Intestinal Absorption of Sodium and Potassium in the Laying Fowl

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Abstract
The relationships between the Na+ and K+ concentrations in the intestinal contents and their net absorption were studied in laying hens fed diets containing either 0.15 or 0.07% sodium (high Na and low Na, respectively). Yttrium-91 served as a reference substance for measuring the intestinal cation absorption in vivo. In the contents of the various intestinal segments, 46 to 87% of the Na+ and 47 to 97% of the K+ were ultrafilterable. A large net secretion of cations, especially Na+, occurred in the upper gastrointestinal tract, regardless of diet. Na+ absorption occurred along the entire small intestine, whereas K+ was absorbed in the jejunum but secreted in the ileum. In the ileum, Na+ absorption and K+ secretion were greater in the low as compared to the high sodium birds. This resulted in a reciprocal relationship between the concentrations of the two cations in this segment. The relationship between the diffusional driving forces and net cation absorption at the absorption sites indicated that Na+ was always absorbed against, and K+ along its respective gradient. K+ was secreted against its gradient into the ileum. The implications of these results with regard to the in vivo mechanism of cation transport are discussed.