Activity, Concentration, and Lumen-Blood Electrochemical Potential Difference of Calcium in the Intestine of the Laying Hen

Abstract

The driving forces for the intestinal absorption of calcium, consisting of the chemical and electrical gradients between the intestinal lumen and circulation, were evaluated in the laying hen. Those forces were related to the in vivo calcium absorption. The concentration and activity of calcium in ultrafiltrates made from the intestinal contents, were found to be dependent on dietary calcium level and source. Of the calcium sources tested, calcium sulfate supported the highest concentration and activity of calcium in the various intestinal segments. Only part of the variation in those parameters could be explained on the basis of corresponding variations in intralumenal pH. Comparison of the fraction of active calcium (activity/concentration) in the ultrafiltrates, with the theoretical activity coefficient, suggested that most of the ultrafilterable calcium in blood plasma was ionic, but that a considerable portion of the ultrafilterable calcium in the intestine was in a complexed form. The transmural electrical potential (PD) in the laying fowl was similar to that of other animals; it was lowest in the lower jejunum and highest in the colon. The electrochemical potential difference (ECPD) of calcium was calculated from the PD and the activities of blood plasma and intestinal contents. Due to a lower activity of calcium and a high PD in the lower ileum and colon, the ECPD was unfavorable for absorption of calcium in those segments as compared with the duodenum and jejunum. This coincided with an absence of any net calcium absorption in those posterior segments. The differences in the ECPD, could only partially account for the difference among the segments in the in vivo calcium absorption. The possibility of absorption of complexed calcium is discussed.