Effects of cholecalciferol on the translocation of calcium by non-everted ileum in vitro

An apparatus is described that allows perfusion of a non-everted segment of intestine in vitro and the study of the accumulation of substances within the mucosal cells. The translocation of Ca²⁺ by rachitic-chick ileum and the effect of pretreatment with cholecalciferol was investigated, with the following conclusions. (1) Entry of Ca²⁺ across the microvilli into mucosal cells is by diffusion; it does not require metabolic energy or the presence of any other inorganic ions. (2) Pretreatment of the chick with cholecalciferol causes increased permeability of the microvillus to Ca²⁺ in both directions (lumen to cell, cell to lumen). The increased transport brought about by cholecalciferol in vivo can be partially mimicked by sodium dodecyl sulphate added in vitro. (3) The sign and the magnitude of the electrical potential difference prevailing across the ileum does not influence Ca²⁺ transport. (4) Exit of Ca²⁺ from the mucosal cell is temperature-sensitive, requires metabolic energy and Na⁺. (5) Pretreatment with cholecalciferol caused increased movement of Ca²⁺ out of the cell across the basement membranes. This effect of cholecalciferol given in vivo could be markedly increased by the presence of dicyclohexylcarbodi-imide in the perfusion fluid. These observations suggested that cholecalciferol increased Ca²⁺ entry (and exit) at the mucosal surface and also caused Ca²⁺ to be more available to the pump at the serosal surface.