Reliability of a single serum progesterone determination as an indicator of ovulation

Nine normal cycling subjects were monitored for mid-cycle LH surge and a subsequent rise in serum progesterone levels. Frequent samples were then taken for progesterone measurement during 8 h sessions in the early, mid and late luteal phases. These data showed: that progesterone secretion is pulsatile throughout the human luteal phase, with maximum frequency in the mid-luteal phase; that during the mid-luteal phase most subjects had progesterone levels both above and below currently accepted ovulatory thresholds; the use of a single measurement of progesterone in the mid-luteal phase is not always a reliable indicator of ovulation; a threshold greater than 20 nmol/l may yield an unacceptable number of false negative results.

Responses to prolactin secretagogues in oestrogen-treated rats suggest that the defect in prolactin regulation produced by oestrogen is at the level of the pituitary gland

Prolactin responses to pharmacological agents were used to characterize the defect in prolactin regulation which occurs after administration of high doses of oestrogen to rats. Animals with chronically implanted venous cannulae were injected with 2 mg oestradiol benzoate in oil and 2-3 days later prolactin concentrations were measured after injections of saline, thyrotrophin-releasing hormone (TRH), fenfluramine, apomorphine and butaclamol. The responses were compared with those in oil-injected animals. Hyperprolactinaemia in oestrogen-treated animals was unresponsive to apomorphine, but was even more sensitive to dopamine receptor blockade than controls. These results suggest that the lactotrophs in oestrogen-treated animals are already maximally suppressed by endogenous dopamine, though ineffectively. Although there was an increased prolactin response to TRH in oestrogen-treated animals, there was an impaired response to fenfluramine, indicating suppressed serotonergic prolactin-releasing factor mechanisms. Maximal endogenous dopaminergic activity and suppressed prolactin-releasing factor mechanisms are appropriate hypothalamic responses to hyperprolactinaemia. The operation of these responses in the earliest stages of the development of pituitary hyperplasia indicates that oestrogen induces a disturbance of prolactin regulation in the lactotroph, independent of hypothalamic control.

Synchronous secretion of luteinizing hormone and prolactin in the human luteal phase: neuroendocrine mechanisms

Studies of normal luteal phase women have shown that increases in serum LH and PRL are commonly synchronous. This study was designed to investigate the possible neuroendocrine mechanism(s) underlying this phenomenon. Six normal women were studied during the midluteal phase of 2 cycles. In the first cycle, they had blood samples collected at 15-min intervals for 6 h on 3 occasions during which time they received an infusion of normal saline or naloxone (1 mg/h) or a bolus of metoclopramide (10 mg, iv). In a second cycle, they received GnRH in increasing iv doses of 1, 10, and 50 micrograms at 2-h intervals. During the saline infusion, 11 of the 16 serum LH pulses (69%) were accompanied by an increase in serum PRL, and in 5 of the subjects, the first pulse of LH was synchronous with that of PRL (P = 0.0015). Naloxone increased the number of LH pulses from 16 to 20 and the number of PRL pulses from 12 to 16, all of which were synchronous with LH pulses. Administration of metoclopramide caused a substantial increase in PRL and a loss of further PRL pulsatility; however, LH pulsatility remained unaffected. Even after the smallest dose of GnRH (1 microgram), there was an increase in serum PRL [basal level, 11.8 +/- 2.1 (+/- SE) micrograms/liter; peak level, 16.5 +/- 3.3 micrograms/liter] as well as LH and FSH. The increase in serum PRL was, unlike the gonadotropin response, maximal after the 10-microgram dose of GnRH (peak level, 23.2 +/- 6 micrograms/liter) and did not increase further after the 50-micrograms dose (peak level, 18.5 +/- 2.4 micrograms/liter). These studies demonstrate that there is a PRL response to GnRH in the luteal phase and suggest that the observed synchrony in LH and PRL secretion at this time results from a physiological response of both the gonadotrope and the lactotrope to endogenous GnRH.

Influence of prolactin on serum androgens in normoprolactinaemic women

Previous studies in hyperprolactinaemic women have led to conflicting views about the influence of prolactin (PRL) on serum dehydroepiandrosterone sulphate (DS) levels. This study was designed to examine the effect of a reduction of normal levels of circulating prolactin on the serum DS concentration in twenty ovulating women treated with bromocriptine for 4 months in a double-blind crossover study. The women were recruited after initial studies of hypothalamic-pituitary, gonadal and adrenal function were assessed as being normal. Blood samples were collected in the early follicular and mid-luteal phases of both placebo and bromocriptine treated cycles, and were assayed for LH, FSH, PRL, DS, oestradiol (E2), testosterone (T), androstenedione (A) and sex hormone binding globulin (SHBG). The only significant changes were in serum prolactin which decreased from a mean level of 12.6 (s.e.m. = 0.38) to 9.7 (s.e.m. = 0.36) micrograms/l (P less than 0.0025), and DS which decreased from 4.9 (s.e.m. = 0.23) to 4.5 (s.e.m. = 0.22) mumol/l (P much less than 0.0005) after treatment with bromocriptine. Although the mechanism of this phenomenon is not clear, this study suggests that PRL may have a physiological role in modulating the tonic secretion or metabolism of DS.

The effects of ovariectomy and estrogen treatment on the dopamine inhibition of gonadotropin and prolactin release

The suppressive action of dopamine (DA) on circulating gonadotropin and PRL levels is found to be positively correlated with their basal serum concentrations. Thus, DA inhibition is greater in ovariectomized (agonadal) women than in normal women on day 2 of their menstrual cycles, and the reverse is observed for PRL. Pretreatment of agonadal subjects with estrogen lowers the basal levels of LH and FSH and thereby proportionately decreases the suppressive effects of DA. Estrogen treatment elevates the basal level of PRL in agonadal subjects and also the PRL-inhibiting effect of DA. These findings indicate that castration and estrogen treatment significantly influence the inhibitory effect of DA on gonadotropin and PRL release.

Effects of a dopamine antagonist on the release of gonadotropin and prolactin in normal women and women with hyperprolactinemic anovulation

Administration of a dopamine (DA) antagonist, metoclopramide (MCP) resulted in dose-related acute increments of circulating levels of LH and FSH in patients with hyperprolactinemic anovulation due to pituitary microadenoma but not in normal cycling women during the early follicular phase. Concomitant PRL responses to MCP in hyperprolactinemic patients were 1/10 those observed in the cycling women. These findings suggest a relative DA excess at the hypothalamic LRF neurons and a relative DA deficiency at the adenoma lactotroph of hyperprolactinemic patients as compared to cycling women.

Inhibition of gonadotropin and prolactin release by dopamine: effect of endogenous estradiol levels

The influence of endogenous estradiol (E2) levels on gonadotropin and PRL sensitivity to dopamine (DA) infusion (4 micrograms/kg/min) was assessed at different stages of the follicular phase of the menstrual cycle. Basal LH and FSH levels were comparable in day 2 and day 12 subjects, and despite a 4-fold increase in E2 concentration, the inhibition of LH by DA was small and quantitatively similar and there was no discernible effect on FSH in either group. In marked contrast, day 14 subjects with an elevated basal LH level exhibited a dramatic increase in the sensitivity of LH and FSH to DA inhibition. Further, a remarkable rebound release for LH but not FSH occurred on the termination of DA infusion. There was a significant correlation between basal LH and response to DA (r = 0.979). This unique increase in response to DA at a time when hypothalamic LRF secretion is assumed to be elevated suggests that DA may exert its effect by inhibiting LRF release. The inhibition of PRL release by DA is correlated with endogenous E2 levels (r equal 0.685) as well as basal PRL levels (r = 0.878). Rebound release of PRL occurs in all three groups of women on termination of the DA infusion, but the magnitude was greatest in Day 14 subjects with the highest endogenous E2 levels. These data suggest that while E2 seems to augment the sensitivity of PRL inhibition by DA, its does not seem to directly influence gonadotropin sensitivity to DA inhibition. The selective hypersensitivity of both LH and FSH to DA observed on the day before midcycle LH peak is consistent with a reduction in LRF neuronal inhibition by tuberoinfundibular DA neurons at this time.

Prolactin secretion by metoclopramide in man

Six men and nine women were given intravenous injections of 2.5 mg of metoclopramide to assess its potential as a stimulus to prolactin release. Following the administration of metoclopramide, there was prompt increase in serum prolactin to a peak response of 38.2 +/- 3.9 ng/ml in men and 103 +/- 10.2 ng/ml in women. The prolactin response to metoclopramide in men was compared with the response to 400 mug of TRH in 10 men. The peak response after TRH was 22.4 +/- 2.2 ng/ml, which was significantly less than that observed after metoclopramide. Pretreatment with 500 mg of L-dopa suppressed the prolactin response to metoclopramide in 6 men to a mean response of 16.3 +/- 4.3 ng/ml. We have concluded that metoclopramide is a safe, reliable, and potent stimulus of prolactin secretion and exerts this effect by blocking dopamine receptors in the hypothalamus and decreasing prolactin inhibiting factor. It is free of side effects and is a useful alternative to chlorpromazine.

Hyperprolactinaemia: its physiology and investigation

Since the establishment of a sensitive and specific radioimmunoassay for serum prolactin levels, very rapid progress has been made in the understanding of the physiology of prolactin and its role in human disease. Hyperprolactinaemia is now known to be a common finding in many conditions. Up to 20% of women with unexplained functional secondary amenorrhoea have an elevated serum prolactin level. Reduction of serum prolactin levels to normal, by removal of a prolactin-secreting tumour or by treatment with bromoergocryptine, results in a restoration of normal menstrual cycles and fertility. This paper outlines the control of prolactin secretion and discusses the mechanisms which result in secondary amenorrhoea. A new method of investigation of hyperprolactinaemia is proposed which localizes the site of abnormality which results in an elevation of serum prolactin levels.

A combined test of anterior pituitary reserve

Determination of the functional reserve of the anterior pituitary gland requires the serial measurement of hormones released from the pituitary by specific stimuli. Individually, insulin-induced hypoglycaemia, thyrotrophin releasing hormone (TRH) and gonadotrophin releasing hormone (GnRH) have proved to be reliable stimuli for the release of the various pituary hormones. The maximum incremental hormone response of growth hormone, cortisol, luteinising hormone, follicle stimulating hormone, thyrotrophin, triiodothyronine and prolactin has been assessed after the simultaneous administration of insulin, TRH and GnRH in six normal subjects. These responses have been compared to previously determined responses in a normal population when insulin, TRH or GnRH have been given separately. There were no significant differences in the peak response or the maximum incremental response of any hormone. The test has been used to assess the pituitary reserve of 54 patients with disorders of the hypothalamic-pituitary axis, and five patients have been assessed both before and after hypophysectomy. The test is found to be convenient, sensitive and reliable in the assessment of pituitary reserve.