Influence of infused prolactin on hormone binding to tissue slices

Short-term regulation of prolactin receptors in the liver, mammary gland and kidney of pregnant and lactating rats infused with ovine prolactin or human growth hormone

Short-term regulation of prolactin (PRL) receptors was studied in ketamine-anaesthetized 18-day pregnant or 7-day lactating female rats, by infusing them with various doses of oPRL or human growth hormone (hGH) for 0-3 h and measuring the binding of [125I]oPRL of [125I]hGH to the microsomal fractions prepared from the liver, mammary gland and kidneys of animals sacrificed at various states of infusion. Our main findings are: In pregnant rats, only 30% of liver receptors are unoccupied and infusion with 25 micrograms/h for 3 h of either oPRL of hGH decreased both free and total receptors by 22-30% while infusion with 250 micrograms/ml caused an additional decrease only in the free receptors. In the mammary gland and the kidney of pregnant rats, all receptors seem to be unoccupied; infusion with 25 micrograms/ml had none or a slight elevating effect on the number of both free and total receptors in the mammary gland but caused a significant 3-fold increase in the kidney; infusion with 250 micrograms/ml, however, resulted in a slight decrease in the mammary gland and a significant decrease in the kidney in both total and free receptors. In the liver of the lactating rats, there was no significant difference between the number of free and total receptors, but in mammary gland, specific binding to the total receptor was higher than to the free ones indicating partial occupancy; infusion with 25 micrograms/ml caused a significant decrease in free and total liver receptors without a remarkable change in the mammary gland and some decrease (by infusion with hGH only) in the kidney. In all cases, the changes in the specific binding resulted from the increase or decrease in receptor number and not from the change in receptor-hormone affinity. In almost all cases, infusion with oPRL or hGH yielded similar results. Infusion with both hormones did not affect the level of the endogenous rat prolactin. In conclusion, our results indicate the short-term regulation of PRL receptors by exogenous hormones is a complicated process which is affected by the level of the infused hormone, physiological state of the animal and may yield, simultaneously, different or even opposite changes in receptor number in various organs.

Down-regulation of prolactin receptors in the liver, mammary gland and kidney of female virgin rat, infused with ovine prolactin or human growth hormone

Down regulation of prolactin (PRL) receptors resulting from i.v. infusion of oPRL or human growth hormone (hGH) into female virgin rats was demonstrated. A decrease of over 85% in the number of free receptors was achieved within 15 - 30 min using infusion of oPRL or hGH at 25 micrograms/h and remained at this level until the end of infusion. Ovine growth hormone or recombinant bovine growth hormone at ten-fold higher concentration had no effect at all. The decrease in the specific binding resulted from a lower number of binding sites and not from change in the dissociation constants. The decrease in the total receptors in the liver was more gradual and leveled off at 40 - 50% of the initial value. Our results suggest that a change in blood PRL or hGH level may lead to a new steady state in the number, occupancy and distribution of prolactin receptors.

Prolactin receptors in primary cultures of carcinogen-induced rat-mammary tumors

7,12-Dimethylbenz[alpha]anthracene-induced rat mammary tumors were dissociated with collagenase and hyaluronidase and placed into primary culture. In most cultures, specific binding of 125I-labeled ovine prolactin was (i) lower than that for the original tumors unless bovine prolactin (1 microgram/ml) had been added to the dissociation medium, and (ii) varied with the type of growth medium used. The level of prolactin binding in cultured cells was relatively constant for the first 7-10 days. Prolactin binding in cultured cell homogenates was maximal at pH 7.0, proportional to cell protein, specific for prolactin, and reached a steady state by 12 h at 22 degrees C. The half-maximum inhibition of 125I-labeled prolactin binding by unlabeled prolactin was 100 ng/ml for cells grown in 5-1000 ng of prolactin/ml. After prolactin was removed from the growth medium, the level of available binding sites progressively increased, reached a maximum at 48 h and then declined. At 48 h, the dissociation constant for prolactin binding (Kd approximately 1 x 10(-10) M) was comparable to that in tumors. In some cultured tumors, a 48-h treatment with 0.5 or 1.0 ng of prolactin/ml caused an apparent increase in the level of prolactin binding. Prolactin increased DNA synthesis and its removal caused a reduction in [3H]estradiol and [3H]-R5020 binding to cultured cell cytosols.

Progesterone and oestrogen receptors in human breast cancer

Receptors for progesterone were found in 27% of 98 human breast tumours, and for oestrogen in 57% of 191 tumours. With one exception, progesterone receptors were found only in tumours which also contained oestrogen receptors. Levels of oestrogen receptor in positive tumours rose significantly with patient age whereas progesterone receptors were unchanged. Progesterone receptor levels were lower in lymph node metastases than in primary tumours, and oestrogen receptor levels were lower in large tumours (greater than 5 cm diameter) compared to small lesions. Receptor levels were not significantly correlated with circulating concentrations of either oestrogen or prolactin. The implications of receptor measurements in assessing hormone responsiveness of breast tumours are discussed.

Effect of the association time of in vivo bound prolactin on the [125I]prolactin receptor assays of female rat livers

Significantly (P less than 0.01) reduced 125I-labeled ovine prolactin binding (mean, --22%), as a result of an ether anesthesia-reduced rise in serum prolactin, was observed in plasma membrane preparations of liver samples of female rats taken after 5 min of etherization when compared to samples taken from the same animals during the first minute of etherization. This reduction in assayable receptors occurred after 1 h but not 2 h of assay incubation time. Significantly (P less than 0.05) reduced 125I-labeled ovine prolactin binding (mean, --16%) was also observed in liver samples exposed to a 30-min ether-induced rise in serum prolactin when compared to liver samples taken during the first minute of etherization. In contrast, this reduction was apparent at assay incubation times of 1, 2 and 4 h but not at 10 h. These results suggest that serum prolactin can bind to prolactin receptors in vivo and partially block subsequent 125I-labeled ovine prolactin receptor assay. In addition, these data provide evidence that a complex time-dependent binding of prolactin may occur in the plasma of the female rat liver.