Evidence for a specific estradiol binding site on rat pituitary membranes.

Using adenohypophyses from normal female rats, we demonstrate that estradiol binds pituitary membranes to one homogeneous population of sites with high affinity [dissociation constant (Kd) = 0.041 +/- 0.014 nM; n = 6] and low capacity [maximum binding (Bmax) = 13.6 +/- 5.6 fmol/mg protein]. The binding is thermolabile. Association experiments show that the best experimental conditions are an overnight incubation at 0 C. When the amount of proteins is increased more than 0.3 mg/ml of membrane suspension, binding is rapidly nonlinear. The presence of 0.5 M leupeptin does not improve the binding. Extensive washing of the membranes does not decrease the amount of sites, indicating that the binding is not loosely attached to the membranes. Parenthetically, it should be noted that the membrane fraction was devoid of the cytosolic enzyme marker, lactate dehydrogenase. Binding is specific for estrogenic compounds. When 100% specific binding was determined in the presence of 10(-6) M diethylstilbestrol, 17 beta-estradiol, estrone, and estriol displaced total binding by 110, 80, and 75%, respectively. Neither 4-OH-tamoxifen nor dihydrotestosterone, progesterone, or cortisol displaced the binding. Taken together, these data argue in favor of the presence of specific membrane recognition sites for estradiol in the rat pituitary.

Modifications of the high and low affinity pituitary domperidone-binding sites in chronic estrogenized rats.

The effect of chronic estrogen treatment on the anterior pituitary domperidone-binding sites was studied in female rats. The rats were implanted from 1-6 months with a Silastic capsule containing 17 beta-estradiol. The Feldman analysis of [3H]domperidone binding to anterior pituitary membranes of control or estrogenized rats revealed the presence of two sites. The binding characteristics of the higher affinity site were identical for both groups (Kd of the high affinity site, 0.30-0.45 nM; maximum number of binding sites of the high affinity site, 74-95 fmol/mg protein); however, those of the lower affinity site were affected by the estrogen treatment: the Kd of the low affinity site increased from 17.4 +/- 3.2 to 41.5 +/- 9 (+/- SE) nM, and the maximum number of binding sites of the low affinity site increased from 214 +/- 22 to 343 +/- 35 fmol/mg protein. Thus, in chronic estrogenized rats, the total number of binding sites was increased by 54%. These changes, induced by chronic estrogenization, were reversible, since 2 weeks after removal of the 17 beta-estradiol pellet, the binding characteristics were no longer different from those observed in control rats. In contrast to chronic estrogen treatment, ovariectomy reduced markedly the total number of [3H]domperidone-binding sites in anterior pituitary membranes (-70%). Feldman analysis revealed that this reduction resulted from the complete disappearance of the low affinity sites in those membranes. No significant change in the binding characteristics of the high affinity site was detected in ovariectomized rats. Since estradiol induces a decrease in the anterior pituitary content of dopamine, a denervation supersensitivity-like mechanism might be responsible for the increase in pituitary domperidone-binding sites in estrogenized rats. Conversely, a hyposensitivity mechanism could be implicated in the decrease in the total number of the pituitary domperidone-binding sites in ovariectomized rats, since pituitary dopamine levels are increased in those animals. Whether the antidopaminergic properties of estrogen are also involved in this modulation after chronic estradiol treatment requires further investigation.

In vitro and in vivo antagonistic regulation by estradiol and progesterone of the rat pituitary domperidone binding sites: correlation with ovarian steroid regulation of the dopaminergic inhibition of prolactin secretion in vitro.

The influences of in vivo and in vitro estradiol (E2) and progesterone (P) treatments on the characteristics of [3H]domperidone binding to intact and ovariectomized (OVX) rat pituitary membranes were analyzed and compared to the modulation by these steroids of dopamine (DA) inhibition of PRL secretion in vitro from intact and OVX rat pituitaries. Using intact rat pituitaries, high and low affinity binding sites for domperidone were detected; the dose-dependent DA inhibition curve of PRL secretion was biphasic (range, 10(-13) - 10(-10) M DA, IC50 = 6 X 10(-12) M; range, 10(-10) - 10(-6) M DA, IC50 = 2 X 10(-8) M). Using OVX rat pituitaries, only the high affinity sites for domperidone were detected, and the dose-dependent DA inhibition curve of PRL secretion was monophasic (range, 10(-10) - 10(-6) M DA, IC50 = 10(-8) M). E2 and P did not modify the characteristics of the high affinity sites either after in vivo treatment or when directly added to the in vitro binding assay. However, using in vivo and in vitro tests, a modulation of the low affinity sites by E2 and P was demonstrated. When E2 is in excess and P levels are low or undetectable, these sites are not detectable, and P is able to restore there presence. A parallelism has been established between this antagonistic E2 and P regulation and the modulation of DA inhibition of PRL secretion (range, 10(-13) - 10(-10) M DA). When intact rat pituitaries are perifused in the presence of 10(-8) M E2, the biphasic dose-dependent inhibition curve of the control is changed into the monophasic curve of the OVX rat pituitaries. Conversely, when OVX rat pituitaries are perifused in the presence of 10(-6) M P, the monophasic curve of the control is changed into the biphasic curve of the intact rat pituitaries. Thus, the DA inhibition in the range 10(-13) - 10(-10) M might result from an interaction between DA and the low affinity site for domperidone. In summary, the biological regulation of PRL by DA at the pituitary level may be mediated by two different DA sites, one being submitted to an antagonistic E2 and P regulation directly at the membrane level. The consequence of this regulation is that, whereas E2 decreases the sensitivity of the cell to DA, P is necessary for a normal DA response of the lactotroph.

Altered balance between thyrotropin-releasing hormone and dopamine in prolactinomas and other pituitary tumors compared to normal pituitaries.

We measured TRH and dopamine (DA) concentrations in prolactinomas and other pituitary tumors in order to further understand the roles of these two factors in the hormone hypersecretion and growth of these tumors. The mean TRH concentration (by RIA) in 16 prolactinomas was 247 +/- 92 (+/- SE) fmol/mg cell protein (range, 10-1297), near that found in normal pituitary tissue. The prolactinoma TRH content did not correlate with the patient's tumor size or plasma PRL level. By contrast, DA assayed by high pressure liquid chromatography was present in normal pituitary tissue (7.3 +/- 3.5 pmol/mg cell protein), but was very low or undetectable in the prolactinomas (23 fmol/mg cell protein or less). 3,4-Dihydroxyphenylacetic acid, also assayed by high pressure liquid chromatography, was undetectable in both normal pituitary tissue and prolactinomas. This imbalance between TRH and DA content also was found in GH-secreting and nonsecreting adenomas. The TRH content in 18 GH-secreting tumors (24 +/- 6 fmol/mg) was considerably lower than that in the prolactinomas (P less than 0.001). In 8 nonsecreting adenomas, the mean TRH concentration was 109 +/- 28 fmol/mg, about half of that in the prolactinomas. In those 2 types of adenomas, DA also was nearly undetectable (less than or equal to 73 fmol/mg cell protein). We conclude that the imbalance between TRH and DA contents in prolactinomas compared to those in normal pituitary tissue might participate in the mechanisms leading to hypersecretion of PRL and the growth of all types of pituitary adenomas.

Somatostatin receptors in human growth hormone and prolactin-secreting pituitary adenomas.

[125I-Tyr]Somatostatin [( 125I-Tyr]SRIH) binding was found in 11 GH-secreting pituitary adenomas [Kd = 0.46 +/- 0.15 (+/- SE) nM; maximum binding, 165 +/- 35 fmol/mg protein). This binding was specific, since it was displaced by somatostatin-14 (SRIH-14), N-Tyr-SRIH-14, and SRIH-28. In contrast, a number of peptides and drugs not structurally related to SRIH, such as bombesin, dopamine, LHRH, met-enkephalin, naloxone, neurotensin, secretin, substance P, TRH, or vasoactive intestinal peptide, did not affect [125I-Tyr]SRIH binding. [125I-Tyr]SRIH specific binding also was found in PRL-secreting pituitary adenomas. The kinetic characteristics of the specific binding were similar to those of GH-secreting adenomas. However, maximal binding was one quarter that of GH-secreting adenomas (37 +/- 9 fmol/mg protein). In contrast, nonsecreting (chromophobe) tumors were devoid of any specific binding. Finally, in acromegaly, the density of [125I-Tyr]SRIH-binding sites in the adenomas was negatively correlated with plasma GH levels before surgery (r = -0.80). This suggests that somatostatinergic control is involved in GH secretion in acromegalic patients.

Evidence of dopamine receptors in human growth hormone (GH)-secreting adenomas with concomitant study of dopamine inhibition of GH secretion in a perifusion system.

Dopamine (DA) is known to influence human GH release both in vivo and in vitro. The direct control of GH secretion at the pituitary level has been observed using human GH-secreting pituitary adenomas in organ culture, but no dose-response relationship between DA and GH inhibition was demonstrated. Such a dose-response relationship was obtained using GH-secreting adenomatous cells in a perifusion column. The IC50 of DA was 5 X 10(-8) M. Binding studies with [3H]apomorphine on human GH-secreting pituitary adenoma membranes demonstrated the presence of a dopaminergic receptor (Kd = 0.73 +/- 0.24 nM; maximum binding, 18.5 +/- 3.6 fmol/mg protein), which presumably mediates the effect of DA on GH release. In comparison to prior studies with PRL-secreting adenomas, several differences were apparent: 1) the maximum inhibition of GH release by DA is lower than that of PRL (50% vs. 80%), 2) the maximum number of binding sites is much lower (5-20 times) in GH-secreting adenomas than in PRL-secreting adenomas, and 3) [3H]domperidone does not show specific binding on GH-secreting adenomas but does on PRL-secreting adenomas. These experimental results indicate that the dopaminergic control of GH secretion is different from that of PRL secretion, as DA is less effective on GH release than on PRL release from human pituitary adenomas.

Evidence of receptors for thyrotropin-releasing hormone in human prolactin-secreting adenomas.

Specific receptors for TRH were demonstrated on cellular membranes from 8 out of 10 human PRL-secreting adenomas. One binding site was characterized. The mean (+/- SD) value for the dissociation constant (Kd) from 8 adenomas was 68 +/- 7 nM. The maximal number of binding sites varied from 1 adenoma to another (from 50-1200 fmol/mg protein). In 7 of the 8 patients, there was little or no in vivo plasma PRL response to TRH although receptors for TRH were present on their tumors. It is thus suggested that in vivo absence of stimulatory effect of TRH in human PRL-secreting adenomas is not due to the absence of TRH receptors.

Normal and adenomatous human pituitaries secrete thyrotropin-releasing hormone in vitro: modulation by dopamine, haloperidol, and somatostatin.

TRH is present in human normal pituitaries and in pituitary adenomas. In this study we demonstrated that the same tissues can release TRH in vitro. Fragments from seven normal pituitaries (10-15 mg/syringe) and dispersed cells from eight prolactinomas, four GH-secreting and two nonsecreting adenomas (1-3 x 10(6) cells/syringe) were perifused using a Krebs-Ringer culture medium. After 1 h of equilibration the perifusion medium was collected every 2 min (1 mL/fraction) for 3 h. TRH, PRL, and GH were measured by RIA under basal conditions and in the presence of 10(-10) to 10(-6) mol/L dopamine (DA), alone or concomitant with haloperidol, or in the presence of 10(-10) or 10(-6) mol/L somatostatin. Both normal pituitary fragments and pituitary adenomatous cells (from all types of adenomas studied) spontaneously released TRH in vitro. TRH was detected in the perifusion medium either immediately after the end of the equilibration period or 30-60 min later. The molecular identity of TRH was assessed by high pressure liquid chromatography. There was no difference in the profile and the rate of TRH secretion between normal and tumoral tissues, and no correlation was found between the level of TRH release and that of PRL or GH secretion. DA stimulated TRH release from normal pituitaries and from PRL- and GH-secreting adenomas at doses as low as 10(-10) mol/L. A concomitant decrease in PRL and GH release was observed from adenomatous cells and in one case of normal tissue. Haloperidol (10(-7) mol/L) antagonized the effect of 10(-8) mol/L DA on both TRH and PRL secretion in normal pituitary and in prolactinomas. DA had no effect on TRH release from two nonsecreting tumors. The amounts of TRH released during 1 h of perifusion were 60-1640 pg/2 mg wet wt tissue in normal pituitaries and 54-2174 pg/10(6) cells in adenomas; these values were very high compared to those precedently reported within the tissues. These results indicate that pituitary cells can release TRH in vitro and suggest that TRH might be synthesized in situ. We suggest that TRH could act on pituitary hormone secretion and/or cell proliferation via a paracrine and/or an autocrine mechanism.

Absence of receptors for thyrotropin (TSH)-releasing hormone in human TSH-secreting pituitary adenomas associated with hyperthyroidism.

In patients with TSH-secreting pituitary adenomas associated with hyperthyroidism, TSH secretion usually does not respond to exogenous TRH stimulation. To determine the basis for this unresponsiveness, we studied TRH binding to the membranes of two such TSH-secreting pituitary adenomas. The patients, a 28-yr-old man and a 60-yr-old woman, had clinical and biochemical hyperthyroidism with increased serum TSH levels (15.7 and 14 mU/L, respectively; normal range, 1.1-7.2) and alpha-subunit to TSH molar ratios greater than 1 (2.4 and 1.7, respectively). Neither patients had an increase in serum TSH in response to TRH (200 micrograms, iv). Immunocytochemistry of the two adenomas, removed by transsphenoidal surgery showed a pure population of thyrotropic cells. Binding studies were performed by incubation of tumor cell membrane suspensions with increasing amounts of [3H]TRH. Two PRL-secreting adenomas and one normal human pituitary were used as controls. The characteristics of the TRH-binding sites from the control tissues were similar to those previously reported (Kd, 56, 30, and 49 nM; maximum binding, 187, 46, and 94 fmol/mg protein, respectively). In contrast, no specific TRH binding was found in the two TSH-secreting adenomas. We conclude that the unresponsiveness of TSH after TRH administration is related to the absence of specific TRH-binding sites in these thyrotropic tumors.

Evidence of thyrotropin-releasing hormone (TRH) and TRH-binding sites in human nonsecreting pituitary adenomas.

Immunoreactive TRH (IR-TRH) and TRH-binding sites were sought in nonsecreting pituitary adenomas. [3H]TRH bound specifically to cellular membranes from 11 of 12 such adenomas studied, with a dissociation constant (Kd) of 50 +/- 5 (+/- SEM) nmol/L and a maximum number of binding sites of 76 +/- 16 fmol/mg membrane protein (range, 32-229 fmol/mg protein). IR-TRH was detected in all 8 of the tumors in which it was sought. The identity of the IR-TRH was verified by high pressure liquid chromatography. The tumor IR-TRH concentration varied from 45-248 fmol/mg cell protein (mean, 109 +/- 28 fmol/mg cell protein), about half that in normal human pituitary (229 +/- 55 fmol/mg protein). There was no correlation between the number of binding sites and the IR-TRH content. The role of TRH in nonsecreting pituitary adenomas is unknown at this time.

Immunoreactive thyroliberin (TRH) precursor forms in human hypothalamus and anterior pituitary tissues.

Immunoreactive (IR) proTRH forms were characterized in human hypothalamic tissue with two antisera raised against a hepta- and a decapeptide containing the TRH progenitor sequence (-Gln-His-Pro-Gly-). A similar study was performed in human normal and adenomatous anterior pituitaries, tissues in which TRH synthesis has been previously suggested. IR-proTRH was found in all the samples ranging from 42-775 fmol/mg proteins. Size exclusion chromatography identified a major 25-35 kDa form and a minor 4-8 kDa form. The existence of the major form was confirmed by immunoblotting. The results suggest that both human hypothalamic and normal or adenomatous anterior pituitary tissues synthesize similar IR-proTRH forms.

Evidence of thyrotropin-releasing hormone (TRH) gene expression in rat anterior pituitaries and modulation by estrogens of TRH-like immunoreactivity and TRH-elongated peptide contents.

TRH gene expression in the anterior pituitary has previously been reported in the human in vivo and in the rat in vitro. Until now, modulation of this synthesis with glucocorticoids and thyroid hormones has been observed in rats. The present study demonstrates for the first time that the TRH gene is also expressed, in vivo, in the rat anterior pituitary and that anterior pituitary TRH-like immunoreactivity (TRH-LI) and elongated forms of the immediate TRH progenitor sequence (TRH-elongated peptide) contents are also modulated by estrogens (E2). To investigate the presence of proTRH mRNA in the rat anterior pituitary, total RNA was reverse transcribed (RT) and the RT products were then amplified by PCR. Treatments with E2 were performed on intact and ovariectomized (OVX) rats for 2 months. TRH-LI was measured by RIA with an antibody which did not recognize the TRH-like peptide. pGlu-Glu-Pro-NH2 (< EEP-NH2) (cross-reactivity < 0.1%) and was characterized further as TRH-LI by HPLC. TRH-elongated peptides were measured by EIA and characterized by Sephadex G-50 chromatography and immunoblotting (molecular mass 25-35 kDa). The plasma prolactin levels and the pituitary sizes were increased by E2 treatment in both intact and OVX rats. Anterior pituitary TRH-LI increased in intact E2-treated rats compared with intact rats (82.7 +/- 19.0 versus 39.6 +/- 3.6 fmol/mg protein; means +/- S.E.M.; P < 0.001). This increase was greater when E2 was administered to OVX rats (599.0 +/- 98.4 after E2 treatment versus 58.6 +/- 3.6 fmol/mg protein: P < 0.001). In intact rats, anterior pituitary TRH-elongated peptide contents were not modified by E2 treatment while they were significantly decreased in OVX E2-treated rats (144.6 +/- 8.8 versus 223.7 +/- 9.5 fmol/mg protein; P < 0.001). These results demonstrate TRH gene expression in the rat anterior pituitary in vivo and suggest that E2 treatment is responsible for an increase in anterior pituitary TRH-LI, together with a decrease in TRH-elongated peptide contents.

Thyrotropin-releasing hormone (TRH) binding sites and thyrotropin response to TRH are regulated by thyroid hormones in human thyrotropic adenomas.

In order to see whether, in thyrotropic adenomas with thyrotoxicosis, plasma thyroid hormones regulate the thyrotropin-releasing hormone (TRH) binding sites and the thyrotropin (TSH) response to TRH, we investigated: the presence of TRH binding sites in two cases of thyrotropic adenomas associated with hyperthyroidism and in one case of thyrotropic adenoma secondary to thyroid failure: and the in vitro effect, in a perifusion system, of triiodothyronine (T3) on the response of TSH to TRH in three cases of TSH-secreting adenomas associated with hyperthyroidism. The TRH binding sites were absent in the adenomas associated with high levels of circulating thyroid hormones, whereas they were present in the adenoma secondary to primary thyroid failure (Kd = 47 nmol/l, Bmax = 40 nmol/kg membrane proteins). In vitro, the three adenomas spontaneously released TSH in the perifusion medium (1.49 +/- 0.06 (mean +/- SEM), 7.25 +/- 0.12 and 16.73 +/- 0.36 mIU.l-1 x 10(6) cells-1 x 2 min-1) and exhibited an ample TSH response to 10(-7) mol/l TRH pulses. In two cases, tumoral secretion of fragments was compared with those of fragments maintained since the time of surgical removal in the presence of 10(-8) mol/lT3. The TSH responses to TRH were abolished in the presence of T3 in these two cases. We conclude that thyrotropic adenomas associated with hyperthyroidism are still controlled in vivo by T3. In particular, T3 regulates the TSH response to TRH, probably via a down-regulation of the TRH binding sites.

Somatostatin and regulation of prolactin secretion.

In addition to its classical growth hormone (GH) inhibiting action, somatostatin (SRIF) inhibits prolactin (PRL) secretion in man and rat under specific endocrine conditions. Furthermore, SRIF counteracts the thyrotropin releasing hormone (TRH) and vasoactive intestinal peptide (VIP) stimulated prolactin release from rat adenohypophysis in vitro. Two criteria are needed to demonstrate a physiological role of SRIF in PRL control: specific receptors must be present on prolactin secreting cells, and antagonization of endogenous SRIF must affect PRL secretion in vitro. In fact [125I]N--Tyr--SRIF binds to membranes not only of human GH-secreting adenomas, but also of prolactinomas. Specific binding characteristics are comparable in both cell types, but the density of sites in PRL-secreting adenomas is only one-quarter that in GH-secreting adenomas. In contrast, non-PRL-secreting chromophobe adenomas are devoid of specific binding. On the other hand, administration of SRIF antisera (SRIF-AS) affects both GH and PRL secretion in starved rats (a model in which pulsatile GH secretion is abolished); a marked increase in PRL plasma levels occurs, but the needed SRIF-AS concentration is higher than that for GH disinhibition. This demonstrates that endogenous SRIF may exert a negative control over PRL secretion, although lactotroph cells appear less sensitive to SRIF than somatotrophs. Since the apparent affinity of SRIF binding sites is similar on both GH and PRL secreting cells, at least in human tumor tissues, a lower density of SRIF receptors on PRL cells could account for this reduced responsiveness. Alternatively, different coupling mechanisms may be involved in the two cell types.

Biochemical and functional alterations of central GABA receptors during chronic estradiol treatment.

The characteristics of GABA and benzodiazepine receptors were examined in the hippocampus, striatum and cerebral cortex of female rats at various times (up to 9 months) after the subcutaneous implantation of an estradiol pellet (10 mg). A significant decrease in the Bmax of the high-affinity binding of [3H]muscimol to membranes from these 3 regions was detected as soon as one week after the implantation. Although the characteristics of the high-affinity binding of [3H]flunitrazepam remained unaffected during the whole treatment, the stimulatory effect of GABA (and muscimol) on this binding was significantly reduced by estrogenization. The changes in GABA receptor binding appeared functionally relevant since the elevation of striatal acetylcholine levels normally induced by the peripheral administration of muscimol (5 mg/kg) was significantly lower in estradiol-treated than in control female rats. In contrast to that observed in intact female rats, the implantation of estradiol in hypophysectomized animals did not affect the characteristics of [3H]muscimol binding to hippocampal, striatal and cortical membranes. [3H]muscimol binding was also unchanged in female rats implanted with estradiol and treated chronically with bromocriptine for 3 weeks. Since both hypophysectomy and the chronic administration of bromocriptine suppressed the hyperprolactinemia normally induced by estrogenization, the down-regulation of central GABA receptors very likely involved prolactin in intact animals implanted with 17-beta-estradiol.

Neuropeptides of anterior pituitary origin. Autocrine or paracrine functions?

Several neuropeptides classically associated with the hypothalamus have been found in the anterior pituitary. The question arises whether they are locally synthesized and if they play a paracrine or autocrine role on pituitary cell functions. Using normal and tumoral human pituitaries we found neuropeptides (TRH, SRIH, GHRH) and dopamine in variable quantities according to the nature of the tissue. They were all present in normal pituitaries, while stimulatory hormones (TRH and GHRH) were predominantly found in tumoral tissue, implying an imbalance of pathophysiological importance between the stimulatory and inhibitory control of hypophyseal hormones (PRL and GH) in pituitary adenomas. Both normal and tumoral pituitaries released TRH, SRIH and GHRH in large amounts suggesting their local synthesis. The in situ synthesis was demonstrated for SRIH by the evidence of SRIH mRNA, the detection of SRIH immunoreactivity in peculiar cells and the presence of SRIH precursor. The possible role of these pituitary neuropeptides was suggested for instance by the negative correlation found in vitro between SRIH and GH secretions. Moreover, neuropeptides could interact with each other. Indeed DA stimulated TRH release while PRL secretion decrease at the same time. Pulses of TRH had differential effects on SRIH release according to the nature of the tissue as TRH inhibited SRIH release from adenoma while it stimulated SRIH release from normal pituitary. Concerning the effects of SRIH and GHRH on GH secretion, there was an endogenous regulatory pattern comparable to that observed in rat portal blood vessels. Pulses of GHRH induced GH secretion only when endogenous SRIH release was not stimulated.(ABSTRACT TRUNCATED AT 250 WORDS)

[Paracrine regulation of anterior pituitary hormones by neuropeptides]. / Régulation paracrine des hormones antéhypophysaires par les neuropeptides.

The hypothalamus is the source of neuropeptides which, being secreted into the portal system, control the synthesis and the secretion of the anterior pituitary hormones. Besides the well characterized hypothalamic central control and the hormonal peripheral control, recent studies have shown, in the anterior pituitary, the expression, among many other regulatory factors, of neuropeptides that are identical to those produced by the hypothalamus and that seem involved in the local control of anterior pituitary functions through autocrine or paracrine mechanisms. The presence of the neuropeptide mRNAs, precursors and mature forms of the peptides in anterior pituitary tissues as well as the secretion of the mature peptides argue in favor of the intrinsic ability of the normal and tumoral anterior pituitary to express neuropeptides. This expression of neuropeptides occurring in tissues bearing functional receptors for these ligands, anterior pituitary control could rely, at least in part, on endogenous neuropeptides acting locally. Correlations between neuropeptide contents in the anterior pituitary and the plasma levels of anterior pituitary hormones suggest that neuropeptides of anterior pituitary origin can play a local regulatory role, complementary of the classical hypothalamic central control. In the normal anterior pituitary which remains under hypothalamic control, it is presently difficult to evaluate the relative importance of the local and central control. However, anterior pituitary hyperplasia and pituitary tumors represent two models in which the specific contribution of the local control is easier to define.

[Neuropeptides of anterior pituitary origin]. / Neuropeptides d'origine antéhypophysaire.

Several neuropeptides classically associated with the hypothalamus have been found in the anterior pituitary. The question arises whether they are locally synthesized and if they play a paracrine or autocrine role on pituitary hormone secretion. Using normal and tumoral human pituitaries we found neuropeptides (TRH, SRIH, GHRH) and dopamine in variable quantities according to the nature of the tissue. They were all present in normal pituitaries, while stimulatory hormones (TRH and GHRH) were predominantly found in tumoral tissue, implying an imbalance of pathophysiological importance between the stimulatory and inhibitory control of hypophyseal hormones (PRL and GH) in pituitary adenomas. Both normal and tumoral pituitaries released TRH, SRIH and GHRH in large amounts suggesting their local synthesis. The in situ synthesis was demonstrated for SRIH by the evidence of SRIH mRNA, the detection of SRIH immunoreactivity in peculiar cells and the presence of SRIH precursor. The possible role of these pituitary neuropeptides was suggested for instance by the negative correlation found in vitro between SRIH and GH secretions. Moreover neuropeptides could interact on each other. Indeed DA stimulated TRH release while PRL secretion decreased at the same time. Pulses of TRH had differential effects on SRIH release according to the nature of the tissue as TRH inhibited SRIH release from adenoma while it stimulated SRIH release from normal pituitary. Concerning the effects of SRIH and GHRH on GH secretion, there was an endogenous regulatory pattern comparable to that described in rat portal blood vessels. Pulses of GHRH induced GH secretion only when endogenous SRIH release was not stimulated.(ABSTRACT TRUNCATED AT 250 WORDS)

A new method to measure portal and hepatic blood flow using taurocholate in the rat.

Hepatic artery and portal vein blood flows were assessed separately in anesthetized rats using radiolabeled taurocholate. The ratio of portal vein blood flow to total hepatic blood flow was calculated from measurements of taurocholate concentrations of portal vein, hepatic artery and hepatic vein, and of taurocholate hepatic extraction ratio. Animals were administered [14C]taurocholate intraperitoneally as a label of the taurocholate pool 24 hr before the experiments. In steady-state conditions, serum [14C]taurocholate activities were used as an index of taurocholate relative concentration and the hepatic extraction ratio of taurocholate was estimated using [3H]taurocholate infused into the systemic blood stream. Total hepatic blood flow was estimated by the Fick's principle using indocyanine green. The mean hepatic blood flow was 13.9 ml X min-1 (S.E. 1.9), and the mean ratio of portal blood flow to hepatic blood flow was 72.2% (S.E. 6.7). It is concluded that taurocholate may be used to assess separately portal venous and hepatic arterial blood flow in the rat.

Correlative studies between the presence of thyrotropin-releasing hormone (TRH) receptors and the in vitro stimulation of growth-hormone (GH) secretion in human GH-secreting adenomas.

Specific receptors for TRH were characterized on cellular membranes of 6 out of 13 somatotrophic adenomas obtained from acromegalic patients. These receptors had the same dissociation constant (Kd: 62 +/- 10 nM) as those found in human PRL-secreting adenomas, but their maximal number of binding sites (Bmax: 76 +/- 24 fmol/mg of protein) was six fold smaller. A good correlation was found between the presence of TRH receptors and the in vitro TRH-induced stimulation of GH secretion. The increase in GH release varied from 25 to 200%. It was thus concluded that these receptors are functional. However, why only some of the human somatotrophic adenomas possess TRH receptors and respond to TRH in vitro needs further investigations.