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The effect of hyperprolactinemia induced by median eminence lesions (MEL) and ACTH and glucocorticoid replacement on prolactin (Prl) receptors was studied in the adrenal, isolated Langerhans islets and the liver. Adult rats were ovariectomized 15 days before MEL and they were divided in the following groups: 1) SHAM: injected with saline solution 3 times in alternate days; 2) MEL: saline solution; 3) MEL + ACTH: 50 micrograms: 10 IU/rat, s.c. (Synacthen) and 4) MEL + DEXA: 10 micrograms/rat (dexamethasone). For measuring total lactogenic binding sites an in vitro treatment of the membrane fraction with 4M MgCl2 was used. MEL originated a significant increase in Prl serum levels, which was not altered by injections of ACTH or dexamethasone. In contrast, serum corticosterone (B) levels in MEL rats were significantly lowered, and it was restored by ACTH. Unexpectedly, B levels increased when dexamethasone was administered to MEL rats. Prl receptors were diminished in the adrenal gland and Langerhans islets from MEL animals, as compared with the SHAM group. ACTH and glucocorticoid administration did not affect the pancreatic Prl receptors, while the adrenal gland exhibited a further lowering of Prl binding sites during ACTH treatment. Since no effect was found when dexamethasone was injected, a possible direct action of ACTH is suggested. On the other hand, Prl receptors were induced in the liver by MEL, and this action was abolished by dexamethasone and ACTH. Binding affinity in every tissue studied remained unchanged. Our data suggest that endogenous Prl is able to regulate its own receptors not only in the liver, but also in the adrenal gland and pancreatic islets.

The role of prolactin on some ovarian functions was studied in collagenase-dispersed luteal cells obtained from PMSG/hCG-primed rats. The in vitro effect of ovine prolactin (oPrl) on luteal cell function was assayed. This hormone produced a dose-dependent increase of progesterone production and an additive effect on hCG stimulation. oPrl had no effect on cAMP production. Chronic effects of prolactin were studied in sulpiride (S), bromocriptine (Br) and oPrl-treated rats. Serum levels of prolactin were significantly higher in S-treated animals whereas Br administration rendered undetectable values. Serum progesterone was reduced in Br-treated animals and LH levels were similar in all groups studied. In vitro studies demonstrated a marked reduction of hCG stimulation of progesterone and cAMP production by luteal cells from hypoprolactinemic animals, while a significant increase was observed in hyperprolactinemic states. oPrl and S treatment significantly increased ovarian LH binding sites while a reduction was observed in Br-treated rats. These data suggest that luteal cell function is regulated by circulating levels of prolactin and that this hormone has some direct effect on the steroidogenic process.

Membrane preparations of collagenase-dispersed Langerhans islets of female Wistar rats exhibit specific binding sites for 125I-labelled ovine prolactin (125I-oPrl). Almost negligible binding was detected in islets of male animals. The binding is a saturable and time-temperature dependent process, equilibrium being reached after 16 h incubation at 0 degrees C. The bound oPrl is not displaceable by hFSH, hLH, bGH or hGH. In contrast with other cell fractions, the 12,000 g pellet accounts for more than 80% of the specific binding of 125I-oPrl. Scatchard plots of data obtained in saturation studies indicate a single class of binding sites with Ka = 0.21 x 10(10)M-1. Protein and phospholipid moieties are essential for the receptor activity, since after trypsin or phospholipase C digestions marked loss of binding was verified. In islets of streptozotocin diabetic rats a marked reduction in the number of binding sites was observed. These findings may suggest that some of the actions of prolactin on endocrine pancreas could be explained by its specific interaction with islet cell membranes.

Specific receptors for iodine-labelled human prolactin ([125I]hPrl) are present in membrane preparations of the rat ventral prostate. The binding is saturable with an apparent association constant (Ka) of 2.2 X 10(9) M-1 and a binding capacity of about 1 pmol/100mg prostatic tissue. The binding of [125I]hPrl is inhibited by hPrl, ovine Prl (otprl) and human growth hormone, but not by ovine FSH or LH. Serum from rats having Prl-producing pituitary tumors caused a displacement of the [125I]hPrl from the receptors, and the displacement curve was parallel with that of the hPrl standard. Treatment of immature rats with varying doses of dihydrotestosterone propionate (10-5000 microng) causes a dose-dependent stimulation of Prl receptors calculated both as binding sites per mg of membrane protein and as binding sites per prostate. Androgen stimulation of prostatic Prl receptors increases the tissue sensitivity for circulating Prl and may be one reason for the known increases in endogenous cAMP levels in prostatic tissue after androgen treatment in vivo.

The role of prolactin (Prl) in the regulation of Prl binding to its specific binding sites was studied in the Langerhans islets, adrenal gland and liver of adult ovariectomized female rats. Animals were sc injected twice daily during 10 days with ovine Prl (1 mg/kg BW), sulpiride (30 mg/kg BW) and bromocriptine (3 mg/kg BW). At the end of the treatment period, the animals were killed and serum was collected for Prl assay. Total Prl binding sites were measured in the membrane fraction of tissue by desaturating the occupied membrane receptors in vitro with 4M MgCl2. Serum levels of Prl were significantly higher in sulpiride-treated animals, whereas bromocriptine administration rendered undetectable values. Prolactin and sulpiride treatment significantly reduced Prl binding to the adrenal gland and Langerhans islets, whereas it greatly increased Prl binding to the liver. On the other hand, bromocriptine increased Prl binding sites in the adrenal gland and Langerhans islets, but in the liver caused no apparent effect. The binding affinity (Ka) in each tissue remained unchanged under the different experimental conditions. In addition, the binding of Prl to pancreas islets membranes was lower in late pregnancy when compared with control rats. All of these data provide strong evidence in favor of a role for Prl in regulating the number of its own tissue binding sites.

The present study examined the effects of streptozotocin-induced diabetes on prolactin (Prl) secretion and its correlation with estrogen receptor levels in the anterior pituitary and hypothalamus. Prl was measured in adult ovariectomized rats and after estradiol treatment (10 micrograms estradiol benzoate (Eb) 48, 24 and 1 h before experiments) or acute TRH administration (4 micrograms/kg body weight). Substantial decreases in estradiol- and TRH-induced Prl release were observed in diabetic rats. Insulin therapy was able to restore this response. Measurement of nuclear estradiol receptors by exchange assay in the pituitary of Eb-treated rats revealed a significant reduction in receptor levels in the diabetic group and a restoration to normal values in insulin-treated diabetic rats. Similar results were obtained by measuring total pituitary receptor content (cytosolic plus nuclear receptors). No significant changes were observed in nuclear hypothalamic estradiol receptors. However, the number of total hypothalamic estradiol receptors was diminished in diabetic rats although the translocation was proportionally greater in these animals. These results indicate that the disrupted reproductive functions described in streptozotocin diabetic rats may be due, at least in part, to deficiencies in Prl secretion and pituitary estradiol action.

Specific receptors for [125I]hPrl (human prolactin) are present in membrane preparations of rat testis. The receptors are specific for lactogenic hormones (prolactin and human growth hormone) but do not bind gonadotropins. The prolactin receptors are localized exclusively in the interstitial cell tissue, and are not present in membrane preparations from isolated seminiferous tubules. The localization of prolactin receptors interstitial tissue suggests that the effect of prolactin on LH/hCG-stimulated testosterone production is due to a direct effect of prolactin of Leydig cells.

Rat adrenal prolactin receptors possess the same hormonal specificity as those in the prostate gland and liver, but are less stable during storage and after freezing. There is a gradual decrease in specific prolactin binding to the adrenal during sexual maturation in male rats; maximum binding capacity of 980 fmol/mg protein is at 25 days of age decreasing to approximately 100 fmol/mg protein at day 90. Prolactin receptors in the prostate are high at 25 days of age (700 fmol/mg protein), decrease sharply by day 30 (180 fmol/mg protein) and then gradually increase. Ovariectomy resulted in a significant rise in total prolactin binding in the adrenal gland, while the administration of oestradiol or testosterone reduced the binding, the reverse of changes in prolactin binding in the liver. Only oestrogen increased serum levels of prolactin in female rats. Ovine prolactin (500 micrograms) given to female rats resulted in a rapid increase over a period of 2-8 h total prolactin receptors in the adrenal, and these then decreased to normal levels, indicating a possible positive regulation of prolactin receptors by homologous hormone.

In prepubertal male rats, the injection of bromocriptine (Br) for 10 days caused an increase in adrenal weight (Br 0.75 mg/kg BW (Br I): 2.83%; Br 1.5 mg/kg BW (Br II): 12.1% and Br 3 mg/kg BW (Br III): 24.7%), and this effect was only significant at the highest dose. Sulpiride (S, 30 mg/kg BW/day) for 10 days produced a significant decrease in adrenal weight (18.6%), whereas ovine prolactin (oPRL) administered at doses of 0.5 or 5 mg/kg BW/day for 10 had no effect on this parameter. The action of these drugs on corticosterone serum levels was for Br III a 50.6% increase and for S a 29.2% decrease. Bromocriptine caused a significant increment of cytosolic available estrogen receptors C: 7.65 +/- 0.36 (SE); Br I: 10.2 +/- 0.36; Br II: 11.0 +/- 0.23 and Br III: 13.3 +/- 0.35) and total lactogenic receptors in the adrenal gland (C: 125.2 +/- 2.84; Br I: 203.8 +/- 4.43; Br II: 213.1 +/- 7.58 and Br III: 251.3 +/- 10.4), and this effect was dose-related. oPRL diminished adrenal estrogen receptors only at the highest dose used (C: 11.1 +/- 1.73; PRL 5: 8.2 +/- 0.75) as did S (C: 11.2 +/- 1.84 and S: 5.2 +/- 1.07); while the former originated a marked decrease in lactogenic adrenal binding sites at both doses (C: 198.7 +/- 12.2; PRL 0.5: 52.9 +/- 5.00 and PRL 5: 38.8 +/- 4.76), S also had a highly significant diminution over these receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

(125I)-ovine prolactin (oPRL) binding was found in several brain areas of the toad, Bufo arenarum Hensel. The olfactory bulb, cerebral hemispheres, and both dorsal and ventral mesencephalic regions showed saturable, high affinity, (125I)-oPRL binding, ranging between 5.6 to 29.9 fmol/mg protein, while the association constant (Ka) by Scatchard analysis was between 4.0 to 8.7 x 10(9) M-1. This binding was compared with the Scatchard plot of the kidney, which has been already described by other groups, and gave 41.7 fmol/mg protein and Ka 2.5 x 10(9) M-1. Liver showed no binding and in the cerebral hemispheres (125I)-oPRL was not displaced by non-lactogenic hormones, indicating that binding was hormone and tissue specific.