On the role of extracellular Ca2+ for prolactin release and adenosine 3′:5′-monophosphate formation induced by thyroliberin in cultured rat pituitary cells

In cultured rat pituitary tumour cells (GH₃ cells) the absence of extracellular Ca⁺⁺ or addition of NaEGTA reduced spontaneous prolactin (PRL) release and abolished the stimulatory effect of thyroliberin (TRH). Readdition of CaCl₂, but not of equimolar concentrations of MgCl₂ increased spontaneous hormone release, and restored the effect of TRH. The calcium ionophore, A-23187, induced PRL release during normal calcium conditions, but not when an excess NaEGTA was present. TRH increased cyclic AMP accumulation in the presence and the absence of extracellular calcium. The effect of TRH on PRL release and cyclic AMP formation occured concomitantly with an increased efflux of ⁴⁵Ca²⁺. Intracellular electrophysiological recordings from the same single cells before and after TRH activation showed increased frequency and duration of the Ca²⁺ dependent action potentials. We conclude that TRH elevates the Ca²⁺ influx which depends on the depolarizing action current, and this effect is probably linked to formation of cyclic AMP and PRL release.     

Thyrotropin releasing hormone

Summary Thyrotropin releasing hormone (TRH) acutely stimulates release of thyrotropin (TSH) and prolactin from anterior pituitary cells. A considerable number of studies have been performed with neoplastic and nonneoplastic pituitary cells in culture to elucidate the sequence of intracellular events involved in this action. Although cyclic AMP was suggested as an intracellular messenger, it has been demonstrated that TRH stimulation of hormone release can be dissociated from changes in cyclic AMP concentration, thereby supporting the contention that cyclic AMP is not a required mediator. In contrast, stimulation of hormone release by TRH requires Ca²⁺ and it seems likely that Ca²⁺ is the intracellular coupling factor between TRH stimulation and hormone secretion. TRH has been shown to stimulate ⁴⁵Ca²⁺ efflux from preloaded pituitary cells. Enhanced ⁴⁵Ca²⁺ efflux is thought to reflect an increase in the free intracellular Ca²⁺ concentration which leads to hormone release; however, the source of this Ca^2– is uncertain. Results are reviewed from a series of experiments in pituitary cells which attempt to determine the pool (or pools) of Ca²⁺ that is affected by TRH. These include the following: the effects of decreasing the extracellular Ca^2– concentration on hormone release stimulated by TRH; the effect of TRH on cellular Ca²⁺ as monitored by chlortetracycline; the effects of TRH on Ca²⁺ influx; the effects of the organic Ca²⁺ channel blocking agents, verapamil and methoxyverapamil, on TRH-stimulated hormone release; and the effects of TRH on plasma membrane potential difference and on Ca²⁺-dependent action potentials. Based on these data, separate hypotheses of the early events in TRH stimulation of hormone release in mammotropes and thyrotropes are proposed. In mammotropes, TRH is thought to stimulate prolactin release optimally by elevating the free intracellular Cat+ concentration by mobilizing cellular Ca^2– only. In contrast, in thyrotropes under normal physiological conditions, TRH is thought to stimulate TSH release by mobilizing Ca² from a cellular pool (or pools) and to augment this effect by also inducing influx of extracellular Ca²⁺ through voltage-dependent channels in the plasma membrane.     

Thyrotropin-releasing hormone stimulates the activity of the rat thyrotropin beta-subunit gene promoter transfected into pituitary cells

In order to investigate the molecular mechanism(s) by which TRH regulates the biosynthesis of TSH, we are studying the effects of TRH on the expression of the TSH subunit genes (alpha and TSH beta). To study the structure-function relation of TRH stimulation of the activity of the single rat TSH beta gene, chimaeric plasmids were constructed. The 5'-flanking region of the rat TSH beta gene including exon 1 (5'-untranslated region) was inserted into a promoterless, modified pBR, chloramphenicol acetyltransferase (CAT) expression vector. After transfection, specific TSH beta promoter activity was evident in both TRH-responsive pituitary-derived GH3 and primary pituitary cell cultures. To determine potential regulation of TSH beta promoter-directed activity in these cells by TRH, cells were incubated with media containing TRH (10(-7) to 10(-11) M) for 1 to 48 h. TRH stimulated a 1.5- to 3-fold increase in TSH beta promoter activity. Concomitant with an increase in CAT activity was an anticipated increase in PRL synthesis in the GH3 cells in response to TRH. The TRH effect on the TSH beta gene was specific; no increase in CAT activity was detected for TKCAT (thymidine kinase of herpes simplex virus promoter), pBRCAT (no promoter), or TSH beta CAT (3'-5'-orientation). Similar results were obtained using primary pituitary cell cultures. Deletion mutation analysis indicated that TRH sensitivity was detected in a 1.1 kilobase, but not in a 0.38 kilobase TSH beta gene fragment suggesting that the TRH responsive element(s) resides at least in part within the 700 base pairs of the 5'-flanking sequence.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cyclic AMP level of human thyroid cells in monolayer culture. TSH induced refractoriness to TSH action.

Thyroid cells from euthyroid patients with Graves' disease were cultured in a chemically defined medium. The cells preserved the ability to respond to TSH with 8-fold increase in cyclic AMP concentration. This cyclic AMP response to TSH was diminished by prior exposure of cells to TSH. The decrease in cyclic AMP response to TSH induced to TSH was reversible, was not associated with a similar decrease to cyclic AMP response to PGE1, and could not be attributed to increased phosphodiesterase activity or to decreased adenyl cyclase activity. The partial resistence to TSH stimulation of thyroid cells previously exposed to TSH may be due to changes in the TSH receptor, possibly caused by TSH itself.     

Inhibitory effects of cysteamine on neuroendocrine function

The action of cysteamine on anterior pituitary hormone secretion was studied in vivo using conscious, freely moving male rats and in vitro using anterior pituitary cells in monolayer culture. Administration of 500 micrograms cysteamine into the lateral cerebral ventricles of normal rats caused the complete inhibition of pulsatile GH secretion for a minimum of 6 h. This treatment also significantly decreased plasma concentrations of LH for at least 6 h in orchiectomized rat, TSH in short-term (0.5 month) thyroidectomized rats, and PRL in long-term (6 months) thyroidectomized rats. The in vivo stimulation of GH, LH, TSH and PRL with their respective releasing hormones 60 min after administration of cysteamine was not different from the response observed in rats pretreated with saline except for PRL where cysteamine pretreatment significantly inhibited the expected PRL increase. In vitro, 1 mM cysteamine decreased basal and TRH stimulated PRL release while not affecting basal or stimulated GH, LH, TSH and ACTH secretion. These data demonstrate the dramatic and wide-ranging effects of cysteamine on anterior pituitary hormone secretion. This action appears to be mediated through hypothalamic pathways for GH, LH and TSH and through a pituitary pathway for PRL.     

Effect of repeated stimulation by thyrotropin-releasing hormone (TRH) on thyrotropin and prolactin secretion in perfused euthyroid and hypothyroid rat pituitary fragments

The previously reported refractoriness of pituitary response to thyrotropin-releasing hormone (TRH) stimuli was investigated here in an in vitro perfusion system using pituitary tissue from euthyroid and hypothyroid rats. Thyroid-stimulating hormone (TSH) and prolactin (PRL) responses to TRH (28 pmol) were significantly greater in hypothyroid tissue compared with euthyroid. Hypothyroid tissue showed a reduction in response to two consecutive stimuli in both TSH and PRL, however the TSH decline in response was more marked than PRL. Euthyroid tissue showed no significant decline in response to TRH. An increase in the dose of TRH (112 pmol), administered to euthyroid tissue, resulted in increased TSH and PRL response, but no decline in response to sequential stimuli was observed. Three consecutive stimuli by TRH (28 pmol) of hypothyroid tissue resulted in a consistent decline in TSH response. The decline in PRL response only reached statistical significance by the third stimulation. Euthyroid and hypothyroid pituitary tissue was subjected to sequential depolarising stimulation with KCl (50 mumol). Euthyroid tissue showed no decline in response in either TSH or PRL. In hypothyroid tissue only, the decline in TSH response reached statistical significance. This decline in TSH response was significantly smaller than the decline in response observed in hypothyroid tissue stimulated with TRH. Refractoriness of hypothyroid pituitary tissue to repeated TRH stimuli is reported here. Our data suggest that the decline in hormonal response cannot be explained solely on the basis of tissue depletion.     

In vitro release of prolactin by thyrotropin-releasing hormone: influence of dopamine, thyroxine, and cycloheximide.

An acute incubation procedure, using explanted normal rat hemipituitaries pretreated with fresh plasma obtained from pituitary donor animals, was employed to further investigate the in vitro stimulation of prolactin (PRL release by thyrotropin-releasing hormone (TRH). Pretreatment with dopamine (0.1 microgram/ml) caused a 30-50% decrease in the amount of PRL released into incubation media; the inhibitory effect of dopamine was not reversed by treatment with 0.5-6.0 ng. TRH, although these TRH concentrations consistently stimulated PRL release from pituitaries not exposed to dopamine. Treatment with thyroxine (10(-6) to 10(-5) M) showed a competitive inhibition of thyrotropin release by TRH (0.5 ng), but was without effect on TRH-stimulated PRL release. Cycloheximide (100 microgram/ml) blocked a net increase in PRL levels. TRH, nevertheless, significantly increased PRL release in the presence of cycloheximide. The results indicate that neither dopamine nor thyroxine compete with TRH in causing PRL release, and that the TRH stimulation of PRL release is unrelated to ongoing levels of hormone synthesis.     

Plasminogen activator production by parietal endoderm cells: Stimulation by a protein from pituitary

It has been shown previously that dibutyryl cyclic AMP increases the production of plasminogen activator in mouse parietal endoderm cells. This fact suggested that the production of plasminogen activator by parietal endoderm cells may be under the control of a hormone acting via adenylate cyclase. We have cultured rat parietal endoderm cells in the absence of serum and show that they respond to dibutyryl cyclic AMP with an increase in plasminogen activator production and a change in morphology. We describe the existence of a compound from pituitary which is capable of stimulating plasminogen activator secretion in these cells. Relatively impure preparations of ovine and bovine TSH contain significant amounts of activity, whereas more highly purified preparations of TSH, and all other pituitary hormones tested, are inactive, indicating that the factor is not a known pituitary hormone. The active compound was characterized using ovine and bovine TSH as a source, and it is macromolecular and proteinaceous, and depends on protein synthesis for its effect. The stimulation is enhanced by methylisobutylxanthine, a phosphodiesterase inhibitor, suggesting that the event is mediated by cyclic AMP. This observation leads to the prediction that the coaddition of dibutyryl cAMP and the active compound at nonsaturating concentrations should be additive. Instead, the stimulation is synergistic, and depends on the addition of dibutyryl cyclic AMP first when the compounds are added sequentially. Finally, we show that mouse teratocarcinoma cells chemically induced to differentiate to a cell type indistinguishable from parietal endoderm respond to a source of the compound by increasing plasminogen activator production.     

Interaction of calcium and cyclic nucleotides in thyroliberin-stimulated prolactin release

The object of the present study was to determine the relative importance of Ca++ and cyclic nucleotides as “second messengers” in thyroliberin (TRH)-mediated prolactin (PRL) release in the GH₃ and GH₄ rat pituitary tumor cell lines. PRL, cyclic adenosine 3': 5'-monophosphate (cAMP), and cyclic guanosine 3': 5'-monophosphate (cGMP) were measured by radioimmunoassay (RIA) following TRH stimulation. TRH increased PRL release and cAMP levels in GH₃ and GH₄ cells, but cGMP increases were variable. Treatment with 1 mM theophylline increased PRL release and raised cAMP and cGMP. Addition of TRH to theophylline-pretreated cells produced further significant increases in PRL release without any additional increases in cAMP and cGMP. Co++, a Ca++ antagonist, abolished TRH-induced PRL release in a dose-dependent manner. The Co++ inhibition was partially reversed by Ca++ in GH₃ or GH₄ cells. Furthermore, the Ca++ ionophore A23187 stimulated PRL release. We conclude that Ca++ is the primary “second messenger” for TRH-mediated PRL release from GH₃ or GH₄ cells.     

Effects of VIP, TRH, GABA and dopamine on prolactin release from superfused rat anterior pituitary cells

Effects of VIP, TRH, dopamine and GABA on the secretion of prolactin (PRL) from rat pituitary cells were studied in vitro with a sensitive superfusion method. Dispersed anterior pituitary cells were placed on a Sephadex G-25 column and continuously eluted with KRBG buffer. Infusion of TRH (10(-11) - 10(-8)M) and VIP (10(-9) - 10(-6)M) resulted in a dose-related increase in PRL release. LHRH (10(-8) - 10(-5)M) had no effect on PRL release. On the other hand, infusion of dopamine (10(-9) - 10(-6)M) and GABA (10(-8) - 10(-4)M) suppressed not only the basal PRL release from dispersed pituitary cells but also the PRL response to TRH and VIP. The potency of TRH to stimulate PRL release is greater than that of VIP, and the potency of dopamine to inhibit PRL secretion is stronger than that of GABA on a molar basis. These results indicate that TRH and VIP have a stimulating role whereas dopamine and GABA have an inhibitory role in the regulation of PRL secretion at the pituitary level in the rat.     

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 hypothalamic-releasing hormones on LH, FSH, and prolactin in pituitary monolayer cultures.

Four-day-old pituitary monolayer cultures were incubated with various hypothalamic releasing hormones. Rat hypothalamic extract stimulated the release of LH, FSH, and PRL by these cultures in a dose-related fashion. Synthetic LH-RH stimulated the release of LH and FSH but not of PRL. Synthetic TRH increased the release of PRL but had no effect on LH or FSH. At 10(-8) M, somatostatin did not affect any of the three adenohypophyseal hormones. Incubation with DBcAMP or theophylline also stimulated PRL release without any detectable effect on LH and FSH release. These data suggest the involvement of cyclic AMP--adenylate cyclase system in the mechanism of PRL release, but their involvement in gonadotropin release requires further studies.     

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.     

17β-Estradiol modulates the prolactin secretion induced by TRH through membrane estrogen receptors via PI3K/Akt in female rat anterior pituitary cell culture

Considering that estradiol is a major modulator of prolactin (PRL) secretion, the aim of the present study was to analyze the role of membrane estradiol receptor-α (mERα) in the regulatory effect of this hormone on the PRL secretion induced by thyrotropin-releasing hormone (TRH) by focusing on the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) pathway activation. Anterior pituitary cell cultures from female rats were treated with 17β-estradiol (E(2), 10 nM) and its membrane-impermeable conjugated estradiol (E(2)-BSA, 10 nM) alone or coincubated with TRH (10 nM) for 30 min, with PRL levels being determined by RIA. Although E(2), E(2)-BSA, TRH, and E(2)/TRH differentially increased the PRL secretion, the highest levels were achieved with E(2)-BSA/TRH. ICI-182,780 did not modify the TRH-induced PRL release but significantly inhibited the PRL secretion promoted by E(2) or E(2)-BSA alone or in coincubation with TRH. The PI3K inhibitors LY-294002 and wortmannin partially inhibited the PRL release induced by E(2)-BSA, TRH, and E(2)/TRH and totally inhibited the PRL levels stimulated by E(2)-BSA/TRH, suggesting that the mER mediated the cooperative effect of E(2) on TRH-induced PRL release through the PI3K pathway. Also, the involvement of this kinase was supported by the translocation of its regulatory subunit p85α from the cytoplasm to the plasma membrane in the lactotroph cells treated with E(2)-BSA and TRH alone or in coincubation. A significant increase of phosphorylated Akt was induced by E(2)-BSA/TRH. Finally, the changes of ERα expression in the plasmalemma of pituitary cells were examined by confocal microscopy and flow cytometry, which revealed that the mobilization of intracellular ERα to the plasma membrane of lactotroph cells was only induced by E(2). These finding showed that E(2) may act as a modulator of the secretory response of lactotrophs induced by TRH through mER, with the contribution by PI3K/Akt pathway activation providing a new insight into the mechanisms underlying the nongenomic action of E(2) in the pituitary.     

The involvement of cyclic-3',5'-AMP in the release of hormones from the anterior pituitary in vitro.

DBcAMP significantly increased the release of GH but not of LH, FSH, TSH, or PRL, except in the presence of hypothalamic extract when it augmented the release of LH, FSH, and GH, reversed the inhibition of PRL, but did not further influence TSH release. Theophylline increased release of GH and PRL while inducing increased tissue content of cAMP without consistently increasing the release of TSH, LH, or FSH. Hypothalamic extractor K+-stimulated hormone rel-ase was consistently and significantly potentiated by theophylline. Neither hypothalamic extract, increased [K+], or synthetic TRH and LRH were able to raise tissue content of cAMP while producing their expected effects on hormone release. Cholera enterotoxin produced a highly significant increase in tissue content of the cyclic nucleotide but increased the release of GH only, and not that of LH, FSH, TSH, or PRL. DBcAMP was able to lower the threshold concentration of K+ required to stimulate release of GH, LH, and FSH and also to augment K+-stimulated release to the higher levels induced by the hypothalamic releasing hormones. It did not augment K+-induced release of TSH.     

Evidence for differential synthesis of thyroxine and tri-iodothyronine by cultured human thyroid cells following exposure to thyrotrophin or dibutyryl cyclic AMP.

Cultured human thyroid cells treated with thyrotrophin (TSH) or dibutyryl cyclic AMP release more tri-iodothyronine (T3) and thyroxine (T4) than unsupplemented cells. Column chromatography was used to investigate the secretion of newly-synthesised 125-I labelled T3 and T4 from cells cultured with 125-I and TSH or dibutyryl cyclic AMP. Radioimmunoassays were used to determine total T3 and T4 release from cells cultured with unlabelled iodide.Iodothyronines released after TSH addition contained more 125-I than those released after dibutyryl cyclic AMP. This increase in 125-I was primarily in “new” T4. Release of “new” T3, however, was increased more by dibutyryl cyclic AMP than by TSH. Dibutyryl cyclic AMP and TSH were comparable in their stimulation of total T3 and total T4 release.Interpretation of these observations suggests that TSH and dibutyryl cyclic AMP may differ in some aspects of their in vitro effects on cellular iodination and iodothyronine coupling systems.     

Stimulation of guanosine 3',5'-cyclic monophosphate accumulation in rat anterior pituitary gland in vitro by synthetic somatostatin

Synthetic somatostatin stimulated cyclic GMP accumulation with dose dependency (10 ng/ml – 10 μg/ml in a dose examined) in rat anterior pituitary gland in vitro. The stimulation of cyclic GMP levels in the gland was observed after 2 min incubation with somatostatin. Cyclic AMP production induced by TRH or PGE₁ was supressed by this GH release inhibiting factor, while cyclic GMP concentration in the gland was elevated. The present results seem to suggest that inhibitory effect on GH release by somatostatin in anterior pituitary gland is mediated through change in concentration of cyclic AMP and cyclic GMP in the target cells.     

Significant qualitative differences exist between thyrotropin and prolactin secretory dynamics induced by pituitary cell swelling

Cell swelling produced by a variety of techniques is a potent stimulus intensity-related inducer of an immediate secretory burst of thyroid-stimulating hormone (TSH) and prolaction (PRL) secretion from anterior pituitary cells. A 2-min "square wave" exposure to either hyposmolarity or isotonic urea induced stimulus intensity-correlated TSH and PRL secretory bursts peaking within 3 min, but the PRL zenith occurred 1 min later than that of TSH. With continuous exposure to these stimuli, TSH secretion rapidly decreased and remained only slightly above the unstimulated rate after 5 min. PRL secretion fell to and remained below the unstimulated level after 10 min. After stopping the stimulus, another secretory burst ("off" response) occurred with PRL, but not with TSH. A progressive "ramp" increase in stimulus intensity over 18 min induced a corresponding gradual increase in TSH secretion; there was a progressive depression, rather than increase, in PRL secretion during the stimulus ramp, with an off response secretory burst when the stimulus was discontinued. Removal of extracellular Ca2+ or addition of verapamil to the medium did not alter the dynamics of hyposmolarity-induced TSH secretion, but markedly altered those of PRL secretion; there was no off response PRL secretion and a hyposmolar ramp induced a corresponding gradual increase in PRL secretion, with a return to baseline after removing the stimulus. The dramatic qualitative differences in the response of the thyrotroph and lactotroph may reflect differences between the cell types in the size of secretory vesicles, membrane potential, the mechanism of exocytosis, and/or the role of Ca2+ influx across the plasmalemma.     

Thyroid stimulating hormone and prolactin secretion: reduced sensitivity to TRH-stimulated prolactin release after midpregnancy in rats

Prolactin (PRL) and thyroid stimulating hormone (TSH) plasma concentrations were measured during the latter part of the dark period in early and mid-late pregnancy in the rat. On Days 4-5 and 7-8 of pregnancy, plasma PRL concentrations surged between 22:00 and 06:00 hr and TSH values increased between 22:00 and 02:00 hr. While the TSH pattern was maintained during the second-half of pregnancy, surges in PRL release ceased and PRL levels remained at less than 10 ng/ml. The effects of thyrotropin releasing hormone (TRH) administration on PRL and TSH secretion were then measured to determine whether the second-half of pregnancy is associated with a decrease in sensitivity to an agent that can stimulate PRL release. Injection (iv) of cannulated pregnant rats with a low dosage (20 ng) of TRH stimulated a twofold increase in plasma TSH during both early (Days 5-9) and later (Days 14-18) pregnancy but did not change plasma PRL levels. Treatment with a high dosage (2 micrograms) of TRH induced a sixfold rise in plasma TSH during both phases of gestation. The higher dose of TRH also stimulated elevations in plasma PRL during early and mid-late pregnancy; however, both the absolute increase in the amount of PRL in plasma and the percentage increase over baseline levels were greater from Days 5-9 than from Days 14-16 of gestation. These data indicate that the neuroendocrine sensitivity to factors that stimulate PRL secretion changes as pregnancy progresses, and suggest that nocturnal secretion of PRL and TSH during pregnancy may be regulated, in part, by a common trophic factor.     

Effects of phorbol ester on GH, TSH and PRL release by superfused rat adenohypophysis

The present study was undertaken to examine the effects of 12-0-tetradecanoyl-phorbol-13-acetate (TPA), one of the potent tumor promoting agents, on GH, TSH and PRL release by rat adenohypophyseal dispersed cells and fragments, using a superfusion technique. TPA (10(-6) to 10(-5) M) stimulated GH release from acutely dispersed rat adenohypophyseal cells. Neither TSH nor PRL was affected, but both were increased by TRH in a dose-dependent fashion (10(-9) to 10(-7) M). In fragments, TPA (10(-8) to 10(-6) M) elicited a dose-related release of GH. Exposure of the fragments to 10(-6) M TPA for 5 min promptly caused a 5-fold increase in GH release which continued for at least 40 min after stopping the stimulation. The addition of somatostatin (SRIF) (10(-7) M) decreased basal GH release and abolished GH release induced by 10(-6) M TPA. In contrast to GH, neither TSH nor PRL release was affected by TPA, but both were stimulated by TRH. These results indicate 1) that GH release is more sensitive to stimulation with TPA in normal rat anterior pituitaries in vitro than the release of TSH and PRL, and 2) that SRIF abolishes TPA-induced GH release.