Ca-stimulated ATPase in brush border and basolateral membranes of rat duodenum with high affinity sites for Ca ions

Abstract

THE small intestine plays an important part in the calcium homeostasis of the body, but the mechanism by which calcium is absorbed is poorly understood. In young rats, active calcium transport occurs primarily in the duodenum and is strongly dependent on the vitamin D status of the animal1,2. In brush border membranes (BBM) as well as in basolateral membranes (BLM) the presence of Ca2+-ATPase activity has been reported3–5. Moreover, a good correlation is found between Ca2+-ATPase activity and net calcium transport in different segments of rat small intestine6. These observations suggest a role for Ca2+-ATPase in calcium absorption. The electrochemical potential of Ca2+ in intestinal cells suggests that Ca2+ influx is passive, whereas extrusion of Ca2+ must be active6. Inherent in a role for Ca2+-ATPase in calcium transport is the requirement that Ca2+-ATPase be stimulated by concentrations occurring in the cytosol (≤10−5M). So far, Ca2+-ATPase activities in small intestine have been assayed in the presence of 2 to 40 mM Ca2+ (refs 3–5). We report here that Ca2+-ATPases in BBM and BLM of rat duodenum have Km values for Ca2+ activation of 1.1 and 0.5 µM respectively. Also Ca2+-ATPase activity below 5 µM Ca2+ is higher in BLM than in BBM fragments. This asymmetrical distribution of Ca2+-ATPase activity may provide a mechanism for net calcium transport from lumen to blood.