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SUMMARY

The prolactin concentration in the dog pituitary gland was determined by isoelectric focusing of adenohypophysial extract in polyacrylamide gels followed by densitometry of the isolated stained hormone band. Dogs of both sexes and various ages (excluding newborn pups and weanlings) were studied. The bitches comprised animals at different stages of the oestrous cycle and also included a small number of pregnant, lactating or ovariectomized animals.

Low pituitary prolactin concentrations were found in males, sexually immature females and dioestrous females. Concentrations about 1·5 times as high occurred in oestrous, metoestrous (luteal) and ovariectomized females. Post-partum lactating bitches had the highest pituitary prolactin concentrations, these being double those occurring at dioestrus. With the exception of relatively high concentrations in ovariectomized bitches, these results are in good agreement with findings in the rat, mouse and rabbit. The persistence of high pituitary prolactin levels throughout metoestrus was believed to be associated with differences between the canine and murine reproductive cycle. Age did not influence pituitary prolactin levels in either males or females.

Semen samples were collected from 35 men and the levels of prolactin in semen and seminal plasma were measured. There was no significant difference in prolactin concentrations between the two fluids (t = 0·333, P > 0·7). There was also no correlation between the prolactin concentration and the kinematic viscosity of the semen (r = 0·065, P > 0·7).

ABSTRACT

There is evidence that prolactin (PRL) influences gastrointestinal function. However, the sites at which prolactin exerts these effects are not known. A monoclonal antibody was therefore generated against the rabbit mammary gland prolactin receptor (MAb 218) and used to study the distribution of the prolactin receptor in the rabbit gastrointestinal tract (GIT) by immunohistochemistry. MAb 218 is an IgG 1 κprecipitating antibody which precipitates major affinity cross-linked mammary gland prolactin receptor subunits of molecular masses 45 and 80 kDa. It has an affinity of 0·8 × 10⁹ mol/l for the prolactin receptor and does not react with GH or insulin receptors in precipitation assays. MAb 218 immunoreactivity was observed in classical prolactin target cells such as mammary gland epithelium, and this immunoreactivity was abolished by preincubation of MAb 218 with purified prolactin receptor but not by preincubation with purified GH receptor.

In the GIT, the most intense immunoreactivity was associated with the oesophageal epithelium, chief (zymogenic) cells of the fundic mucosa, pancreatic islets of Langerhans and surface epithelial cells of the duodenum and jejunum. Other specific elements of the GIT were immunoreactive at lower levels or were immunonegative. No immunoreactivity was observed in these locations with a control monoclonal antibody (MAb 50·8) of identical isotype to 218.

To support the immunohistochemical findings, rabbit gastric mucosal membranes were used to show the presence of lactogen-specific binding. Scatchard analysis of ¹²⁵I-labelled human GH binding to the gastric mucosal membranes with rat prolactin as displacing ligand yielded an affinity constant (K _a) of 1·0 ± 0·2 × 10⁹ mol/l with a capacity of 3·5 ± 0·4 fmol/mg protein. Affinity cross-linking and sodium dodecyl sulphate-polyacrylamide gel electrophoresis of the gastric receptor revealed lactogenic hormone-binding subunits of molecular masses 43, 68 and 83 kDa. The 68 kDa subunit was not seen in rabbit mammary gland or ovarian tissue, and may be unique to gastric mucosa.

In conclusion, we have demonstrated the presence of a high affinity lactogenic receptor in specific epithelial cell subpopulations of the GIT. This localization of the prolactin receptor in the GIT will assist in further functional assignment of prolactin to gastrointestinal physiology.

Journal of Endocrinology (1993) 139, 371–382

ABSTRACT

The interaction between prolactin and zinc was examined in vitro in the human prostate gland. The results indicated that prolactin did not modulate the acute uptake of zinc into benign prostatic hypertrophy tissue whereas zinc, in contrast, increased the uptake of prolactin into the prostate gland. Our study further showed that the augmented uptake of prolactin by zinc was partly due to an increase in the non-specific binding properties of the peptide hormone. We were also able to demonstrate that the specific binding of ¹²⁵I-labelled human prolactin to the receptor was reduced in the presence of zinc by a competitive mechanism.

J. Endocr. (1984) 102, 73–76

Radioimmunoassay techniques have revealed very high levels of prolactin in the range 1·2–10 μg/ml in samples of human amniotic fluid (AF) during the 10th-20th weeks of pregnancy and also a gradual increase in the level of prolactin in the maternal blood serum throughout pregnancy (Hwang, Guyda & Friesen, 1971; Friesen, Hwang, Guyda, Tolis, Tyson & Myers, 1972; Tyson, Hwang, Guyda & Friesen, 1972). Between the 10th and 20th week of gestation the ratio of AF prolactin: serum prolactin ranges from 22:1 to 227:1 (Friesen et al. 1972). As pregnancy advances the AF prolactin levels fall but always remain higher than the maternal serum prolactin values (Friesen et al. 1972; Tyson et al. 1972).

It was possible that human prolactin might well occur in AF in a form which was immunoreactive (and so detectable by radioimmunoassay) but which was biologically inactive. Therefore the lactogenic activity of human AF has been investigated

SUMMARY

Immunoreactive rat prolactin was detected in rat milk. Increasing volumes of milk decreased the percentage of ¹³¹I-labelled rat prolactin bound to antiserum in a manner parallel to the decreases produced by increasing log doses of standard hormone and blood serum. Rats suckling litters of 4, 8 or 12 pups were milked at various stages of lactation and prolactin concentrations (ng/ml) in milk varied significantly both with stage of lactation and with litter size. Mean prolactin levels were maximal on day 4 to day 15 and they decreased sharply near the end of lactation. At every stage of lactation litter size tended to influence prolactin concentrations with larger litters always being associated with more prolactin per millilitre of milk. Intraperitoneal injection of 1 mg bovine prolactin into lactating rats promptly caused the appearance of high levels of this foreign hormone in rat milk without significantly affecting the concentration of the native hormone.

Prolactin has been separated from growth hormone (GH) in extracts from sheep and cattle pituitaries, but not when human pituitaries were used. The prolactin activity found in most human growth hormone (HGH) preparations (Li & Liu, 1964; Forsyth, Folley & Chadwick, 1965) supports the hypothesis that HGH and human prolactin (H-Pr) are biologically and immunologically related and suggests that both activities reside in the same molecule (Irie & Barrett, 1962). However, Chen & Wilhelmi (1964) have reported that prolactin activity can be separated from that of GH by chromatography. Moreover, Pasteels, Brauman & Brauman (1963), using tissue cultures of human pituitary cells, found a rapid decrease of GH activity in the medium but an increase in prolactin activity. The present report describes the immunological properties of a human prolactin preparation (Apostolakis, 1965) more potent than any previous preparation.

The activities of batches XVI-R₈ and XVII-R₈ of human prolactin

SUMMARY

Many investigations of the regulation of prolactin synthesis and release are based on single plasma prolactin determinations. The purpose of the present experiment was to ascertain whether groups of rats (i.e. young or adult, male or female animals, being either intact, gonadectomized or gonadectomized and treated with oestrone), differing in age and/or endocrine status, will react to a single dose of perphenazine by an acute release of pituitary prolactin in proportion to their initial plasma prolactin levels.

No consistent relation existed between the classification of the twelve groups of rats into three categories of basal plasma prolactin levels (i.e. < 20, 25–50, > 125 ng/ml) and their response to perphenazine. Even though all groups showed a highly significant increase of plasma prolactin levels the magnitude of the maximum prolactin response at 30 min varied greatly within the groups of one category and thus was not related to the initial prolactin levels.

The effect of 14 days of oestrone treatment in increasing plasma prolactin levels in gonadectomized animals was greatest in young and adult male rats, less in young females and not significant in adult females. The results obtained after perphenazine treatment in the latter group made it clear that the effect of oestrogen treatment on prolactin release can be completely blocked by increasing synthesis and/or release of the prolactin-release inhibiting factor (PIF). Since perphenazine induces decrease of pituitary prolactin and a concomitant increase of plasma prolactin levels through lowered PIF-action, the positive effect of oestrogens on prolactin release (as observed in gonadectomized male and young female rats) apparently is caused by a different mode of action. The implications of these findings for the regulation of prolactin release, as affected by the endocrine status of the rat, is discussed.

Moreover, comparison of prolactin lost from the pituitary and gained in the circulation of the experimental animals, with amounts of prolactin that were observed to disappear from plasma during the experiment, provided suggestive evidence that the capacity to synthesize and/or eliminate prolactin, after a sudden provoked release of the hormone, differed among the groups.

The rates of synthesis by the pituitary, of release from the pituitary into the circulation as well as of elimination of the hormone from the circulation (equally involved in determining actual plasma levels) are thought, therefore, to be far more important for the elucidation of prolactin regulation than single plasma prolactin determinations.