Intestinal Calcium Transport and Calcium Extrusion Processes at the Basolateral Membrane ,

Abstract

The intestinal absorption of calcium has been proposed to occur by the transcellular transfer of Ca²⁺ through the enterocyte proper and between the cells of the intestinal epithelium, i.e., the paracellular path. Attention in this report is given to the transcellular models of Ca²⁺ absorption and, more specifically, the Ca²⁺ extrusion events occurring at the basolateral membrane. These extrusion processes include the operation of an ATP-dependent Ca²⁺ pump and a Na⁺/Ca²⁺ exchanger, as well as exocytosis as the terminal event in a proposed vesicular transport mechanism. Evidence for the presence of an ATP-dependent Ca²⁺ pump at the basolateral membrane is documented and illustrated with biochemical and immunological data from studies on the avian intestinal basolateral membrane. As shown immunohistochemically, the Ca²⁺ pump was primarily localized on the enterocyte basolateral membrane. The ATP-dependency and vitamin D enhancement of Ca²⁺ uptake by isolated basolateral membrane vesicles are shown. Western blot analysis of intestinal mucosa, by using a monoclonal antibody produced against the erythrocyte Ca²⁺ pump, indicated that the number of pump units is increased by 1,25-dihydroxycholecalciferol. The possible involvement of calbindin-D_28K as a direct stimulator of the Ca²⁺ pump is discussed, and the quantitative relationship between Ca²⁺ transport rates and Ca²⁺ pumping activity has been estimated. Information related to the basolateral membrane Na⁺/Ca²⁺ exchanger and the vesicular transport model of Ca²⁺ absorption is also briefly reviewed.