Fate of milk calcium in the gastrointestinal tract of suckling rats

Abstract

In the rat, as in many other mammals, the majority of milk Ca is associated with casein micelles. Because luminal proteolysis is immature in the suckling rat, we investigated the hypothesis that milk Ca remains in the macromolecular form as it traverses the stomach and upper small intestine. When intestinal contents of rat pups were examined 1 h after gavage with 45Ca-milk, 98% of the 45Ca was found to be in a soluble form. Surprisingly, chromatography on Sephadex G-10 showed it to have a molecular weight in the range of that of Ca2+. Analysis of gastric contents showed the stomach to be the initial site of release of Ca from the casein micelles. Although only 45% of the 45Ca was in a soluble form in stomach contents, 100% of this soluble 45Ca behaved as Ca2+. Comparison with the fate of 32P from milk confirmed the suggestion that most of the Ca is released as Ca2+. In vitro studies suggested that this release is the result of simple dilution or the combined action of intrinsic milk enzymes and dilution. Because milk Ca appears to enter the small intestine as Ca2+, we presume its mechanism of absorption in suckling animals is the same as that already established using simple salts of Ca. Nevertheless, the slow release from the bound form may be important in establishing the physiological pattern of Ca absorption in the infant. Implications of this for the supplementation of milks for low-birth-weight human infants is discussed.