Jantarajit W, Thongon N, Pandaranandaka J, Teerapornpuntakit J, Krishnamra N, Charoenphandhu N. Prolactin-stimulated transepithelial calcium transport in duodenum and Caco-2 monolayer are mediated by the phosphoinositide 3-kinase pathway. Am J Physiol Endocrinol Metab 293: E372-E384, 2007. First published May 8, 2007; doi:10.1152/ajpendo.00142.2007 has been shown to stimulate intestinal calcium absorption but the mechanism was still unknown. This study aimed to investigate the mechanism and signaling pathway by which PRL enhanced calcium transport in the rat duodenum and Caco-2 monolayer. Both epithelia strongly expressed mRNAs and proteins of PRL receptors. Ussing chamber technique showed that the duodenal active calcium fluxes were increased by PRL in a dose-response manner with the maximal effective dose of 800 ng/ml. This response diminished after exposure to LY-294002, a phosphoinositide 3-kinase (PI3K) inhibitor. Caco-2 monolayer gave similar response to PRL with the maximal effective dose of 600 ng/ml. By nullifying the transepithelial potential difference, we showed that the voltage-dependent paracellular calcium transport did not contribute to the PRL-enhanced flux in Caco-2 monolayer. In contrast, the calcium gradient-dependent paracellular transport and calcium permeability were increased by PRL. Effects of PRL on Caco-2 monolayer were abolished by PI3K inhibitors (LY-294002 and wortmannin), but not by inhibitors of MEK (U-0126) or JAK2 (AG-490). To investigate whether the PRL-enhanced paracellular transport was linked to changes in the epithelial charge selectivity, the permeability ratio of sodium and chloride (P Na/PCl) was determined. We found that PRL elevated the P Na/PCl in both epithelia, and the effects were blocked by PI3K inhibitors. In conclusion, PRL directly and rapidly stimulated the active and passive calcium transport in the rat duodenum and Caco-2 monolayer via the nongenomic PI3K-signaling pathway. This PRL-enhanced paracellular calcium transport could have resulted from altered charge selectivity. charge selectivity; dilution potential; paracellular transport; prolactin receptor; tight junction; transcellular transport AS ONE OF THE CALCIUM-REGULATING HORMONES during pregnancy and lactation, prolactin (PRL) has been shown to stimulate intestinal calcium absorption (38), thereby protecting against development of negative calcium balance during these reproductive periods. Further investigations in nonmated female rats also revealed stimulatory actions of PRL on intestinal calcium absorption (11, 54), especially in the duodenum which was the most efficient site for calcium transport (21,32). Although the presence of PRL receptor (PRLR) proteins in duodenal enterocytes of rats was controversial, expression of rat PRLR (rPRLR) transcripts in the duodenal mucosa, demonstrated by in situ hybridization technique (45), implicated a direct action of PRL on the duodenal epithelial cells.Calcium traversed the duodenal epithelium by both active and passive pathways with the former being neglig...
Chronic metabolic acidosis results in a negative calcium balance as a result of bone resorption and renal calcium loss. However, reports on the changes in intestinal calcium transport have been controversial. The present investigation therefore aimed to study the effects of chronic metabolic acidosis induced by 1.5% NH(4)Cl administration on the three components of duodenal calcium transport, namely, solvent drag-induced, transcellular active, and passive paracellular components, in rats using an in vitro Ussing chamber technique. The relative mRNA expression of genes related to duodenal calcium transport was also determined. We found that 21-day chronic metabolic acidosis stimulated solvent drag-induced and transcellular active duodenal calcium transport but not passive paracellular calcium transport. Our results further demonstrated that an acute direct exposure to serosal acidic pH, in contrast, decreased solvent drag-induced calcium transport in a pH-dependent fashion but had no effect on transcellular active calcium transport. Neither the transepithelial resistance nor duodenal permeability to Na(+), Cl(-), and Ca(2+) via the passive paracellular pathway were altered by chronic metabolic acidosis, suggesting that widening of the tight junction and changes in the charge-selective property of the tight junction did not occur. Thus the enhanced duodenal calcium transport observed in chronic metabolic acidosis could have resulted from a long-term adaptation, possibly at the molecular level. RT-PCR study revealed that chronic metabolic acidosis significantly increased the relative mRNA expression of duodenal genes associated with solvent drag-induced transport, i.e., the beta(1)-subunit of Na(+)-K(+)-ATPase, zonula occludens-1, occludin, and claudin-3, and with transcellular active transport, i.e., transient receptor potential vanilloid family Ca(2+) channels 5 and 6 and plasma membrane Ca(2+)-ATPase isoform 1b. Total plasma calcium and free ionized calcium and magnesium concentrations were also increased, whereas serum parathyroid hormone and 1alpha,25-dihydroxyvitamin D(3) levels were not changed. The results indicated that 21-day chronic metabolic acidosis affected the calcium metabolism in rats partly through enhancing the mRNA expression of crucial duodenal genes involved in calcium absorption, thereby stimulating solvent drag-induced and transcellular active calcium transport in the duodenum.
Charoenphandhu N, Nakkrasae L, Kraidith K, Teerapornpuntakit J, Thongchote K, Thongon N, Krishnamra N. Two-step stimulation of intestinal Ca 2ϩ absorption during lactation by longterm prolactin exposure and suckling-induced prolactin surge.
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