Transport of electrolytes and water in the upper jejunum of the fowl in vivo perfusion

Candidate genes of the transcellular and paracellular calcium absorption pathways in the small intestine of laying hens

To meet the high calcium (Ca) demand during eggshell biomineralization (2 g of Ca per egg), laying hens develop specific metabolic regulations to maintain Ca homeostasis. The intake of Ca, its solubilization, and absorption capacity are enhanced at sexual maturity (SM). A better knowledge of the intestinal Ca transporters involved in their variations at this stage could indicate new nutritional strategies to enhance Ca digestive utilization. Transcellular Ca absorption pathway and its major player calbindin-D 28 K (CALB1) mediate a saturable transport, which has been extensively described in this model. Conversely, a contribution by the paracellular pathway involving non-saturable Ca transport through intercellular tight junction has also been suggested. The aim of the present study was to identify candidate genes of these two pathways and their patterns of expression, in immature pullets (12, 15, and 17 wk old) and mature laying hens (23 wk old) in the duodenum, jejunum, and ileum. Using RT-qPCR, this study identifies 3 new candidate genes for transcellular, and 9 for paracellular Ca transport. A total of 5 candidates of the transcellular pathway, transient receptor potential cation channels subfamily C member 1 (TRPC1) and M member 7 (TRPM7); CALB1 and ATPase plasma membrane Ca2+ transporting 1 (ATP2B1) and ATPase plasma membrane Ca2+ transporting 2 (ATP2B2) were enhanced with age or after SM in the duodenum, the jejunum or all 3 segments. A total of 4 candidates of the paracellular pathway Claudin 2 (CLDN2) and tight junction proteins 1, 2, and 3 (TJP1, TJP2 and TJP3) increased in the small intestine after SM. Additionally, CALB1, ATP2B2, and CLDN2 were overexpressed in the duodenum or the jejunum or both segments after SM. The enhanced expression of candidate genes of the paracellular Ca pathway after SM, supports that the non-saturable transport could be a mechanism of great importance when high concentrations of soluble Ca are observed in the intestinal content during eggshell formation. Both pathways may work cooperatively in the duodenum and jejunum, the main sites of Ca absorption in laying hens.

Ontogenetic development of proline intestinal transport in the domestic fowl

1. The proline disappearance from the jejunal and ileal lumen of chickens aged from 1 day to 15 weeks were studied using a perfusion method "in vivo". 2. A decrease in proline transport was observed from younger to older animals. The 2 intestinal segments showed different behaviours, the 1-day and 1-week-old animals showed the same value in the jejunum, whereas in the ileum a progressive decrease in proline transport was observed from the first week of life. 3. The differences observed in the 2 segments could be attributed to the different rates of growth of the jejunum and ileum with age; the jejunum showed a peak of growth in the second week of life whereas the ileum showed a peak of growth in the first week. Proline transport in the jejunum decreased until the fifth week and remained constant thereafter.

Involvement of 1,25-dihydroxycholecalciferol in the short- and long-term increase of intestinal calcium absorption in laying hens: stimulation by gonadal hormones is partly independent of 1,25-dihydroxycholecalciferol

The short- and long-term effects of 1,25-dihydroxycholecalciferol (1,25(OH)2D3) and its interactions with sexual steroids on Ca absorption were studied in hens. A single injection of 375 ng 1,25(OH)2D3 did not increase intestinal calcium transport in D-repleted hens within 24 hr of administration. However, five daily injections of 1 alpha-(OH)D3(0.5 micrograms/kg), a low Ca intake, or oestrogen-testosterone (O.T) did increase calcium absorption in D-repleted immature pullets (measured by in vivo perfusion), mainly by increasing the diffusional component of Ca transport. O.T. further increased the stimulation by 1 alpha-(OH)D3 of calcium absorption and of duodenal CaBP concentration. At higher doses the stimulation by 1 alpha-(OH)D3 of calcium transport estimated by in situ ligated loop procedure was dose dependent and was not saturable at daily doses of less than 3 micrograms/kg. When Ca transport was stimulated by 2 micrograms/kg of 1 alpha-(OH)D3, an additional effect of O.T. was observed again without any increase in plasma levels of 1,25(OH)2D3. Both oestrogen and testosterone were necessary for this effect on Ca transport. This is evidence for an indirect effect on Ca transport, as both steroids are also necessary to induce medullary bone. It is concluded that O.T. increased intestinal absorption of calcium, first by stimulating 25(OH)D3 1 alpha-hydroxylase activity but also by another mechanism which is independent of 1,25(OH)2D3 metabolism.