Kinetic characteristics of calcium absorption and secretion by rat colon

Abstract

The kinetic characteristics of Ca active transport in rat descending colon were determined by measuring unidirectional transmural Ca fluxes in vitro. The absorptive flux from mucosa to serosa (Jm .fwdarw. s) was saturable, with a calculated affinity (Kt) of Ca for the transport system of 1.6 mM and a maximal transport capacity (Vmax) of 133 nmol.cntdot.cm-2.cntdot.h-1. The administration of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3) increased Jm .fwdarw. s by increasing Vmax to 236 nmol.cntdot.cm-2.cntdot.h-1 without changing Kt (1.8 mM). The secretory flux from serosa to mucosa (Js .fwdarw. m) was not saturable and was not increased by 1,25(OH)2D3. Mannitol, a marker of transepithelial extracellular flux, underwent net absorption in the absence of electrochemical gradients, and its Jm .fwdarw. s and Js .fwdarw. m were not altered by 1,25(OH)2D3 administration. Addition of 11 mM DD-glucose to the bathing medium consistently increased Ca Js .fwdarw. m and mannitol Jm .fwdarw. s and Js .fwdarw. m. Glucose reduced net Ca absorption except when Na was removed from the medium. Ca Js .fwdarw. m varied linearly with mannitol Js .fwdarw. m over the range of medium Ca from 0.125 to 5.0 mM. The behavior of Ca absorption by descending colon is compatible with a carrier-mediated, active-transport mechanism, whereas Ca secretion occurs by a nonsaturable process via a predominately paracellular pathway.