Fig. 2
Communication between different organs in lactation. Suckling stimulates the hypothalamus to secrete oxytocin and prolactin. Both suckling and prolactin inhibit the pulse center of gonadotropin-releasing hormone (GnRH), which suppresses luteinizing hormone (LH) and follicle-stimulating hormone (FSH). This decreases the levels of progesterone and estradiol. In the mammary glands, prolactin stimulates milk synthesis and secretion. Prolactin increases parathyroid hormone-related protein (PTHrP) secretion by mammary epithelial cells and its release in maternal circulation. The decrease of steroid hormones along with serotonin-induced PTHrP secretion by prolactin promotes bone resorption and liberates calcium in circulation. Circulating calcium in turn acts on the mammary gland calcium-sensing receptor to control PTHrP secretion. If the delivery of calcium to the gland decreases, then the resulting lower systemic calcium levels reduce receptor expression. This leads to increased PTHrP secretion by mammary epithelial cells and secretion into the maternal circulation to increase the supply of calcium from skeletal stores. PTHrP promotes renal calcium conservation. In lactating rodents, intestinal calcium absorption is increased. Continuous line arrows induced pathways; dotted lines inhibited pathways