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 animal^1,2. In brush border membranes (BBM) as well as in basolateral membranes (BLM) the presence of Ca²⁺-ATPase activity has been reported^3–5. 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 K_m 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.