TRH-stimulated TSH Release Research Articles

Effect of medroxyprogesterone acetate on thyrotropin secretion in adult and old female rats.

Steroid hormones have been implicated in the modulation of TSH secretion; however, there are few and controversial data regarding the effect of progesterone (Pg) on TSH secretion. Medroxyprogesterone acetate (MPA) is a synthetic alpha-hydroxyprogesterone analog that has been extensively employed in therapeutics for its Pg-like actions, but that also has some glucocorticoid and androgen activity. Both hormones have been shown to interfere with TSH secretion. The objective of the present study was to investigate the effects of MPA or Pg administration to ovariectomized (OVX) rats on in vivo and in vitro TSH release and pituitary TSH content. The treatment of adult OVX rats with MPA (0. 25 mg/100 g body weight, sc, daily for 9 days) induced a significant (P<0.05) increase in the pituitary TSH content, which was not observed when the same treatment was used with a 10 times higher MPA dose or with Pg doses similar to those of MPA. Serum TSH was similar for all groups. MPA administered to OVX rats at the lower dose also had a stimulatory effect on the in vitro basal and TRH-induced TSH release. The in vitro basal and TRH-stimulated TSH release was not significantly affected by Pg treatment. Conversely, MPA had no effect on old OVX rats. However, in these old rats, ovariectomy alone significantly reduced (P<0.05) basal and TRH-stimulated TSH release in vitro, as well as pituitary TSH content. The results suggest that in adult, but not in old OVX rats, MPA but not Pg has a stimulatory effect on TSH stores and on the response to TRH in vitro.

Dose-dependent effects of 17-beta-estradiol on pituitary thyrotropin content and secretion in vitro.

We studied the basal and thyrotropin-releasing hormone (TRH) (50 nM) induced thyrotropin (TSH) release in isolated hemipituitaries of ovariectomized rats treated with near-physiological or high doses of 17-beta-estradiol benzoate (EB; sc, daily for 10 days) or with vehicle (untreated control rats, OVX). One group was sham-operated (normal control). The anterior pituitary glands were incubated in Krebs-Ringer bicarbonate medium, pH 7.4, at 37 degrees C in an atmosphere of 95% O2/5% CO2. Medium and pituitary TSH was measured by specific RIA (NIDDK-RP-3). Ovariectomy induced a decrease (P < 0.05) in basal TSH release (normal control = 44.1 +/- 7.2; OVX = 14.7 +/- 3.0 ng/ml) and tended to reduce TRH-stimulated TSH release (normal control = 33.0 +/- 8.1; OVX = 16.6 +/- 2.4 ng/ml). The lowest dose of EB (0.7 microgram/100 g body weight) did not reverse this alteration, but markedly increased the pituitary TSH content (0.6 +/- 0.06 microgram/hemipituitary; P < 0.05) above that of OVX (0.4 +/- 0.03 microgram/hemipituitary) and normal rats (0.46 +/- 0.03 microgram/hemipituitary). The intermediate EB dose (1.4 micrograms/100 g body weight) induced a nonsignificant tendency to a higher TSH response to TRH compared to OVX and a lower response compared to normal rats. Conversely, in the rats treated with the highest dose (14 micrograms/100 g body weight), serum 17-beta-estradiol was 17 times higher than normal, and the basal and TRH-stimulated TSH release, as well as the pituitary TSH content, was significantly (P < 0.05) reduced compared to normal rats and tended to be even lower than the values observed for the vehicle-treated OVX group, suggesting an inhibitory effect of hyperestrogenism. In conclusion, while reinforcing the concept of a positive physiological regulatory role of estradiol on the TSH response to TRH and on the pituitary stores of the hormone, the present results suggest an inhibitory effect of high levels of estrogen on these responses.

Uptake of 3,3',5,5'-tetraiodothyroacetic acid and 3,3',5'- triiodothyronine in cultured rat anterior pituitary cells and their effects on thyrotropin secretion

We compared the uptake, metabolism, and biological effects of tetraiodothyroacetic acid (Tetrac) and rT3 in anterior pituitary cells with those of T4 and T3. Cells were isolated from adult male Wistar rats and cultured for 3 days in medium with 10% fetal calf serum. Uptake was measured at 37 C in medium with 0.1% BSA for [125I]Tetrac (200,000 cpm; 240 pM) and [125I]T4 (100,000 cpm; 175 pM) or with 0.5% BSA for [125I]rT3 (100,000 cpm; 250 pM) and [125I]T3 (50,000 cpm; 50 pM). The free fraction of Tetrac was 1% that of T4 (in medium with 0.1 and with 0.5% BSA), and the free fraction of rT3 was half that of T3. Uptake of the four tracers increased sharply up to 1 h of incubation and then leveled off. Expressed as femtomoles per pM free hormone, uptake at equilibrium was 1.16 +/- 0.16 (n = 6) for Tetrac, 0.15 +/- 0.01 (n = 6) for T4, 0.023 +/- 0.003 (n = 6) for rT3, and 0.21 +/- 0.02 (n = 6) for T3. Cell-associated radioactivity after incubation for 24 h with [125I]Tetrac was represented for 15% by [125I]Triac; after incubation with [125I]T4 for 15-20% by [125I]T3, after incubation with [125I]rT3 for 6% by [125I]3,3'-T2, while [125I]T3 was still for 98% [125I]T3. Exposure of cells for 2 h to 100 nM TRH stimulated TSH release by 90-135%. Tetrac was effective in reducing this response at a free concentration of 0.05 pM, but rT3 was effective only at a free concentration of 16 nM. A free Tetrac concentration of 5 pM was equally effective as 50 pM free T4 in reducing the TSH response to TRH. In human serum, Tetrac was exclusively bound to T4-binding prealbumin. The free Tetrac fraction was 0.001% in control subjects and rose 2- to 12-fold in patients with nonthyroidal illness. As uptake of [125I]Tetrac in the pituitary was higher than that of T4 and T3, and it was more potent than T4 in reducing TSH release, Tetrac may be of potential significance for the regulation of TSH secretion in vivo.

Effect of loop diuretics and nonsteroidal antiinflammatory drugs on thyrotropin release by rat anterior pituitary cells in vitro

The close inverse-feedback relationship between serum free thyroxine (T 4) and thyrotropin (TSH) is altered in some patients receiving therapeutic doses of drugs such as furosemide, fenclofenac, and diphenylhydantoin. We therefore examined the effect of nonsteroidal antiinflammatory drugs (NSAID), diuretics, and diphenylhydantoin on TSH release in rat anterior pituitary cells in primary culture. TSH content of the culture medium was measured at 22 hours at 37°C either with or without thyrotropin-releasing hormone ([TRH] 10 nmol/L) i either with or without thyrotropin-releasing hormone ([TRH] 10 nmol/L) in medium containing 0.5% bovine serum albumin. The mean basal TSH release by pituitary cells was 6.2 ± 1.2 ng/mL (n = 10) and was not influenced by unlabeled triiodothyronine ([T 3] 100 nmol/L) or any of the drugs tested at ≤ 400 μmol/L, except ethyacrynic acid. TRH 10 nmol/L increased mean TSH release by 346% ± 95% (n = 10). T 3 1 and 100 nmol/L inhibited TRH-stimulated TSH release by 24% and 31%, respectively ( P < .001), whereas TRH-stimulated TSH release was inhibited by 100 μmol/L meclofenamic acid (29%), fenclofenac (28%), furosemide (24%), and diphenylhydantoin (48%) ( P < .001 v TRH alone). Meclofenamic acid and furosemide (100 μmol/L) did not significantly alter the inhibitory effect of T 3 1 nmol/L on TRH-stimulated TSH release. These in vitro studies suggest that meclofenamic acid, fenclofenac, furosemide, and diphenylhydantoin could influence TSH release by attenuating the TSH response to TRH. This effect may influence T 4-TSH relationships when these agents are used in vivo.

Effect of gastrin-releasing peptide (GRP) and grp antagonists on TSH secretion from rat isolated pituitaries

It has previously been demonstrated that gastrin releasing peptide (GRP), a bombesin-like peptide, inhibited TSH release “in vivo”. In this study, we have shown that GRP acts directly at the pituitary gland, inhibiting basal and TRH-stimulated TSH release from incubated rat anterior pituitary glands. This effect was observed at the highest GRP concentration (10 −5 M), but not at the lower concentrations (10 −7 and 10 −9 M). Incubation of the glands with two antagonists of GRP (d-Phe 8-GRP and Gly 6-GRP) induced an increase of basal TSH secretion. We suggest a physiological role of locally produced bombesin-like peptides in the control of TSH release. Another antagonist (Ala 6 — GRP) did not change TSH secretion. This result suggests the existence of different subtypes of GRP receptors in the anterior pituitary gland.

Pituitary and peripheral thyroid hormone responses to thyrotropin-releasing hormone during sustained sleep deprivation in freely moving rats.

Sleep deprivation is associated with poor cognitive ability and impaired physical health, but the ways in which the brain and body become compromised are not understood. In sleep-deprived rats, plasma total T4 and T3 concentrations decline progressively to 78% and 47% below baseline values, respectively, brown adipose tissue 5'-deiodinase type II activity increases 100-fold, and serum TSH values are unknown. The progressive decline in plasma thyroid hormones is associated with a deep negative energy balance despite normal or increased food intake and malnutrition-like symptoms that eventuate in hypothermia and lethal systemic infections. The purpose of the present experiment was to evaluate the probable causes of the low plasma total T4 during sleep deprivation by measuring the free hormone concentration to minimize binding irregularities and by challenging the pituitary-thyroid axis with iv TRH to determine both 1) the pituitary release of TSH and 2) the thyroidal response of free T4 (FT4) and free T3 (FT3) release to the TSH increment. Sleep-deprived rats were awake 91% of the total time compared with 63% of the total time in yoked control rats and 50% of the total time during the baseline period. Cage control comparison rats were permitted to sleep normally. Sustained sleep deprivation resulted in a decline from baseline in plasma FT4 of 73 +/- 6% and FT3 of 45 +/- 12%, which were similar to the declines in total hormone concentrations observed previously; nonstimulated TSH was unchanged. In the yoked and cage control groups, FT4 also declined, but much less than that of the sleep-deprived group. The relative changes in free compared with total hormone concentrations over the study were also less parallel than those in the sleep-deprived group. The plasma TSH response to TRH was similar in all groups across experimental days. The plasma FT4 and FT3 concentrations in sleep-deprived rats increased after TRH-stimulated TSH release to an extent comparable to control values. Taken together, low basal FT4 and FT3 hormone concentrations and unchanged TSH and thyroidal responses to TRH suggest a pituitary or hypothalamic contribution to the hypothyroxinemia during sleep deprivation.

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

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

Uptake of 3,3',5,5'-tetraiodothyroacetic acid and 3,3',5'-triiodothyronine in cultured rat anterior pituitary cells and their effects on thyrotropin secretion.

We compared the uptake, metabolism, and biological effects of tetraiodothyroacetic acid (Tetrac) and rT3 in anterior pituitary cells with those of T4 and T3. Cells were isolated from adult male Wistar rats and cultured for 3 days in medium with 10% fetal calf serum. Uptake was measured at 37 C in medium with 0.1% BSA for [125I]Tetrac (200,000 cpm; 240 pM) and [125I]T4 (100,000 cpm; 175 pM) or with 0.5% BSA for [125I]rT3 (100,000 cpm; 250 pM) and [125I]T3 (50,000 cpm; 50 pM). The free fraction of Tetrac was 1% that of T4 (in medium with 0.1 and with 0.5% BSA), and the free fraction of rT3 was half that of T3. Uptake of the four tracers increased sharply up to 1 h of incubation and then leveled off. Expressed as femtomoles per pM free hormone, uptake at equilibrium was 1.16 +/- 0.16 (n = 6) for Tetrac, 0.15 +/- 0.01 (n = 6) for T4, 0.023 +/- 0.003 (n = 6) for rT3, and 0.21 +/- 0.02 (n = 6) for T3. Cell-associated radioactivity after incubation for 24 h with [125I]Tetrac was represented for 15% by [125I]Triac; after incubation with [125I]T4 for 15-20% by [125I]T3, after incubation with [125I]rT3 for 6% by [125I]3,3'-T2, while [125I]T3 was still for 98% [125I]T3. Exposure of cells for 2 h to 100 nM TRH stimulated TSH release by 90-135%. Tetrac was effective in reducing this response at a free concentration of 0.05 pM, but rT3 was effective only at a free concentration of 16 nM. A free Tetrac concentration of 5 pM was equally effective as 50 pM free T4 in reducing the TSH response to TRH. In human serum, Tetrac was exclusively bound to T4-binding prealbumin. The free Tetrac fraction was 0.001% in control subjects and rose 2- to 12-fold in patients with nonthyroidal illness. As uptake of [125I]Tetrac in the pituitary was higher than that of T4 and T3, and it was more potent than T4 in reducing TSH release, Tetrac may be of potential significance for the regulation of TSH secretion in vivo.

GnRH-Induced Desensitization of in Vitro Luteinizing Hormone Secretion in the Turtle, Trachemys scripta

Gonadotropin-releasing hormone (GnRH)-induced desensitization of pituitary luteinizing hormone (LH) secretion in female turtles (Trachemys scripta) was investigated by in vitro superfusion of whole or hemipituitaries with the endogenous forms of the peptide. In sub-adults, LH secretion was rapidly attenuated desensitized) by both prolonged and pulsatile treatments of chicken II GnRH (cII-GnRH), but pulsatile GnRH administration was more effective than prolonged administration in stimulating LH release. Prolonged administration of chicken I GnRH (cI-GnRH) also desensitized LH secretion; cI-GnRH was slightly less potent than cII-GnRH. GnRH-induced attenuation of LH secretion was not the result of tissue deterioration or depletion of relesable LH: GnRH responsiveness of desensitized pituitaries was fully restored after 4 hr of in vitro recovery, and further LH secretion was induced by tetraethylammonium chloride, a membrane depolarizing agent, in the desensitized glands; TRH-stimulated TSH release did not show desensitization in the same glands. Pituitaries from most adult (vitellogenic) females failed to respond to GnRH challenges; however, when responsive, adult pituitaries also desensitized under prolonged GnRH stimulation and were more resistant to GnRH-induced desensitization under pulsatile GnRH treatments. Current results show that adult T. scripta typically display lower GnRH sensitivity than the subadults; the latter are like mammals, birds, and goldfish in their susceptibility to desensitization by both forms of endogenous GnRH.

The role of somatostatin and/or dopamine in basal and TRH-stimulated TSH release in food-restricted rats

The present study was carried out to examine the role of endogenous dopamine and somatostatin in the mechanisms involved in the restricted feeding-induced inhibition of TSH secretion in rats. GH secretion was examined in parallel. Restricted feeding by 50% or 75% was associated with a decrease in the pituitary and circulating levels of TSH and GH in both untreated and TRH-treated groups (p less than 0.001), the changes being proportional to the feeding level. Intravenous injections of the dopamine antagonists, domperidone or haloperidol, failed to affect the magnitude of the differences in plasma TSH and GH levels among control and food-restricted groups, indicating that dopaminergic mechanisms had little effect on the regulation of TSH and GH secretion during restricted feeding in rats. Cerebroventricular injection of somatostatin anti-serum resulted in a marked increase in plasma TSH and GH levels in all the experimental groups (p less than 0.001). The increase in plasma GH and TSH induced by somatostatin anti-serum was greater in rats fed a 25% diet than in either controls or rats fed 50% of the diet; the values for the latter two groups were also different (p less than 0.001). The decreased TSH and GH values in somatostatin anti-serum-treated food restricted rats as compared with those in control animals on somatostatin anti-serum or normal rabbit serum can probably be attributed to the decreased available pituitary TSH and GH pools. The data indicate that long-term restricted feeding affects anterior pituitary function in rats, presumably reflecting alterations in the secretion of an inhibiting hormone, somatostatin.

Pyridostigmine, an Acetylcholinesterase Inhibitor, Stimulates Growth Hormone Release, but has no Effect on Basal Thyrotrophin or Adrenocorticotrophin Levels, or the Thyrotrophin Response to Thyrotrophin‐Releasing Hormone

Abstract Pyridostigmine, an acetylcholinesterase inhibitor, stimulates growth hormone (GH) release and is thought to act by inhibiting hypothalamic somatostatin release. There are few data concerning the effect of pyridostigmine on other pituitary hormones apart from GH. We have studied the effect of pyridostigmine on basal GH, thyrotrophin (TSH), prolactin, adrenocorticotrophin and cortisol release, and thyrotrophin-releasing hormone (TRH)-stimulated TSH and prolactin release, in two studies involving nine healthy male subjects. Pyridostigmine stimulated GH release in all subjects but had no effect on adrenocortocotrophin or cortisol levels, or basal or TRH-stimulated TSH and prolactin levels. There are some data to suggest that somatostatin inhibits TRH-stimulated TSH release. Our findings, however, suggest that either endogenous somatostatin tone has little effect on the TSH response to TRH compared to its effects on GH or pyridostigmine acts through a mechanism other than altering somatostatin tone. Pyridostigmine did not alter adrenocorticotrophin or cortisol levels in the presence of a clear action on GH release, providing further evidence that the previously reported effects of cholinergic drugs on cortisol release are stress-related.

Effect of Diabetes Mellitus on Thyrotropin Release from Isolated Rat Thyrotrophs

The effect of streptozotocin-induced diabetes on thyrotropin (TSH) secretion was studied in vitro in male rats, using purified thyrotrophs isolated by centrifugal elutriation. The development of diabetes was associated with a significant fall in serum T4 (4.9 +/- 1.5 vs. 1.2 +/- 0.7 micrograms/dl, control vs. diabetic), T3 (59 +/- 16 vs. 20 +/- 9 ng/dl) and TSH (53 +/- 22 vs. 23 +/- 18 microU/ml). In vitro, basal TSH release was lower in cells from diabetic rats (1570 +/- 248 microU/10(6) cells) than in controls (2612 +/- 765 microU/10(6) cells) after 48 h in culture. Furthermore, thyrotropin-releasing hormone (TRH)-induced TSH release was decreased about 30% and the intracellular TSH content was also reduced 45% in diabetic rat thyrotrophs compared with controls. The addition of 200 microU/ml of insulin in the incubation media did not change the TRH-stimulated TSH release, but it decreased the basal TSH release by 42%. Preincubation of thyrotrophs with glucose (25 mM) did not impair the basal or TRH-stimulated TSH release. In the same experimental conditions, no effect of insulin or glucose was seen on the basal or TRH-stimulated TSH release from thyrotrophs of diabetic rats. Preincubation of thyrotrophs with 10, 100, or 1000 nM of T4 for 48 h decreased basal TSH release by 27%, 45%, and 48%, respectively, and reduced TRH-induced TSH release by 46%, 48%, and 55%; 100 nM T4 also reduced TRH-induced TSH release from diabetic thyrotrophs.(ABSTRACT TRUNCATED AT 250 WORDS)

EFFECT OF DOPAMINE AND A DOPAMINE D‐1 RECEPTOR AGONIST ON PULSATILE THYROTROPHIN SECRETION IN NORMAL WOMEN

The inhibitory effect of a pharmacological dose of dopamine and the specific dopamine D-1 receptor agonist fenoldopam on basal and pulsatile TSH secretion was investigated in normal women. The TSH response to fenoldopam and subsequent releasing hormone administration was also studied. Six women received placebo or dopamine infusion (4.0 micrograms/kg min) for 17 h. After 9 h, blood samples were collected every 10 min between 0800 and 1600 h for measurement of TSH. Eight women received 8-h (0900-1700 h) infusions of either fenoldopam (0.5 micrograms/kg min) or placebo. After 7 h of infusion 10 micrograms TRH, 5 micrograms GnRH and 25 micrograms CRF was given i.v. Blood samples were collected every 10 min. Dopamine infusion as well as fenoldopam infusion significantly reduced both mean basal TSH secretion and TSH pulse frequency compared with corresponding control infusions (P less than 0.05). However, while the effect on TSH pulsatility was comparable (P greater than 0.05), the percentage decrease in basal TSH levels after 16 h of dopamine infusion was 51 +/- 16% (mean +/- SD) and after 7 h of fenoldopam infusion 19 +/- 12% (P less than 0.05). Neither of the drugs affected TSH pulse amplitude and fenoldopam did not influence TRH-stimulated TSH release (P greater than 0.05). The results suggest that dopamine D-1 receptors are involved in modulation of TSH pulsatility probably at the hypothalamic level. It is argued that dopaminergic inhibition of basal TSH secretion and TSH pulsatility is predominantly regulated through dopamine D-2 receptors at the pituitary level, and through D-1 receptors at the hypothalamic level, respectively.

The effect of beta-endorphin on basal and TRH-stimulated TSH release in conscious male rats.

The inhibitory effect of beta-endorphin (EP) or other opioids on TSH secretion is, in contrast to their stimulating properties on PRL release, still a matter of debate. In the present study a dose of 1 microgram beta-EP injected intracerebroventricularly (IVT) in unstressed conscious male rats, though highly effective on PRL release, did not affect basal TSH levels, nor the TRH-induced TSH secretion. The previously reported inhibition of TSH release by opioids may therefore be an effect only seen when pharmacological doses are used.

Neuroendocrine regulation of thyrotropin release in cultured human pituitary cells.

The regulation of TSH release in man was investigated using cell cultures derived from human pituitaries obtained within 24 h of accidental death. TRH stimulated TSH release in a dose-dependent manner. The ED50 was 2.9 +/- 0.6 (+/- SEM) nmol/L, similar to that reported for rat pituitary cell cultures. The release of TSH was calcium dependent, since the calcium channel antagonist verapamil inhibited TRH-stimulated TSH release, and the calcium ionophore A23187 stimulated TSH release. 12-O-Tetradecanoyl-phorbol-13-acetate stimulated TSH secretion, while dibuytryl cAMP had no effect. Epinephrine and serotonin stimulated TSH release, and dopamine and somatostatin inhibited TRH-stimulated TSH release. These findings have directly demonstrated that the regulation of TSH secretion by hypothalamic neuropeptides and biogenic amines in the human pituitary is similar to that in the rat. The development of a tissue culture system to study thyrotrophs from postmortem human pituitaries provides the means for detailed studies of the regulation of TSH secretion in man.

Evidence for a dual effect of gamma-aminobutyric acid on thyrotropin (TSH)-releasing hormone-induced TSH release from perifused rat pituitaries.

The effects of gamma-aminobutyric acid (GABA) on the spontaneous and TRH-induced TSH release were investigated in vitro on perifused rat pituitaries. The dynamic pattern of TSH release was measured in response to a 6-min pulse of TRH (10 nM) with or without GABA addition. GABA had no effect on spontaneous TSH release but exhibited a dual effect on TSH-stimulated release according to the dose (as calculated by the induced-basal ratio): a potentiation of the TSH response to TRH at the lowest concentrations tested (less than or equal to 10 nM) and an inhibition for GABA concentrations equal or higher than 100 nM. The GABA potentiation was mimicked by muscimol (10 microM) and isoguvacine (10 nM) but not by baclofen (1 microM). Bicucullin (1 microM) or picrotoxin (1 microM) added 15 min before GABA was unable to reverse the GABA potentiation of the TSH response, although SR 95103 (1 and 10 microM), a specific GABA A antagonist, partially or totally antagonized this response. Diazepam (7 nM) was able to potentiate the TSH response by 216% when GABA was added to the system at a concentration (60 nM) which does not modify by itself the TSH response. The inhibitory effect of GABA (100 nM) was completely abolished by bicucullin (1 microM), by picrotoxin (1 microM), and by SR 95103 (1 microM). Picrotoxin not only blocked the inhibitory action of GABA but significantly (P less than 0.05) potentiated the TSH response to TRH. Our data suggest a dual GABA-ergic control of TRH-stimulated TSH release directly on the pituitary, probably mediated by two different kinds of GABA receptors: a GABA A receptor site mediating the inhibitory effect and a nonclassical GABA A receptor site of higher affinity for its stimulatory action.

Cholinergic mediation of growth hormone secretion induced by thyrotropin-releasing hormone in cirrhotic patients.

In order to evaluate the possible involvement of muscarinic cholinergic receptors in the GH response to TRH in patients with liver cirrhosis, 8 males with post-hepatitic cirrhosis and 11 males with post-alcoholic cirrhosis were primed with the anticholinergic agent pirenzepine and tested with TRH. In addition, 10 male patients affected by piecemeal necrosis were tested in a similar manner. High basal concentrations of GH were found in all groups. None of the patients with piecemeal necrosis responded to TRH, whereas in patients with post-hepatitic and in post-alcoholic cirrhosis, TRH induced a significant rise in GH levels. The priming with pirenzepine (40 mg given iv 10 min before TRH) completely blocked the TRH-induced GH increase, but did not affect the TRH-induced TSH release. These data suggest that a muscarinic cholinergic pathway is involved in the anomalous response of GH to TRH in patients with liver cirrhosis. The lack of effect of pirenzepine on the TRH-stimulated TSH release suggests that the muscarinic cholinergic mediation is peculiar for the effect of TRH on GH secretion.

Benzodiazepines inhibit thyrotropin (TSH)-releasing hormone-induced TSH and growth hormone release from perifused rat pituitaries.

The perifusion technique was used to investigate the action of diazepam (DZ), a benzodiazepine molecule known to compete for TRH receptor binding in rat pituitary, on TRH-induced TSH and GH release. The release kinetics for the two hormones from quartered pituitaries were measured in response to a 6-min pulse of TRH (10 nM), without or with DZ addition for a period of 30 min before and during the TRH pulse, plus an additional 15-min period. The dynamic patterns of TSH and GH release in response to TRH were characterized by a rapid increase in hormone release, declining slowly over the next 20 min. The rate of release represented 2.98 +/- 0.02 (+/- SE) and 1.75 +/- 0.06 times the corresponding basal level for TSH and GH, respectively, when evaluated over the first 15 min of the response to TRH. Addition of increasing doses of DZ suppressed the stimulatory effect of TRH in a dose-related manner, with an ID50 of 3 nM for both TSH and GH. The maximal effect of DZ was obtained with a concentration of 10 nM for both hormones. Ro 15-1788 (100 nM), a selective antagonist of the central type of benzodiazepine-binding sites (added to the perifusion system 30 min before DZ and then during the whole period of DZ perifusion), completely abolished (P less than 0.01) the inhibitory effect of DZ (10 nM) on the TRH-induced TSH and GH responses. When added alone before the TRH pulse, Ro 15-1788 had no effect on the TSH response to TRH. In contrast, PK 11,195 (100 nM), a selective antagonist of the nonneuronal benzodiazepine-binding sites, was unable to abolish the inhibitory action of DZ on TRH-stimulated TSH release. In addition, the effects of four other types of benzodiazepine (flurazepam, chlordiazepoxide, midazolam, and medazepam), all tested at a 10-nM concentration, corroborated these findings. Furthermore, DZ inhibition of the TSH response was nullified by picrotoxin (1 microM), but not by bicuculline (1 microM), two gamma-aminobutyric acid antagonists that had no effect, by themselves, on this response. For comparison, the effect of DZ (10 nM) was also tested on the release of GH in response to human GH-releasing factor-(1-44)-NH2 (10 nM) and was found to be ineffective.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of TSH-stimulated thyroid hormone release and potentiation of TRH-stimulated TSH release by indomethacin in perifusion systems of rat thyroids and pituitaries.

Using indomethacin (Ind), a prostaglandin synthesis inhibitor, in vivo experiments in rats and in vitro experiments with perifusion systems of rat thyroids and pituitaries were conducted. After 35 days of intragastric infusion of Ind, serum TSH levels were markedly increased, the thyroid was swollen and, as a consequence, T3 and T4 levels were normal. The T3 release from perifused rat thyroids under continuous stimulation with 10 mU/ml TSH was inhibited significantly (p less than 0.01) by 1.0 X 10(-6) M Ind. On the other hand, the TSH release from perifused rat pituitaries under TRH stimulation was enhanced conspicuously by Ind. It was concluded that Ind decelerated thyroid hormone release from the thyroid and accelerated TSH release from the pituitary in perifusion systems.

Preparation of thyrotroph cells from adult male rat pituitary glands by centrifugal elutriation.

To study the metabolism of thyrotrophs and dynamics of TSH secretion in vitro, it is desirable to have a highly enriched population of thyrotrophs. For that purpose, centrifugal elutriation, a recently developed cell isolation method based on the size and density of cells, was used to prepare thyrotrophs from a cell suspension of adult male rat pituitary cells. Trypsin-dispersed cells (4-8 X 10(7] were loaded into the elutriation rotor (Beckman, JE-6) operating at 2800 rpm. Twelve cell fractions were collected at variable rotor speed (2000-2800 rpm) and increasing medium flow rate (10-103 ml/min). Cell recovery was 77-98%. The viability of the cells after elutriation was 90-95% based on trypan blue exclusion. Each fraction was analyzed for TSH, GH, and PRL content and for TRH-stimulated TSH release by RIA. Thyrotrophs were found predominantly in fractions 8-11 (flow rate 38-75 ml/min) based on TSH RIA. The mean TSH concentration in these fractions was 56 +/- 13.6 (+/- SD) microU/10(3) cells compared with 7.6 +/- 3.8 microU/10(3) cells in the initial cell suspension, representing a 7- to 8-fold enrichment of the thyrotrophs. Incubation with 20 nM TRH for 3 h increased the TSH release of cells eluted in fractions 8-11 by 3- to 5-fold; there was no significant increase in TSH release in fractions 3-6. Centrifugal elutriation may be used to prepare a uniform highly enriched thyrotroph fraction with excellent recovery from a suspension of rat pituitary cells. This technique should be valuable for study of the metabolism of thyrotrophs.