Abstract
The presence of an Na +/Ca 2+ exchange system in basolateral plasma membranes from rat small intestinal epithelium has been demonstrated by studying Na + gradient-dependent Ca 2+ uptake and the inhibition of ATP-dependent Ca 2+ accumulation by Na +. The presence of 75 mM Na + in the uptake solution reduces ATP-dependent Ca 2+ transport by 45%, despite the fact that Na + does not affect Ca 2+-ATPase activity. Preincubation of the membrane vesicles with ouabain or monensin reduces the Na + inhibition of ATP-dependent Ca 2+ 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 Ca 2+ for intravesicular Ca 2+ binding sites. In the presence of ouabain, the inhibition of ATP-dependent Ca 2+ 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 Ca 2+ in the absence of ATP can be enhanced by an Na + gradient (Na + inside > Na + outside). This Na + gradient-dependent Ca 2+ uptake is further stimulated by an inside positive membrane potential but abolished by monensin. The apparent affinity for Ca 2+ of this system is below 1 μM. In contrast to the ATP-dependent Ca 2+ transport, there is no significant difference in Na + gradient-dependent Ca 2+ uptake between basolateral vesicles from duodenum, midjejunum and terminal ileum. In duodenum the activity of ATP-driven Ca 2+ uptake is 5-times greater than the Na +/Ca 2+ exchange capacity but in the ileum both systems are of equal potency. Furthermore, the Na +/Ca 2+ 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 +/Ca 2+ exchange system while it doubles the ATP-driven Ca 2+ pump activity.
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Schedule a callMore From: Biochimica et Biophysica Acta (BBA) - Biomembranes
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