The absorption of calcium from the intestine.

From studies of loss of radiocalcium from the intestinal lumen of rats it was well known that the absorption of calcium occurs mainly in the proximal small intestine (Harrison & Harrison, 195 I ; Nicolaysen, 195 I) . A similar observation was made in the pig (Moore & Tyler, 1955). In the chick, according to Coates & Holdsworth (1961), however, calcium absorption appeared to occur over the whole length of the small intestine. The recent advances in the use of everted sacs provided a useful tool to study the absorption of calcium. Thus Schachter & Rosen (1959), Rasmussen (1959) and others could show that active transport of calcium against a concentration gradient can be achieved in everted sacs of the rat duodenum, but not of ileum, and the capacity of this transport mechanism decreased with distance from the pylorus. Anaerobic conditions, low temperature, dinitrophenol or other enzyme poisons effectively inhibited the active transport (Schachter & Rosen, 1959; Schachter, Dowdle & Schenker, 1960; Wassermann, 1960). Additions of glucose or fructose improved greatly the active absorption, while mannose, galactose, amino acids or some fatty acids had no marked effect (Schachter & Rosen, 1959). T h e age of the animals influenced the active transport in everted sacs; younger rats had a more efficient transport mechanism than older animals (Kimberg, Schachter & Schenker, 1961). These effects were more marked when a low-calcium diet was given (Kimberg et al. 1961). When rats were given a vitamin D-deficient diet the active transport in everted sacs was significantly diminished and could be restored by in vivo dosing with vitamin D (Schachter & Rosen, 1959; Dowdle, Schachter & Schenker, 1960; Kimberg et al. 1961). The response to vitamin D had a certain lag period of about 8-10 h. A biological assay based on the technique of everted sacs has been suggested by Schachter, Kimberg & Schenker (1961). Addition of vitamin D in vitro did not restore the impaired absorption, but recently Schachter, Kowarski & Finklestein (1964) reported that cholecalciferol placed directly into in vivo loops of rat duodenum in vitamin D-deficient animals increased markedly the subsequent ( 5 h later) uptake