Effect of cellular cholesterol changes on insulin secretion by tumor cell lines

Glucose and cell swelling induce insulin secretion by alternative signaling pathways. Swelling-induced secretion is in most systems independent of calcium and various mediators of glucose stimulation. Comparison of two insulinoma tumor cell lines revealed surprising difference; INS-1E cells in contrast to INS-1 cells and isolated rat pancreatic islets do not respond to hypotonicity in the presence of calcium. To delineate the role of cholesterol the effect of its extraction or addition on the insulin secretion in response to glucose and cell swelling was compared. INS-1E cells have significantly higher cholesterol content than INS-1 cells (58.5 ± 2.9 and 46.3 ± 2.5 mg chol/mg prot respectively). After cholesterol desorption by 1.0, 5.0 and 10.0 mM of carboxymethyl-β-cyclodextrin, methyl-β-cyclodextrin, or 2-hydroxypropyl-β- cyclodextrin the response to hypotonicity in INS-1E cells emerged. On the contrary, supplementation of INS-1 cells with cholesterol inhibited their response to cell swelling. Cyclodextrin pretreatment inhibited glucose-induced insulin secretion from INS-1 cells while INS-1E cells were more resistant to their effect.

CONCLUSION

Cellular cholesterol content substantially affects secretory process; both high and low levels could be inhibitory. Absence of swelling-induced insulin secretion in INS-1E cells despite adequate response to glucose is related to their high cholesterol content. Optimal cholesterol concentration is different for either type of stimulation; swelling-induced mechanism is more sensitive to higher cholesterol content. The difference is likely to reflect involvement of sequential type exocytosis after cell swelling. Sensitivity of secretory processes suggests that either hypercholesterolemia or excessive effort to decrease plasma cholesterol in patients could have adverse effect on insulin secretion.

TSH Response to 30 Min Stay in Sauna in the Morning and during Evening Hours

The response of plasma TSH to 30 min stay in sauna was compared in the morning and in the evening. Both in the morning and in the evening plasma TSH was significantly elevated after sauna, with more prolonged response in the evening. This difference resembles the different reactivity of TSH to exogenous TRH administration in various times of day.

Thyrotropin-releasing hormone in rat heart: effect of swelling, angiotensin II and renin gene

AIM

This study was performed to examine thyrotropin-releasing hormone (TRH) secretion and regulation in rat heart.

METHODS

Expression of prepro-TRH gene in left atrium and left ventricle was studied by RT-PCR. TRH secretion from slices of left auricle and left ventricle in response to cell swelling (induced by hypotonic medium or ethanol in isosmotic medium), angiotensin II and losartan and their combinations was studied.

RESULTS

RT-PCR revealed two times higher prepro-TRH expression in left auricle than left ventricle. In transgenic rats with extra copy of mouse renin gene a marked increase of prepro-TRH expression in the heart was noted but the relative difference between left atrium and left ventricle persisted. The swelling stimulated TRH release from both left auricle and left ventricle and this stimulation could not be inhibited by bumetanide. Angiotensin II (10 nmol L-1) added into medium significantly decreased basal secretion of TRH. The inhibiting effect of Angiotensin II was prevented by 1 micromol L-1 losartan, an angiotensin II AT1 receptor blocker. When angiotensin II and hypotonicity were applied simultaneously, swelling-induced secretion persisted.

CONCLUSION

TRH secretion from heart slices has attributes of regulated secretion--depending on the stimulus it could be either stimulated or inhibited. Renin positively affects prepro-TRH expression in the heart. Angiotensin II inhibits TRH secretion from heart tissue by a mechanism involving AT1 receptors. Swelling-induced TRH secretion overrides inhibitory effect of angiotensin II. Swelling could be a useful tool when natural or pharmacological secretagogue is unknown. Peptides and proteins released by swelling could be mediators of local and remote ischaemic preconditioning protecting from subsequent ischaemia.

The Effect of Swelling on TRH and Oxytocin Secretion From Hypothalamic Structures

1. Cell swelling induces exocytosis of material stored in secretory vesicles resulting in a secretory burst of peptidic hormones or enzymes from various types of cells including endocrine cells and neurons. We have previously shown that swelling-induced exocytosis possesses limited selectivity; hypotonic medium evokes TRH but not oxytocin release from hypothalamic paraventricular nucleus (PVN) and neurohypophysis (NH). 2. It is the aim of this study to ascertain whether the swelling-induced oxytocin secretion could be unmasked by the inhibition of specific osmotic response using Ca(2+)-free medium and GdCl(3), an inhibitor of stretch activated channels. 3. Oxytocin release from the PVN was stimulated by the hypotonic medium only in the presence of 50 or 100 microM GdCl(3.) Oxytocin release from supraoptic nucleus (SON) was also stimulated by the Ca(2+)-free hypotonic medium in the presence of GdCl(3). Oxytocin secretion from the NH was not stimulated even in the presence of GdCl(3), both in Ca(2+) containing and Ca(2+)-free medium. TRH response to swelling-inducing stimulus was not affected by the presence of GdCl(3). 4. An intranuclear oxytocin secretion to hyposmotic stimulation within the PVN and the SON could be unmasked by the inhibiting specific response by GdCl(3). At these conditions general secretory response to swelling-inducing stimuli emerged. Secretion of oxytocin from the NH was not affected by any of these treatments. 5. Peptides and proteins released after cell swelling can play an important role in the pathophysiology of ischemia and could be mediators of local or remote preconditioning. Disruption of mechanosensitive gating in magnocellular neurosecretory cells could result in an inadequate secretory response (e.g. stimulation instead of inhibition and vice versa) of hormones engaged in water and salt metabolism regulation.

Effect of neonatal streptozotocin and thyrotropin-releasing hormone treatments on insulin secretion in adult rats

Neonatal STZ (nSTZ) treatment results in damage of pancreatic B-cells and in parallel depletion of insulin and TRH in the rat pancreas. The injury of B-cells is followed by spontaneous regeneration but dysregulation of the insulin response to glucose persists for the rest of life. Similar disturbance in insulin secretion was observed in mice with targeted TRH gene disruption. The aim of present study was to determine the role of the absence of pancreatic TRH during the perinatal period in the nSTZ model of impaired insulin secretion. Neonatal rats were injected with STZ (90 microg/g BW i.p.) and the effect of exogenous TRH (10 ng/g BW/day s.c. during the first week of life) on in vitro functions of pancreatic islets was studied at the age 12-14 weeks. RT-PCR was used for determination of prepro-TRH mRNA in isolated islets. Plasma was assayed for glucose and insulin, and isolated islets were used for determination of insulin release in vitro. The expression of prepro-TRH mRNA was only partially reduced in the islets of adult nSTZ rats when compared to controls. nSTZ rats had normal levels of plasma glucose and insulin but the islets of nSTZ rats failed to response by increased insulin secretion to stimulation with 16.7 mmol/l glucose or 50 mmol/l KCl. Perinatal TRH treatment enhanced basal insulin secretion in vitro in nSTZ animals of both sexes and partially restored the insulin response to glucose stimulation in nSTZ females.

Effect of immobilization on in vitro thyrotropin-releasing hormone release from brain septum in wild-type and corticotropin-releasing hormone knock-out mice

There is considerable evidence linking alcohol consumption, sedation, and thyrotropin-releasing hormone (TRH) in the brain septum. We have shown that ethanol in clinically relevant concentrations can in vitro induce TRH release from the septum by a mechanism involving neuronal swelling. Corticotropin-releasing hormone-deficient (CRH-KO) mice serve as an interesting model to help us understand the role of CRH in the regulation of different neuroendocrine systems. The aim of this study was to compare TRH release activity in the brain septum at basal and stress conditions in CRH-KO mice and their wild-type (WT) littermates. Experimental mice were decapitated immediately or 3 h after single (2 h) or repeated (seven times for 2 h daily) immobilization stress. The brain septum was immediately cut out and incubated to measure basal-, ethanol-, and hyposmosis-stimulated TRH release in vitro. Ethanol in isosmotic medium or hyposmotic medium stimulated TRH release from mice septal explants from WT and CRH-KO mice. The response was disturbed immediately after immobilization and recovered 3 h later. Our results show that immobilization stress transiently affects the TRH system in brain septum. Inborn absence of CRH does not affect septal TRH and its response to ethanol before and 3 h after immobilization.

Cell swelling induced secretion of TRH by posterior pituitary, hypothalamic paraventricular nucleus and pancreatic islets: effect of L-canavanine

The aims of this study were to test if ethanol induces thyrotropin-releasing hormone (TRH) secretion in vitro from the posterior pituitary and hypothalamic explants by a mechanism involving cell swelling, and to characterize the pathway of stimulated secretion. Ethanol, at a concentration of 80 mM, stimulated the release of TRH from the posterior pituitary, the hypothalamic paraventricular nucleus, the median eminence, and the brain septum, when administered only in isosmolar but not in hyperosmolar medium. This indicates the involvement of a cell swelling-inducing mechanism. L-canavanine in a concentration of 3 mM, increased the basal and hyposmosis-induced TRH secretion from the posterior pituitary and the paraventricular nucleus, and both basal and ethanol-induced TRH secretion from isolated pancreatic islets. This indicates the presence of both constitutive and regulatory secretory pathways. Our results suggest that cell swelling induces exocytosis from clathrin coated granules.

The osmotic component of ethanol and urea action is critical for their immediate stimulation of thyrotropin-releasing hormone (TRH) release from rat brain septum

There is considerable evidence linking alcohol consumption and sedation and TRH in the brain septum. Moreover, innate septal TRH concentration is inversely related to the degree of ethanol preference. Recently we demonstrated in rats that four-week ethanol drinking increased the septal TRH content by 50 %. We had shown previously that ethanol induces neuronal swelling, which is known to evoke the secretion of hormones, peptides and amino acids from various types of cells. We have therefore explored the effect of hyposmotic medium and of 80 and 160 mM ethanol and 80 mM urea (both permeant molecules) in isosmotic and hyperosmotic (preventing cell swelling) media on the in vitro release of TRH by the rat septum. Lowering medium osmolarity resulted in a hyposmolarity-related increase in TRH secretion. Both ethanol and urea stimulated TRH release only in isosmolar solution. Our data indicate that ethanol in clinically relevant concentrations can induce TRH release from the septum by a mechanism involving neuronal swelling.

Body mass, plasma leptin, glucose, insulin and C-peptide in offspring of diabetic and non-diabetic mothers

OBJECTIVE

The aim was to investigate the relationship between body mass index (BMI), plasma leptin, glucose, insulin and C-peptide levels in the offspring of diabetic mothers (DM) and non-diabetic healthy mothers (HM).

DESIGN

Seventy-two offspring (37 girls and 35 boys, age 4-20 years) of DM were investigated in a prospective study. Those 14-16 years old (Tanner stage II-IV) were compared with age-matched offspring of HM (33 girls and 33 boys).

RESULTS

BMI strongly correlated with plasma leptin concentration in the offspring of both DM and HM children. There were higher BMI and plasma leptin and glucose levels in DM than in HM children. There was no difference in plasma insulin or C-peptide levels between HM and age-matched DM children. There was a highly significant positive correlation between plasma leptin and C-peptide in boys of DM.

CONCLUSIONS

The higher plasma leptin found in the offspring of DM reflects their higher BMI. A moderately high but still normal glycemia might be a preclinical sign of insulin resistance or other disturbance of glucoregulation.

Regulation of hormone secretion by acute cell volume changes: Ca(2+)-independent hormone secretion

Exocytosis of intravesicular material should help a cell meet a relative extracellular hyposmotic challenge by expanding the plasmalemma through fusion with vesicular membrane. Cell swelling evokes an immediate secretory burst of hormones stored in secretory vesicles with dynamics indistinguishable from those induced by specific secretagogues. Hormone secretion induced by cell swelling is not associated with a rise in cAMP, IP(3), or prostaglandins, and it is not depressed by inhibition of stretch mechano-receptors or aquaporin channels. In contrast to most types of regulated secretion, that induced by cell swelling in normal cells does not require a rise in intracellular Ca(2+) through opening L-type Ca(2+) channels. However, such Ca(2+) influx is essential for cell-swelling induced secretion in tumor-derived pituitary cells. Cell swelling induces universal secretion of exocytotic material. The response of cells specialized in osmoregulation is, however, different. Possible physiological significance: Consistent stimulation of secretion occurs with a 4% hyposmolar challenge. It is likely that fluctuations in osmotic pressure with resultant cell volume changes have a significant regulatory role in hormone secretion. Released hormones could also play an important role in the pathophysiology of ischemia. Exocytosis itself does not have an essential role in volume regulation.

Comparison of pancreatic and hypophysiotropic TRH systems

The thyrotropin-releasing hormone (TRH) is a molecule with widespread distribution through many organ systems. The function of TRH is probably not identical in each system so that TRH synthesis and secretion may be unique for each system under specific experimental conditions. The present study was designed to explore the common and diverse features of the regulation of TRH encoded with the same gene in two different organs: hypophysiotropic hypothalamus and pancreatic islets. During in vitro incubation, the TRH content in hypothalamic structures remained stable while that in isolated pancreatic islets increased sharply. In contrast to the pancreatic islets, exposure to different concentrations of D-glucose did not affect TRH release from the hypothalamic paraventricular nucleus or median eminence. This divergence in the regulation of the hypophysiotropic and pancreatic TRH systems may be related to differences in the role of TRH produced in these tissues.

Four-week ethanol drinking increases both thyrotropin-releasing hormone (TRH) release and content in rat pancreatic islets

Ethanol exerts profound effects on the endocrine and exocrine pancreas. Some effects of chronic alcohol consumption on insulin secretion in response to glucose load are similar to those of TRH gene disruption. TRH is present in insulin-producing B-cells of the islets of Langerhans; its role in this location is still not fully explored. To examine the possible effect of long-term in vivo ethanol treatment on pancreatic TRH we compared three groups of rats: a 10% (wt:vol) ethanol-drinking group (E), absolute controls (AC) and pair-fed (PF) group with solid food intake corresponding to that of E. The fluidity of pancreatic membranes was not affected by chronic in vivo exposure of rats to ethanol, but was significantly decreased in PF group. Four-week treatment resulted in significantly higher TRH content in isolated islets of the E group and increased basal and 80 mM isotonic ethanol-induced secretion compared to AC and PF. Plasma levels of insulin, C-peptide, IGF-I, and glycemia were, however, not affected by ethanol treatment. Cell swelling, which can be induced by the presence of permeants (e.g. ethanol) in an isotonic extracellular medium, is a strong stimulus for secretion in various types of cells. In the present study, isosmotic ethanol (40, 80, and 160 mM) induced dose-dependent release of TRH and insulin from adult rat pancreatic islets in vitro. The same concentrations were not effective when applied in a hyperosmotic medium (addition of ethanol directly to the medium), thus indicating the participation of cell swelling in the ethanol-induced secretion. In conclusion, chronic ethanol treatment significantly affected pancreatic TRH and this effect might be mediated by cell swelling. The role of these changes in the profound effect of ethanol on the endocrine and exocrine pancreas remains to be established.

Glucose stimulates and insulin inhibits release of pancreatic TRH in vitro

OBJECTIVE

Pancreatic TRH is present in insulin-producing B-cells of the islets of Langerhans. There is fragmentary evidence that it may be involved in glucoregulation. The aim of our present study was to analyze how glucose and insulin affect TRH secretion by the pancreatic islets.

DESIGN

Isolated pancreatic islets were incubated with different concentrations of glucose, insulin and glucagon, and TRH release was measured.

RESULTS

In the present study, 6 and 12mmol/l d-glucose caused significant TRH release from isolated adult rat pancreatic islets when compared with that in the presence of the same concentrations of biologically ineffective l-glucose. Thirtymmol/l d-glucose was also ineffective, but this was not due to depression of secretion by hyperosmolarity since isosmotic compensation for the high glucose addition did not restore its stimulatory effect. Five micromol/l dibutyryl cyclic 3',5'-adenosine monophosphate (db-cAMP) increased both basal and glucose-stimulated TRH release, but this effect was not seen with 50micromol/l db-cAMP. Stimulation of phosphodiesterase by imidazole resulted in decreased basal but not glucose-stimulated release of TRH. Glucagon (10(-7)mol/l) did not affect either basal or glucose-stimulated release of TRH, while insulin (10(-7) and 10(-6)mol/l) inhibited both.

CONCLUSION

Our present data showing that glucose stimulates and insulin inhibits pancreatic TRH release are compatible with the possibility that this substance may play a role in glucoregulation.

Hypo-osmolarity stimulates and high sodium concentration inhibits thyrotropin-releasing hormone secretion from rat hypothalamus

The hypothalamic paraventricular nucleus, representing cell bodies in which thyrotropin-releasing hormone is synthesized, and the median eminence, representing nerve terminals, were incubated in vitro. Various hypo- and hyperosmotic solutions were tested to determine osmotic sensitivity of thyrotropin-releasing hormone secretion. High KCl (56 mM) causing membrane depolarization was used as a non-specific control stimulus to induce thyrotropin-releasing hormone secretion. A 30% decrease of medium osmolarity (from 288 to 202 mOsmol/l) increased thyrotropin-releasing hormone secretion from both the paraventricular nucleus and median eminence. A 30% decrease of medium NaCl content by its replacement with choline chloride did not affect basal thyrotropin-releasing hormone secretion. Increasing medium osmolarity with biologically inactive L-glucose did not affect basal or KCl-induced thyrotropin-releasing hormone secretion from either structure. Medium made hyperosmotic (350-450 mOsmol/l) by increasing the NaCl concentration resulted in a dose-dependent decrease of basal thyrotropin-releasing hormone secretion and abolished KCl-induced thyrotropin-releasing hormone secretion. If an osmotically equivalent amount of choline chloride was substituted for NaCl, there was no effect on thyrotropin-releasing hormone secretion, indicating a specific action of Na+. This study indicates a specific sensitivity to high concentrations of Na+ ions of both thyrotropin-releasing hormone-producing parvocellular paraventricular neurons and thyrotropin-releasing hormone-containing nerve terminals in the median eminence.

Chronic ethanol drinking and food deprivation affect rat hypothalamic-pituitary-thyroid axis and TRH in septum

Because chronic ethanol ingestion may perturb thyroid function, we evaluated the effect of 4-wk of oral 10% ethanol ingestion on the hypothalamic-pituitary-thyroid (HPT) axis and septal thyrotropin-releasing hormone (TRH) in 200-g male Wistar rats. Animals were divided into three groups: absolute control receiving tap water and food ad libitum; ethanol group receiving food ad libitum and 10% ethanol as the sole source of drinking fluid; pair-fed group receiving tap water and an amount of food corresponding to the consumption of ethanol group. After 4-wk of treatment, the body weight of the ethanol group was 7% and of the pair-fed rats 19% lower than that of the absolute controls. Both chronic ethanol treatment and food deprivation produced a decrease in plasma thyroid-stimulating hormone (TSH). Pair-fed rats also had a lower plasma T3. Type I iodothyronine 5'-deiodinase activity in the liver was increased in the pair-fed and even more in the ethanol-treated group. The content and secretion in vitro of TRH from the hypothalamic paraventricular nucleus and median eminence were unchanged. TRH content in the septum was increased in both the ethanol and pair-fed groups. TRH secretion from the septum in vitro was lower in the pair-fed, but unchanged in the ethanol group. These data suggest that 4-wk of peroral ethanol intake affects thyroid function mostly at the extrahypothalamic level and that there is a contribution of concomitant food deprivation. Both ethanol treatment and food deprivation increased TRH content in the septum.

Four-week ethanol intake decreases food intake and body weight but does not affect plasma leptin, corticosterone, and insulin levels in pubertal rats

Long-term intake of ethanol decreases food intake and inhibits growth in experimental rats. The aim of this study was to determine the effect of 4-week oral ethanol ingestion on plasma leptin and adrenal function. Male 45-day-old Wistar rats were divided into three groups: absolute control (AC), ethanol (E) administered 10% (wt/vol) ethanol instead of tap water, and pair-fed (PF) given an amount of food corresponding to the food intake of E animals. E rats consumed less pelleted diet (74% cumulative total intake); however, this caloric deficit was compensated by ethanol ingestion. Net water intake in E animals was 76% of that in the control groups. The body growth of both E and PF rats was stunted compared with AC animals, but E rats were heavier than PF rats. The plasma leptin level was similar in E and AC and decreased in PF animals. There were no differences in plasma osmolality or glycemia among the three groups. Plasma insulin was decreased in PF compared with both AC and E rats. Plasma corticosterone was not affected by ethanol, but was increased in the food-restricted (PF) group. Although there were no differences in basal adrenal corticosterone production in vitro, there was a slightly higher response to corticotropin (ACTH) in E rats. We conclude that drinking 10% ethanol decreased the dietary intake and body growth. These changes were not mediated by plasma leptin changes. Although alcohol ingestion and its energy content theoretically normalized the total energy intake and prevented the decrease of plasma leptin, the growth of young rats was inhibited. Drinking 10% ethanol instead of tap water for 4 weeks did not stimulate basal adrenal activity.

Thyroid hormone receptor occupancy and biological effects of 3,5,3,-L-triiodothyronine (T3) in GH4C1 rat pituitary tumour cells

The GH4C1 pituitary cell line, an excellent model for a thyroid hormone action study, was used for determination of the relationship between thyroid hormone receptor occupancy and intensity of cell proliferation, prolactin (PRL) production, thyrotropin (TSH) inhibition and 3,5,3,-L-triiodothyronine (T3) receptor down-regulation. Nuclear receptor population was progressively occupied by T3 in concentrations ranging from 0.025 to 10.0 nM T3. Bmax ranged from 0.029 fmol/10(6) cells at the lowest T3 concentration to Bmax = 12.51 fmol/10(6) cells at the highest concentration. Each of the observed biological events is operative within distinct dose-response ranges in cultured GH4C1 cells. The maximal biological response (except the TSH inhibition and T3 receptor down-regulation) does not require the occupation of the whole nuclear receptor population by T3 and the intensity of none of the responses studied was directly proportional to thyroid hormone receptor occupancy.

Long-term action of potassium bromide on the rat thyroid gland

Male rats fed by a standard diet with determined of bromine and iodine content were exposed to a 133-day oral administration of KBr (100, 200, 400 mg Br-/l drinking water). Their thyroid glands showed increased growth of the epithelial cells reflected by a microfollicular rearrangement of the parenchyma due to proliferation of very small follicles with a low or zero content of colloid. Morphometric analysis of thyroids of Br(-)-exposed animals revealed a significant decrease in the volume of intrafollicular colloid and marked increase in the number of the smallest follicles (areas up to 100 and 100-300 micron 2). In addition, the nuclei of thyrocytes showed an increased number of mitoses. The vascularization was increased as well. In the blood plasma of the Br(-)-exposed animals the T4 concentration was significantly decreased in dependence on the bromine concentrations. Thyroglobulin immunoreactivity in the colloid of Br(-)-exposed animals decreased after administration of 400 mg Br-/l drinking water. Increasing concentrations of Br- in the drinking water caused an increased bromine concentration in the thyroid, a decreased iodine content and a decreased I/Br molar ratio. The changes in the rat thyroid caused by long-term administration of 100 mg Br-/l were similar to hyperplastic parenchymal goitre and were comparable to those induced in previous experiments by the same bromine concentration administered over a 16- and 66-day period respectively.

Potassium bromide and the thyroid gland of the rat: morphology and immunohistochemistry, RIA and INAA analysis

The increasing environmental concentration of bromine has resulted in attempts to obtain information on its possibly deleterious effect on humans, particularly on a major target organ of this halogen i.e. the thyroid gland. In order to establish the morphological and functional effects of bromine on the thyroid, we have performed experiments on male rats which, in addition to a standard diet with an estimated iodine/bromine content, were fed for periods of 16 and 66 days with the small quantities of bromide expected to be encountered in the environment (10, 50 and 100 mg of Br-/l in drinking water). This treatment induced growth of the follicular epithelial component and microfollicular tissue rearrangement, a reduction of intrafollicular colloid, an increase in the height of the follicular cells and the number of mitoses, and it enhanced vascularization. Image analysis revealed a significant reduction in the volume of colloid, despite the accompanying rise in the number of minute follicles. The immunohistochemical positivity of the thyroglobulin fell in the microfollicular colloid of the exposed animals, although this was affected to a lesser extent in the larger follicles. The concentration of bromine in the thyroid increased with the amount of bromine intake, while at the same time the molar ratio of iodine/bromine decreased. The plasma level of T4 was lowered after both 16 and 66 days of treatment, but the T3 level only after 66 days treatment. The level of TSH did not exhibit any significant change. The observed changes, which have a parenchymatous goitre-like character, may have a direct relevance for human medicine, since the concentrations of bromide chosen in these experiments are readily encountered in the environment.

Both iso- and hyperosmotic ethanol stimulate release of hypothalamic thyrotropin-releasing hormone despite opposite effect on neuron volume

Previous studies have indicated that isosmolar, but not hyperosmolar, ethanol induces in vitro gonadotropin-releasing hormone secretion from the basal hypothalamus, presumably by causing cell swelling. Moreover, ethanol reduces secretion of another hypothalamic neuropeptide vasopressin. We have studied the acute effect of ethanol on specific hypophysiotropic basal and K+-stimulated thyrotropin-releasing hormone secretion in vitro especially in relation to cell swelling. Isosmotic 40-160 mM ethanol increased thyrotropin-releasing hormone release from the hypothalamic paraventricular nucleus and median eminence in a dose-dependent manner. Both a 30% decrease of osmolarity and isosmotic 80 mM ethanol induced 12% swelling of hypothalamic neurons. Hyperosmotic 80 mM or 160 mM ethanol induced release of thyrotropin-releasing hormone from both hypothalamic structures but did not cause cell swelling (80 mM) or even induced cell shrinkage (160 mM). Depletion of medium Ca2+ did not affect thyrotropin-releasing hormone secretion caused by either isosmotic or hyperosmotic ethanol. Our data indicate that both iso- and hyperosmotic ethanol stimulated release of hypophysiotropic thyrotropin-releasing hormone despite opposite effects on neuron volume. The mechanism of ethanol action appears complex and variable depending on the type of cell and neuropeptide affected.

Hyposmolar medium and ethanol in isosmotic solution induce the release of thyrotropin-releasing hormone (TRH) by isolated rat pancreatic islets

Cell swelling induced by hypotonic medium or small isotonic permeant molecules results in an immediate secretory response in various types of cells. We have expanded exploration of this phenomenon by examining the effect of either isotonic ethanol or hyposmotic medium on the release of TRH by freshly isolated islets of Langerhans in static incubation and perifusion. Ethanol (40, 80 or 160 mM in isotonic solution) dose-dependently evoked the release of TRH by statically incubated islets. The dynamics of TRH release induced by 80 mM isotonic ethanol or 30% hypotonic medium were similar to those induced by 50 mM KCl, with the highest secretion rate during the first 5 min of incubation irrespective of the duration of stimulation. Ca2+ depletion of the incubation medium abolished the response to 50 mM KCl but did not diminish the response to 80 mM isotonic ethanol. We conclude that osmotic stimuli known to induce cell swelling also induce release of TRH by isolated pancreatic islets.

Presence of atriopeptin-like immunoreactivity in human and rat milk

Maternal milk is a significant source of hormones and other bioactive substances. They might be involved either in the control of mammary gland function or in the regulation of growth and development of the neonate. Atriopeptin (atrial natriuretic factor, ANF) is a peptide with strong diuretic, natriuretic and vasorelaxant actions, and it has been suggested to play an important role in the circulatory adaptation to extrauterine life. The aim of this study was to determine whether ANF is present in maternal milk, using radioimmunological analysis. The levels of ANF-like substance in human milk were found to be in the range of 0.3-3.0 pg/ml, those in rat milk between 37-117 pg/ml. The measured concentrations of ANF were proportional to the volume of the extracted milk. Serial dilutions of the extracts yielded curves which were not totally parallel to the human alpha-ANF standard curve. Our data indicate that, during the first days after delivery, ANF levels in human milk are higher than those in later periods of lactation. This pilot study provides the first description of the presence of atriopeptin in milk. Though a detailed characterization of milk ANF-like immunoreactivity is needed, a biological significance of present findings seems possible.

In vitro effects of sodium selenite on nuclear 3,5,3'-triiodothyronine (T3) receptor gene expression in rat pituitary GH4C1 cells

The present study was undertaken in order to investigate the effects of sodium selenite on: 1. The growth of rat pituitary GH4C1 cells; 2. The nuclear T3 receptor gene expression; 3. The cytoplasmic protein phosphorylation; and 4. The prolactin secretion in rat pituitary GH4C1 cell line. Sodium selenite (up to 2.5 microM) has no inhibitory effect on GH4C1 cell proliferation as well as the prolactin secretion. On the other hand, 0.5 microM sodium selenite significantly decreases the rate of mRNA synthesis and/or degradation of both, the alpha 1 form of the T3 receptor (TR alpha 1) and the alpha 2 isoform of the T3 receptor. At 1 microM of sodium selenite, significant changes in the electrophoretic profile of low molecular mass cytoplasmic proteins were found, moreover, sodium selenite (1 microM) also considerably affects phosphorylation of a higher molecular mass proteins. The results based on the in vitro experiments suggest that sodium selenite may affect specific processes at the pretranslational level as well as it may also take part in processes of posttranslational modification of protein(s), the cell vitality and the cell growth remaining unchanged.

Different regulation of thyrotropin releasing hormone content and release in paraventricular nucleus (PVN) and median eminence (ME) of rat hypothalamus during in vitro incubation

TRH is present throughout the central nervous system possessing many different functions. Only TRH synthesized in the hypothalamic PVN and transported to the ME regulates anterior pituitary secretion of thyrotropin. To investigate this hypophysiotropic system, we have developed a method using dissected rat PVN and ME, representing mostly cell bodies and nerve terminals respectively, of the same TRH system. Tissues were incubated for four 30 min periods each in Locke's medium with alternatively normal and high KCl concentration. Repeated KCl-induced membrane depolarization resulted in significantly increased TRH release from both tissues (15% of TRH content) which was dependent on Ca2+ influx. Some important differences were found in the regulation of TRH in the PVN and ME: Comparison of TRH content in the tissue before and after incubation with the amount of secreted TRH showed new production of TRH during incubation in the ME, but not in the PVN. Frequent medium replacement during depolarization revealed that TRH secretion at the level of the ME, but not the PVN, is probably inhibited by some substance released during incubation. These data suggest that there is a different regulation of TRH in the PVN and ME and that TRH secreted by the isolated PVN may come mostly from the perikarya and represent paracrine neurohormone secretion.

Thyroid function and plasma immunoglobulins in subjects with Down's syndrome (DS) during ontogenesis and zinc therapy

Thyroid function parameters and immunoglobulin concentrations in sera of outpatients with Down's syndrome (DS, n = 110) of different ages (DS1 = 1-9 years; DS2 = 6-15; DS3 = 15-35) were compared with those of age-matched controls (n = 110). Although mean serum TSH was higher in all DS groups, thyroid hormone concentrations were significantly lower only in DS3. In DS1, a notable frequency rate of high T4 and T3 was found. Serum concentrations of thyroxine binding globulin (TBG) were significantly higher in all DS groups. Free T4 and T3 indexes, calculated as the ratio of total hormone: TBG concentrations, were lower in all DS groups. IgA serum concentrations were significantly higher in all DS groups, IgA was higher in DS1 and DS2. Serum zinc levels were lower in all DS groups. Repeated examination after one year revealed lower T4 and higher TSH in DS patients treated with zinc during this interval as compared to values observed before treatment. Our results suggest a high occurrence rate of complex immune and endocrine disorders with thyroid dysregulation in DS patients, with zinc deficiency playing a considerable role.

Pancreastatin-like immunoreactivity in the pancreas of newborn rats

Pancreastatin (PST), a novel COOH-terminally alpha-amidated peptide is a part of Chromogranin A molecule. Precursor processing implies also the final amidation step dependent on peptidylglycine alpha-amidating monooxygenase (PAM). High activity of PAM as well as Thyroliberin (TRH, another alpha-amidated peptide) concentration and biosynthesis were reported to be very high in neonatal rat pancreas. We followed the concentration of PST-like immunoreactivity in rat pancreas during ontogenesis. High perinatal PST concentration, resembling that of previously reported for PAM activity and TRH concentration was found. These findings suggest that perinatal PAM activation may affect a broader spectrum of pancreatic peptides.

[Hormones and bioactive substances in milk--rudiments or physiologic signals?]

Human milk and milk of lower mammals contains hormones, growth factors, as well as secondary messengers. It can be supposed that in the past this composition secured a better survival for the offspring in a certain phase of development. The question is, what role do the factors play in the physiological development today. They certainly affect the mammary gland at its own right, there is evidence about a positive effect on the intestinal mucosa of infants, even on the regulation of hormonal secretion. Hence, they represent signals sent by the maternal organism and affect the reaction of infants. Whether these effects are physiologically significant in animals and especially in humans still remains to be proved. A very rapidly progressing research promises many interesting and surprising findings in the near future. Objective evaluation of effects of milk composition on the offspring neither allows us to ignore the possibility of negative consequences in postreproductive age, since these signs are not being eliminated by natural selection during evolution.

Search for optimal age for weaning. Ten-year prospective study

The effect of infant nutrition on some clinical and biochemical parameters was studied from birth in a longitudinal prospective study. The data covering the period up to 7 years were reported previously. At the age 10 years 80 children were examined. Two children have been regularly followed up because of high blood pressure. Both had been breast-fed for less than one week and are obese at present. Surprisingly serum total cholesterol was the highest in the group breast-fed for more than 6 months. There were no pronounced differences in the levels of lipoprotein cholesterol carriers. The incidence of a high (over 6.0) atherogenic index (total chol - HDL chol) x APOB/APOA1 x HDL chol) and cholesterolemia over 4.8 mmol/l was significantly (chi 2 test) higher in the group breast-fed for over 6 months. Thyroid function parameters were not affected by the duration of breast-feeding. We conclude that besides undoubtedly favorable effects of breast-feeding, some functions might be also adversely affected if it lasts for more than 6 months. For justified recommendation of optimal duration of breast-feeding all aspects should be considered and more data collected.

Passive immunization and hypothalamic peptide secretion

Passive immunization is a common approach used to eliminate the biological activity of an endogenous substance by its binding to a specific antibody (Ab). Surprisingly little information has been gathered on the mechanisms involved. Moreover, the possibility that immunoneutralization could affect also the secretion of the antigen itself has been mostly ignored. To study hypothalamic neuropeptide secretion under the condition of passive immunization, labeled and unlabeled monoclonal antibody (MoAb) against arginine vasopressin (AVP) was injected intravenously. After 2 h a similar amount of 125I-MoAb was found in hypophyseal portal and peripheral (femoral artery) plasma, showing a distribution volume of 73.2 ml/kg. Assessment of the MoAb dilution in the same plasma samples from the binding studies revealed substantially higher dilutions (800-5,700 ml/kg). Such a MoAb dilution (saturation) would be attained by the binding of 130-290 pmol AVP/ml plasma. The calculated amount of plasma AVP decreased by one half within the interval from 2 to 24 h after Ab injection, similarly as did the 125I-MoAb content. Intravenous injection of polyclonal corticotropin-releasing hormone (CRH) Ab resulted in a decrease of plasma adrenocorticotropin and corticosterone levels. After 24 h the dilution of the Ab in portal plasma exceeded two times that in peripheral plasma. CRH concentrations of 0.6-2.5 pmol/ml were found by specific radioimmunoassay after its dissociation from the Ab in plasma. The CRH concentration was higher in portal than in peripheral plasma and was related to the amount of the Ab injected. CRH mRNA levels in the paraventricular nucleus were significantly increased in CRH Ab as compared with normal rabbit serum injected rats.(ABSTRACT TRUNCATED AT 250 WORDS)

TRH analogs with 4-nitro- and 4-N-butyloxycarbonylamino-1-methyl-2-pyrolylcarboxylic acid in position 1. Synthesis and biological properties

Two new analogs of thyrotropin-releasing hormone (TRH), obtained by the replacement of the L-pyroglutamic acid residue with 4-nitro- or 4-N-butyloxycarbonylamino-1-methyl-2-pyrolylcarboxylic acid (analogs 1, 3), and three related derivatives, in which also the L-histidine residue was replaced with L-norvaline (analogs 2, 4) or L-norleucine (analog 5), were synthesized and tested for endocrine and central nervous system (CNS) activity. The replacement of the L-pyroglutamic acid residue with 4-nitro-1-methyl-2-pyrolylcarboxylic acid (analogs 1 and 2) resulted in the separation of the endocrine from the direct CNS activity. The effect of these analogs on the sleeping time, rectal temperature and breathing frequency, was either the same or greater than that of TRH. However, neither the correlation between the binding of analogs to TRH receptors in the brain nor their activity on the CNS parameters measured was found. Analogs 3, 4 and 5, containing 4-N-butyloxycarbonylamino-1-methyl-2-pyrolylcarboxylic acid in place of L-pyroglutamic acid, were inactive.

Hormones and bioactive substances in milk: a rudiment or a message?

The composition of milk is the result of an evolutionary process with both positive and negative natural selection mechanisms where variants with components supporting offspring development from birth to reproduction (but not later) are preferred. Almost all known hormones and many other bioactive substances are present in milk of various species. Some of them are synthesized by the mammary gland, the others are passively or selectively transported (and eventually modified) from blood into milk. Hormones in milk are of importance for the regulation of the functions of the mammary gland itself. There are indications that they affect (sometimes permanently) both functions of suckling GIT and other functions after their resorption during the defined developmental periods. The definite role of these messages for further offspring development remains to be established.

Two novel thioamide analogues of TRH with selective activity on CNS

TRH analogues containing C-terminal tioamide group and norvaline ([Nva2, Prot3] TRH) or norleucine ([Nle2, Prot3] TRH) in position 2 were synthesized and tested for hormonal and central nervous system (CNS) activities. Receptor binding studies revealed that the analogues neither bind to pituitary nor to brain TRH receptors. Accordingly, no TSH releasing activity was recorded. However, both analogues significantly affected sleeping time and breathing frequency. Dissociation of endocrine effects from those on the CNS of [Prot3] TRH was achieved with the replacement of histidine2 by aliphatic amino acids. The presence of central histidine is not essential for the analogues to be active on the CNS.

Late effects of breast-feeding and early weaning: seven-year prospective study in children

The effect of breast feeding on some clinical and thyroid function parameters was studied in a prospective longitudinal study from birth up to 7 years of age. At the ages 1-7 years, the obesity rates observed in children breast-fed for less than 3 months were substantially higher than in children who had been breast-fed over longer intervals. Mean age when obesity was reported was similar in all groups (4-5 years). The rates of respiratory tract diseases were found to be highest in children which had been breast-fed for less than 2 weeks. Breast-feeding for more than 6 months had a protective effect against diseases of the gastrointestinal tract. The longitudinal follow-up revealed biphasic changes of thyroid hormones and TSH in sera with a nadir at 2-3 years, followed by an increase at the end of preschool age. Duration of breast-feeding did not affect profoundly these parameters at the ages 1-7 years. Surprisingly, during late preschool age (5-6 years) total serum cholesterol increased with the age at weaning. The atherogenic index in 6-year-old children was most favourable in the group breast-fed over more than 1 but less than 3 months. This was due to the highest levels of HDL-cholesterol in this group. We conclude that the age at weaning may be important for the later development of children.

Thyroliberin (TRH) and TRH free acid (TRH-OH) present in milk do not originate from local synthesis in mammary gland

Hypothalamic hormones represent a peculiar group of hormones present in milk in surprisingly high concentrations. High levels of these neuropeptides raised the question of their origin. The hypothesis suggesting local synthesis of TRH in the mammary gland was, therefore, tested. Acid extracts of human milk contained TRH and TRH-OH immunoreactivity. RIA determinations at various purification steps revealed that only a part of the immunoreactivity may represent authentic peptides. No high molecular weight TRH precursor could be demonstrated upon a sequential enzymatic treatment of human milk and rat mammary gland extracts. Exploration of rat mammary gland tissue for TRH mRNA showed that the TRH gene is not expressed in the mammary gland. Rat mammary gland homogenates were able to deamidate exogenous TRH to TRH-OH.

CONCLUSION

TRH is not synthesized in the mammary gland via a high molecular weight precursor. It is likely that the TRH-free acid in milk (demonstrated for the first time in this product) originates from TRH deamidation in mammary gland cells during TRH transport from the blood.

[The effect of 1-p-bromphenyl-5-mercapto-1,2,3,4-tetrazole (Br FMT) on thyroid gland function in rats]

The effects of the novel potential thyrostatic agent 1-p-bromphenyl-5- mercapto-1,2,3,4-tetrazole (Br-FMT) on the serum levels of thyroxine, thyrotropic hormone (TSH), the content of cyclic adenosine monophosphate (cAMP) in the thyroid gland, the body weight and the weight of the thyroid gland in liver transaminases and the white blood picture in Wistar strain rats were investigated. The effect of Br-FMT was compared with the effect of the well-known thyrostatic agent and goitrogen ethylester of 3-methyl-2-thio-4-imidazoline-1- carboxylic acid, carbimazole (Spofa) and with the control group, which received placebo only. The drugs tested were administered to animals in the dose do 7.5 mumol/animal via a gastric tube for the period of one month. Br-FMT and carbimazole decreased the level of serum thyroxine in a statistically significant manner. The serum level of TSH was evidently decreased after Br-FMT; it was not changed after administration of carbimazole in the given dose. The content of cAMP in the thyroid gland was significantly increased only after carbimazole. The weight of the thyroid gland was not significantly changed in any group under study, though after carbimazole the mean value was higher by a quarter as compared with the control group. The body weight and white blood picture were not significantly changed in all groups under study. ALT and AST values were evidently lower after carbimazole and Br-FMT, most probably due to the hypothyroid state of the animals.(ABSTRACT TRUNCATED AT 250 WORDS)

[Comparison of the results of radioimmunologic and hemagglutination methods in the determination of antibodies against thyroglobulin and the effect of these antibodies on thyroglobulin serum levels]

The following methods have been introduced at our institute: labeling of human thyroglobulin (H-Tg) with the radioiodine 125I by the lactoperoxidase method, radioimmunologic method for serum H-Tg determination by means of rabbit antiserum to H-Tg prepared at our institute, and radioimmunologic method for the determination of antibodies to thyroglobulin. Sera from 15 patients with different thyropathies were examined by the given methods. In the first part of the work the quality of 125I labeled H-Tg was studied. The maximum binding by antiserum was found to be substantially decreased as early as two weeks following labeling, The second part of the study presents our first experience with comparing our RIA method and the hemagglutination method for TgAb determination. The results yielded by the two methods did not differ significantly. The level of serum H-Tg is falsely affected by the competition of TgAb autoantibodies with the first rabbit antibody to H-Tg at recipitation by means of the second antibody. It is therefore important to establish the titer of autoantibodies before actual serum H-Tg determination. This approach is of importance e.g. in following up patients with malignant goiter. The introduced methods are a contribution to diagnosis and management of patients with thyropathies.

Thyrotropin releasing hormone in the pancreas of newborn rats from streptozotocin-treated mothers

The effect of maternal diabetes (induced by i.p. injections of 40-50 mg/kg BW Streptozotocin on the day of mating) on TRH in the pancreas of newborn rats was studied. Determination of peptide alpha amidation activity and TRH precursor level on the day of birth revealed decreased biosynthesis of TRH resulting in profoundly (10 times) lower pancreatic TRH and TRH-OH concentrations in pups of diabetic rats. Pancreatic His-Pro-diketopiperazine (His-Pro-DKP) remained unaffected by maternal diabetes. The depression of pancreatic TRH was less profound 24 h later, and even elevated TRH was measured in the pancreas of pups of diabetic mothers on postnatal day 5. Short term postnatal starvation or nursing of intact pups by the diabetic foster mother did not affect pancreatic TRH. It could be postulated that postnatal TRH development in the rat pancreas is retarded by maternal diabetes, while His-Pro-DKP remains unaltered.

Thyroid hormone levels in cow maternal and fetal sera during last trimester of pregnancy

Thyroid hormone levels were studied in 51 paired pregnant cow and fetal calf serum samples (fetal age 7-9 months). Thyroxine and rT3 levels were substantially higher and those of T3 distinctly lower in calf fetuses as compared to respective mothers. An increase of T3 has been detected in calf sera at the fetal age 8 to 8.5 months. These results suggest some prenatal maturation of thyroxine metabolism in the calf.

Exchange transfusion in premature newborns: effect of maturity on thyrotropin and thyroxine responses

The effects of exchange transfusion on plasma T4 and TSH were studied in two groups of premature newborns to evaluate the effect of maturation on the reactivity of the pituitary-thyroid axis. In newborns with a birth weight between 1900-2500 g (gestational age 35.4 +/- 0.45 weeks) the responses of both hormones were essentially similar to those reported previously for mature newborns (a profound decrease during the procedure and an increase 24 h later). In a group of newborns with a birth weight below 1900 g (average 1673 +/- 55, gestational age 32.7 +/- 0.72 weeks), however, the secondary increase in plasma T4 at 24 h after the procedure was absent. At this time both T4 and TSH levels were significantly lower than in those in heavier newborns. It is concluded that the ability to respond to exchange transfusion by an increase in plasma thyroxine at 24 h after the procedure matures at the gestational age between 32 and 35 weeks.

Effects of sauna and glucose intake on TSH and thyroid hormone levels in plasma of euthyroid subjects

The effect of sauna on thyroid function parameters and its modification by glucose was studied in young euthyroid male volunteers. A 30-minute stay in sauna resulted in an increase in plasma TSH; the response was exaggerated if glycemia had been increased by oral glucose intake at the beginning of the experiment. Plasma rT3 also increased in sauna, this response was, however, blunted by the higher glycemia. TSH response to sauna was definitely present in young men (aged 20 to 25) and absent in middle-aged ones (50 to 55). To explore the mechanism of the effect of increased glycemia, TRH tests were performed and dopamine infusions were administered with and without glucose pretreatment. Increased glycemia did not affect TSH and T3 response to TRH in young volunteers; however, 90 minutes after the administration, plasma rT3 levels were significantly lower in glucose pretreated subjects than in those receiving TRH injections after water pretreatment. Simultaneous infusion of glucose prevented the inhibitory effect of dopamine infusion on plasma TSH. It was concluded that glucose directly modulates the effect of sauna on plasma TSH at a suprapituitary level, while the inhibiting effect of glucose on plasma rT3 response to sauna and TRH is probably mediated by the insulin effect on thyroid hormone metabolism.

Human milk does not degrade TRH

We have found previously that TRH is accumulated in rat milk in biologically active form. TRH was reported to be present in high concentrations in human milk too. These findings together with the absence of TRH degrading activity in plasma of newborns suggest a possible physiological role of the neurohormone coming from milk. We studied, therefore, TRH degrading activity of human milk. TRH incubated in vitro with 100 microliters human milk (4 days and 4 months after delivery) in 0.01 mol l-1 phosphate saline buffer (pH 7.6) with 1% gelatine (total volume 0.3 ml) at 37 degrees C was not degraded during 2 hours as revealed by specific RIA. The addition of the same amount of milk to adult human plasma did not affect intensive TRH degradation. We conclude that human milk does not contain TRH degrading enzymes nor their inhibitors.

Effect of breast-feeding on infant thyroid activity: 3 year follow up--longitudinal study

The effect of breast-feeding on serum thyroid hormones and TSH was studied in a longitudinal study from birth up to 3 years. T4 was found to be significantly higher in breast-fed than in weaned infants at the age of 4 and 6 months and so was rT3 at the age 4 months. The body weight of breast-fed girls at the age 4 and 6 months was lower than that of formula-fed ones. Breast-feeding at the age 9 months resulted in a decreased serum TSH. Serum T3 of infants weaned before the end of the first week of life was higher than in other groups of infants at the age of 10 weeks. Serum T4 of children 1-2-year-old correlated positively with the age at weaning, provided that they had been breast-fed at least for 60 days. Serum rT3 of 2-year-old children also correlated with the age of weaning. Serum TSH at the age of 3 years was higher in children who had been weaned during first 60 days of life than in those weaned later. It is concluded that breast-feeding possess some immediate effects on thyroid function parameters. Some effects were still detected at the age of 1-3 years.

Role of thyrotropin-releasing hormone in thyroid-stimulating hormone and growth hormone regulation during postnatal maturation in female Wistar rats

The role of endogenous thyrotropin-releasing hormone (TRH) in the control of pituitary thyroid-stimulating hormone (TSH) and growth hormone (GH) secretion was studied during postnatal maturation in female Wistar rats. Half of the sucklings in each litter was treated intraperitoneally with either specific rabbit antiserum against TRH or normal rabbit serum (0.1-0.3 ml according to age). All animals were decapitated after 2 h. The presence of anti-TRH activity was checked as a binding of labelled TRH with plasma of the experimental animals. Immunoneutralization of endogenous TRH resulted in a decrease of plasma TSH in 3- to 15-day-old female pups as compared to control littermates. No effect of TRH antibody injection was seen at the ages of 1, 21, 30 and 70 days despite the presence of excess antibody in the plasma. A profound effect of TRH antibody on plasma TSH was seen again at the age of 100 days. Plasma GH in the same animals exhibited a paradoxical increase after TRH immunoneutralization at the age of 5 and 8 days, a decrease was found at the age of 21 days. It was concluded that hypothalamic TRH control of TSH secretion matures early in Wistar rats. Hypothalamic secretion of TRH at the ages of 1, 21, 30, and 70 days is low and(or) its role in TSH regulation is masked by other regulating factors. TRH may play a dual role in the regulation of GH secretion during the postnatal period.

TRH analogue with C-terminal thioamide group: rapid degradation by plasma and its biological effects

L-pyroglutamyl-L-histidyl-L-proline thioamide--([Prot3]TRH), a new TRH analogue, has been previously found to have the same binding affinity to adenopituitary receptors as well as TSH and alpha-MSH releasing activities as native TRH. In this paper we report also the same time course of TSH response after i.p. injection of this compound (10 micrograms kg-1) to rats. Binding affinity to specific receptors in rat amygdala, cortex (frontal lobes), hypothalamus, striatum (order according to decreasing affinity) of both peptides was also similar. In contrast to TRH, however, [Prot3]TRH in doses 0.5 and 5 mg kg-1 i.p. did not affect sleeping time and breathing frequency in the rats during barbiturate anaesthesia. Surprisingly, human plasma degraded the new analogue much faster (T1/2 8.5 min) than native TRH (T1/2 30 min). [Prot3]TRH was also degraded faster in plasma of adult rats. Plasma of 6-day-old rat pups failed to degrade both peptides. It was concluded that the substitution of proline amide, for proline thioamide group in TRH molecule did not change binding affinity to receptors in the central nervous system, but decreased biological effectiveness in CNS and substantially decreased the resistance to degradation in human and rat plasma.

Maturation of the inhibitory response of growth hormone secretion to ether stress in postnatal rat

To study the maturation of inhibitory influences on growth hormone (GH) secretion the effect of ether stress on plasma GH levels was studied during postnatal ontogenesis in female rats. Ether stress did not affect plasma GH levels in 1-day-old pups. A distinct decrease of plasma GH was found in 3- and 9-day-old pups, and the response was prevented by treatment of 3-day-old animals with somatostatin antiserum. No effect of ether stress on plasma GH was noted in 12-, 15-, 18- and 21-day-old rats. Treatment of intact 12-day-old pups with the somatostatin antiserum increased plasma GH level under basal conditions. The inhibitory effect of ether stress on plasma GH was noted again at the age 30 days and in adult animals. It is concluded that the hypothalamus of 3-day-old rats is able to release enough somatostatin to inhibit GH secretion after stress. At the period 12-18 days a phase of pituitary refractoriness was noted: ether stress as well as TRH injection (our previous observation) fail to affect plasma GH in female pups, probably due to high somatostatin secretion under basal conditions and (or) low capacity of pituitary to release GH. It is suggested that regulation of GH secretion is not mature until after the 21st day of life.