Relation between the Lumen-Blood Electrochemical Potential Difference of Calcium, Calcium Absorption and Calcium-binding Protein in the Intestine of the Fowl

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Abstract
The mechanism of calcium absorption by the intestine in vivo was investigated in both chicks and laying hens. Calcium activity, measured in ultrafiltrates of intestinal contents and blood plasma, and the electrical potential (PD) between the intestinal lumen and circulation, were used to calculate the electrochemical potential difference of calcium (ECPD) between lumen and blood. The calcium level of the hens' diets was 3.94, 1.76, or 0.60%; that of the chicks' diets was 1.11, 0.71 or 0.31%. The same diets were also used to measure calcium absorption in laying hens with the aid of yttrium-91, and calcium retention in chicks by bone analysis. Included were also measurements of the calcium-binding protein (CaBP) in the duodenal mucosa. In hens, the ECPD in the calcium-absorbing intestinal segments, the duodenum and jejunum, decreased as dietary calcium decreased but remained always positive. Thus, absorption always proceeded in the direction of the driving forces (downward). The ECPD in the chick upper intestine was positive with the two higher calcium diets, but became negative in the entire intestine with the 0.31% calcium diet. Thus, absorption proceeded downward with the 1.11 and the 0.71% calcium diets, but upward (against the driving forces) with the 0.31% calcium diet. It was concluded that, under conditions of calcium restriction in chicks, active transport may assume an important role in the overall process of calcium absorption. The CaBP was higher in laying hens than in chicks; it was independent of dietary calcium in the hen for the range of intake studied, but in chicks it increased as dietary calcium was reduced below 0.71%. The possible association of the CaBP with the adaptation to low calcium intakes is discussed. A net inflow of calcium was observed in the ileum of the hens fed the 3.94% calcium diet but not with diets containing less calcium. Phosphate absorption decreased progressively with increased dietary calcium supplementation. The duodenum was identified as the major site of this antagonism.