Intestinal calcium transport in the chicken

The regulation of intestinal calcium transport is a subject of continuing controversy. Most investigations on this subject have involved use of the laboratory rat or layer-type chicken. The research reported in this thesis was conducted in 0- to 21-d-old rapidly growing broiler cockerels, mainly using in situ intestinal loop preparations. Intralumenal test solutions typically containing 75 mM each of 45CaCl 2 and 3H-mannitol were used to delineate the cellular and paracellular fractions of in situ calcium transport. At this concentration, about 75 to 85% of calcium transport from duodenal loops was shown to occur via the paracellular pathway. In contrast, calcium transport from distal ileal loops occurred entirely via the paracellular pathway and was not influenced by 1,25-(OH)2D3 administration. Further, mannitol disappearance from in situ intestinal loops was not affected by vitamin D-dependent mechanisms. In balance studies, calcium absorption in intact birds occurred primarily by a nonsaturable process when solubility was not a limiting factor, thereby supporting the in situ data that showed the predominance of paracellular calcium transport. Mannitol absorption and secretion were demonstrated to be nonsaturable suggesting both occur by paracellular mechanisms. The efficiency of paracellular transport tended to remain unchanged with age, in the duodenal, distal jejunal, and distal ileal loops of 0- to 14-d-old broiler chicks. The efficiency was 1.5 to 2 fold greater in the duodenal compared to the distal jejunal or distal ileal loops. The capacity of paracellular transport in these intestinal regions increased with age. It is concluded that when intralumenal calcium concentration is high and the solubility is not a limiting factor, intestinal calcium transport, both in in situ loops and in intact rapidly growing young broiler cockerels occurs largely by a paracellular process, which can take place in the absence of vitamin D.