Neuropeptidergic control of cyclic AMP accumulation in human thyroid cell

Somatostatin (SRIF), cholecystokinin (CCK), gastrin and substance P, as single agents, do not influence baseline cellular cAMP levels in human thyroid cultures. SRIF inhibits TSH-induced cAMP accumulation in human thyroid cell, while CCK, gastrin and substance P do not modify cAMP response to TSH. Vasoactive intestinal peptide (VIP) increases cellular cAMP levels in human thyroid cultures and its effect is additive to increases produced by norepinephrine (NE) and isoproterenol (ISO). Neither SRIF nor the other tested peptides influence adrenergic and VIP-ergic cAMP stimulation.     

Comparison of helodermin, VIP and PHI in pancreatic secretion and blood flow in dogs

Helodermin, VIP and PHI, which share a high degree of homology with secretin, have been identified in the gut but their physiological role is unknown. In this study 3 series of tests were carried out to determine the actions of helodermin, VIP and PHI on pancreatic secretion in 6 conscious dogs and amylase release from the dispersed canine pancreatic acini and to correlate the alterations in pancreatic secretory and circulatory effects in 24 anesthetized dogs. Helodermin, VIP and PHI infused i.v. in graded doses (12.5-200 pmol/kg.h) resulted in a dose-dependent increase in pancreatic HCO3 secretion reaching, respectively, 100%, 7% and 2% of secretin maximum. When combined with constant dose infusion of CCK-8 (100 pmol/kg.h), helodermin but not VIP or PHI augmented dose-dependently the HCO3 secretion. When added in various concentrations (10(-10)-10(-5)M) to the incubation medium of dispersed pancreatic acini only helodermin but not VIP or PHI increased dose-dependently amylase release reaching about 50% of CCK-8 maximum. In anesthetized dogs, the pancreatic blood flow (PBF) measured by electromagnetic blood flowmetry showed an immediate and dose-dependent increase following the injections of various doses of helodermin, VIP, PHI and secretin, the peak blood flow preceding by about 1 min the peak secretory stimulation. This study shows that helodermin resembles secretin in its potent pancreatic HCO3 stimulation but differs from VIP or PHI which are poor secretagogues but potent vasodilators. We conclude that if tested peptides are released in the gut, helodermin, like secretin, may be involved in the hormonal stimulation of exocrine pancreas, whereas VIP and PHI may serve mainly as vasodilators in the pancreatic circulation.     

Helodermin-like peptides in noradrenaline cells of adrenal medulla

Helodermin, a VIP/secretin-like peptide, was first isolated from the venom of the lizard Gila monster. Small amounts of helodermin-like peptides have since been detected in many mammalian tissues. Notably high concentrations were demonstrated in the thyroid gland, and immunocytochemical studies revealed intense helodermin-like immunostaining in thyroid C cells and medullary thyroid carcinoma cells. In the present study, we examined the adrenal gland of mouse, rat and pig for the presence of helodermin-like peptides. Using an antiserum raised against lizard helodermin immunostaining was observed in the noradrenaline-producing cells of the adrenal medulla in all 3 species. Radioimmunoassay revealed high concentrations of helodermin-like peptides in the mouse and rat adrenal. The concentrations in the pig adrenal could not be determined because of a non-parallel dilution curve. Upon high-performance liquid chromatography, the immunoreactive material in extracts of mouse and rat adrenals eluted in one major peak, close to the elution position of lizard helodermin.     

Is helodermin produced by medullary thyroid carcinoma cells and normal C-cells? Immunocytochemical evidence

Helodermin is a VIP/secretin-like 35-amino acid peptide originally isolated from the venom of the lizard Gila monster. Recently, helodermin-immunoreactive material was demonstrated in mammalian salivary glands, brain and gut. In the present study 8 human medullary thyroid carcinomas as well as 4 normal thyroid glands were examined immunocytochemically for the presence of helodermin using an antiserum raised against helodermin-(5-35) that does not cross-react with VIP or secretin. Cells displaying helodermin-like immunoreactivity were found in all tumours examined except one. On the whole the helodermin-immunoreactive cells had the same distribution as those storing calcitonin, suggesting coexistence of the two peptides in most of the tumour cells. Also normal human C-cells displayed helodermin immunoreactivity. The results suggest that a peptide chemically related to helodermin is a constituent of human medullary thyroid carcinoma cells as well as of normal C-cells.     

Effects of ketamine on the cholecystokinin,somatostatin, substance P,and thyrotropin releasing hormone in discrete regions of rat brain

Intraperitoneal injection of ketamine (100 mg/kg body weight) significantly reduces the levels of cholecystokinin (CCK) somatostatin (SRIF), and substance P (SP)-like immunoreactivity in various regions of rat brain. No significant change in thyrotropin releasing hormone (TRH)-like immunoreactivity was observed. Neuropeptide systems may be involved in the neuropharmacologic effects of ketamine.     

Increase of thyrotropin response to thyrotropin-releasing hormone (TRH) and TRH release in rats during pregnancy

Regulation of thyrotropin (TSH) release by thyrotropin releasing hormone (TRH) in the anterior pituitary gland (AP) of pregnant rats was studied. The pregnant (day 7, 14, and 21) and diestrous rats were decapitated. AP was divided into 2 halves, and then incubated with Locke's solution at 37 degrees C for 30 min following a preincubation. After replacing with media, APs were incubated with Locke's solution containing 0, or 10 nM TRH for 30 min. Both basal and TRH-stimulated media were collected at the end of incubation. Medial basal hypothalamus (MBH) was incubated with Locke's medium at 37 degrees C for 30 min. Concentrations of TSH in medium and plasma samples as well as the cyclic 3':5' adenosine monophosphate (cAMP) content in APs and the levels of TRH in MBH medium were measured by radioimmunoassay. The levels of plasma TSH were higher in pregnant rats of day 21 than in diestrous rats. The spontaneous release of TSH in vitro was unaltered by pregnancy. TRH increased the release of TSH by AP, which was higher in pregnant than in diestrous rats. Maternal serum concentration of total T3 was decreased during the pregnancy. The basal release of hypothalamic TRH in vitro was greater in late pregnant rats than in diestrous rats. After TRH stimulation, the increase of the content of pituitary cAMP was greater in late pregnant rats than in diestrus animals. These results suggest that the greater secretion of TSH in pregnant rats is in part due to an increase of spontaneous release of TRH by MBH and a decrease of plasma thyroid hormones. Moreover, the higher level of plasma TSH in rats during late pregnancy is associated with the greater response of pituitary cAMP and TSH to TRH.     

Effects of orexin A on thyrotropin-releasing hormone and thyrotropin secretion in rats.

Effects of orexin A on secretion of thyrotropin-releasing hormone (TRH) and thyrotropin (TSH) in rats were studied. Orexin A (50 microg/kg) was injected iv, and the rats were serially decapitated. The effects of orexin A on TRH release from the rat hypothalamus in vitro and on TSH release from the anterior pituitary in vitro were also investigated. TRH and thyroid hormone were measured by individual radioimmunoassays. TSH was determined by the enzyme-immunoassay method. The hypothalamic TRH contents increased significantly after orexin A injection, whereas its plasma concentrations tended to decrease, but not significantly. The plasma TSH levels decreased significantly in a dose-related manner with a nadir at 15 min after injection. The plasma thyroid hormone levels showed no changes. TRH release from the rat hypothalamus in vitro was inhibited significantly in a dose-related manner with the addition of orexin A. TSH release from the anterior pituitary in vitro was not affected with the addition of orexin A. The findings suggest that orexin A acts on the hypothalamus to inhibit TRH release.     

Thyroid dysfunction in adult long-term survivors after hemapoeitic stem-cell transplantation (HSCT).

Thyroid function was evaluated in 72 adult survivors (41 females and 31 males) at 16 to 56 years of age, 1.5 years mean time (range 0.2 - 9.8) after hemapoeitic stem cell transplantation (HSCT) with no known prior history of thyroid dysfunction. Thyroid stimulating hormone (TSH) and free thyroxin levels (FT4) were determined before and after stimulation with thyrotropin releasing hormone (TRH). Conditioning regimens for HSCT did not include TBI. Overt hypothyroidism (basal TSH > 8 microIU/ml, FT4 < 0.8 ng/dl) was observed in 6% of male patients and 5% of female patients; subclinical hypothyroidism (basal TSH 4 - 8 microIU/ml, low normal FT4 0.8 - 1.9 ng/dl) was observed in 13% of males and 5% of females. A significant number of euthyroid patients (40% males and 54% females) with normal basal TSH and FT4 levels overresponded to TRH stimulation; the finding being statistically significant (p < 0.005). A heavy TSH response after TRH stimulation indicates compensated subclinical dysfunction of the thyroid gland. Chemotherapy-only conditioning regimens may have an adverse effect on thyroid gland function not always detected by determination of basal TSH and FT4 levels. This finding warrants long-term evaluation of thyroid function in HSCT patients.     

Serum free thyrotropin subunit in congenital isolated thyrotropin deficiency

In two patients with congenital isolated thyrotropin (TSH) deficiency, serum TSH determined by a sensitive immunoradiometric assay (IRMA) was consistently undetectable. The basal levels of serum free TSH-alpha subunit (TSH-alpha) determined by a specific radioimmunoassay (RIA) were elevated in the hypothyroid state, and decreased to the undectable level during displacement therapy with thyroid hormone. The serum free TSH-alpha significantly increased following intravenous administration of thyrotropin releasing hormone (TRH). Serum free TSH-beta subunit (TSH-beta) was undectable. These findings suggest that TSH deficiency in this disease is not due to absence of thyrotroph in the pituitary gland or deficiency of TSH-alpha, but to abnormalities of the TSH-beta gene.     

Effects of various drugs on thyrotropin secretion in rats

The effects of alpha-neoendorphin, kyotorphin, melatonin or diphenylhydantoin (DPH) on thyrotropin-releasing hormone (TRH) and thyrotropin (TSH) release in rats were studied. alpha-neoendorphin (1.0 mg/kg), kyotorphin (1.0 mg/kg), melatonin (2.5 mg/kg) or DPH (75 mg/kg) was injected iv or ip, and the rats were serially decapitated. TRH, TSH and thyroid hormone were determined by radioimmunoassay. The hypothalamic immunoreactive (ir-TRH) contents decreased significantly after melatonin injection, but not after alpha-neoendorphin, kyotorphin or DPH. The plasma ir-TRH concentrations decreased significantly after DPH injection, but not after alpha-neoendorphin, kyotorphin or melatonin. The plasma TSH levels decreased significantly in a dose-related manner with a nadir at 10 min. after melatonin, at 30 min. after DPH and at 40 min. after alpha-neoendorphin or kyotorphin injection. The plasma thyroid hormone levels did not change significantly after these drugs injection. The plasma ir-TRH and TSH responses to cold were inhibited by these drugs, but the plasma TSH response to TRH was not influenced. In the L-DOPA- or 5-hydroxy-tryptophan (5-HTP)-pretreated group, the inhibitory effect of alpha-neoendorphin or kyotorphin on TSH levels was prevented, but not in the haloperidol- or para-chloprophenylalanine (PCPA)- pretreated group. In the haloperidol- or PCPA-pretreated group, the inhibitory effect of melatonin on TSH levels was prevented, but not in the L-DOPA- or 5-HTP-pretreated group. These drugs alone did not affect plasma TSH levels in terms of the dose used. The inactivation of TRH immunoreactivity by hypothalamus or plasma in vitro after these drugs injection did not differ from that of the control.(ABSTRACT TRUNCATED AT 250 WORDS)     

Specific labelling by [125I]helodermin of high-affinity VIP receptors in rat liver membranes

Helodermin, a newly isolated peptide from Gila Monster venom, is structurally related to VIP and secretin. When used as radioligand, [125I]helodermin bound rapidly and reversibly to crude rat liver membranes, the dissociation being accelerated by GTP. Competition binding curves of [125I]helodermin and [125I]VIP with unlabelled peptides showed the following order of decreasing affinity: VIP greater than helodermin greater than secretin greater than hpGRF(1-29)-NH2. The shape of binding curves and of concurrent adenylate cyclase activation is compatible with the specific labelling, by [125I]helodermin, of a class of high-affinity VIP receptors that is capable to stimulate adenylate cyclase.     

Substances resembling peptides of the vertebrate gonadotropin system occur in the central nervous system of Periplaneta americana L.: Immunocytological and some biological evidence

Cockroach cerebrum, retrocerebral complex and suboesophageal ganglion were studied by immunocytochemistry to detect the presence of luteinizing hormone (LH), follicle stimulating hormone (FSH), gonadoliberin (LHRF), thyroid stimulating hormone (TSH) and thyrotropin releasing hormone (TRH) immunoreactive neurons. Gonadotropin system (LH, FSH, LHRF) immunoreactive perikarya and/or nerve fibres were encountered in various regions. In contrast, no labeling could be observed with antisera to either hormone of the vertebrate thyrotropin system (TSH, TRH). Preliminary data indicate that extracts of corpora cardiaca, which by immunocytochemical methods are shown to contain abundant immuno-LH-like material, are capable of stimulating testosterone secretion in mouse Leydig cells (positive LH bioassay). Functional and evolutionary aspects of the presence of vertebrate peptides in insects are discussed.     

Maturation of the pituitary-thyroid axis during the perinatal period.

To clarify the maturation process of the pituitary-thyroid axis during the perinatal period, thyrotropin (TSH) response to thyrotropin releasing hormone (TRH) and serum thyroid hormone levels were examined in 26 healthy infants of 30 to 40 weeks gestation. A TRH stimulation test was performed on 10 to 20 postnatal days. Basal concentrations of serum thyroxine (T4), free thyroxine (free T4) and triiodothyronine (T3) were positively correlated to gestational age and birth weight (p less than 0.001-0.01). Seven infants of 30 to 35 gestational weeks demonstrated an exaggerated TSH response to TRH (49.7 +/- 6.7 microU/ml versus 22.1 +/- 4.8 microU/ml, p less than 0.001), which was gradually reduced with gestational age and normalized after 37 weeks gestation. A similar decrease in TSH responsiveness to TRH was also observed longitudinally in all of 5 high responders repeatedly examined. There was a negative correlation between basal or peak TSH concentrations and postconceptional age in high responders (r = -0.59 p less than 0.05, r = -0.66 p less than 0.01), whereas in the normal responders TSH response, remained at a constant level during 31 to 43 postconceptional weeks. On the other hand, there was no correlation between basal or peak TSH levels and serum thyroid hormones. These results indicate that (1) maturation of the pituitary-thyroid axis is intrinsically controlled by gestational age rather than by serum thyroid hormone levels, (2) hypersecretion of TSH in preterm infants induces a progressive increase in serum thyroid hormones, and (3) although there is individual variation in the maturation process, the feedback regulation of the pituitary-thyroid axis matures by approximately the 37th gestational week.     

Somatostatin inhibition of growth hormone secretion in an adult bird: the domestic fowl

1. The intravenous (i.v.) infusion of somatostatin (SRIF, 1.0 microgram/kg per min) promptly (within 5 min) reduced the growth hormone (GH) concentration in the plasma of conscious adult chickens. 2. The GH concentration progressively declined throughout a 60-min period of SRIF infusion, but was dramatically increased above pre-infusion levels within 5 min of SRIF withdrawal and maintained at an elevated level for at least 30 min afterwards. 3. Sodium pentobarbitone-anaesthesia lowered the basal GH concentration to levels comparable with those in conscious birds infused with SRIF. When administered to anaesthetized birds, exogenous SRIF was unable to further reduce the GH concentration and unable to induce 'rebound' GH release. 4. While thyrotropin releasing hormone (TRH, 10 micrograms/kg) increased the GH concentration in both conscious and anaesthetized birds, only the GH response in the anaesthetized birds was diminished by SRIF infusion. 5. Rebound GH secretion following the termination of SRIF infusion was observed in both conscious and anaesthetized birds injected with TRH. 6. These results demonstrate that SRIF can inhibit basal and TRH-stimulated GH secretion in adult domestic fowl and indicate that anaesthesia disrupts the normal control of GH releases.     

Pertussis toxin blocks the inhibitory effects of somatostatin on cAMP-dependent vasoactive intestinal peptide and cAMP-independent thyrotropin releasing hormone-stimulated prolactin secretion of GH3 cells

It was shown that somatostatin (SRIF) inhibited cAMP-dependent vasoactive intestinal peptide (VIP)-stimulated prolactin (PRL) release by a GH3 clonal strain of rat pituitary tumor cells and decreased basal PRL secretion and inhibited PRL release in response to thyrotropin releasing hormone (TRH) whose action was independent of prior synthesis of cAMP. Pretreatment of these cells with pertussis toxin prevented SRIF's inhibitory effects on basal and TRH-stimulated hormone secretion as well as its VIP-stimulated responses. The blockade of SRIF's inhibitory effect on the actions of TRH or VIP was dependent on both the duration of preincubation and concentration of the toxin and was correlated with the ability of the toxin to catalyze the ADP-ribosylation of the 39,000-Da membrane protein. It is likely that this pertussis toxin substrate is involved in signal transduction of SRIF on cAMP-dependent actions of VIP and cAMP-independent action of TRH. However, the mechanism of SRIF's action on TRH is not clear, since SRIF did not affect the intracellular responses by TRH, neither intracellular Ca2+ mobilization nor the increase of 1,2-diacylglycerol formation following the breakdown of polyphosphoinositides.     

TRH-like peptides in prostate gland and other tissues

This minireview is aimed to recapitulate the occurrence of TRH-like peptides in the prostate gland and other tissues and to discuss their known functions in the organism. The hypothalamic thyrotropin-releasing hormone (TRH) was the first chemically defined hypophyseotropic hormone with the primary structure pGLU-HIS-PRO.NH2. However, the presence of extrahypothalamic TRH-immunoreactive peptides was reported in peripheral tissues including the gastrointestinal tract, placenta, neural tissues, male reproductive system and certain endocrine tissues. It was supposed that this TRH immunoreactivity can partially originate from TRH-homologous peptides and that these peptides have significant cross-reactions with the antibody specific against authentic TRH. This assumption was confirmed by the identification of prostatic TRH immunoreactivity as pyroGLU-GLU-PRO.NH2 using fast atom bombardment mass spectrometry and gas phase sequence analysis. TRH-like peptides are characterized by substitution of the basic amino acid histidine (related to authentic TRH) for neutral or acidic amino acids, such as glutamic acid, phenylalanine, glutamine or tyrosine. The physiological role of TRH-like peptides in peripheral tissues is not precisely known, but they possess a C-terminal amide group which is characteristic for many biologically active peptides. The occurrence of these peptides in the male reproductive system can influence male fertility. They are also closely related to circulating thyroid and steroid hormones. There might be an important connection of TRH-like peptides to the prostatic local autocrine/paracrine network mediated by extrahypothalamic TRH immunoreactivity corresponding to TRH-like peptides and extrapituitary thyrotropin (TSH) immunoreactivity also found in the prostatic tissue. A similar system of intraepithelial lymphocyte hormonal regulation due to the local paracrine network of TRH/TSH has been described in the gastrointestinal tract. The local network of TRH-like peptides/TSH may be involved in possible regulation of prostatic growth.     

The pituitary-thyroid axis in patients with pituitary disorders

The pituitary-thyroid axis of 12 patients, exposed to transsphenoidal pituitary microsurgery because of nonfunctioning adenomas (6), prolactinomas (3) and craniopharyngioma (1), or to major pituitary injury (1 apoplexy, 1 accidental injury), was controlled more than 6 months following the incidents. The patients did not receive thyroid replacement therapy and were evaluated by measurement of the serum concentration of thyroxine (T4), 3,5,3'-triiodothyronine (T3), 3,3',5'-triiodothyronine (rT3), T3-resin uptake test and thyrotropin (TSH, IRMA method) before and after 200 micrograms thyrotropin releasing hormone (TRH) iv. The examination also included measurement of prolactin (PRL) and cortisol (C) in serum. Apart from 1 patient with pituitary apoplexy all had normal basal TSH levels and 9 showed a significant TSH response to TRH. Compared to 40 normal control subjects the 12 patients had significantly decreased levels of T4, T3 and rT3 (expressed in free indices), while the TSH levels showed no change. Five of the patients, studied before and following surgery, had all decreased and subnormal FT4I (free T4 index) after surgery, but unchanged FT3I and TSH. The levels of FT4I were positively correlated to both those of FT3I and FrT3I, but not to TSH. The TSH and thyroid hormone values showed no relationship to the levels of PRL or C of the patients exposed to surgery. It is concluded that the risk of hypothyroidism in patients exposed to pituitary microsurgery is not appearing from the TSH response to TRH, but from the thyroid hormone levels.     

Effects of helodermin on fetal rat bone metabolism in vitro

Helodermin belongs to the VIP family of polypeptides. Recent in vivo data suggest that helodermin-like peptides might be involved in the regulation of calcium metabolism. We show that helodermin specifically binds to a secretin-type receptor in osteoblast-like cells from fetal rat calvaria and increases the basal and PTH-stimulated cAMP concentration of these cells. In organ cultures of fetal rat calvaria, helodermin strongly inhibits bone matrix apposition and augments PTH-induced bone resorption. Helodermin-like peptides may thus be capable of enhancing the direct effects of PTH on bone metabolism.     

Calcium modifies the effects of thyroliberin and somatostatin on hormone release from cell cultures of the rat anterior pituitary

Role of calcium (Ca2+) in the effects of thyroliberin (TRH) and somatostatin (SRIF) on the release of growth hormone (GH), prolactin (PRL) and thyroid stimulating hormone (TSH) from the rat adenohypophyseal cells in primary monolayer cultures has been studied. Decrease of extracellular Ca2+ diminished the stimulatory effects of TRH on TSH and PRL release. Ca2+ is also an important factor in the mechanism of SRIF action. Data obtained in the experiments with high Ca2+ levels in the medium indicate that some antagonistic interrelationship exists between Ca2+ and SRIF. These results suggest that the participation of cAMP alone is not sufficient for stimulus-secretion coupling. Another messenger, namely Ca2+, is necessary for the effects of hypothalamic hormones. On the other hand, the contribution of Ca2+ to the secretory process in mammotrophs, somatotrophs and thyrotrophs is not equal. PRL and TSH secretion is more dependent on the presence of extracellular Ca2+ than the release of GH.