Effect of Extracellular Calcium Depletion on Epithelial Structure and Function in Rabbit Ileum: A Model for Selective Crypt or Villus Epithelial Cell Damage and Suggestion of Secretion by Villus Epithelial Cells

Abstract

The epithelium of mammalian small intestine can be conveniently divided into two compartments: the crypt and the villus. These compartments are not only structurally distinct but also may be functionally distinct with regard to sodium and chloride ion transport. Villus epithelial cells are thought to be responsible for electrolyte absorption and crypt cells for secretion of ions and water. We show that exposure of rabbit ilea] mucosal sheets in vitro to bathing solutions depleted of calcium with 1 mM ethyleneglycol-bis (β-aminoethylether)-N,N'-tetraacetic acid on either the mucosal or the serosal surface produces selective damage of one epithelial compartment while leaving the other structurally intact. Thus these preparations represent a potential in vitro model for studying the relative contribution of crypt and villus epithelium to net transepithelial active ion transport. The effects of selective mucosal or serosal extracellular calcium depletion on epithelial structure were studied in detail using light and electron microscopy and freeze-fracture techniques. The functional significance of the structural abnormalities produced in these models was evaluated by measuring unidirectional sodium and chloride ion fluxes and serosal theophylline-dependent and mucosal glucose-dependent increases in short-circuit current. Serosal depletion of extracellular calcium produced diffuse damage to crypt epithelial cells, which consisted of vacuolization, detachment from the basement membrane, and dissolution of tight junctions, but which produced no detectable structural damage to villus epithelial cells. These structural alterations produced a decrease in basal shortcircuit current, an increase in transepithelial conductance and in net sodium and chloride ion absorption, no effect on the glucose-induced increase in short-circuit current and a significant decrease in the theophylline-induced short-circuit current response. In contrast, mucosal depletion of extracellular calcium resulted in vacuolization of a large portion of villus epithelial cells with concomitant detachment from the basement membrane and tight junction dissolution, but produced no structural defect in crypt epithelial cells. These alterations were associated with an increase in transepithelial conductance, no significant change in basal net sodium and chloride ion transport, and significant decreases in both glucose and theophylline-induced short-circuit current response. Furthermore, the decrease in theophylline-induced short-circuit current responses seen in tissues with selective villus damage but structurally intact crypts was at least as great as that seen with selective crypt damage. These data support the concept that crypt epithelium actively secretes and villus epithelium actively absorbs sodium and chloride ions. In addition, however, they suggest that a population of epithelial cells on the intestinal villi may also have the capacity to secrete electrolytes.