Alpha 1-adrenoreceptors on intact rat anterior pituitary cells: correlation with adrenergic stimulation of thyrotropin secretion

Plasma prolactin, thyroid-stimulating hormone, melanocyte-stimulating hormone, and adrenocorticotropin responses to thyrotropin-releasing hormone in mares treated with detomidine and butorphanol

Stress or excitement is a concern when performing endocrine tests on fractious horses. Sedation may be a solution; however, perturbation of test results may preclude useful information. Thyrotropin-releasing hormone (TRH) is a known stimulator of prolactin, thyroid-stimulating hormone (TSH), melanocyte-stimulating hormone (MSH), and ACTH. Thyrotropin-releasing hormone-induced ACTH is a diagnostic tool for the assessment of endocrinopathies such as pituitary pars intermedia dysfunction. It is unknown if drugs commonly used for sedation alter endocrine responses. The objective of this study was to assess the effects of detomidine (DET) and butorphanol on endocrine responses to TRH. Nine light horse mares were used in a replicated 3 × 3 Latin square with the following treatments: saline, DET, and detomidine + butorphanol (DET/BUT), all administered intravenously at 0.01 mg/kg BW. A 1-wk washout period was allowed between phases, all of which were performed in December. Blood samples were collected at -10 and 0 min before treatment and 5 and 10 min post-treatment. Administration of 1 mg TRH occurred 10 min post-treatment, and blood sampling continued 5, 10, 20, and 30 min post-TRH. Data were analyzed by ANOVA as a replicated Latin square with repeated sampling. Plasma prolactin increased (P < 0.0001) after TRH in all groups, rapidly peaking at 5 min in drug-treated mares and 40 min in saline-treated mares. The peak prolactin response to TRH was 2-fold higher (P < 0.0001) in saline-treated mares compared with those drug-treated. A peak rise in plasma TSH was observed in DET/BUT-treated mares 10 min after TSH and was greater (P ≤ 0.007) compared with DET- and saline-treated mares. Plasma MSH was stimulated (P = 0.001) by DET and DET/BUT before TRH, and the peak MSH response to TRH was greater (P < 0.0001) in drug-treated mares, although not hastened as observed with prolactin and TSH. A peak rise in ACTH was observed in drug-treated mares 5 min after administration of TRH, whereas a peak rise was observed in control mares 10 min post-TRH and was almost 2-fold lower (P = 0.05) than the peak observed in DET and DET/BUT-treated mares. Basal ACTH concentrations were not affected by DET or DET/BUT, indicating that sedation with these compounds may be achieved when needing to measure basal plasma ACTH. Treatment with DET and DET/BUT did alter the prolactin, TSH, MSH, and ACTH responses to TRH; therefore, the use of these drugs may not be advisable when assessing endocrine responses to TRH stimulation.

Beta1-adrenoceptors in rat anterior pituitary may be constitutively active. Inverse agonism of CGP 20712A on basal 3',5'-cyclic adenosine 5'-monophosphate levels

Catecholamines directly stimulate GH, ACTH, and prolactin secretion from rat anterior pituitary through the beta(2)-adrenoceptor (AR). We recently showed that gonadotrophs express the beta(1)-AR and that glucocorticoids drastically increase its mRNA expression level. The present investigation explores whether beta(1)-ARs are functionally coupled to adenylate cyclase. In anterior pituitary cell aggregates, the highly selective beta(1)-AR antagonists CGP 20712A and ICI 89,406-8a attenuated isoproterenol-stimulated cAMP accumulation, but no agonist action of norepinephrine could be detected. Remarkably, CGP 20712A inhibited basal cAMP levels by its own for at least 50%, an action that tended to be more effective in dexamethasone-supplemented medium. The latter effect was abolished by the beta-AR antagonist carvedilol, but not by other beta-AR antagonists. Pretreatment with pertussis toxin abolished the action of CGP 20712A on basal cAMP. CGP 20712A also attenuated isoproterenol-induced cAMP accumulation in the gonadotroph cell lines alphaT3-1 and LbetaT2, but not in the somatotroph precursor cell line GHFT and the folliculo-stellate cell line TtT/GF. However, in LbetaT2 cells CGP 20712A did not inhibit basal cAMP levels by its own. The present data suggest that beta(1)-AR in the anterior pituitary is positively coupled to adenylyl cyclase but is constitutively active in a pertussis toxin-sensitive manner. CGP 20712A may act as an inverse agonist with approximately 50% negative intrinsic activity, suggesting that the beta(1)-AR significantly contributes to basal adenylate cyclase activity in the pituitary.

Beta1-adrenoceptor expression in rat anterior pituitary gonadotrophs and in mouse alphaT3-1 and LbetaT2 gonadotrophic cell lines

The rat anterior pituitary expresses beta(2)-adrenoceptors (ARs) on somatotrophs, lactotrophs, and corticotrophs. The present study investigates whether beta(1)-ARs exist in the anterior pituitary, in which cell type(s) they are found, and whether they are regulated by glucocorticoids. As determined by quantitative RT-PCR and Western immunoblotting, the rat anterior pituitary expressed beta(1)-AR mRNA and protein. Unlike the beta(2)-AR, expression decreased to very low levels after 5-d aggregate cell culture but was strongly up-regulated in a dose- and time-dependent manner by dexamethasone (DEX). Glucocorticoids attenuated isoproterenol-induced down-regulation of beta(1)-AR mRNA levels. As examined by immunofluorescence confocal microscopy, beta(1)-AR immunoreactivity was detected in a subpopulation of gonadotrophs, but not in somatotrophs, lactotrophs, corticotrophs, thyrotrophs, or folliculo-stellate cells. beta(1)-AR-immunoreactivity cells were often surrounded by cup-shaped lactotrophs. Consistent with these findings, beta(1)-AR mRNA was considerably more abundant in the gonadotrophic alphaT3-1 and LbetaT2 cell lines than in the GHFT, GH3, and TtT/GF cell lines. DEX did not affect expression level in the cell lines. DEX also failed to up-regulate beta(1)-AR mRNA levels in aggregates from a subpopulation enriched in large gonadotrophs obtained by gradient sedimentation. In contrast, excessive DEX-dependent up-regulation of beta(1)-AR mRNA was found in a subpopulation enriched in small nonhormonal cells. The present data indicate that beta(1)-AR is expressed in a subpopulation of gonadotrophs with a topographical relationship to lactotrophs. However, the glucocorticoid-induced up-regulation does not seem to occur directly in the gonadotrophs but within (an)other unidentified cell type(s), or is transduced by that cell type on gonadotrophs.

Effect of acute high dose dobutamine administration on serum thyrotrophin (TSH)

OBJECTIVE

The interpretation of thyroid function tests in the setting of severe illness is often complicated by concomitant drug administration which may independently produce changes in thyroid hormone concentrations or even secondary hypothyroidism. Although the effects of dopamine on TSH are well established, the effects of dobutamine, another drug commonly used in the setting of severe illness, on TSH are unknown. The aim of the study was to establish the effect(s) of acute high dose dobutamine on serum TSH concentration.

DESIGN AND PATIENTS

Thirty subjects undergoing dobutamine stress echocardiogram were compared to twenty controls. Serum TSH was determined between the hours of 0800 and 1000 h at baseline, at maximum dobutamine infusion (20-50 micrograms/kg/min and 15 minutes after stopping dobutamine.

MEASUREMENTS

Serum TSH concentration was measured using a third generation chemiluminescent assay.

RESULTS

TSH concentration decreased with time in both dobutamine and control subjects and there was an additional statistically significant effect of dobutamine treatment to decrease TSH. TSH concentration remained within the normal range in all subjects who started with normal TSH concentration and remained above normal in the three dobutamine-treated subjects with elevated TSH at baseline. The dobutamine-associated decrease in TSH was still present 15 minutes after discontinuing dobutamine.

CONCLUSIONS

These results indicate that acute high dose dobutamine lowers TSH by an unknown mechanism. Additional study with prolonged dobutamine infusion is needed to establish the steady state level and physiological consequences of dobutamine-inhibited TSH.

Thyrotropin and prolactin responses to TRH in delusional (psychotic) and nondelusional depressed patients

The thyrotropin (TSH) and prolactin (PRL) responses to protirelin (TRH) were studied in 62 female unipolar depressive patients, 24 delusional (psychotic) and 38 nondelusional (Diagnostic and Statistical Manual [DSM]-III-R criteria). The TSH response to TRH was blunted (≤6 μlU/mL) in ten of the 24 delusional (41.6%) and in 18 of the 38 nondelusional (47.4%) patients, the difference being not significant. The TSH and PRL responses to TRH, calculated as area under the curve, were not different between the two groups. Similar results were obtained when the response patterns were compared by repeated measure analysis of covariance.

Influence of partial sleep deprivation on the secretion of thyrotropin, thyroid hormones, growth hormone, prolactin, luteinizing hormone, follicle stimulating hormone, and estradiol in healthy young women

The influence of partial sleep deprivation during the second half of the night on the secretion of thyroid stimulating hormone (TSH), thyroxin (T4), free T4 (fT4), triiodothyronine (T3), prolactin (PRL), growth hormone (GH), luteinizing hormone (LH), follicle stimulating hormone (FSH), and estradiol (E2) was investigated in 10 healthy young women. Blood samples were drawn at hourly intervals over a 64-hour period (i.e., 3 consecutive days and nights). During night 2, all subjects were awakened at 1:30 a.m. During partial sleep deprivation, TSH concentrations increased significantly and remained elevated throughout the following day. Levels of T4, fT4, and T3 were enhanced during the partial sleep deprivation hours only, and changes in these hormones seemed to be independent of TSH. PRL levels decreased, LH and E2 concentrations increased, and GH and FSH secretion remained unchanged during partial sleep deprivation. This pattern of change of different endocrine axes during partial sleep deprivation resembles those seen after total sleep deprivation, suggesting that similar neurochemical changes are induced by both forms of antidepressant therapy. The late evening GH peak occurred almost exclusively before the onset of sleep. Partial sleep deprivation did not influence the chronobiological profiles of any of the hormones investigated. The chemical changes underlying these alterations are speculated to involve enhancement of central norepinephrine and dopamine activity with a concomitant increase in the activity of the sympathetic nervous system.

Adrenergic control of the secretion of anterior pituitary hormones

The hypothalamic hypophysiotrophic neurones are densely innervated by adrenergic and noradrenergic nerve terminals. Activation of alpha 1-adrenoceptors located in the brain stimulates the secretion of ACTH, prolactin and TSH. The effects of the alpha 1-adrenoceptors seem to be exerted on hypothalamic neurones that secrete vasopressin, CRH-41 and TRH. These mechanisms are important in the physiological control of the secretion of ACTH and TSH in humans. alpha 2-Adrenoceptors are not involved in the control of secretion of these hormones under basal conditions in humans. However, alpha 2-adrenoceptors exert an inhibitory effect that acts as a negative feedback mechanism, limiting excessive secretion of these hormones. There is no convincing evidence for the involvement of beta-adrenoceptors in the control of the secretion of these three hormones in humans. Studies on cultured anterior pituitary cells suggested that adrenaline and noradrenaline may influence the secretion of ACTH, prolactin and TSH directly at the level of the pituitary. However, these effects are not demonstrable in humans, and are likely to be due to alterations in the pituitary adrenoceptors during culture. In the case of growth hormone, activation of alpha 2-adrenoceptors located in the brain stimulates secretion of this hormone both by increasing the secretion of GHRH and by inhibiting the secretion of somatostatin. Activation of beta-adrenoceptors inhibits the secretion of growth hormone via an increase in the secretion of somatostatin. The effects of the central alpha 2- and beta-adrenoceptors are important in the physiological control of growth hormone secretion in humans. A considerable amount of evidence implicates brain alpha 1-adrenoceptors in the control of secretion of the gonadotrophins in experimental animals, but, despite intensive study, no convincing evidence has been found in humans of reproductive age.

Activation of central alpha 1-adrenoceptors in humans stimulates secretion of prolactin and TSH, as well as ACTH

In normal male volunteers, intravenous infusions of the alpha 1-adrenergic agonist methoxamine stimulated the secretion of prolactin, thyroid-stimulating hormone (TSH), and adrenocorticotropic hormone (ACTH), and the effects were abolished by pretreatment with the alpha 1-antagonist prazosin. To investigate the site of action of methoxamine, its effects were compared with those of equipotent doses of norepinephrine, an alpha 1-agonist that reaches the pituitary gland and the median eminence after an intravenous infusion but, unlike methoxamine, does not cross the blood-brain barrier. Norepinephrine did not stimulate secretion of prolactin, TSH, or ACTH, suggesting that the stimulant alpha 1-adrenoceptors are located in the central nervous system and not directly on the pituitary gland or in the periphery. The alpha 2- and beta-adrenoceptor agonist properties of norepinephrine could not account for the differences from methoxamine, as pretreatment with prazosin did not modify hormone concentrations after norepinephrine. Methoxamine had no behavioral stimulant effects, as judged by visual analog scales that were sensitive to physiological changes in behavioral arousal. In four patients with hypothalamic dysfunction but responsive pituitary corticotrophs, methoxamine had no stimulant effect on the secretion of ACTH, confirming that the alpha 1-adrenoceptors that stimulate ACTH secretion are not located directly on the pituitary. None of the drugs had an effect on the secretion of growth hormone or the gonadotrophins.

Alpha-adrenergic inhibition of thyrotropin-releasing hormone-induced prolactin secretion in GH4C1 cells is associated with a depressed rise in intracellular Ca2+

alpha-Adrenergic receptors are present on the plasma membrane of normal anterior pituitary cells and alpha-adrenergic agonists may play a role in the secretion of corticotropin (ACTH) and thyrotropin (TSH). However, alpha-adrenergic involvement in prolactin (PRL) secretion is uncertain. We have therefore examined this question in the PRL-secreting clonal rat pituitary tumor-derived GH4C1 cells. Norepinephrine (NE), an alpha-adrenergic agonist, had no effect on basal PRL secretion but abolished thyrotropin-releasing hormone (TRH)-induced PRL secretion in a dose-dependent manner (EC50 100 nM). NE also significantly suppressed the TRH-stimulated rise in [Ca2+]i. Phentolamine (PA), a non-selective alpha-adrenergic antagonist, reversed the inhibitory effect of NE on both the TRH-stimulated PRL secretion and [Ca2+]i rise. NE did not inhibit the rise in PRL secretion or [Ca2+]i induced by depolarizing 30 mM K+, 30% hyposmolarity or BAY K-8644, a specific L-type Ca2+ channel agonist. The inhibitory effect of NE on TRH-induced PRL and [Ca2+]i changes was also present when Ca2+ influx was prevented by removing medium Ca2+ or by blocking L-type Ca2+ channels with 2 microM nifedipine. The TRH-stimulated first-phase rise in [Ca2+]i in GH4C1 cells is believed to result primarily from release of sequestered Ca2+ from an intracellular pool through the activation of inositol 1,4,5-trisphosphate (IP3) and this [Ca2+]i spike stimulates PRL secretion. Our data thus suggest that GH4C1 cells have alpha-adrenergic receptors and that alpha-adrenergic agonists either suppress IP3 generation or block IP3 release of sequestered intracellular Ca2+.

Thyrotropin responses to TRH and MHPG excretion before and after an electroconvulsive therapeutic course in depressed patients

1. TSH response to TRH, and urinary MHPG were investigated before and after an ECT course in 12 female patients with endogenous depression. 2. The changes caused by ECT treatment on these parameters were not significant. 3. A positive correlation (r = 0.75, p = 0.005) was found between the changes in TSH response and the changes in urinary MHPG excretion.

Neuroendocrinological investigations during sleep deprivation in depression. II. Longitudinal measurement of thyrotropin, TH, cortisol, prolactin, GH, and LH during sleep and sleep deprivation

Thyrotropin (TSH), thyroxin (T4), triiodothyronine (T3), free T3 (fT3), cortisol, prolactin, and human growth hormone (HGH) were measured every 2 hr during a night of sleep, the following day, and a night of sleep deprivation (SD) in 14 patients with major depressive disorder. In subgroups fT4 (n = 5), reverse T3 (rT3), and luteinizing hormone (LH) (n = 6) were also investigated. Significant increases in TSH, T4, fT4, T3, fT3, rT3, and cortisol and decreases in prolactin levels occurred during the night of SD, compared to the pattern during the night of sleep. The pre-SD T4 and T3 levels of the responders to SD were already higher than in the nonresponders, and increased less during SD. The cortisol and HGH concentrations of the responders rose higher during SD than those of the nonresponders. Changes in TSH and prolactin were not correlated to clinical response. Analysis of possible neurochemical mechanisms underlying this "pattern" of changes in different endocrine profiles suggests that enhanced noradrenergic activity might play a role in the changes in TSH, cortisol, thyroid hormones, and possibly HGH secretion during SD, and increased dopaminergic tone probably induced the decline in prolactin levels. Additional effects of the serotonergic system cannot be excluded at present. In conclusion, the data suggest that enhanced noradrenergic activity of the locus coeruleus stimulates alpha and/or beta adrenergic receptors in depressed patients during SD. This mechanism could well be involved in the antidepressant effect of this therapy.

Activity of the hypothalamic-pituitary-thyroid axis during exposure to cold

It seems clear from the studies reviewed here that there is adequate evidence to support the concept of a biphasic response of the thyroid gland to cold as first postulated by Moll et al. (1972). The initial response to acute exposure to cold begins at the level of the hypothalamus as a result of either neural stimuli from skin and other areas and/or blood of somewhat lower than normal temperature reaching the hypothalamus (Andersson et al., 1963). As a result, the secretion of norepinephrine and/or dopamine may increase, and serotonin and/or somatostatin may decrease. The net result of these is an increase in the release of TRH from the hypothalamus. This, in turn, stimulates the cascade for the release of TSH from the anterior pituitary gland and thyroid hormone from the thyroid gland. Moll et al. (1972) postulated the lack of a feedback limb in this acute phase, and, indeed, this may be the case. It is possible, however, that certain hormones, such as somatostatin, norepinephrine, T3, and T4 could act in the capacity of feedback inhibitors. Additional experiments will be required to assess this possibility. The transitional link between the acute (less than 1 day) and chronic (greater than 1 day) phases of the response of the thyroid gland to cold could be T4 itself. An increase in the concentration of T4 in plasma has been reported to increase peripheral deiodination of T4 to T3 by kidneys and liver of rats. There are no studies at present to indicate that hepatic conjugation can be increased by elevation of plasma levels of T4 and T3. If it can, these responses would provide adequate reasons as to why peripheral metabolism of thyroid hormones increases during chronic exposure to cold. The time-course for these changes to occur needs to be studied in greater detail to establish the sequence of events following acute exposure to cold. The latter may also increase urinary excretion of T4 and T3 in man, but not the rat. This suggests that another aspect of exposure to cold needing additional study is measurement of the binding affinities of T4 and T3 for their transport proteins during exposure to cold as compared to affinities prior to exposure to cold. If binding affinities are reduced, the amount of free hormones would increase and, consequently the likelihood of being excreted into urine and conjugated by the liver would also increase.(ABSTRACT TRUNCATED AT 400 WORDS)

Multifactorial control of pituitary hormone secretion: the "wheels" of the brain

An attempt is made to deal with the complexity of the nerve fibers in the median eminence. Visual aids are presented in the shape of "wheels" that depict a dynamic interplay of neurochemicals which result in the release of hormones from the anterior pituitary gland. The multiplicity of neurochemicals in the median eminence is perceived to be responsible for the integrated control of pituitary hormone releasing factors.

The circadian rhythms of thyrotrophin and prolactin secretion

As with other anterior pituitary hormones, the secretion of both thyrotrophin (TSH) and prolactin (PRL) displays a circadian variation with different patterns for each hormone. In recent years there has been a substantial increase in the understanding of the neuroregulation of TSH and PRL. However the primary events involved in the generation of their circadian rhythms remains unclear. Regulatory pathways comprise two major groups: central factors, where the control is exerted by the central nervous system via the hypothalamus and peripheral factors, which include all extra CNS mechanisms. The first group is represented mainly by neuropeptides and neurotransmitters controlling TSH and PRL release, whereas the second one comprises both physical phenomena such as variations in plasma volume or postural changes and hormonal influences arising from target glands such as the adrenal, the thyroid and the gonads.

Alpha-adrenergic stimulation of growth hormone release in perifused rat anterior pituitary reaggregate cell cultures

It has been previously shown that beta-adrenergic agonists stimulate growth hormone (GH) release from perifused rat anterior pituitary cell aggregates cultured in serum-free defined medium for 5-6 days. The present data show that the natural beta-agonist epinephrine (EPI) stimulates GH release through an additional alpha-adrenergic mechanism. The involvement of this mechanism was suggested by the following findings. The intrinsic activity of EPI, at concentrations greater than 10 nM, was significantly higher than that of isoproterenol (ISO) in stimulating GH release. Phenylephrine (PHE), a specific agonist of alpha 1-adrenergic receptors, provoked a significant rise of GH release. The effect was concentration dependent between 100 nM and 10 microM. The stimulatory effect of EPI and PHE was lowered, respectively blocked, by low concentrations of the potent alpha 1-adrenergic antagonist prazosin. The EPI-induced GH release could be totally blocked only by administration of both alpha- and beta-receptor antagonists. Under the same conditions and using concentrations up to 1 microM, the alpha 2-agonist clonidine had no or only a slight stimulatory effect; dopamine had no effect. Administration of both PHE and ISO resulted in a more than additive stimulation of GH release. The effectiveness of PHE but not clonidine, together with the high potency of the alpha 1-blocker prazosin suggest that the alpha-adrenergic receptor is predominantly of the alpha 1-subtype. When tested on days 1, 3, 6 and 8 in culture, both alpha- and beta-adrenergic responses appeared to be higher in the presence of the glucocorticoid dexamethasone compared to the responses in the absence of dexamethasone.(ABSTRACT TRUNCATED AT 250 WORDS)

TRH stimulation test and depression

A thyrotropin-releasing hormone (TRH) stimulation test was performed in 52 male inpatients with major depressive disorder. Twenty-nine percent of the 52 subjects had a delta thyroid-stimulating hormone (delta TSH) less than 5 microU/ml. The cerebrospinal fluid (CSF) amine metabolites, 3-methoxy-4-hydroxyphenylglycol (MHPG), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5HIAA), were measured in 29 subjects, and a dexamethasone suppression test (DST) was performed in 48 subjects. Of the three CSF amine metabolites, only MHPG correlated significantly with baseline TSH and none correlated with delta TSH. The baseline TSH correlated positively with the TSH response at 30 minutes. Neither baseline TSH nor delta TSH correlated with cortisol levels before or after dexamethasone. The correlation between CSF MHPG and serum TSH suggests a relationship between central norepinephrine and baseline TSH.

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.

Effect of low-dose dopamine infusion on basal and stimulated TSH and prolactin concentrations in man

Dopamine (DA) infused at pharmacological doses in man inhibits thyrotrophin (TSH) secretion, although the physiological significance of this observation is unclear. The effect of low-dose DA infusion (0.1 microgram/kg/min) on TSH and prolactin (PRL) concentrations during stimulation with thyrotrophin releasing hormone (TRH) in normal male subjects is reported. Six subjects were given intravenous DA or placebo infusions for 165 min on separate days. A bolus of TRH (7.5 micrograms) was given at + 90 min, followed by infusion of the tripeptide (750 ng/min) for 45 min during both DA and placebo studies. In all subjects TRH administration caused a small rise in TSH which was partially inhibited by DA (peak 5.73 +/- 0.85 mU/l vs 4.58 +/- 1.09, P less than 0.05). PRL response to TRH was almost totally inhibited by DA (620 +/- 164 mU/l vs 234 +/- 96, P less than 0.05); integrated TSH and PRL responses to TRH were similarly inhibited by DA. Circulating plasma DA concentration during infusion of the catecholamine was 3.46 +/- 1.00 ng/ml, which is within the range reported in pituitary stalk plasma of other species. These data support the hypothesis that DA is a physiological modulator of TSH secretion in normal man. Major differences in the time course of TSH and PRL responses to TRH, and in the suppressive effect of DA on these responses suggest that there are fundamental differences in stimulus-secretion coupling for TRH and the lactotroph and thyrotroph.

Relationships between the circadian rhythms of TSH, prolactin and cortisol in surgically treated microprolactinoma patients

Pharmacological doses of glucocorticoids inhibit TSH release both in vivo and in vitro and since the circadian rhythms of TSH and cortisol show a reciprocal relationship, the hypothesis has been advanced that changes in cortisol levels may be a primary determinant of circadian TSH changes. We have tested this hypothesis by studying the relationship between circadian cortisol and TSH rhythms in subjects before and during blockade with metyrapone. Seven patients were studied during their routine post-operative assessment following selective transethmoidal adenomectomy for microprolactinomas. PRL levels were restored to normal (less than 420 mU/l) in all patients by surgery (pre-op: 930-2752 mU/l, post-op: 33-376 mU/l) and the patients also had normal pituitary function in other respects. Blood was sampled hourly for 24 h before and on the third day of treatment with metyrapone (250 mg, 2 hourly). In order to compare circadian rhythms, hormonal data were subjected to cosinor analysis which involved fitting of the data with a cosine function using the method of least squares. The 6% cross reactivity of the cortisol antibody with 11-deoxycortisol was taken into account during the calculation of results. All subjects showed a normal cortisol rhythm which was strikingly blunted during metyrapone treatment. Group mean (+/- SD) TSH mesors, amplitudes and acrophases for control and metyrapone treated subjects were 1.5 +/- 0.26, 1.29 +/- 0.48; 0.46 +/- 0.26, 0.23 +/- 0.13 and -49 degrees +/- 9.8 degrees; -62 degrees +/- 2.7 degrees respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Circulating catecholamine, thyrotrophin, thyroid hormone and prolactin responses of normal subjects to acute cold exposure

The responses of circulating catecholamines, TSH, thyroid hormones and prolactin to 30 min of acute cold exposure (4 degrees C) were measured in eight normal volunteers over a 2 h period. There was a rise in circulating noradrenaline, TSH, T4 and T3 levels and a fall in circulating prolactin in the subjects studied, but no change in circulating adrenaline levels nor any alteration in the T4/T3 ratio. The thyroid axis of normal individuals can respond rapidly to acute cold exposure. In addition, the increased plasma noradrenaline levels accompanied by unaltered adrenaline levels suggest that the stimulus exerted by cold does not evoke a generalised stress response, but rather that the sympathetic nervous system is selectively stimulated.