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.     

Receptors and neurohormones in human pituitary adenomas

In order to go further into the pathogenesis of human pituitary adenomas, we studied receptors for neurohormones (thyroliberin, TRH; dopamine, DA; somatostatin, SRIH), for estradiol and epidermal growth factor (EGF) thought to influence hormone secretion and/or cell growth. The following results were obtained: (1) the receptors listed above, with the exception of EGF receptors in the adenomas, are present in normal pituitary tissue and in prolactin (PRL)- and growth hormone (GH)-secreting adenomas; (2) they are functional and their affinities are not different in normal or tumoral tissues; (3) their density is variable and depends on the type of secreting adenoma (GH or PRL), the size of the tumor and the plasma level of the hormone which is secreted, and (4) in nonsecreting adenomas, only TRH receptors are found with characteristics identical to those observed in secreting adenomas. We also showed that TRH is contained in normal and tumoral pituitary tissues. TRH and SRIH are released in vitro from adenomatous cells in large amounts, suggesting their possible synthesis by the pituitary. In both cases a local regulation is observed. TRH release is stimulated in the presence of DA while SRIH is inhibited in the presence of TRH. This neuropeptide release may be implicated in the pituitary hormone regulation through a paracrine or an autocrine mechanism. Thus, the neurohormone receptors found in pituitary adenomas should be dependent on a more complex regulation than it has been envisaged till now.     

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.     

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.     

The mechanisms by which vasoactive intestinal peptide (VIP) and thyrotropin releasing hormone (TRH) stimulate prolactin release from pituitary cells

The effect of vasoactive intestinal peptide (VIP) on prolactin (PRL) secretion from pituitary cells is reviewed and compared to the effect of thyrotropin releasing hormone (TRH). These two peptides induced different secretion profiles from parafused lactotrophs in culture. TRH was found to increase PRL secretion within 4 s and induced a biphasic secretion pattern, while VIP induced a monophasic secretion pattern after a lag time of 45–60 s.The secretion profiles are compared to changes in adenylate cyclase activity, production of inositol polyphosphates, changes in intracellular calcium concentrations and changes in electrophysiological properties of the cell membrane.Abbreviations AC adenylate cyclase - DG diacyglycerol - GH growth hormone - GTP guanosine trisphosphate - G_i GTP binding proteins that mediate inhibition of adenylate cyclase and that are pertussis toxin sensitive - G_s GTP binding protein that mediates stimulation of adenylate cyclase - GH cells clonal rat pituitary tumor cells producing PRL and/or growth hormone - GH₃ GH₄C₁ and GH₄B6 subclones of GH cells - PKA protein kinase A - PKC protein kinase C - PLC phospholipase C - PRL prolactin - TPA 12-O-tetradecanoyl phorbol 13-acetate - TRH thyrotropin releasing hormone - VIP vasoactive intestinal peptide     

Growth hormone response to thyrotropin-releasing hormone in liver cirrhosis: unique alteration in anterior pituitary responsiveness to hypothalamic hormones

Patients with chronic liver diseases were evaluated for: 1) the ability of somatostatin to affect the thyrotropin-releasing hormone (TRH) induced growth hormone (GH) rise; 2) the competence of luteinizing-hormone releasing hormone (LH-RH) to release GH; 3) the non-specific releasing effect of TRH and LH-RH on other anterior pituitary (AP) hormones. In 6 patients, infusion of somatostatin (100 micrograms iv bolus + 375 micrograms i.v. infusion) completely abolished the TRH (400 micrograms i.v.)-induced GH rise; in none of 12 patients, of whom 7 were GH-responders to TRH, did LH-RH (100 micrograms i.v.) cause release of GH; 4) finally, LH-RH (12 patients) did not increase plasma prolactin (PRL) and TRH (7 patients) did not evoke a non-specific release of gonadotropins. It is concluded that: 1) abnormal GH-responsiveness to TRH is the unique alteration in AP responsiveness to hypothalamic hormones present in liver cirrhosis; 2) the mechanism(s) subserving the altered GH response to TRH is different from that underlying the TRH-induced GH rise present in another pathologic state i.e. acromegaly, a condition in which the effect of TRH escapes somatostatin suppression and LH-RH evokes GH and PRL release.     

Bombesin stimulates prolactin secretion from cultured rat pituitary tumour cells (GH4C1) via activation of phospholipase C

Bombesin (BBS) stimulated prolactin (PRL) secretion from monolayer cultures of rat pituitary tumour cells (GH4C1) in a dose-dependent manner with half maximal and maximal effect at 2 nM and 100 nM, respectively. No additional stimulatory effect on PRL secretion was seen when BBS was combined with thyroliberin (TRH) used in concentrations known to give maximal effects, while the effects of BBS and vasoactive intestinal peptide (VIP) were additive. Using a parafusion system, BBS (1 microM) was found to increase PRL secretion within 4 s and the secretion profiles elicited by BBS and TRH (1 microM) were similar. Both BBS and TRH increased inositoltrisphosphate (IP3) as well as inositolbisphosphate (IP2) formation within 2 s. BBS also induced the same biphasic changes in the electrical membrane properties of GH4C1 cells as TRH, and both peptides caused a rapid and sustained increase in intracellular [Ca2+]. These results suggest that BBS stimulates PRL secretion from the GH4C1 cells via a mechanism involving the immediate formation of IP3 thus resembling the action of TRH.     

Isolation and characterization of rat-mouse somatic cell hybrids secreting growth hormone and prolactin

Interspecific somatic cell hybrid clones have been isolated and characterized in order to study growth hormone (GH) and prolactin (PRL) gene expression. Rat pituitary tumor cells (GH3, 69 chromosomes) secreting rat GH and PRL were grown for 48 h together with nonhormone secreting, aminopterin-sensitive murine fibroblast cells (LMTK-, 55 chromosomes) and fused using polyethylene glycol. Resultant heterokaryons were selected in hypoxanthine-aminopterin-thymidine (HAT) medium and cloned. Five clones produced rat GH and PRL. Hormone-producing hybrids morphologically resembled the mouse parent fibroblast. Hybrids grew in monolayers and contained 80-142 chromosomes, and marker chromosomes for both rat (small submetacentric) and mouse (bi-armed and large true metacentric) were identified. The interspecific nature of the hybrids was further confirmed by the presence of both rat and mouse adenosine deaminase and superoxide dismutase isozymes. Using specific antisera and indirect immunoperoxidase staining, both hybrid clones and GH3 rat parental cells stained positively for rat GH and PRL, while the murine fibroblast parental cells were negative. Hormone production by the hybrids has been sustained for over twenty subcultures; secretion rates were initially 150 ng PRL and 321 ng GH/10(6) cells/24 h and are currently 100 ng PRL and 90 ng GH/10(6) cells/24 h. Parental GH3 rat cells secreted 720 ng PRL and 660 ng GH/10(6) cells/24 h. Exposure of hybrids to KCl (50 mM) resulted in acute stimulation of rat PRL, but not rat GH release, and long-term incubation with thyrotropin-releasing hormone (TRH, 80 nM) stimulated PRL secretion. Hormone-dependent modulation of PRL secretion was transferred to the hybrid cell thus enabling the model to be used in studying regulation of PRL gene expression.     

Regulated expression of the prolactin gene in rat pituitary tumor cells

Prolactin (PRL) gene expression in three strains of GH cells (rat pituitary tumor cells) has been quantitated by measurement of: (a) intracellular and extracellular PRL, (b) cytoplasmic translatable PRL-specific mRNA (mRNAPRL), and (c) molecular hybridization of cytoplasmic poly(A) RNA to cDNAPRL (DNA complementary to mRNAPRL). Three GH cell lines utilized in this investigation were a PRL-producing (PRL+) strain, GH4C1, a PRL nonproducing 5-bromo-deoxyuridine resistnat (PRL- BrdUrdr) strain, F1BGH12C1, and a new strain, 928-9b, derived by fusion of PRL+ cells with a nuclear monolayer of the PRL-, BrdUrdr GH cell strain. PRL production is a characteristic of 928-9b cells, but the level of PRL production (2-4 micrograms/mg protein/24 h) is much lower than that of the PRL+ strain, GH4C1 (15-25 micrograms/mg protein/24 h). Levels of cytoplasmic translatable mRNAPRL and cytoplasmic PRL-RNA sequences quantitated with a cDNAPRL probe were also much lower in 928-9b as compared to the PRL+ parent. PRL-RNA sequences could not be detected in the PRL- strain. Thyrotopin-releasing hormone (TRH) stimulates PRL synthesis about threefold and inhibit a growth hormone (GH) synthesis 72% in the PRL+ strain. TRH has no effect on the synthesis of either PRL or GH in the 928-9b strain, although TRH receptors could be detected in these cells. Stimulation of PRL synthesis in the PRL+ strain by TRH could be correlated with increases in levels of cytoplasmic translatable mRNAPRL and increases in cytoplasmic PRL-RNA sequences. These results demonstrate that the graded expression of the PRL gene at the basal level, and in response to TRH, is caused by the regulated production of specific mRNA, i.e., mRNAPRL in these three GH cell strains.     

Secretion-stimulating and secretion-inhibiting hormones stimulate high-affinity pertussis-toxin-sensitive GTPases in membranes of a pituitary cell line

Different peptide hormones influence hormone secretion in pituitary cells by diverse second messenger systems. Recent data indicate that luteinizing-hormone-releasing hormone (LHRH) stimulates and somatostatin inhibits voltage-dependent Ca2+ channels of GH3 cells via pertussis-toxin-sensitive mechanisms [Rosenthal et al. (1988) EMBO J. 7, 1627-1633]. In other pituitary cell lines, somatostatin has been shown to cause a pertussis-toxin-sensitive decrease in adenylate cyclase activity, and LHRH and thyrotropin-releasing hormone (TRH) stimulate phosphoinositol lipid hydrolysis in a pertussis-toxin-independent manner. Whether stimulation of Ca2+ influx by TRH is affected by pertussis toxin is not known. In order to elucidate which of the hormone receptors interact with pertussis-toxin-sensitive and -insensitive G-proteins, we measured the effects of LHRH, somatostatin and TRH on high-affinity GTPases in membranes of GH3 cells. In control membranes, both LHRH and TRH stimulated the high-affinity GTPase by 20%, somatostatin by 25%. Maximal hormone effects were observed at a concentration of about 1 microM. Pretreatment of cells with pertussis toxin abolished pertussis-toxin-catalyzed [32P]ADP-ribosylation of 39-40-kDa proteins in subsequently prepared membranes and reduced basal GTPase activity. The toxin also reduced by more than half the increases in GTPase activity induced by LHRH and TRH; stimulation of GTPase by somatostatin was completely suppressed. Stimulation of adenylate cyclase by vasoactive intestinal peptide (VIP) was not impaired by pretreatment of cells with pertussis toxin. Somatostatin but not LHRH and TRH decreased forskolin-stimulated adenylate cyclase activity. The results suggest that the activated receptors for LHRH and TRH act via pertussis-toxin-sensitive and -insensitive G-proteins, whereas effects of somatostatin are exclusively mediated by pertussis-toxin-sensitive G-proteins.     

Somatostatin lowers the cytosolic free Ca2+ concentration in clonal rat pituitary cells (GH3 cells)

Changes in the cytosolic free Ca2+ concentration, [Ca2+]i, have been proposed to mediate the regulation of the secretion of pituitary hormones by hypothalamic peptides. Using an intracellularly trapped fluorescent Ca2+ probe, quin2, [Ca2+]i was monitored in GH3 cells. Somatostatin lowers [Ca2+]i in a dose dependent manner from a prestimulatory level of 120 +/- 4 nM (SEM, n = 13) to 78 +/- 9 nM (n = 5) at 10(-7)M; the effect is half maximal at 2 X 10(-9) M somatostatin. The decrease in [Ca2+]i occurs rapidly after somatostatin addition and a lowered steady state [Ca2+]i is maintained for several minutes. Somatostatin does not inhibit the rapid rise in [Ca2+]i elicited by thyrotropin releasing hormone (TRH) and can still cause a decrease in [Ca2+]i in the presence of TRH (10(-7)M). Concomitantly with its action on [Ca2+]i somatostatin causes hyperpolarization of GH3 cells assessed with the fluorescent probe bis-oxonol. The lowering of [Ca2+]i by somatostatin is however not only due to reduced Ca2+ influx through voltage dependent Ca2+ channels, since it persists in the presence of the channel blocker verapamil. These results suggest that somatostatin may exert its inhibitory action on pituitary hormone secretion by decreasing [Ca2+]i.     

Characterization of phorbol ester- and diacylglycerol-stimulated secretion in permeable GH3 pituitary cells. Interaction with Ca2+

Prolactin (PRL) release in permeable GH3 pituitary cells was stimulated by the protein kinase C activators 12-O-tetradecanoylphorbol 13-acetate (TPA) and 1-oleoyl-2-acetyl-sn-glycerol (OAG). Both agents stimulated secretion at 10 nM Ca2+, but higher [Ca2+] (greater than 0.1 microM) potentiated TPA and OAG action. Maximal potentiation occurred at 1 microM calculated free Ca2+, and a similar value was obtained when the cytoplasmic [Ca2+] was measured with the Ca2+-sensitive dye Quin 2. Release of a secretory sulfated proteoglycan was also stimulated by TPA and OAG in permeable GH3 cells, with characteristics similar to those for PRL release. Trifluoroperazine, polymyxin B, neomycin, and 8-(diethylamino)octyl-3,4,5-trimethoxybenzoate all inhibited both TPA- and Ca2+-stimulated PRL release, but in each case the half-maximal inhibitory concentrations were approximately 2-fold higher for TPA-stimulated release compared to Ca2+-stimulated release. Thyrotropin-releasing hormone (TRH) and guanosine 5'-Q-thiotriphosphate, which stimulate polyphosphoinositide breakdown in permeable cells, were found to be only weak stimulators of PRL release, compared to TPA and exogenous diacylglycerol. However, a much stronger effect of TRH was seen if cells were briefly treated with TRH prior to permeabilization. PRL release from TRH-pretreated permeable cells resembled TPA- and OAG-stimulated secretion, with [Ca2+] greater than 0.1 microM potentiating the effect of TRH pretreatment. These studies support the hypothesis that PRL release in GH3 cells can be stimulated directly by a diacylglycerol-activated secretory mechanism whose activity is modulated by [Ca2+].     

Monolayer cultures of dispersed cells from bovine anterior pituitary: responses to synthetic hypophysiotropic peptides.

Cells were dispersed from bovine anterior pituitary glands, by digestion with collagenase, and cultured. After 4 days the cell monolayers were incubated with fresh medium containing synthetic hypophysiotropic peptides for 2, 6, or 20 h, and hormone released into the medium was estimated by radioimmunoassay. After 2 h, thyroid releasing hormone (TRH) stimulated the release of thyroid-stimulating hormone (TSH) up to eightfold, and of prolactin (PRL) and follicle-stimulating hormone (FSH) about twofold at a minimal effective concentration of 1 ng/ml; enhanced growth hormone (GH) release was not apparent until 20 h, and release of luteinizing hormone (LH) and adrenocorticotrophic hormone (ACTH) was unaffected. Luteinizing hormone releasing hormone (LH-RH) enhanced release of LH maximally (three- to fourfold) during a 2 h incubation and was effective at 0.1 ng/ml; FSH release was significantly enhanced by about 50% above control level. Growth hormone release inhibiting hormone (GH-RIH)(somatostatin) showed significant effects only in the 20 h incubation; GH release was inhibited by 50% and release of PRL was slightly, but significantly, enhanced. Pituitary cell monolayers apparently permit maximal expression of releasing activities inherent in the hypothalamic hormones.     

Ca2(+)-independent secretory mechanism of thyrotropin-releasing hormone (TRH) involves protein kinase C in rat pituitary cells

Thyrotropin-releasing hormone (TRH) stimulates biphasic prolactin (PRL) secretion from rat pituitary GH3 cells. The pretreatment of cells with EGTA (100 microM) plus arachidonic acid (15 microM), a condition which decreased TRH-responsive intracellular Ca2+ pools, eliminated the activity of TRH on burst PRL secretion (2 min) but did not alter that on sustained PRL secretion (30 min). However, the treatment of cells with EGTA, arachidonic acid and H-7 (300 microM), a potent inhibitor of protein kinase C (PKC), almost completely suppressed the activity of TRH for sustained PRL secretion. In cells down-modulated for PKC, TRH abolished this Ca2(+)-independent sustained PRL secretion. These results suggest that TRH acts through a separate, Ca2(+)-independent secretory mechanism, besides by modulating the Ca2(+)-dependent mechanism and that PKC is involved in this Ca2(+)-independent secretory pathway.     

Differential involvement of protein kinase C in basal versus acetylcholine-regulated prolactin secretion in rat anterior pituitary cells during aging

Although it is well known that plasma concentration of prolactin (PRL) increases during aging in rats, how the anterior pituitary (AP) aging per se affects PRL secretion remains obscure. The objectives of this study were to determine if changes in the pituitary PRL responsiveness to acetylcholine (ACh; a paracrine factor in the AP), as compared with that to other PRL stimulators or inhibitors, contribute to the known age-related increase in PRL secretion, and if protein kinase C (PKC) is involved. We also determined if replenishment with aging-declined hormones such as estrogen/thyroid hormone influences the aging-caused effects on pituitary PRL responses. AP cells were prepared from old (23-24-month-old) as well as young (2-3-month-old) ovariectomized rats. Cells were pretreated for 5 days with diluent or 17beta-estradiol (E(2); 0.6 nM) in combination with or without triiodothyronine (T(3); 10 nM). Then, cells were incubated for 20 min with thyrotropin-releasing hormone (TRH; 100 nM), angiotensin II (AII; 0.2-20 nM), vasoactive intestinal peptide (VIP; 10(-9)-10(-5) M), dopamine (DA; 10(-9)-10(-5) M), or ACh (10(-7)-10(-3) M). Cells were also challenged with ACh, TRH, or phorbol 12-myristate 13-acetate (PMA; 10(-6) M) following PKC depletion by prolonged PMA (10(-6) M for 24 h) pretreatment. We found that estrogen priming of AP cells could reverse the aging-caused effects on pituitary PRL responses to AII and DA. In hormone-replenished cells aging enhanced the stimulation of PRL secretion by TRH and PMA, but not by AII and VIP. Aging also reduced the responsiveness of cells to ACh and DA in suppressing basal PRL secretion, and attenuated ACh inhibition of TRH-induced PRL secretion. Furthermore, ACh suppressed TRH-induced PRL secretion mainly via the PMA-sensitive PKC in the old AP cells, but via additional mechanisms in young AP cells. On the contrary, basal PRL secretion was PKC (PMA-sensitive)-independent in the old AP cells, but dependent in the young AP cells. Taken together, these results suggest differential roles of PMA-sensitive PKC in regulating basal and ACh-regulated PRL responses in old versus young AP cells. The persistent aging-induced differences in AP cell responsiveness to ACh, DA, TRH, and PMA following hormone (E(2)/T(3)) replenishment suggest an intrinsic pituitary change that may contribute, in part, to the elevated in vivo PRL secretion observed in aged rats.     

Effect of [D-Trp6]LHRH infusion on prolactin secretion by perifused rat pituitary cells

The effect of a superactive agonistic analog of luteinizing hormone-releasing hormone (LHRH), [D-Trp6]LHRH on prolactin (PRL) secretion by perifused rat pituitary cells was investigated. Constant infusion of [D-Trp6]LHRH (0.5 ng/min) for 2-3 h elicited a significant decrease in PRL secretion by these cells. This decrease in PRL release started ca. 30 min after the beginning of the infusion with the LHRH analog and lasted up to 1.5-2 h. [D-Trp6]LHRH significantly stimulated luteinizing hormone (LH) secretion during the first 30 min of peptide infusion; thereafter, LH levels began to return to control values. In animals pretreated in vivo with 50 micrograms of [D-Trp6]LHRH (s.c.) 1 h before sacrifice, PRL secretion by the rat pituitary cell perifusion system was significantly lower than vehicle-injected controls throughout the entire [D-Trp6]LHRH infusion period. On the other hand, thyrotropin-releasing hormone (TRH)-stimulated PRL secretion was slightly, but significantly imparied by [D-Trp6]LHRH infusion, while dopamine (DA) inhibition of PRL release was unaffected by this same treatment. These results reinforce previous observations of a modulatory effect of [D-Trp6]LHRH, probably mediated by pituitary gonadotrophs, on PRL secretion by the anterior pituitary. In addition, our findings suggest that basal PRL secretion by the lactotroph may be dependent on a normal function of the gonadotroph. The collected data from this and previous reports support the existence of a functional link between gonadotrophs and lactotrophs in the rat pituitary gland.     

Culture requirements for optimal expression of 1α,25-dihydroxyvitamin D3-enhanced thyrotropin secretion

Summary An effect of the hormone, 1α,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] on hormone secretion by normal rat pituitary cells was investigated in vitro. Based on previous findings using GH₄C₁ cells, dispersed anterior pituitary cell cultures were prepared and maintained in serum-free conditions for up to 6 d. Under these circumstances, there was no effect of 1,25(OH)₂D₃ to alter medium or cell-associated levels of thyrotropin (TSH), prolactin (PRL), or growth hormone (GH). Cultures maintained under these conditions had lower medium and cell-associated hormone levels and lesser responses to agonists than cultures maintained in serum-supplemented medium. In the presence of 10% charcoal-treated fetal bovine serum, treatment with 10⁻⁸ M 1,25(OH)₂D₃ for 24 h selectively increased TRH (10⁻¹⁰ to 10⁻⁷ M)-induced TSH secretion (P<0.001), with maximal enhancement observed at 10⁻⁹ M TSH-releasing hormone (TRH). Enhancement of TSH secretion by 1,25(OH)₂D₃ was detected after 15 min exposure to TRH. There was no effect on agonist-induced PRL or GH secretion or on cell-associated hormone levels. The effect was evident after 24 h treatment with 1,25(OH)₂D₃, and decreased thereafter. Several other steroid hormones had no effect on 10⁻⁹ M TRH-induced TSH secretion. These data contrast with the effect of 1,25(OH)₂D₃ in GH cells. They suggest that 1,25(OH)₂D₃ may act selectively in the normal pituitary to modulate TSH secretion.     

Growth hormone and prolactin secretion in hypophysial stalk-transected pigs as affected by growth hormone and prolactin-releasing and inhibiting factors.

Control of growth hormone (GH) and prolactin (PRL) release was investigated in hypophysial stalk-transected (HST) and stalk-intact pigs by determining the effects of analogs of GH-releasing factors (GHRF), somatostatin (SRIF), arginine, thyrotropin-releasing hormone, alpha-methyl-rho-tyrosine, and haloperidol. HST and control gilts were challenged with intravenous injections of human pancreatic GHRF(1-40)OH, thyrotropin-releasing hormone, and analogs of rat hypothalamic GHRF. HST animals remained acutely responsive to GHRF by releasing 2-fold greater quantities of GH than seen in controls. This occurred in spite of a 38% reduction in pituitary gland weight and a 32 and 55% decrease in GH concentration and total content. During SRIF infusion, GH remained at similar basal concentrations in HST and control gilts, but increased immediately after stopping SRIF infusion only in the controls. Releasable pituitary GH appears to accumulate during SRIF infusion. GHRF given during SRIF infusion caused a 2-fold greater release of GH than seen in animals receiving only GHRF. Arginine increased (P less than 0.05) GH release in controls, but not in HST gilts, which suggests that it acts through the central nervous system. Basal PRL concentrations were greater (P less than 0.05) in HST gilts than in control gilts. TRH acutely elevated circulating PRL (P less than 0.001) in HST gilts, suggesting that it acts directly on the pituitary gland. Haloperidol, a dopamine receptor antagonist, increased circulating PRL in controls but not in HST animals. alpha-Methyl-rho-tyrosine did not consistently increase circulating PRL, however, suggesting that it did not sufficiently alter turnover rate of the tyrosine hydroxylase pool. The results indicate that the isolated pituitary after HST remains acutely responsive to hypothalamic releasing and inhibiting factors for both GH and PRL release in the pig.