Two-step stimulation of intestinal Ca(2+) absorption during lactation by long-term prolactin exposure and suckling-induced prolactin surge.

During pregnancy and lactation, the enhanced intestinal Ca(2+) absorption serves to provide Ca(2+) for fetal development and lactogenesis; however, the responsible hormone and its mechanisms remain elusive. We elucidated herein that prolactin (PRL) markedly stimulated the transcellular and paracellular Ca(2+) transport in the duodenum of pregnant and lactating rats as well as in Caco-2 monolayer in a two-step manner. Specifically, a long-term exposure to PRL in pregnancy and lactation induced an adaptation in duodenal cells at genomic levels by upregulating the expression of genes related to transcellular transport, e.g., TRPV5/6 and calbindin-D(9k), and the paracellular transport, e.g., claudin-3, thereby raising Ca(2+) absorption rate to a new "baseline" (Step 1). During suckling, PRL surge further increased Ca(2+) absorption to a higher level (Step 2) in a nongenomic manner to match Ca(2+) loss in milk. PRL-enhanced apical Ca(2+) uptake was responsible for the increased transcellular transport, whereas PRL-enhanced paracellular transport required claudin-15, which regulated epithelial cation selectivity and paracellular Ca(2+) movement. Such nongenomic PRL actions were mediated by phosphoinositide 3-kinase, protein kinase C, and RhoA-associated coiled-coil-forming kinase pathways. In conclusion, two-step stimulation of intestinal Ca(2+) absorption resulted from long-term PRL exposure, which upregulated Ca(2+) transporter genes to elevate the transport baseline, and the suckling-induced transient PRL surge, which further increased Ca(2+) transport to the maximal capacity. The present findings also suggested that Ca(2+) supplementation at 15-30 min prior to breastfeeding may best benefit the lactating mother, since more Ca(2+) could be absorbed as a result of the suckling-induced PRL surge.

Prolactin directly enhances bone turnover by raising osteoblast-expressed receptor activator of nuclear factor kappaB ligand/osteoprotegerin ratio.

Hyperprolactinemia leads to high bone turnover as a result of enhanced bone formation and resorption. Although its osteopenic effect has long been explained as hyperprolactinemia-induced hypogonadism, identified prolactin (PRL) receptors in osteoblasts suggested a possible direct action of PRL on bone. In the present study, we found that hyperprolactinemia induced by anterior pituitary transplantation (AP), with or without ovariectomy (Ovx), had no detectable effect on bone mineral density and content measured by dual-energy X-ray absorptiometry (DXA). However, histomorphometric studies revealed increases in the osteoblast and osteoclast surfaces in the AP rats, but a decrease in the osteoblast surface in the AP+Ovx rats. The resorptive activity was predominant since bone volume and trabecular number were decreased, and the trabecular separation was increased in both groups. Estrogen supplement (E2) fully reversed the effect of estrogen depletion in the Ovx but not in the AP+Ovx rats. In contrast to the typical Ovx rats, bone formation and resorption became uncoupled in the AP+Ovx rats. Therefore, hyperprolactinemia was likely to have some estrogen-independent and/or direct actions on bone turnover. Osteoblast-expressed PRL receptor transcripts and proteins shown in the present study confirmed our hypothesis. Furthermore, we demonstrated that the osteoblast-like cells, MG-63, directly exposed to PRL exhibited lower expression of alkaline phosphatase and osteocalcin mRNA, and a decrease in alkaline phosphatase activity. The ratios of receptor activator of nuclear factor kappaB ligand (RANKL) and osteoprotegerin (OPG) proteins were increased, indicating an increase in the osteoclastic bone resorption. The present data thus demonstrated that hyperprolactinemia could act directly on bone to stimulate bone turnover, with more influence on bone resorption than formation. PRL enhanced bone resorption in part by increasing RANKL and decreasing OPG expressions by osteoblasts.

High physiological prolactin induced by pituitary transplantation decreases BMD and BMC in the femoral metaphysis, but not in the diaphysis of adult female rats.

High physiological prolactin (PRL) stimulated intestinal calcium absorption and renal calcium uptake in mammals. Previous histomorphometric study revealed a significant increase in bone turnover in the trabecular part of the PRL-exposed long (cortical) bone; however, whole-bone densitometric analysis was unable to demonstrate such effect. We therefore studied differential changes in bone mineral density (BMD) and contents (BMC) of the femoral diaphysis and metaphysis in adult female rats exposed to high PRL induced by anterior pituitary (AP) transplantation. The estrogen-dependent effects of PRL on the femur were also investigated. We found that chronic exposure to PRL had no effect on BMD or BMC of the femoral diaphysis, which represented the cortical part of the long bone. It is interesting that 7 weeks after an AP transplantation, BMD and BMC of the femoral metaphysis were significantly decreased by 8% and 14%, respectively. Ovariectomy (Ovx) for 2, 5, and 7 weeks also decreased BMD and BMC in the femoral metaphysis, but not in the diaphysis. However, the AP transplantation plus Ovx (AP+Ovx) produced no additive effects. Nevertheless, 2.5 microg/kg 17beta-estradiol (E2) supplementation abolished the osteopenic effects of both Ovx and AP+Ovx on the femur. As for the L5-6 vertebrae, BMD and BMC were not affected by PRL exposure, but were significantly decreased by Ovx and AP+Ovx, and such decreases were completely prevented by E2 supplementation. It could be concluded that high physiological PRL induced a significant osteopenia in the trabecular part, i.e., the metaphysis, of the femora of adult female rats in an estrogen-dependent manner. Since PRL had no detectable effect on the vertebrae, the effects of PRL on bone appeared to be site-specific.