Kinetic properties of Na+/Ca2+ exchange in basolateral plasma membranes of rat small intestine

The presence of an Na⁺/Ca²⁺ exchange system in basolateral plasma membranes from rat small intestinal epithelium has been demonstrated by studying Na⁺ gradient-dependent Ca²⁺ uptake and the inhibition of ATP-dependent Ca²⁺ accumulation by Na⁺. The presence of 75 mM Na⁺ in the uptake solution reduces ATP-dependent Ca²⁺ transport by 45%, despite the fact that Na⁺ does not affect Ca²⁺-ATPase activity. Preincubation of the membrane vesicles with ouabain or monensin reduces the Na⁺ inhibition of ATP-dependent Ca²⁺ uptake to 20%, apparently by preventing accumulation of Na⁺ in the vesicles realized by the Na⁺-pump. It was concluded that high intravesicular Na⁺ competes with Ca²⁺ for intravesicular Ca²⁺ binding sites. In the presence of ouabain, the inhibition of ATP-dependent Ca²⁺ transport shows a sigmoidal dependence on the Na⁺ concentration, suggesting cooperative interaction between counter transport of at least two sodium ions for one calcium ion. The apparent affinity for Na⁺ is between 15 and 20 mM. Uptake of Ca²⁺ in the absence of ATP can be enhanced by an Na⁺ gradient (Na⁺ inside > Na⁺ outside). This Na⁺ gradient-dependent Ca²⁺ uptake is further stimulated by an inside positive membrane potential but abolished by monensin. The apparent affinity for Ca²⁺ of this system is below 1 μM. In contrast to the ATP-dependent Ca²⁺ transport, there is no significant difference in Na⁺ gradient-dependent Ca²⁺ uptake between basolateral vesicles from duodenum, midjejunum and terminal ileum. In duodenum the activity of ATP-driven Ca²⁺ uptake is 5-times greater than the Na⁺/Ca²⁺ exchange capacity but in the ileum both systems are of equal potency. Furthermore, the Na⁺/Ca²⁺ exchange mechanism is not subject to regulation by 1α,25-dihydroxy vitamin D-3, since repletion of vitamin D-deficient rats with this seco-steroid hormone does not influence the Na⁺/Ca²⁺ exchange system while it doubles the ATP-driven Ca²⁺ pump activity.