact that the levels of both mRNA-S14 and ME were low. This dissociation raises the possibility that the S14 protein participates in lipogenesis in the neonate and adult but not in the fetus.

Aging↗
PubMed1987

An analysis of the interrelationship of nuclear and plasma triiodothyronine in the sea lamprey, lake trout, and rat: evolutionary considerations.

We have studied the interrelationship of plasma and hepatic nuclear T3 in two lower vertebrates, lake trout and sea lamprey. Specific nuclear T3-binding sites were observed in liver of both species. Binding affinities for T3 and several analogs in lamprey and trout were similar to those in rat liver. The findings of identical sedimentation coefficients, Stokes radii, and mol wt suggest a high degree of homology of the receptor molecule among these species. In lamprey ammocoete, both total (approximately 26 nM) and free (approximately 0.12 nM) T3 concentrations were 7- to 36-fold higher than in the trout or rat. The concentration of free T4 was 4 times greater than that in rat plasma despite somewhat lower total T4 levels. This resulted from a 6 times lesser binding of T4 by ammocoete plasma. Total and free T3 and T4 levels fell sharply during transformation to the adult form. In trout, a 27 times greater concentration of free T3 in hepatic nuclei than in cytosol, previously documented for rat liver, gave evidence of the early evolution of an active transport process between these compartments. Analysis of [125I]T3 interchange in trout indicated the existence of two kinetically distinct compartments within the liver, an outer compartment in rapid equilibrium with plasma, and a slowly equilibrating compartment decaying with a t1/2 of 3.7 h. T3 associated with the nuclear receptor appeared to be in equilibrium with the slower hepatic compartment. Despite the apparent resemblance of T3 receptors in these species to those in the rat, administration of T3 (200 micrograms/100 g BW for 4 days) to trout caused no increase in hepatic oxygen consumption or the activities of malic enzyme or alpha-glycerophosphate dehydrogenase. Further, such treatment produced no consistent changes in hepatic mRNA activity profiles analyzed by two-dimensional electrophoresis of in vitro translational products. As suggested by the high free T3 concentration in the larval lamprey ammocoete, T3 in early vertebrates may have a primarily developmental function, manifested over a restricted time frame.

Animals↗
PubMed1987

Diurnal variation in hepatic expression of the rat S14 gene is synchronized by the photoperiod.

We have analyzed the factors responsible for the circadian variation in rat hepatic mRNA-S14. Regulation of this sequence, which is found in lipogenic tissues and encodes a protein (S14) believed to be associated with fatty acid synthesis, is an excellent model of the interaction of thyroid hormone and dietary factors at the hepatocellular level. The mRNA exhibits a 3-fold diurnal variation (peak, approximately 2000 h; nadir, 0800 h) in ad libitum feeding rats on a 12-h light, 12-h dark photoschedule. We studied the effects of the photoschedule, periodic food intake, hypophysectomy, and induction by thyroid hormone (T3) on the mRNA-S14 rhythm. Adaptation to feeding restricted to either light or dark periods for 15 days did not greatly affect the diurnal rhythm. Photoreversal resulted in a 180 degrees phase shift, whereas the rhythm persisted in the presence of constant light. Oscillation continued around a higher baseline after a receptor-saturating dose of T3 in both normal and hypophysectomized rats. Our results indicate primary entrainment of the mRNA-S14 diurnal rhythm to the photoperiod, rather than to periodic food intake. Moreover, the circadian regulatory signal, which probably originates in the central nervous system, appears capable of antagonizing a maximal T3-inductive stimulus and does not originate in the pituitary gland. Persistence of the oscillation in constant light rules out circulating melatonin as the mediator. Synchronization of the rhythm by the photoschedule suggests that neuroendocrine factors are important determinants of rhythmic changes in hepatic gene expression.

Animals↗