EFFECT OF DOPAMINE INFUSION ON SERUM PROLACTIN CONCENTRATION IN NORMAL AND HYPERPROLACTINAEMIC SUBJECTS

The serum prolactin response to intravenous dopamine infusion (5μg.kg⁻¹ min⁻¹) was measured in twenty-one healthy subjects, in seven hyperprolactinaemic patients without evidence of a pituitary tumour, and in twenty-one patients with prolactinomas. Mean serum prolactin values were significantly suppressed in all three groups, without any significant difference between the degree of suppression. A decrease of serum prolactin to below 50% of basal values occurred in fifteen healthy subjects, in four patients without evidence of pituitary tumour, and in fourteen patients with prolactinomas. These findings demonstrate that most human prolactin-secreting pituitary adenomas are normally suppressible by exogenously administered dopamine and that dopamine infusion is not able to distinguish between tumorous and non-tumorous hyper-prolactinaemia. Since intravenously infused dopamine is believed to inhibit prolactin secretion by acting at pituitary level, it is suggested that a normal functioning of pituitary dopamine receptors is maintained in most human prolactinomas.     

EFFECT OF TWO SEROTONIN ANTAGONISTS ON PROLACTIN AND THYROTROPHIN SECRETION IN MAN

The effects of serotoninergic blockade on prolactin and thyrotrophin secretion in man was evaluated by determining the basal and TRH-stimulated serum prolactin and TSH concentrations in normal volunteers before and after a 3 days course of cyproheptadine or methergoline administration. Cyproheptadine, a serotonin antagonist with antihistaminic, anticholinergic and antidopaminergic properties as well, did not affect prolactin secretion, while it reduced the serum TSH response to TRH; methergoline, a specific blocker of central serotonin receptors, decreased basal and TRH-induced serum prolactin levels, without affecting TSH secretion. These results support the existence of serotoninergic stimulatory influences on human prolactin release, while suggesting that human TSH secretion is not modulated by serotoninergic inputs.     

THE PARTICIPATION OF HYPOTHALAMIC DOPAMINE IN MORPHINE-INDUCED PROLACTIN RELEASE IN MAN

In order to assess the role of dopamine in opiate-induced prolactin secretion, morphine alone or in combination with the dopamine blocker metoclopramide, or the L-aromatic aminoacid decarboxylase inhibitor benserazide, was administered to a group of normal adult men. Morphine (10 mg) stimulated prolactin release in all subjects; however, the effect was totally abolished when 10 mag metoclopramide or 200 mg benserazide were given before the opiate agonist. The prolactin releasing effect of a sub-maximal metoclopramide dose (1 mg) was potentiated by morphine. In vitro, benserazide was totally inactive in stimulating prolactin release by isolated anterior pituitary cells. Moreover, benserazide failed to alter the inhibiting action of dopamine on prolactin release. The data suggest that opiates stimulate prolactin release in man by acting through dopaminergic mechanisms.     

THE INTERACTION OF TRH AND DOPAMINERGIC MECHANISMS IN THE REGULATION OF STIMULATED PROLACTIN RELEASE IN MAN

The manner by which dopaminergic and TRH mechanisms interact to control PRL release is not known. Whilst dopamine receptor antagonists and TRH both release PRL, it is not known if the PRL released by these two mechanisms reflects similar aspects of physiological control, or if PRL responses to these mechanisms of release can be dissociated. We addressed this question by studying the PRL responses to maximal stimulatory dose of TRH and domperidone (a DA receptor antagonist), which were administered sequentially, simultaneously or separately on different occasions. Six normal volunteers undertook three sets of studies: (1) standard PRL stimulation tests to 400 micrograms TRH, 5 mg domperidone or simultaneous TRH/domperidone administration, (2) domperidone bolus-infusion study in which either 5 mg domperidone or 400 micrograms TRH was administered i.v. at 120 min during a 240 min infusion of domperidone (50 micrograms/min) which was preceded by a 5 mg i.v. bolus of the drug, and (3) TRH bolus-infusion study in which domperidone or TRH was administered i.v. at 120 min during a 240 min infusion of TRH (0.4 micrograms/min) which was preceded by a 400 micrograms i.v. bolus of the drug. In Study 1, simultaneous TRH/domperidone administration induced an incremental rise in PRL (5195 +/- 940 mIU/l) which was significantly greater (P less than 0.0005) than with either domperidone (3730 +/- 825 mIU/l) or TRH (1335 +/- 300 mIU/l) alone. In study 2, TRH administration at 120 min resulted in a significant rise (P less than 0.01) in PRL (delta PRL 960 +/- 232 mIU/l) whilst the second dose of domperidone did not, thus suggesting that the initial bolus and subsequent infusion had resulted in complete DA receptor blockade. In Study 3, domperidone administered at 120 min induced a marked rise in PRL (delta PRL 3609 +/- 963 mIU/l). In contrast, the corresponding TRH stimulus resulted in a small rise (delta PRL 142 +/- 32 mIU/l) suggesting that the PRL release induced by the initial bolus and subsequent infusion had been near maximal. Thus, TRH is able to induce significant PRL release in the presence of maximal DA receptor blockade, and domperidone, in the presence of maximal TRH stimulation, is also capable of inducing significant PRL release. These observations together with the ability of TRH/domperidone to induce a greater PRL response than either agent alone, suggest that each stimulus has a specific releasing action on a fraction of intracellular PRL which is not accessible to the other.(ABSTRACT TRUNCATED AT 400 WORDS)     

THE EFFECT OF VASOPRESSIN INFUSION ON GLUCOSE METABOLISM IN MAN

Studies on intact animals and isolated rat hepatocytes have shown that arginine vasopression (AVP) stimulates glycogen phosphorylase to break down glycogen and raise plasma glucose concentrations. Since no similar work has been performed on healthy human adults, the effect of moderate (25 pmol/min) and high (75 pmol/min) dose AVP infusion on plasma glucose, intermediary metabolites, glucose kinetics, and circulating glucagon and insulin concentrations was investigated. After AVP infusion, plasma glucose rose from 4.9 +/- 0.1 to a peak of 5.7 +/- 0.2 mmol/l (P less than 0.001), but no changes in blood lactate, pyruvate, alanine, glycerol or 3-hydroxybutyrate concentrations were observed. The glucose rise was accounted for entirely by an increase in the rate of appearance of glucose from 11.19 +/- 0.43 to 13.38 +/- 0.63 mu mol/kg/min (P less than 0.001). Infusion of AVP also increased plasma glucagon concentrations from 38 +/- 8 to 79 +/- 20 pg/l (P less than 0.01). The hyperglycaemic effect of AVP may be mediated solely by stimulation of glucagon release, but we cannot exclude direct stimulation of glycogen phosphorylase activity.     

THE EFFECT OF OXYTOCIN INFUSION ON ADENOHYPOPHYSEAL FUNCTION IN MAN

The responses of the adenohypophyseal hormones adrenocorticotrophin (ACTH), growth hormone (GH), thyroid stimulating hormone (TSH), prolactin, luteinizing hormone (LH) and follicle stimulating hormone (FSH) to sub-maximal doses of hypothalamic releasing factors were studied in six lean male volunteers (age 23-35 years) with and without infusions of oxytocin (OXT). OXT infusion (mean plasma concentration 133.6 +/- 2.6 pmol/l) completely inhibited the plasma ACTH responses to corticotrophin releasing hormone (CRH) (saline, peak increment ACTH 1.61 +/- 0.75 pmol/l; OXT, peak increment ACTH - 0.04 +/- 0.28 pmol/l; P less than 0.05). OXT infusion had no significant effect on the GH response to growth hormone releasing hormone (GHRH), the TSH and prolactin responses to thyrotrophin releasing hormone (thyroliberin, TRH) or the LH and FSH responses to gonadotrophin releasing hormone (luteoliberin, GnRH). The data support a role for OXT in the modulation of ACTH secretion in man.     

INFUSION OF GRADED CONCENTRATIONS OF SOMATOSTATIN IN MAN: PHARMACOKINETICS AND DIFFERENTIAL INHIBITORY EFFECTS ON PITUITARY AND ISLET HORMONES

The present study was undertaken to determine the plasma levels of somatostatin-like immunoreactivity (SLI) during constant infusion of graded concentrations of synthetic somatostatin-14 (S-14); to determine the half-life (t1/2) and metabolic clearance rate (MCR) of SLI; to correlate the plasma SLI levels with the degree of inhibition of pituitary and islet hormone secretion and to establish whether the plasma SLI levels capable of inhibiting pituitary and islet hormone secretion fall into the physiological range. Four normal subjects on separate occasions were each infused with saline or S-14 (25,50 and 75 micrograms/h) at a constant rate for 2 1/2 h. Thirty min following the infusions, TRH (200 micrograms) and arginine (0.5 g/kg) were given i.v. Blood samples were drawn every 15 min for measurement of GH, TSH, insulin, glucagon and SLI (by RIA of acid-ethanol extracted plasma) and at rapid intervals for 10 min after stopping the infusions for measurement of SLI disappearance. During S-14 infusions, plasma SLI rose rapidly, reached a plateau from 15-150 min and declined rapidly on cessation of the infusions with a mean t 1/2 of 2.72 +/- 0.45 min. Mean plateau SLI levels were: 149 +/- 3 pg/ml (25 micrograms/h), 465 +/- 35 pg/ml (50 micrograms/h), and 1244 +/- 71 pg/ml (75 micrograms/h). SLI was cleared rapidly but the MCR exhibited a dose-dependent decrease from 3225 +/- 699 ml/min for the 25 micrograms infusion to 1249 +/- 241 ml/min for the 75 micrograms/h infusion (P less than 0.05). The 25 micrograms/h infusion dose produced near-maximal suppression of GH secretion and inhibited insulin secretion but not TSH or glucagon secretion. The intermediate dose significantly inhibited GH, TSH, and insulin but not glucagon whereas the 75 micrograms/h infusion suppressed all four hormones. In six normal subjects endogenous plasma SLI rose from a basal value of 32.5 +/- 4.9 pg/ml to 75.5 +/- 9.0 pg/ml following ingestion of a mixed meal. This level was 50% of that resulting from the 25 micrograms/h infusion and which suppressed GH almost completely. We concluded that: Infused S-14 is cleared rapidly and decays with a short t 1/2; S-14 inhibits its own MCR; The somatotrophs are the most sensitive to S-14 inhibition, followed by the thyrotrophs and the B-cells (approximately equally) followed by the A-cells; Fluctuations in plasma SLI occurring physiologically may influence GH and possibly other S-14 sensitive cells by an endocrine mechanism.     

EFFECT OF FENFLURAMINE ON PROLACTIN SECRETION IN OBESE PATIENTS: EVIDENCE FOR SEROTONINERGIC REGULATION OF PROLACTIN IN MAN

The present investigation was carried out to study the effect of serotoninergic stimulation on prolactin (PRL) secretion in man. Fenfluramine (60 mg, orally), an anorexiant drug which under acute circumstances stimulates the serotoninergic system, was administered to eight obese patients. Compared with placebo, drug administration increased PRL significantly (P less than 0.05 at 180 and 300 min, P less than 0.01 at 240 min). No significant changes were observed after fenfluramine in blood pressure, plasma aldosterone (PA), plasma cortisol, plasma renin activity, serum electrolytes or growth hormone. Since it has been reported that dopaminergic blockade raises PA concentration, the lack of change in PA in obese patients treated with fenfluramine suggests that the observed increase in PRL induced by fenfluramine is likely to be mediated by serotoninergic stimulation.     

THE EFFECT OF PAIN ON PLASMA ARGININE VASOPRESSIN CONCENTRATIONS IN MAN

The effect of pain on plasma AVP concentration in man has previously been studied only during major surgery with general anaesthesia. Plasma AVP concentration (pAVP) and plasma osmolality (pOsm) were measured in thirty-six patients seen in a surgical emergency department complaining of pain and in fifty-one control subjects. No significant difference in pOsm was found, but pAVP was significantly higher in the emergency room patients in pain (M +/- SEM = 4.94 +/- 0.98 pmol/1 compared to 2.31 +/- 0.32 pmol/1 in control subjects, P less than 0.01). In the control subjects, age was found to have a low but significant inverse correlation with pAVP (r = 0.37, P less than 0.01). Chronic smoking was associated with significant elevation of pAVP (3.81 +/- 0.99 pmol/1 in smokers vs. 1.89 +/- 0.28 pmol/1 in non-smokers, P less than 0.02). Neither smoking nor age could account for the difference in pAVP between the pain and control groups. Thus, pain is a non-osmolar factor capable of elevating AVP in conscious man.     

THE DECREASED BASAL AND STIMULATED PROLACTIN LEVELS IN ISOLATED GONADOTROPHIN DEFICIENCY: A CONSEQUENCE OF THE LOW OESTROGEN STATE

Prolactin secretion has been evaluated in seven male and six female patients with isolated gonadotrophin deficiency (IGD). The subjects were challenged with the dopaminergic antagonist, metoclopramide (10 mg) and TRH (200 μg) before, during and after cessation of hormonal treatment. Five females received three consecutive 21-day courses of ethinyl oestradiol (0·1 mg daily) at monthly intervals and the remaining subject conjugated oestrogens (Premarin 0·625 mg daily) according to a similar protocol. Treatment of the males with hCG (pregnyl) 5000 iu twice weekly led to a rise in oestradiol and testosterone levels. Two males were receiving pergonal (human menopausal gonadotrophin) in addition. In the untreated state in both males and females, basal oestradiol and PRL levels were decreased as were the PRL responses to metoclopramide and TRH as compared with normal controls. During treatment in both groups, there was an increase in basal PRL levels as well as PRL response to the two stimuli, which became indistinguishable from the controls. Cessation of treatment was associated with a rapid decrease in basal PRL levels and PRL elevation following metoclopramide and TRH. In contrast to the effect of hCG, the administration of two non-aromatizable androgens (mesterolone and fluoxymesterone) had no effect on basal and TRH-induced PRL secretion. The administration of clomiphene citrate during hCG treatment in one male IGD patient produced a decrease in the basal and stimulated PRL response.
It is concluded that the low basal PRL levels and impaired PRL responses to stimulation are not an inherent component of the syndrome of IGD, but a consequence of the abnormal steroid milieu.     

PROLACTIN AND TSH RESPONSES TO BOTH DOMPERIDONE AND TRH IN NORMAL AND HYPERPROLACTINAEMIC WOMEN AFTER DOPAMINE SYNTHESIS BLOCKADE

A study of the effect of alpha-methyl-1-tryosine (metyrosine) blockade (2 g/d for 2 d) of dopamine (DA) synthesis on the PRL and TSH response to domperidone (DOM) and TRH in normal women and subjects with pathological hyperprolactinaemia is reported. In the normal subjects, there was a marked increase in basal PRL (51.7 +/- 11.1 vs 5.7 +/- 1.0 ng/ml) and the PRL and TSH responses to DOM were abolished. The PRL response to TRH was also reduced. In the hyperprolactinaemic subjects, metyrosine had no effect on basal PRL nor on the virtually non-existent PRL response to DOM, whereas it abolished the exaggerated TSH response. The conclusion is drawn that the response of both PRL and TSH to DA receptor blockers is really dependent upon DA inhibitory tone. A fall in this tone can also be postulated as responsible for the hyporesponsiveness of PRL to DOM frequently observed in pathological hyperprolactinaemia. In addition, the fact that metyrosine also abolished the exaggerated TSH response to DOM shows that the latter is totally dependent on enhanced DA inhibition of the thyrotrophs. Lastly, the reduced PRL response to TRH after metyrosine indicates that DA partly determines the ability of the lactotrophs to respond to TRH.     

DOPAMINE IS A PHYSIOLOGICAL REGULATOR OF THYROTROPHIN (TSH) SECRETION IN NORMAL MAN

Using a sensitive and precise radioimmunoassay for human TSH we have demonstrated significant elevations in serum TSH levels in euthyroid volunteers following administration of the dopamine receptor blocking drug metoclopramide when compared with placebo. The degree of TSH response is significantly greater in females than in males and is sustained over a 3-hour period after a single oral 10 mg dose of metoclopramide. The degree of TSH release after metoclopramide is inversely related to the basal TSH level suggesting that dopamine is a determinant of low daytime TSH levels and is thus implicated in the circadian rhythm of TSH secretion. Pretreatment with 10 mg of metoclopramide orally, one hour before TRH administration leads to significant enhancement of the TSH response to TRH. Our findings provide further evidence for the physiological inhibitory role of dopamine in the control of TSH secretion in normal man. The possible mode of action of dopamine and the clinical implications of this neuroregulatory pathway are discussed.     

EFFECT OF MORPHINE ON CORTISOL AND PROLACTIN SECRETION IN ANOREXIA NERVOSA AND DEPRESSION

Endogenous opioid peptides are involved in feeding regulation, and alterations in opioidergic regulation have been implicated in the pathophysiology of eating disorders. To investigate further this hypothesis, we conducted a placebo-controlled study of the effect of the opiate alkaloid morphine on cortisol and prolactin secretion in six patients with anorexia nervosa and six age-matched healthy volunteers, and compared the results with those obtained in nine depressed patients. Basal cortisol but not basal prolactin levels were elevated in patients with anorexia nervosa and patients with depression. Following the administration of morphine plasma concentrations of cortisol levels declined progressively and at a similar rate in all three groups. The prolactin response to morphine was attenuated significantly in patients with depression. Neither the cortisol and prolactin response to morphine in the anorectic patients nor the cortisol response in the depressed patients we observed in this study suggests altered opiate receptor sensitivity. However, the decreased prolactin response to morphine in depressed patients remains compatible with this hypothesis.     

BASAL AND TRH-STIMULATED PROLACTIN IN PATIENTS WITH PITUITARY TUMOURS

The diagnostic value of the assessment of prolactin (PRL) secretion in reaction to thyrotrophin releasing hormone (TRH) was investigated in twenty-four patients who had all radiological signs of a pituitary tumour and symptoms of hypogonadism and/or galactorrhoea. It was found that in these patients a PRL response to TRH can be present or absent, independent of the size of the tumour and/or the presence of suprasellar extension of the tumour. The measurement of several basal plasma PRL concentrations in combination with lateral tomography of the sella turcica seem to be a good screening for the existence of a prolactin producing pituitary adenoma and no important additional information is obtained by a TRH-test.     

THE EFFECT OF INCREASING DOSES OF INGESTED GLUCOSE ON INSULIN AND GASTRIC INHIBITORY POLYPEPTIDE (GIP) CONCENTRATIONS IN MAN

Gastric inhibitory polypeptide (GIP) is insulinotropic in vivo and in vitro . It is released following glucose ingestion and is a leading candidate as a mediator of the enteroinsular axis. To investigate the GIP response to increasing amounts of oral glucose, and its relationship to glucose levels and insulin secretion, fourteen normal volunteers ingested 25, 50 and 75 g of glucose at random with 5–7 days between each test. Serum insulin, glucose and GIP concentrations were measured and total integrated incremental responses were determined. Peak mean responses to glucose, insulin and GIP occurred at 30 min following each glucose ingestion. There were no significant differences in glucose concentrations at any interval with the varying glucose doses. Mean peripheral insulin concentrations were significantly increased ater 50 or 75 g of glucose as compared with the 25 g dose ( P <0.02). Mean GIP concentrations were significantly greater ( P <0.03) between 15 and 180 min with both the 50 and 75 g glucose stimulus. Total integrated areas under the response curves for glucose, insulin and GIP showed a graded increase in circulating insulin and GIP ( P <0.01) as the amount of ingested glucose was increased from 25 to 75 g. These findings show that increasing doses of glucose stimulated greater levels of GIP and insulin and further support the insulinotropic properties of endogenous GIP in man.     

EFFECT OF METOCLOPRAMIDE ON ALDOSTERONE AND REGULATORY FACTORS IN MAN

To determine the role of known secretagogues in the aldosterone response to dopamine blocking agents, plasma levels of ACTH, angiotensin II, potassium and plasma catecholamines were measured in five normal male subjects before and after intravenous injection of saline or 10 mg metoclopramide. There were no consistent changes in hormone levels after saline injection. After metoclopramide, plasma aldosterone increased three-fold to peak values at 20 min post-injection. A significant increase in aldosterone was observed within 10 min during which period there was no significant change in plasma ACTH, plasma renin activity AII or K. Plasma levels of cortisol, noradrenaline, adrenaline and dopamine showed no significant changes after injection of metoclopramide which induced a ten-fold rise in plasma prolactin. These results provide direct evidence that factors other than the plasma concentration of ACTH, AII and K--or fluctuations in plasma catecholamines--are likely to be responsible for the acute increase in plasma aldosterone after metoclopramide injection in normal man.     

DOPAMINE EFFECTS ON 11-DEOXYCORTICOSTERONE CONCENTRATIONS IN PATIENTS WITH CUSHING''S SYNDROME AND IN VITRO

The effect of dopamine (1 micrograms/kg/min by i.v. infusion) on corticosteroid levels was studied in five patients with cortisol excess caused by increased ACTH secretion. In four of the five subjects dopamine, when compared with placebo infusion, caused plasma concentrations of 11-deoxycorticosterone to rise without affecting levels of cortisol, corticosterone or aldosterone. In a parallel in-vitro study using incubations of bovine adrenal zona glomerulosa cells, dopamine (10(-9) mol/l-10(-5) mol/l) had no effect on basal or ACTH stimulated secretion of aldosterone, cortisol or 11-deoxycorticosterone. However, dopamine (10(-5) mol/l) partially inhibited angiotensin II (10(-7) mol/l) stimulated secretion of aldosterone (4.36 ng/10(6) cells/h +/- 0.28 vs 2.90 +/- 0.20, P less than 0.05) but did not alter 11-deoxycorticosterone secretion i the same incubations. these data suggest that the effect of dopamine on 11-deoxycorticosterone secretion in vivo may not be due to its direct action on the adrenal to inhibit corticosteroidogenesis. An alternative explanation is that dopamine may alter extra-adrenal synthesis of 11-deoxycorticosterone, possibly by increasing substrate availability to tissues capable of 21-hydroxylation reactions.     

THE EFFECT OF PERIPHERAL CATECHOLAMINE CONCENTRATIONS ON THE PITUITARY-ADRENAL RESPONSE TO CORTICOTROPHIN RELEASING FACTOR IN MAN

To evaluate the effect of changes in plasma catecholamines on the pituitary-adrenal response to ovine corticotrophin releasing factor (CRF) in normal man, the response to CRF alone (10 subjects) was compared responses after infusions of adrenaline (6 subjects), noradrenaline (6 subjects) and after oral administration of the alpha 2 agonist clonidine (10 subjects). Compared to control levels, plasma adrenaline and noradrenaline concentrations were increased three- and four-fold respectively by exogenous infusions, whereas plasma noradrenaline was significantly lowered by clonidine. Despite these changes in plasma catecholamine levels, the responses of plasma ACTH, cortisol and aldosterone to CRF did not differ significantly from control (CRF alone) in any of the three studies. Neither clonidine pretreatment nor catecholamine infusions altered basal levels of plasma ACTH, cortisol or aldosterone and no effect of CRF or catecholamine manipulations on plasma arginine vasopressin concentration was observed. These results show that modulation of peripheral plasma catecholamine levels within physiological limits does not affect CRF-stimulated release of ACTH or the adrenal response in normal man.     

EFFECT OF METOCLOPRAMIDE ON SERUM PROLACTIN LEVELS IN HUMANS

The effect of acute and chronic administration of metoclopramide on serum prolactin levels in normal subjects was studied. Metoclopramide 10 mg i.v. induced a prompt rise in serum prolactin levels in all subjects. At 180 min the levels remained high. Prolactin levels were markedly elevated during a 5 day course of treatment with metoclopramide in six subjects. It is suggested that metoclopramide could be used in the functional exploration of the hypothalamic-pituitary axis.