Developmental changes in the mechanisms of duodenal calcium transport in the rat

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Abstract
Duodenal Ca transport was resolved into a saturable and a nonsaturable process by means of an in situ ligated loop procedure applied to Wistar rats at 3, 12, 19, 24, 30, 40, 60, 110 and 150 days of age. All postweaning animals were males that had been placed on a 1.5% Ca, 1.5% P semisynthetic diet. Duodenal Ca-binding protein (CaBP) levels were determined at all ages. The newborn rat had no saturable transport component and no CaBP. Its nonsaturable component was very high. With increasing age the saturable component and CaBP varied biphasically, increasing steeply until the animals were .apprx. 35 days old; thereafter, each decreased to low but detectable values. The nonsaturable component, on the other hand, decreased in near-linear fashion in the 1st 35 days; in animals beyond that age it remained invariant. The difference in age dependence between the satuable and nonsaturable components may be considered to constitute additional evidence for the existence of the 2 transport processes. CaBP and the saturable transport process were highly correlated, further proof that both are vitamin D dependent. Histological studies have revealed the presence of many vacuoles in the intestinal cells of the very young rats; these vacuoles were absent in rats older than 35 days. These vacuoles may be implicated in a pinocytosis-like nonsaturable transport that is superimposed on the nonsaturable, non-vitamin D-dependent Ca transport found in all enterocytes.