title: Subcellular distribution of ATP-dependent calcium transport in rat duodenal epithelium.
abstract: Subcellular fractionation studies were performed to delineate plasma membrane and intracellular membrane populations which might be involved in intracellular Ca2+-homeostasis of rat small intestinal epithelial cells. After a low-speed supernatant fraction had been suspended in 5% sorbitol and subjected to equilibrium centrifugation in a zonal rotor, the Golgi and endoplasmic reticulum markers, galactosyltransferase and NADPH-cytochrome -c reductase, were concentrated in a density region designated Window II. The basal-lateral membrane marker (Na+-K+)-ATPase was concentrated in a higher-density region designated Window III. ATP-dependent Ca2+ transport was equally distributed between the two windows. Several membrane populations could be resolved from each window with good recovery of Ca2+-transport activity by a second density gradient centrifugation step. Second density gradient fractions were subjected to counter-current partitioning in an aqueous polymer two-phase system. Basal-lateral membranes, characterized by an 11-fold enrichment of (Na+-K+)-ATPase, contained ATP-dependent Ca2+-transport activity with Vmax = 3.7 nmol/mg per min and Km = 0.5 microM. A major Golgi-derived population exhibited Ca2+-transport activity with Vmax and Km values similar to those of the basal-lateral membranes. One membrane population, presumed to have been derived from the endoplasmic reticulum, contained Ca2+-transport activity with Vmax = 4 nmol/mg per min and Km = 0.5 microM. In addition to demonstrating that ATP-dependent Ca2+-transport activity has a complex distribution within enterocytes, this study raises the possibility that the basolateral plasma membranes might account for a relatively minor portion of the cell's Ca2+-pumping ability.
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism*; D002458:Cell Fractionation; D002462:Cell Membrane / Q000378:metabolism; D004386:Duodenum / Q000166:cytology / Q000378:metabolism*; D004848:Epithelium / Q000378:metabolism / Q000648:ultrastructure; D007408:Intestinal Absorption; D007425:Intracellular Membranes / Q000378:metabolism; D051381:Rats
chemical_list: D000255:Adenosine Triphosphate; D002118:Calcium