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 [ 125 I]oPRL of [ 125 I]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: (a) In pregnant rats, only 30% of liver receptors are unoccupied and infusion with 25 μg/h for 3 h of either oPRL of hGH decreased both free and total receptors by 22–30% while infusion with 250 μg/ml caused an additional decrease only in the free receptors, (b) In the mammary gland and the kidney of pregnant rats, all receptors seem to be unoccupied; infusion with 25 μg/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 μg/ml, however, resulted in a slight decrease in the mammary gland and a significant decrease in the kidney in both total and free receptors, (c) 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 μg/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, (d) 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, (e) In almost all cases, infusion with oPRL or hGH yielded similar results, (f) 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.