The insulin sensitizing effect of topiramate involves KATP channel activation in the central nervous system.

BACKGROUND AND PURPOSE: Topiramate improves insulin sensitivity, in addition to its antiepileptic action. However, the underlying mechanism is unknown. Therefore, the present study was aimed at investigating the mechanism of the insulin-sensitizing effect of topiramate both in vivo and in vitro. EXPERIMENTAL APPROACH: Male C57Bl/6J mice were fed a run-in high-fat diet for 6 weeks, before receiving topiramate or vehicle mixed in high-fat diet for an additional 6 weeks. Insulin sensitivity was assessed by hyperinsulinaemic-euglycaemic clamp. The extent to which the insulin sensitizing effects of topiramate were mediated through the CNS were determined by concomitant i.c.v. infusion of vehicle or tolbutamide, an inhibitor of ATP-sensitive potassium channels in neurons. The direct effects of topiramate on insulin signalling and glucose uptake were assessed in vivo and in cultured muscle cells. KEY RESULTS: In hyperinsulinaemic-euglycaemic clamp conditions, therapeutic plasma concentrations of topiramate (∼4 µg·mL(-1) ) improved insulin sensitivity (glucose infusion rate + 58%). Using 2-deoxy-D-[(3) H]glucose, we established that topiramate improved the insulin-mediated glucose uptake by heart (+92%), muscle (+116%) and adipose tissue (+586%). Upon i.c.v. tolbutamide, the insulin-sensitizing effect of topiramate was completely abrogated. Topiramate did not directly affect glucose uptake or insulin signalling neither in vivo nor in cultured muscle cells. CONCLUSION AND IMPLICATIONS: In conclusion, topiramate stimulates insulin-mediated glucose uptake in vivo through the CNS. These observations illustrate the possibility of pharmacological modulation of peripheral insulin resistance through a target in the CNS.

Pharmacological modulation of dopamine receptor D2-mediated transmission alters the metabolic phenotype of diet induced obese and diet resistant C57Bl6 mice.

High fat feeding induces a variety of obese and lean phenotypes in inbred rodents. Compared to Diet Resistant (DR) rodents, Diet Induced Obese (DIO) rodents are insulin resistant and have a reduced dopamine receptor D2 (DRD2) mediated tone. We hypothesized that this differing dopaminergic tone contributes to the distinct metabolic profiles of these animals. C57Bl6 mice were classified as DIO or DR based on their weight gain during 10 weeks of high fat feeding. Subsequently DIO mice were treated with the DRD2 agonist bromocriptine and DR mice with the DRD2 antagonist haloperidol for 2 weeks. Compared to DR mice, the bodyweight of DIO mice was higher and their insulin sensitivity decreased. Haloperidol treatment reduced the voluntary activity and energy expenditure of DR mice and induced insulin resistance in these mice. Conversely, bromocriptine treatment tended to reduce bodyweight and voluntary activity, and reinforce insulin action in DIO mice. These results show that DRD2 activation partly redirects high fat diet induced metabolic anomalies in obesity-prone mice. Conversely, blocking DRD2 induces an adverse metabolic profile in mice that are inherently resistant to the deleterious effects of high fat food. This suggests that dopaminergic neurotransmission is involved in the control of metabolic phenotype.

Blocking dopamine D2 receptors by haloperidol curtails the beneficial impact of calorie restriction on the metabolic phenotype of high-fat diet induced obese mice.

Calorie restriction is the most effective way of expanding life-span and decreasing morbidity. It improves insulin sensitivity and delays the age-related loss of dopamine receptor D(2) (DRD2) expression in the brain. Conversely, high-fat feeding is associated with obesity, insulin resistance and a reduced number of DRD2 binding sites. We hypothesised that the metabolic benefit of calorie restriction involves the preservation of appropriate DRD2 transmission. The food intake of wild-type C57Bl6 male mice was restricted to 60% of ad lib. intake while they were treated with the DRD2 antagonist haloperidol or vehicle using s.c. implanted pellets. Mice with ad lib. access to food receiving vehicle treatment served as controls. All mice received high-fat food throughout the experiment. After 10 weeks, an i.p. glucose tolerance test was performed and, after 12 weeks, a hyperinsulinaemic euglycaemic clamp. Hypothalamic DRD2 binding was also determined after 12 weeks of treatment. Calorie-restricted (CR) vehicle mice were glucose tolerant and insulin sensitive compared to ad lib. (AL) fed vehicle mice. CR mice treated with haloperidol were slightly heavier than vehicle treated CR mice. Haloperidol completely abolished the beneficial impact of calorie restriction on glucose tolerance and partly reduced the insulin sensitivity observed in CR vehicle mice. The metabolic differences between AL and CR vehicle mice were not accompanied by alterations in hypothalamic DRD2 binding. In conclusion, blocking DRD2 curtails the metabolic effects of calorie restriction. Although this suggests that the dopaminergic system could be involved in the metabolic benefits of calorie restriction, restricting access to high-fat food does not increase (hypothalamic) DRD2 binding capacity, which argues against this inference.

Growth hormone and ghrelin secretion are associated with clinical severity in Huntington's disease.

BACKGROUND AND PURPOSE: Huntington's disease (HD) is a fatal hereditary neurodegenerative disorder caused by an increased CAG repeat size in the huntingtin gene. Apart from neurological impairment, the disease is also accompanied by progressive weight loss, abnormalities in glucose homeostasis and a higher prevalence of diabetes mellitus, which may partly be caused by disturbed growth hormone (GH) and ghrelin secretion. Therefore, we aimed to perform a detailed analysis of GH and ghrelin secretion in HD patients in relation to clinical signs and symptoms. METHODS: In nine early-stage, medication-free HD patients and nine age-, gender- and body mass index-matched controls, we measured serum GH levels every 10 min for 24 h and assessed ghrelin response to food intake. Multi-parameter auto-deconvolution and approximate entropy analysis were applied to quantify basal, pulsatile, and total GH secretion rates as well as the regularity of GH secretion. RESULTS: We found no significant differences in GH and ghrelin secretion characteristics between HD patients and controls (total GH secretion: 137 +/- 36 vs. 181 +/- 43 mU/l/24 h, respectively; P = 0.439). However, in HD patients, both GH secretion and its irregularity as well as the degree of postprandial ghrelin suppression significantly increased with worsening motor and functional impairment (all P < 0.05). Moreover, postprandial ghrelin suppression also increased with decreasing body weight and higher CAG repeat number (both P < 0.05). CONCLUSIONS: These findings suggest changes in the regulation of GH and ghrelin secretion dynamics in early stage HD patients that could become more prominent in the later stages of the disease.

Expression of the human sodium/iodide symporter (hNIS) in xenotransplanted human thyroid carcinoma.

The uptake of iodide in thyroid epithelial cells is mediated by the sodium/iodide symporter (NIS). The uptake of iodide is of vital importance for thyroid physiology and is a prerequisite for radioiodine therapy in thyroid cancer. Loss of iodide uptake due to diminished expression of the human NIS (hNIS) is frequently observed in metastasized thyroid cancer. So far, no animal model for the study of radioiodine therapy in thyroid cancer has been available. Strategies to restore iodide uptake in thyroid cancer include the exploration of hNIS gene transfer into hNIS defective thyroid cancer. We have performed a stable transfection of hNIS into the hNIS defective follicular thyroid carcinoma cell line FTC133. Stably transfected colonies exhibited high uptake of Na125I, which could be blocked completely with sodium perchlorate. hNIS transfected FTC133 and non-transfected cell lines injected subcutaneously in nude mice formed tumors after 6 weeks. Iodide uptake in the hNIS transfected tumor was much higher than in non-transfected tumor, but a rapid release of radioactivity from the hNIS transfected tumor was observed. Further studies are necessary to investigate the role of hNIS in relation to other thyroid specific proteins in iodide metabolism in thyroid cancer.

The expression of the sodium/iodide symporter is up-regulated in the thyroid of fetuses of iodine-deficient rats.

Is the fetal thyroid already capable to increase its iodide uptake in response to iodine deficiency? To answer this question, we analyzed the expression of the Na(+)/I(-) symporter and several other genes in the thyroid of rat fetuses at 21 d of gestation from control mothers presenting a mild or more severe iodine deficiency. Female rats were placed on a low iodine diet, not supplemented, or supplemented with iodide or perchlorate for 3 months. The maternal and fetal thyroidal iodide uptake was measured 24 h after injection of 10 microCi Na (125)I into the dams. The absolute iodide uptake of the maternal thyroid was unchanged in a low iodine diet, not supplemented, compared with one supplemented with iodide. In contrast, the fetal thyroid absolute iodide uptake of a low iodine diet, not supplemented, and one supplemented with perchlorate was decreased by 70% and 95% compared with that supplemented with iodide. Na(+)/I(-) symporter mRNA was detected in the fetal thyroid of supplemented with iodide and increased about 2- and 4- fold in the thyroid of fetuses from a low iodine diet, not supplemented, and one supplemented with perchlorate, respectively. Na(+)/I(-) symporter expression was induced in the fetal side of the placenta in both a low iodine diet, not supplemented, and one supplemented with perchlorate; in contrast, Na(+)/I(-) symporter mRNA was never detected in the maternal side of the placenta. Fetal thyroid thyroglobulin and type I deiodinase mRNA contents were only significantly increased with a diet supplemented with perchlorate. Glucose transporter 4 mRNA was decreased in the fetal thyroid of both a low iodine diet, not supplemented, and one supplemented with perchlorate compared with one supplemented with iodide. In conclusion, although the up-regulation of Na(+)/I(-) symporter expression in fetal thyroid and placenta in the low iodine diet, not supplemented group did not lead to restoration of a normal absolute iodide uptake, our data show that all adaptive and/or defending mechanisms against iodine deficiency are already present in the fetus.

Beneficial effects of retinoic acid on extracellular matrix degradation and attachment behaviour in follicular thyroid carcinoma cell lines.

The prognosis of patients with metastasised follicular thyroid carcinoma (FTC) is limited, necessitating the search for new treatment options. Beneficial effects of retinoids have been suggested in thyroid cancer and the present study was performed to investigate the effects of retinoic acid (RA) on important determinants of metastatic behaviour in FTC: the disengagement of tumour cells from the primary tumour and the degradation of extracellular matrix, focusing on the role of the plasmin activation system and the integrin and E-cadherin families of attachment molecules. Three FTC cell lines were studied: FTC-133, derived from the primary tumour; and FTC-236 and FTC-238, derived from metastases. FTC cell lines were cultured with 0.1, 1 and 10 microM 13-cis-RA or with the solvent DMSO for 1 and 5 days. Extracellular matrix degradation by these cell lines was studied by assessing the 48-h release of radioactivity from (35)S-methionine labelled extracellular matrix proteins synthesised by the MC3T3 cell line coated onto plastic. The involvement of constituents of the plasmin activation system was investigated by semi-quantitative RT-PCR and zymography. Attachment to extracellular matrix was studied by determining the number of adhering FTC cells to extracellular matrix coated onto plastic, 3 h after seeding. The involvement of attachment molecules was studied by RT-PCR with primers for integrin subclasses and E-cadherin and immunofluorescence for E-cadherin. Five days culturing with 10 microM RA reduced the degradation of extracellular matrix significantly in all cell lines: FTC-133 by 35%, FTC-236 by 74% and FTC-238 by 31%. Zymography revealed diminished activity of urokinase type plasminogen activator (uPA) in FTC-236 and FTC-238, but not in FTC-133 cultured with RA. mRNA expression of the uPA receptor was diminished in FTC-236. In the attachment assay, 10 microM RA for 5 days increased the number of adherent cells to extracellular matrix significantly by 91% in FTC-133, 64% in FTC-236 and 87% in FTC-238. No effects of RA on integrin or E-cadherin mRNA expression were observed. Immunofluorescence, however, revealed enhanced organisation of E-cadherin along the cell membrane by RA treatment. In conclusion, the present study demonstrates beneficial effects of RA on important determinants of metastatic behaviour in FTC cell lines, e.g. decreased degradation of extracellular matrix which may in part be explained by effects on the plasmin activation system and enhanced attachment to extracellular matrix. These findings may add to the explanations for beneficial effects of retinoids in thyroid cancer.

Contribution of 3,5,3'-tri-iodothyronine produced locally from thyroxine in several maternal tissues of the near-term pregnant rat.

3,5, 3'-Tri-iodothyronine(T3)is produced by the thyroid and locally, by monodeiodination of thyroxine (T4), in the peripheral tissues. During pregnancy the thyroid hormone status in rats is altered: plasma and tissue levels of T4 and T3 are decreased. We investigated the effects of pregnancy on the contribution of T3 produced locally in the maternal tissues by administering a continuous infusion of [125I]T4 and [131I]T3. The transport of T4 to almost all maternal organs diminished. Less T was transported from the plasma to brown adipose tissue (BAT), liver, kidney and pituitary, in contrast to the ovary where an increase was found. In BAT and brain the amount of locally produced T3 decreased, despite the known increase in deiodinase type II activity in the brain while in liver the contribution of locally produced T3 remained constant, despite a known increase in deiodinase type I activity during pregnancy. This discrepancy between deiodinase activities and locally produced T3 can be explained by an insufficient availability of T4. Thus, we conclude that in the rat a decrease in maternal T4 concentration, together with the transport of T4 to the feto-placental compartment, results indirectly in a diminished availability of T3 in the maternal organs.

Effects of marginal iodine deficiency on thyroid hormone production, distribution and transport in nonpregnant and near-term pregnant rats.

During pregnancy maternal thyroid hormones are of great importance for normal development of the central nervous system of the fetus. Iodine deficiency of the mother carl result in an impaired development of the fetal brain. In large areas of the world iodine intake is moderately low. To study the effects of marginal iodine deficiency (MID) on the production, distribution, and transport of thyroxine (T4) and 3,5,3'-tri-iodothyronine (T3) in nonpregnant and near-term pregnant rats we performed kinetic experiments (three-compartment analysis). Despite unchanged plasma T4 and T3 during MID, the production and plasma clearance rates of T4 decreased 30% (P = 0.01) in MID nonpregnant (MID-C) rats. For T3, the plasma clearance rate was increased 70% (P = 0.046), while the T3 production was more than doubled (P = 0.042) in MID-C rats. In MID near-term pregnant rats T3 production was decreased 20% (P = 0.04). Hepatic deiodinase type I activity increased during MID in both nonpregnant and pregnant rats. It appears that during MID, rats are able to maintain their euthyroid status. The pronounced increase in transport of T4 from plasma to the fast pool observed in pregnant rats on a normal iodine diet did not occur during MID. In conclusion, in rats MID affects maternal thyroid hormone metabolism, thus influencing the availability of maternal T4 for the fetus.

Iodothyronine deiodinase activities in fetal rat tissues at several levels of iodine deficiency: a role for the skin in 3,5,3'-triiodothyronine economy?

Iodothyronine deiodinases, types I, II, and III (D1, D2, and D3) activities were measured in tissues of fetal rats, at 18 and 21 days of gestation, at several levels of iodine deficiency (ID): mild ID diet (MID) and moderately severe ID, MID + 0.005% perchlorate (MID+P). D2 was present in fetal skin, increased between days 18 and 21, and also in MID and MID+P. In skin, D3 increased during ID at day 18, whereas there was a decrease at day 21. Skin T4 decreased in MID and MID+P, showing an inverse relationship with D2. Skin T3 decreased at day 18 in MID and MID+P but increased at day 21, probably because of the increased D2 and decreased D3, maintaining T3 concentrations. No effect of ID was observed on hepatic D1. D2 increased in brain and brown adipose tissue at day 21 in MID+P. No changes were found in maternal placental D2 and D3, but D2 and D3 increased in the fetal placenta at day 18 in MID+P. A higher level of D2 is present in fetal skin than in the brain. As the activity is increased, in even mild ID (and already at 18 days) it can be concluded that skin D2 is likely to be of considerable physiological importance, at least for fetal thyroid hormone economy, by contributing to the intracellular T3 content of the skin and, possibly, to the plasma T3.

Synthetic flavonoids cross the placenta in the rat and are found in fetal brain.

The synthetic flavonoid EMD-49209 is a potent inhibitor of the in vivo and in vitro binding of thyroxine (T4) to transthyretin (TTR). We studied the distribution of 125I-labeled EMD-49209 in maternal tissues, intestinal contents, and fetal tissues in rats that were 20 days pregnant (from 1 to 24 h after intravenous injection). The percent dose of EMD decreased quickly with time. In maternal brain no radioactive flavonoid could be detected. EMD was excreted very rapidly from the intestines. In the fetal compartment the percent dose of EMD increased with time; after 24 h it contained 17% of the EMD. The flavonoid was found in all fetal tissues investigated and also in the fetal brain. Because TTR concentrations are high in the fetal rat, especially in the brain, the transfer of flavonoid to the fetal brain might be linked to TTR expression. The presence of flavonoid in the fetal brain raises the possibility of an essential interference of flavonoids with the availability of T4 in the fetal compartment.

Maternal thyroxine and 3,5,3'-tri-iodothyronine kinetics in near-term pregnant rats at two different levels of hypothyroidism.

Thyroid hormones are extremely important for development of the fetal central nervous system. Thyroidectomy results in severe hypothyroidism. In this study two levels of maternal hypothyroidism were reached by administration of different amounts of thyroxine (T4) and 3,5,3'-tri-iodothyronine (T3) to thyroidectomized pregnant rats. We examined the production, distribution and transport of T4 and T3 by performing a kinetic experiment (three-compartment analysis) with intact and thyroidectomized near-term pregnant rats which received either very low (Tx + lowTH) or normal (Tx + TH) doses of T4 and T3. Despite administration of normal doses of thyroid hormones, plasma TSH was still elevated in the Tx + TH rats, meaning that these rats were still mildly hypothyroid. The Tx + lowTH rats were markedly hypothyroid, the plasma T4 and T3 levels being very low. In the mildly hypothyroid rats the transport of T4 from plasma to the fast pool and vice versa was decreased compared with intact near-term pregnant rats. This could imply that much less T4 is transported to the feto-placental compartment. Liver type I deiodinase was decreased, resulting in lowered plasma T3 values. In the markedly hypothyroid rats all pools and rates of transport of T4 and T3 were greatly decreased. In conclusion, even mild hypothyroidism, despite normal plasma T4 values, results in significant changes, especially in maternal T4 transport. We suggest that even mild maternal hypothyroidism will have a negative effect on the availability of maternal T4 for fetuses.

Effects of 5,5'-diphenylhydantoin on the metabolic pathway of thyroid hormone in rats.

Treatment of rats with phenytoin (DPH), an anti-epileptic drug, results in lower tissue thyroid hormone (TH) levels and interferes with the metabolic pathway of TH. To test the hypothesis that DPH affects the enterohepatic cycle of TH and, thus, the kinetics of TH turnover, we performed a kinetic experiment (three-compartment analysis) and a steady-state, double-isotope equilibrium experiment in rats treated for 3 weeks with DPH (50 mg/kg body weight per day) and in untreated controls. This included measurements of TH and TH metabolite levels, as well as the activities of enzymes involved in the TH metabolic pathway. DPH treatment resulted in a decrease in the production of thyroxine (T4) (by 25%) and tri-iodothyronine (T3) (by 37%), a decrease in the T3 concentration in all three pools, and a redistribution of T4 from the fast to the slow pool. The amount of T4 increased in intestinal contents and feces by 66% and 71% respectively. Expressed as a fraction of daily TH disposal, fecal loss of T4 was enhanced from 10 to 23% and that of T3 from 16 to 21%. An increase in T4 and T3 UDP-glucuronyltransferase activities was observed, suggesting that the increased fecal loss of T4 and T3 is secondary to an increased biliary output of their glucuronides. The reduced secretion of TH and increased fecal clearance during DPH treatment can lead in the long run to depletion of TH stores.

Different tissue distribution, elimination, and kinetics of thyroxine and its conformational analog, the synthetic flavonoid EMD 49209 in the rat.

The synthetic flavonoids EMD 23188 and EMD 49209, developed as T4 analogs, displace T4 from transthyretin, and in vitro they inhibit 5'-deiodinase activity. In vivo EMD 21388 causes tissue-specific changes in thyroid hormone metabolism. In tissues that are dependent on T3 locally produced from T4, total T3 was diminished. It was not known whether it was the presence of EMD interfering with 5'-deiodinase type II in tissues or the decreased T4 (substrate) availability that caused the lowered T3. To study whether the flavonoids enter tissues and, if this were the case, whether they enter tissues similarly, [125I]EMD 49209 together with [131I]T4 were injected into female rats and rats pretreated with EMD 21388. Tissues were extracted and submitted to HPLC. [125I]EMD 49209 disappeared quickly from plasma and enters peripheral tissues; peak values were reached after 0.25-0.5 h. Then [125I]EMD 49209 appeared in the intestines (after 6 h 40% of the dose). Tissue uptake of [131I]T4 was very rapid. EMD 21388 pretreatment caused an increase in the excretion of [125I]EMD 49209 into the intestines (40% after 0.25 h). The uptake of [131I]T4 increased, but not high enough to ensure normal tissue T4 concentrations. In the 5'-deiodinase type II-expressing tissues, no [125I]EMD 49209 could be detected. We conclude that the decrease in T3 locally produced from T4 is caused by the shortage of T4 as substrate and not to a direct effect of EMD on the activity of 5'-deiodinases I and II.

Effects of marginal iodine deficiency during pregnancy: iodide uptake by the maternal and fetal thyroid.

Iodide uptake by the thyroid is an active process. Iodine deficiency and pregnancy are known to influence thyroid hormone metabolism. The aim of this study was to clarify the effects of iodine deficiency and pregnancy on iodide uptake by the thyroid. Radioiodide was injected intravenously into nonpregnant and 19-day pregnant rats receiving a normal or marginally iodine-deficient diet. The uptake of radioiodide by the thyroid was measured continuously for 4 h. The absolute iodide uptake by the maternal and fetal thyroid glands at 24 h was calculated by means of the urinary specific activity. Pregnancy resulted in a decrease in the absolute thyroidal iodide uptake. Marginal iodine deficiency had no effect on the absolute iodide uptake by the maternal thyroid. The decreased plasma inorganic iodide was compensated by an increase in thyroidal clearance. A similar compensation was not found for the fetus; the uptake of iodide by the fetal thyroid decreased by 50% during marginal iodine deficiency. This can lead to diminished thyroid hormone production, which will have a negative effect on fetal development, especially of the brain.

Effects of 5, 5'-diphenylhydantoin on the thyroid status in rats.

Treatment of rats with phenytoin (DPH), an anti-epileptic drug, results in lower tissue thyroid hormone (TH) levels. To investigate if this is accompanied by tissue hypothyroidism, rats were treated for 3 weeks with DPH (50 mg/kg body wt in food). Thyroid hormone-dependent parameters were measured, and the results were compared to those of control rats and to those of athyreotic rats substituted with thyroxine + triiodothyronine (Tx + TH) to reach the same plasma TH levels as DPH-treated rats. These rats were mildly hypothyroid with regard to their TH and TSH levels and TH-dependent parameters. Both DPH and Tx + TH led to a decrease in plasma thyroxine (T4) and triiodothyronine (T3) (+/-70% of the control). The percentage free T4 was unchanged. Plasma thyrotropin (TSH) was increased only in the Tx + TH rats (sixfold). For DPH rats, pituitary hormone content was not different from the control; growth hormone was lower and TSH was higher in Tx + TH rats. In DPH and Tx + TH rats, an increase in hepatic T4 and T3 uridine-diphosphate glucuronyltransferase activity was found, likewise indicating a change in the metabolic pathway of TH. Hepatic iodothyronine deiodinase (ID) type I activity decreased in Tx + TH rats but did not alter in DPH rats. Hepatic alpha-glycerophosphate dehydrogenase (alpha-GPD) decreased in DPH and Tx + TH rats. Malic enzyme in liver was enhanced in DPH rats. In the brains of DPH rats the level of alpha-GPD activity was raised; in Tx + TH it was lowered. The ID type II activity in the brain was reduced in DPH rats, but ID type III did not change for either group. Total body oxygen consumption increased in DPH rats (13%); it decreased in Tx + TH rats (9%). Our results show that DPH causes changes comparable to mild hypothyroidism. The lack of or a diminished hypothyroid response can be explained as the attenuating agonistic effect of DPH, which is supported by O2 consumption, brain ID type II and alpha-GPD activities. The T4 content was reduced by 30% in thyroid digests; this, together with a reduced T4 secretion, can lead to serious hypothyroxinemia during prolonged DPH treatment.

The effect of GHRH on TSH release in rats in vivo and in vitro.

In man, GHRH has been shown to potentiate the TSH-releasing activity of TRH. To study the way by which GHRH affects TRH-stimulated TSH release, we examined the effect of GHRH (1-29)NH2 on basal and stimulated TSH secretion in intact male rats and superfused dispersed rat pituitary cells. In the intact rats, GHRH(1-29)NH2 potentiated TRH-stimulated TSH release in the evening, but potentiation was not observed in the morning and in dispersed pituitary cells. Basal TSH levels were not changed by GHRH(1-29)NH2. It is concluded that GHRH(1-29)NH2 potentiates the TSH-releasing activity of TRH in the evening in rats possibly through suprahypophyseal disinhibition.

Hypothyroidism retards progressive glomerulosclerosis in the rat by a reduction in food intake.

Hypothyroidism diminishes proteinuria and prolongs survival in several immune models of progressive renal failure. In the well-characterized non-immune model of 5/6 nephrectomy we studied the effects of thyroidectomy (Tx) on the development of proteinuria and glomerulosclerosis (GS). Hypothyroidism was confirmed by lower values of thyroxine in Tx rats compared to sham Tx rats at 9 weeks (12.6 +/- 6.7 nmol/l Tx versus 37.7 +/- 10.8 nmol/l sham Tx) and 12 weeks after operation (7.2 +/- 4.9 nmol/l Tx versus 14.4 +/- 4.1 nmol/l sham Tx). Tx resulted in a reduction in mean arterial blood pressure and proteinuria and a lower incidence of GS (4.2 +/- 3.1% Tx versus 17.1 +/- 10.0% sham Tx) 12 weeks after nephrectomy, along with a decrease in food intake (104 +/- 13 g/week Tx versus 138 +/- 10 g/week sham Tx). In the same experiment a third group of sham Tx rats was pair fed to the Tx rats, resulting in values similar to those of Tx rats for proteinuria and the incidence of GS (6.0 +/- 4.9% pair fed sham Tx). Thyroxine levels at 9 and 12 weeks were comparable to those in sham Tx rats fed ad libitum. No association was found between the incidence of GS and glomerular volume. Studies of the inulin clearance in a second set of experiments showed that glomerular filtration rate and renal plasma flow are lower in hypothyroid rats compared to sham Tx rats. We conclude that hypothyroidism has a renal protective effect due to a decrease in food intake resulting in alterations in renal haemodynamics.

Effect of administration of growth hormone on plasma and intracellular levels of thyroxine and tri-iodothyronine in thyroidectomized thyroxine-treated rats.

We have studied the effects of the administration of GH on plasma levels and peripheral production of tri-iodothyronine (T3) from thyroxine (T4) in thyroidectomized male Wistar rats given a continuous i.v. infusion of T4 (1 microgram/100 g body weight per day) and GH (120 micrograms per day) for 3 weeks. Tracer doses of 131I-labelled T3 and 125I-labelled T4 were added to the infusion. At isotopic equilibrium (10 days after the addition of 125I-labelled T4) the rats were bled and perfused. The plasma appearance rate for T3 was higher (10.6 +/- 1.3 vs 8.4 +/- 2.8 pmol/h per 100 g body weight, P = 0.05) and plasma TSH was lower (246 +/- 24 vs 470 +/- 135 pmol/l, P less than 0.01) in GH-treated rats. The amount of T3 in liver (12.3 +/- 2.8 vs 5.5 +/- 1.7 pmol/g wet weight, P less than 0.01), kidney (11.5 +/- 1.4 vs 6.5 +/- 1.4 pmol/g wet weight, P less than 0.01) and pituitary (8.8 +/- 2.7 vs 4.8 +/- 0.5 pmol/g wet weight, P less than 0.01) was higher than in controls, mainly as a result of an increased local production of T3 from T4, but plasma-derived T3 was also higher in most organs. We found an increased intracellular T3 concentration in the pituitary which may be responsible for the lower plasma TSH concentration in the GH-treated rats. Since the increase in locally produced T3 is found particularly in liver, kidney and pituitary, typical organs that express 5'-deiodinase activity, we suggest that GH acts on thyroid hormone metabolism by stimulating type-I deiodinase activity.

Effects of streptozocin-induced diabetes and food restriction on quantities and source of T4 and T3 in rat tissues.

Diabetes mellitus and fasting are both associated with low plasma thyroid hormone concentrations and loss of body weight. To discriminate between the separate effects of energy shortage and insulin, we studied control rats, diabetic rats (DM), DM rats treated with insulin (DMI), and rats after modified fasting (MF1 and MF2; 70 and 30% of normal daily food intake, respectively). In double-isotopic equilibrium experiments, we determined the tissue thyroxine (T4) and triiodothyronine (T3) concentrations and the contribution of local T4-to-T3 conversion to total T3 in rat tissues; thyroidal T4 and T3 secretion and extrathyroidal T3 production were calculated. In DM and DMI rats, plasma T4 and T3 decreased; in MF1 and MF2 rats, only plasma T4 decreased. Thyroidal T4 secretion decreased, whereas that of T3 remained normal. The decrease in tissue T4 in MF and DM rats paralleled the decrease in plasma T4. Although plasma T3 did not differ in DM and DMI rats, total T3 concentrations in all tissues were not the same due to changed uptake of T3 from plasma and local T4-to-T3 conversion; these changes were not found in several tissues of MF1 and MF2 rats. Our results suggest that the decrease in tissue T4 during diabetes mellitus is due to the decrease in plasma T4 caused by the decreased thyroidal secretion, possibly due to intracellular energy shortage. The changes in tissue T3 during diabetes mellitus are only partly attributable to the same phenomenon; in several tissues, the decrease in T3 seems more related to the lack of insulin.