[Short-term effect of prolactin on its receptor]

It is known that prolactin (PRL) when injected chronically is able to increase the level of its own receptors. We have shown here that, after a single IV injection of PRL to lactating rabbits, total prolactin-receptor levels (measured after in vitro desaturation of injected prolactin by a MgCl2 treatment of the membranes) decreased from 41.6 to 18.4 p. 100 (specific binding) in 6 hrs. The same results were obtained in organ-culture of the rabbit mammary gland in the presence of prolactin; in addition, the down-regulation of prolactin receptors could be counteracted by lysosomotropic agents (chloroquine NH4Cl) in vitro. These results showed that prolactin could, at short term, induce a down regulation of its receptors, and that this effect might be related to an endocytosis of hormone-receptor complex and lysosome degradation.