Plasma aldosterone response to metoclopramide is unmodified by hyperprolactinemia

It has been previously demonstrated that patients with hyperprolactinemia have impaired PRL response to dopaminergic blockade and increased TSH response. Since inhibitory dopaminergic modulation of aldosterone is well established, we have examined whether prolactinoma patients have an altered aldosterone response to dopaminergic blockade. To investigate this possibility we compared the plasma PRL, TSH and aldosterone responses to the dopamine (DA) antagonist metoclopramide (MCP; 10 mg i.v.) in 10 women with prolactinomas and 7 healthy female controls. Basal PRL levels in prolactinoma patients were elevated and showed a blunted rise following MCP. Although basal TSH levels were similar in the 2 groups of subjects, they significantly increased (p = 0.017) in prolactinoma patients while in contrast they did not significantly change in control subjects. Basal supine plasma aldosterone was similar in patients with prolactinomas (0.23 +/- 0.03 nmol/l) and in healthy subjects (0.25 +/- 0.04 nmol/l) and the increased aldosterone concentrations from 15 to 120 min following MCP were not significantly different in prolactinoma patients and in control subjects. It is concluded that in patients with prolactinomas, the alteration in the dopaminergic regulation is specifically related to the lactotroph.     

Surgical treatment of prolactinomas. Short- and long-term results, prognostic factors

All 347 patients surgically treated for a prolactinoma from January 1, 1976 to December 31, 1982, in the neurosurgical ward of Foch Hospital, were retrospectively studied. The frequency of postoperative normalisation of plasma prolactin (PRL) depends on prolactinoma size, preoperative PRL level, duration of first clinical symptom, previous oestroprogestative contraception, and adenoma necrosis. Postoperative PRL values were normalized in 75% of small prolactinomas (grade 0, 1 or 2) with preoperative PRL values less than 200 ng/ml, and clinical duration less than 5 years (n = 102). There was no operative death and minor morbidity (2.7%). Among the 96 patients with postoperative PRL normalisation, operated between 1976 and 1979, 70 were followed up for an average time of 4.4 +/- 0.2 years. 17% of patients had hyperprolactinemia recurrence with a delay of 1.5 +/- 0.4 years. Postoperative PRL levels near the upper normal limit, and weak PRL response to TRH tests were found to be unfavourable prognostic factors for hyperprolactinemia recurrence. Pregnancy did not increase the risk of recurrence, but could reflect genuine long-lasting remission. Selective adenomectomy remains an interesting treatment for prolactinoma, particularly if the adenoma is small, recent and with PRL moderately increased. The frequency of postoperative PRL normalisation after surgery is less than with bromocriptine, but surgery is the only treatment able to achieve a definitive cure with a low iatrogenic risk.     

Role of vasoactive intestinal polypeptide (VIP) in regulating the pituitary function in man

Intramuscular injection of synthetic VIP (200 micrograms) resulted in a rapid increase in plasma prolactin (PRL) concentrations in normal women, which was accompanied by the 4- to 7-fold increase in plasma VIP levels. Mean (+/- SE) peak values of plasma PRL obtained 15 min after the injection of VIP were higher than those of saline control (28.1 +/- 6.7 ng/ml vs. 11.4 +/- 1.6 ng/ml, p less than 0.05). Plasma growth hormone (GH) and cortisol levels were not affected by VIP in normal subjects. VIP injection raised plasma PRL levels (greater than 120% of the basal value) in all of 5 patients with prolactinoma. In 3 of 8 acromegalic patients, plasma GH was increased (greater than 150% of the basal value) by VIP injection. In the in vitro experiments, VIP (10(-8), 10(-7) and 10(-6) M) stimulated PRL release in a dose-related manner from the superfused pituitary adenoma cells obtained from two patients with prolactinoma. VIP-induced GH release from the superfused pituitary adenoma cells was also shown in 5 out of 6 acromegalic patients. VIP concentrations in the CSF were increased in most patients with hyperprolactinemia and a few cases with acromegaly. These findings indicate that VIP may play a role in regulating PRL secretion in man and may affect GH secretion from pituitary adenoma in acromegaly.     

Preclinical Atherosclerosis in Patients with Prolactinoma

ObjectiveThe aim of this study was to evaluate the effect of hyperprolactinemia on body fat, insulin sensitivity, inflammatory markers, and cardiovascular risk in patients with prolactinoma.MethodsThe study included 35 untreated hyperprolactinemic patients with pituitary adenomas, and 36 age-, gender-, and body mass index (BMI)-matched healthy controls without any known disease. Serum glucose, insulin, homeostasis model assessment of insulin resistance (HOMA-IR, lipid profile, high-sensitivity C-reactive protein (hs-CRP), and heart-type fatty acid binding protein (H-FABP) levels were measured. Waist and hip circumference (WC and HC) were measured in all the participants. The body fat percentage was measured, and the visceral fat and abdominal fat percentages were measured via bioelectrical impedance (BIA). In addition, carotid intima media thickness (CIMT) was measured using high-resolution B-mode ultrasound.ResultsThe serum glucose level, HOMA-IR, triglyceride level, and SC were significantly higher in the patient group than in the control group. The hs-CRP level and CIMT were significantly higher in the hyperprolactinemic patients. Visceral and truncal fat percentages were significantly higher in the patients with prolactinoma. H-FABP levels were similar in the 2 groups, and there was a positive correlation between the prolactin (PRL) and H-FABP protein levels.ConclusionsBased on the present findings, hyperprolactinemia is associated with preclinical atherosclerosis and metabolic abnormalities. Patients with hyperprolactinemia might experience cardiovascular disease in the long term. Metabolic control should be achieved in addition to the control of hyperprolactinemia in the clinical management of patients diagnosed with prolactinoma. (Endocr Pract. 2014;20:447-451)     

Paradoxical prolactin response to growth hormone-releasing hormone in a patient with hyperprolactinemia and empty sella.

In a 30-year-old woman with amenorrhea due to hyperprolactinemia, serum PRL increased to twice the basal amount in response to growth hormone-releasing hormone (GHRH). Roentgenological studies revealed no pituitary adenoma but empty sella. Bromocriptine therapy normalized serum PRL and made the paradoxical response to GHRH disappear. The paradoxical response did not occur in any of eight other patients with hyperprolactinemia due to prolactinoma. Although this case is rare, GHRH stimulates PRL as well as GH release remarkably in some cases with hyperprolactinemia without a GH-producing tumor.     

Bone mineral density and bone turnover in hyperprolactinaemia of various origins

INTRODUCTION: Osteopenia and osteoporosis because of hyperprolactinaemia caused by prolactinoma may be followed by an increased risk of fracture. There are no data on the bone effects of functional hyperprolactinaemia. The aim was to assess the influence of hyperprolactinaemia of various origins on bone turnover and density in different skeletal sites. MATERIAL AND METHODS: The study was carried out in 75 women (aged 30.53 +/- 7.8): Group I--32 women with prolactinoma and Group II--43 women with functional hyperprolactinaemia. Both groups of patients were subdivided into those with hypogonadism and those with normal gonadal function. The control group consisted of 29 healthy women aged (33.59 +/- 4.7). In all subjects PRL and bone turnover markers (BAP, OC, ICTP) were studied. BMD measurements (lumbar spine, forearm, proximal femur and total body) were carried out using DXA. RESULTS: Higher PRL concentrations were observed in patients than in controls. The values of bone turnover markers (BAP, ICTP) were shown to be higher in patient groups and subgroups than in controls. In patients with prolactinoma lumbar spine BMD was lower than in patients with functional hyperprolactinaemia and controls. Total body BMD was also lower, albeit to a lesser extent. CONCLUSIONS: Hyperprolactinaemia caused by prolactinoma in women influences bone metabolism unfavourably, more by the impact on the activity of bone turnover markers than on BMD. This provides an opportunity for earlier assessment of bone metabolism disturbances before the BMD changes can be observed. Functional hyperprolactinaemia does not determine such a harmful effect on bone metabolism as hyperprolactinemia due to prolactinoma.     

Effect of zinc administration on thyrotropin releasing hormone-stimulated prolactinemia in healthy men

Previous in vitro studies have demonstrated zinc (Zn⁺⁺) inhibition of basal and of potassium (K⁺) or thyrotropin-releasing hormone (TRH)-stimulated prolactin (PRL) secretion, in a selective, reversible, and dose-dependent manner. Thus, Zn⁺⁺ may regulate physiologically pituitary PRL secretion. Furthermore, studies with patients with uremia, cirrhosis or prolactinoma, have shown the coexistence of hypozincemia and hyperprolactinemia and zinc supplementation did not correct hyperprolactinemia in these patients. In normal individuals Zn⁺⁺ administration produced controversial results on PRL secretion. Here, we investigated whether zinc administration affects TRH-stimulated PRL in healthy men. We found that Zn⁺⁺ administration does not change the TRH-stimulated PRL. Therefore, in normal conditions, Zn⁺⁺ does not inhibit TRH-stimulated prolactinemia. In addition, we found that acute increases of blood PRL and TRH do not alter blood Zn⁺⁺ levels.     

Catecholamines and pituitary function. III. Restoration of the prolactin response to thyrotropin-releasing hormone by low-dose dopamine infusion in women with pathological hyperprolactinemia

Previous studies in Rhesus monkeys have demonstrated that a dopamine (DA) infusion rate of 0.1 microgram/kg X min induces peripheral DA levels similar to those measured in hypophysial stalk blood and normalizes serum prolactin (PRL) levels in stalk-transected animals. We therefore examined the effect of such DA infusion rate on basal and thyrotropin-releasing hormone (TRH)-stimulated PRL secretion in both normal cycling women and women with pathological hyperprolactinemia. 0.1 microgram/kg X min DA infusion fully normalized PRL serum levels in 8 normal cycling women whose endogenous catecholamine synthesis had been inhibited by alpha-methyl-p-tyrosine (AMPT) pretreatment. Furthermore, DA significantly reduced, but did not abolish, the rise in serum PRL concentrations induced by both acute 500 mg AMPT administration and 200 micrograms intravenous TRH injection in normal women. A significant reduction in serum PRL levels in response to 0.1 microgram/kg X min DA, similar to that observed in normal cycling women when expressed as a percentage of baseline PRL, was documented in 13 amenorrheic patients with TRH-unresponsive pathological hyperprolactinemia. However, a marked rise was observed in the serum PRL of the same patients when TRH was administered during the course of a 0.1-microgram/kg X min DA infusion. The PRL response to TRH was significantly higher during DA than in basal conditions in hyperprolactinemic patients, irrespective of whether this was expressed as an absolute increase (delta PRL 94.4 +/- 14.2 vs. 17.8 +/- 14.1 ng/ml, p less than 0.002) or a percent increase (delta% PRL 155.4 +/- 18.9 vs. 17.9 +/- 7.1, p less than 0.0005), and there was a significant linear correlation between the PRL decrements induced by DA and the subsequent PRL responses to TRH. These data would seem to show that the 0.1-microgram/kg X min DA infusion rate reduces basal PRL secretion and blunts, but does not abolish, the PRL response to both TRH and acute AMPT administration. The strong reduction in PRL secretion and the restoration of the PRL response to TRH by 0.1 microgram/kg X min DA infusion in high majority of hyperprolactinemic patients, seem to indicate that both PRL hypersecretion and abnormal PRL response to TRH in women with pathological hyperprolactinemia are due to a relative DA deficiency at the DA receptor site of the pituitary lactotrophs.     

垂体催乳素瘤的病因及发病机制的实验研究

应用在同一只雄性ＳＤ大鼠中,保留原位垂体并在肾囊内植入一异体垂体,同时背部埋植装有１７－β－雌二醇（１７－β－ｅｓｔｒａｄｉｏｌ,Ｅ２）的药泵,经Ｅ２在体作用６０￣１２０天后,其原位垂体和异体移植入肾囊的垂体同时形成２垂体催乳素（ｐｒｏｌａｃｔｉｎ,ＰＲＬ）瘤的动物模型,研究ＰＲＬ瘤的发病机制。结果表明,Ｅ２长期作用可诱发原位垂体和远离下丘脑的移植垂体同时形成ＰＲＬ瘤,并伴高ＰＲＬ血症和ＰＲＬ基因     

Effects of haloperidol and prolactin secreting tumors on cerebrospinal fluid concentrations of prolactin in the female rat.

Studies were conducted to determine the effects of acute and chronic elevations in prolactin (PRL) secretion on serum and cerebrospinal fluid (CSF) PRL concentrations in the female rat. Young female rats showed a dose-dependent increase in serum and CSF PRL in response to haloperidol. A time-course evaluation of serum and CSF PRL levels after haloperidol indicated that serum PRL concentrations increased markedly by 30 min and declined thereafter; while CSF PRL increased more slowly, peaking at 2 to 8 h. In young rats with basal serum PRL levels, CSF PRL was maintained at 0.8 to 2.1% of serum PRL levels. During acute hyperprolactinemia, the CSF to serum PRL ratio increased to about 4%. During chronic severe hyperprolactinemia, induced by the growth of a MtT.W15 tumor, CSF PRL concentrations increased to 75 ng/ml, but this represented only 1.5% of serum PRL concentrations. Collectively, these data indicate that the blood-brain barrier effectively limits access to the brain of circulating PRL.     

The Effect of Raloxifene on Serum Prolactin Level in Patients with Prolactinoma

Objective: To evaluate the effect of raloxifene on prolactin (PRL) levels in addition to dopamine agonist (DA) therapy in patients with prolactinoma.Methods: We conducted a retrospective chart review of 14 patients with prolactinoma on stable dose of DA for 6 months who received raloxifene 60 mg daily, as PRL could not be normalized despite being on fairly high doses of DA. Patients were informed that raloxifene is not approved by the Food and Drug Administration for prolactinoma treatment. PRL level was measured at 1 to 6 months after starting raloxifene and at 1 to 3 months following its discontinuation. Raloxifene was stopped in 8 out of 14 patients after 2 (1 to 6) months of treatment as the absolute change in PRL level was felt to be small.Results: The median age and female/male sex ratios were 50 years (range 18 to 63 years), 6/8 respectively. The baseline DA dose was 3 mg/week (0.5 to 7 mg/week) for cabergoline and 15 mg/day for bromocriptine. Ten patients had an absolute and percentage decrease in PRL of 8.3 ng/mL (1.5 to 54.2 ng/mL) and 25.9% (8 to 55%) from baseline, respectively, after 1 to 6 months on raloxifene treatment. Among 10 patients with a decrease in PRL level, 2 (20%) achieved PRL normalization. Two patients had no change in PRL and two patients had an increase in PRL level by 22.8 ng/mL and 8.8 ng/mL (47% and 23.6%), respectively.Conclusion: Raloxifene was associated with a 25.9% (8 to 55%) decrease in PRL level in 10/14 (71%) patients with prolactinoma who were on stable doses of DA including 2 patients (14%) who achieved normoprolactinemia.Abbreviations: CV = coefficient of variation; DA = dopamine agonist; FSH = follicule-stimulating hormone; LH = luteinizing hormone; PRL = prolactin; PTTG = pituitary tumor transforming gene     

A patient with a prolactinoma associated with an aldosterone producing adrenal adenoma: differences in dopaminergic regulation of PRL and aldosterone secretion.

A patient with a rare combination of prolactinoma and aldosterone producing adrenal adenoma (APA) was reported in relation to studies concerning dopaminergic regulation of PRL and aldosterone secretion. The patient is a 38-year-old female with plasma PRL and aldosterone concentrations (PAC) of 563 ng/ml and 54 ng/dl, respectively. A bolus of 10 mg of metoclopramide significantly increased plasma PRL in 6 normal subjects and in 4 patients with APA, whereas the responses were blunted in 7 patients with prolactinoma and in our patient. The response of aldosterone to metoclopramide was less than that of PRL, but similar in all studied subjects, indicating that the dopaminergic inhibition of aldosterone secretion is less than that of PRL in normal subjects and did not change in patients with APA or prolactinoma. Oral administration of 2.5 mg of bromocriptine suppressed plasma PRL significantly in all the subjects studied, but did not produce any consistent changes in PAC. Discrepancies in the response of PRL and aldosterone to metoclopramide and to bromocriptine suggest a difference in the dopaminergic regulation of PRL and aldosterone secretion in both normal subjects and patients with prolactinoma and APA. It is unlikely that reduced dopaminergic inhibition is the basis for hypersecretion of PRL and aldosterone in our patient.     

Prolactin after baclofen in healthy subjects and prolactinoma patients

Serum prolactin (PRL) levels in basal conditions (two samples) and 30, 60, 90, 120, 150 e 180 minutes after oral administration of baclofen (20 mg) were evaluated in 6 healthy subjects and in 10 patients with prolactinoma. The effect of baclofen (20 mg by mouth) on the PRL secretion cimetidine (400 mg i.v.) or domperidone (20 mg i.v.) induced were evaluated in 9 healthy women by administration of baclofen 60 minutes before cimetidine or domperidone. Baclofen was unable to significantly rise serum PRL levels in healthy subjects and in patients affected by prolactinoma and furthermore did not interfere with PRL rise domperidone induced. On the contrary baclofen decreased PRL rise cimetidine induced. It was concluded that: in basal condition, GABAb receptor don't play an obvious role in modulation of PRL secretion; when H2 istaminergic inhibition on PRL secretion is blocked (at an hypothalamic site), a GABA inhibition, b receptor mediated, on PRL secretion became more clear; the domperidone blockade of hypophysial dopaminergic receptors suggests that GABAb modulation of prolactin secretion don't obtain itself by dopaminergic pathways.     

Macroprolactinemia in childhood and adolescence: a cause of asymptomatic hyperprolactinemia

Asymptomatic hyperprolactinemias associated with altered proportions of molecular forms of circulating prolactin (PRL) have been reported in adults. The scarce references available in children and adolescents prompted us to report our experience in the evaluation and follow-up of patients with macroprolactinemia. We studied 5 patients (1 male and 4 females) aged 11.6-18 years with incidentally discovered asymptomatic hyperprolactinemia. Patients underwent repeated evaluations for a period of 3 months to 8 years, and their PRL levels remained elevated (34.4-516 ng/ml). Structural variants of PRL >/=45 kD ranged between 58.9 and 78.6%. Chromatographic profiles showed increases in Big Big PRL in the 5 cases, ranging between 40 and 72% (normal: 9-21%), and in Big PRL in 3 cases, ranging between 30.0 and 32.6% (normal: 5-25%). Little PRL was decreased in all cases, ranging between 20.6 and 41.1% (normal: 50-90%). In conclusion, upon detection of hyperprolactinemia with no clinical manifestations and no alteration of the remaining endocrine functions, macroprolactinemia should be considered as a possible diagnosis. The confirmed absence of functional alterations during the follow-up would favor a no-treatment approach and at the same time avoid repeating imaging studies.     

Disturbed prolactin responses to dopamine-related substances in patients with acromegaly and hyperprolactinemia

We undertook this study, because conflicting data were reported about the dopaminergic regulation of prolactin (PRL) secretion in patients with acromegaly and hyperprolactinemia. In order to clarify the dopaminergic regulation of PRL secretion in patients with acromegaly and hyperprolactinemia, the effects of nomifensine, a central dopamine agonist, FK 33-824, a centrally antidopaminergically acting agent, and domperidone, a peripheral dopamine antagonist, on plasma PRL in these patients were studied. The results were compared with those observed in normal subjects and hyperprolactinemic patients, with or without a pituitary tumor. Nomifensine did not lower the PRL levels and FK 33-824 did not raise the PRL levels in acromegalic patients. In hyperprolactinemic patients, nomifensine did not lower the PRL levels and FK 33-824 failed to raise the PRL levels. Domperidone did not increase PRL in about a third of acromegalic patients, while TRH increased PRL in the all normoprolactinemic acromegalic patients. These results suggest that in acromegalic patients there may be a disturbance in dopamine related neurotransmission and that such disorders also seem to be present in patients with hyperprolactinemia, with or without a pituitary tumor.     

Catecholamines and pituitary function. VI. Effect of different dopamine doses on TRH-induced prolactin release in women with pathological hyperprolactinemia

The present study was designed to examine the effect of low-dose dopamine (DA) infusion rates (0.02 and 0.1 microgram/kg X min) on both basal and TRH-stimulated prolactin release in normal and hyperprolactinemic individuals. Sixteen normally menstruating women in the early follicular phase of a cycle and 23 hyperprolactinemic patients were studied. 0.1 microgram/kg X min DA was infused in 8 normal women and 15 patients with pathological hyperprolactinemia, while 8 normal controls and 8 patients received 0.02 microgram/kg X min DA TRH (200 micrograms, i.v.) was administered alone and at the 180th min of the 5-hour DA infusion in all controls and patients. A significant reduction in serum PRL levels, which was similar in normal women (-59.5 +/- 4.0%, mean +/- SE) and hyperprolactinemic patients (-48.2 +/- 5.5) was observed in response to 0.1 microgram/kg X min DA. In normal cycling women DA infusion significantly (P less than 0.02) reduced the PRL response to TRH with respect to the basal TRH test (delta PRL 45.0 +/- 7.0 vs. 77.9 +/- 15.4 ng/ml). On the contrary, the PRL response to TRH was significantly higher during 0.1 microgram/kg X min DA than in basal conditions in hyperprolactinemic patients, both in absolute (delta PRL 91.8 +/- 17.6 vs. 38.4 +/- 6.8, P less than 0.03) and per cent (198.5 +/- 67.6 vs. 32.1 +/- 7.5, P less than 0.02) values. A normal PRL response to TRH, arbitrarily defined as an increase greater than 100% of baseline, was restored in 11 out of 15 previously unresponsive hyperprolactinemic patients.(ABSTRACT TRUNCATED AT 250 WORDS)     

BMI and Metabolic Profile in Patients With Prolactinoma Before and After Treatment With Dopamine Agonists

Hyperprolactinemia might be related to weight gain, metabolic syndrome (MS), and insulin resistance (IR). Treatment with dopamine agonist (DA) has been shown to reduce body weight and improve metabolic parameters. The objectives of this study were to determine the prevalence of obesity, overweight, MS, and IR in patients with prolactinoma before and after therapy with DA and to evaluate the relation between prolactin (PRL), body weight, fat distribution, leptin levels, IR, and lipid profile before treatment. In addition, we investigated the correlation of the reduction in PRL levels with weight loss and metabolic profile improvement. Twenty‐two patients with prolactinoma completed 6 months of treatment with DA. These patients were submitted to clinical (BMI, waist circumference, blood pressure (BP)), laboratory evaluation (leptin, glucose, low‐density lipoprotein (LDL)‐cholesterol, and triglyceride (TG) levels) and abdominal computed tomography (CT) before and after treatment. The statistical analyses were done by nonparametric tests. At the beginning of the study, the prevalence of obesity, overweight, MS, and IR was 45, 27, 27, and 18%, respectively. After 6 months of treatment with DA, PRL levels normalized, but no significant difference in BMI was observed. However, there was a significant decrease on homeostasis model assessment of insulin resistance (HOMA_IR) index, glucose, LDL‐cholesterol, and TG levels. This study suggests a possible involvement of prolactinoma on the prevalence of obesity. We should consider that DA may be effective on improving metabolic parameters, and we speculate that a period longer than 6 months of treatment is necessary to conclude whether this drug can interfere in the body weight of patients with prolactinoma.     

HER2/ErbB2 receptor signaling in rat and human prolactinoma cells: strategy for targeted prolactinoma therapy

Dopamine agonist resistance or intolerance is encountered in approximately 20% of prolactinoma patients. Because human epidermal growth factor receptor 2 (HER2)/ErbB2 is overexpressed in prolactinomas and ErbB receptor ligands regulate prolactin (PRL) gene expression, we tested the role of HER2/ErbB2 in prolactinoma hormone regulation and adenoma cell proliferation to assess the rationale for targeting this receptor for prolactinoma therapy. As we showed prolactinoma HER2 overexpression, we generated constitutively active HER2-stable GH3 cell transfectants (HER2CA). PRL mRNA levels were induced approximately 250-fold and PRL secretion was enhanced 100-fold in HER2CA cells, which also exhibited increased proliferation. Lapatinib, a dual tyrosine kinase inhibitor (TKI) of both epidermal growth factor receptor (EGFR)/ErbB1 and HER2, blocked receptor signaling, and suppressed PRL expression more than gefitinib, a TKI of EGFR/ErbB1. Lapatinib also suppressed colony formation in soft agar more than gefitinib. Oral lapatinib treatment caused tumor shrinkage and serum PRL suppression both in HER2CA transfectant-inoculated Wistar-Furth rats and in estrogen-induced Fischer344 rat prolactinomas. In cultured human cells derived from resected prolactinoma tissue, lapatinib suppressed both PRL mRNA expression and secretion. These results demonstrate that prolactinoma HER2 potently induces PRL and regulates experimental prolactinoma cell proliferation. Because pituitary HER2 signaling is abrogated by TKIs, this receptor could be an effective target for prolactinoma therapy.     

Alpha 2-adrenoceptor-mediated inhibition of prolactin release in suckling- or fenfluramine-induced hyperprolactinemia.

It has recently been shown that the specific and selective alpha 2-antagonist idazoxan (IDZ) displays prolactin-lowering activity on hyperprolactinemia induced in the rat either by suckling or serotonergic drugs. In an attempt better to understand the role of alpha 2-adrenoceptors under the above conditions, experiments were carried out to compare the effects of IDZ with that of the classic alpha 2-antagonist yohimbine (YOH), and also of the alpha 2-agonists clonidine (CLO) and B-HT 920, on prolactin (PRL) release during lactation and in hyperprolactinemia induced in male rats by the serotonergic drug fenfluramine (FEN). In lactating rats, both alpha 2-agonists decreased PRL release; this effect was enhanced by prior separation of the animals from their pups for several hours. A decrease of plasma PRL levels was also induced by IDZ but not by YOH, which tended further to increase hyperprolactinemia. In male rats treated with FEN, IDZ and CLO, a significant decrease of plasma PRL was produced, but YOH further enhanced PRL secretion. It is concluded that the alpha 2-agonists tested and also the alpha 2-antagonist IDZ display a unique inhibitory activity on PRL release during suckling or serotonergic-induced hyperprolactinemia.     

Prolactin stimulates the L-type calcium channel-mediated transepithelial calcium transport in the duodenum of male rats

Elevated plasma levels of prolactin (PRL) have been reported in several physiological and pathological conditions, such as lactation, prolactinoma, and dopaminergic antipsychotic drug uses. Although PRL is a calcium-regulating hormone that stimulates intestinal calcium absorption in lactating rats, whether PRL is capable of stimulating calcium absorption in male rats has been elusive. Herein, the transepithelial calcium transport and electrical characteristics were determined in ex vivo duodenal tissues of male rats by Ussing chamber technique. We found that PRL receptors were abundantly present in the basolateral membrane of the duodenal epithelial cells. PRL (200–800 ng/mL) markedly increased the active duodenal calcium transport in a dose-dependent fashion without effect on the transepithelial resistance. The PRL-enhanced active duodenal calcium transport was completely abolished by L-type calcium channel blocker (nifedipine) as well as inhibitors of the major basolateral calcium transporters, namely plasma membrane Ca²⁺-ATPase and Na⁺/Ca²⁺ exchanger. Several intracellular mediators, such as JAK2, MEK, PI3K and Src kinase, were involved in the PRL-enhanced transcellular calcium transport. Moreover, PRL also stimulated the paracellular calcium transport in the duodenum of male rats in a PI3K-dependent manner. In conclusion, PRL appeared to be a calcium-regulating hormone in male rats by enhancing the L-type calcium channel-mediated transcellular and the paracellular passive duodenal calcium transport. This phenomenon could help restrict or alleviate negative calcium balance and osteoporosis that often accompany hyperprolactinemia in male patients.