Evolution of Lactogen Receptors in Selected Rabbit Tissues During Pregnancy

Abstract

The ontogeny of lactogen receptors in brain, adipose, liver, kidney, adrenal gland, mammary gland, ovarian and uterine tissues of pregnant rabbits was evaluated in this study using 125I bovine prolactin as tracer. Brain and adipose tissues were found to have very low receptor numbers throughout pregnancy (less than 20 fmol/mg of protein), while liver and kidney had higher but constant levels of receptor through the same period (200 and 100 fmol/mg of protein, respectively). Mammary gland and adrenal gland tissues exhibited sharp increases in prolactin binding between 15 and 17 days with both having peak receptor binding at 17 days of around 200 fmol/mg of protein. Ovarian and uterine receptor binding increased slowly after day five of pregnancy and reached peak levels of approximately 225 fmol/mg of protein at day 20. Scatchard analysis of the binding of protein in the tissues having increased binding during the course of pregnancy, revealed that its affinity for sites in these tissues was the same at 5 and 20 days of gestation, indicating the rise in binding to be a result of increased numbers of available receptors. Sub-organ localization studies found the binding of prolactin to adrenal gland, ovary and uterus to be essentially located in adrenal cortex, nonluteal ovary and endometrium. Incubation of membranes from each of the tissues showing significant change during pregnancy, from several time points of pregnancy, with 5.0 M MgCl2 produced little change in apparent receptor numbers; suggesting that receptor occupancy levels of endogenous prolactin was low.

Human Myometrial Smooth Muscle Cells are Novel Targets of Direct Regulation by Human Chorionic Gonadotropin1

“…To our knowledge, the presence of GH or its receptors has never been demonstrated in uterus. However, it is known that human myometrium synthesizes decidual-type prolactin [33,34]; prolactin can regulate a number of uterine functions including endometrial proliferation in animals [35,36], and prolactin receptors that can also bind human GH are present in animal uteri [36][37][38][39]. Therefore, the present findings are suggestive of a possible autocrine role for prolactin in human myometrial growth.…”

Modulation of the epithelial barrier by dexamethasone and prolactin in cultured Madin—Darby canine kidney (MDCK) cells

“…Receptors for glucocorticoids and PRL have been unequivocally demonstrated in renal cells (Barash et al, 1986;Grissom and Littleton, 1988;Sharma and Timiras, 1988). The kidney has been pointed out as potential target of these hormones.…”

“…The kidney has been pointed out as potential target of these hormones. Receptors for glucocorticoids and PRL have been unequivocally demonstrated in renal cells (Barash et al, 1986;Grissom and Littleton, 1988;Sharma and Timiras, 1988). Interestingly, it has reported that the kidney itself expresses the PRL gene, and the parietal epithelial cells of Bowman's capsule are the major sites of the de novo synthesis of PRL in the mouse kidney (Sakai et al, 1999).…”

Role of Ovarian Steroids in Development of Uterine Binding Sites for Prolactin in the Ferret1

“…A significant reduction in progesterone levels occurred by Day 35, at which time we also observed a significant reduction in the concentration of uterine PRL binding sites. Uteri of rabbits are also reported to have a higher concentration of unoccupied PRL receptors during pseudopregnancy than estrus [19], and peak concentrations of PRL receptors occur by Day 20 of pregnancy [20]. We hypothesized that the observed changes in PRL binding to ferret uterine membranes occurred in response to a shift in blood levels of progesterone, since plasma concentrations of estrogen and PRL exhibited little or no change throughout pseudopregnancy.…”

“…Many of these events may be crucial in preparing the uterus for blastocyst implantation and successful pregnancy. Specific binding sites for PRL have been demonstrated in uteri of humans [17], rabbits [18][19][20], sheep and cows [21,22], rats [23,24], pigs [25], and mink [26].…”