title: Evidence against parallel operation of sodium/calcium antiport and ATP-driven calcium transport in plasma membrane vesicles from kidney tubule cells.
abstract: The present study aimed to clarify the existence of a Na+/Ca2+ antiport device in kidney tubular epithelial cells discussed in the literature to represent the predominant mechanistic device for Ca2+ reabsorption in the kidney. Inside-out oriented plasma membrane vesicles from tubular epithelial cells of guinea-pig kidney showed an ATP-driven Ca2+ transport machinery similar to that known to reside in the plasma membrane of numerous cell types. It was not affected by digitalis compounds which otherwise are well-documented inhibitors of Ca2+ reabsorption. The vesicle preparation contained high, digitalis-sensitive (Na+ + K+)-ATPase activities indicating its origin from the basolateral portion of plasma membrane. The operation of a Na+/Ca2+ antiport device was excluded by the findings that steep Ca2+ gradients formed by ATP-dependent Ca2+ accumulation in the vesicles were not discharged by extravesicular Na+, and did not drive 45Ca2+ uptake into the vesicles via a Ca2+-45Ca2+ exchange. The ATP-dependent Ca2+ uptake into the vesicles became increasingly depressed with time by extravesicular Na+. This was not due to an impairment of the Ca2+ pump itself, but caused by Na+/Ca2+ competition for binding sites on the intravesicular membrane surface shown to be important for high Ca2+ accumulation in the vesicles. Earlier observations on Na+-induced release of Ca2+ from vesicles pre-equilibrated with Ca2+, seemingly favoring the existence of a Na+/Ca2+ antiporter in the basolateral plasma membrane, were likewise explained by the occurrence of Na+/Ca2+ competition for binding sites. The weight of our findings disfavors the transcellular pathway of Ca2+ reabsorption through tubule epithelium essentially depending on the operation of a Na+/Ca2+ antiport device.
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D006168:Guinea Pigs; D007684:Kidney Tubules / Q000166:cytology*; D007700:Kinetics; D008433:Mathematics; D012964:Sodium / Q000494:pharmacology; D019831:Sodium-Calcium Exchanger; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism
chemical_list: D002352:Carrier Proteins; D019831:Sodium-Calcium Exchanger; D000255:Adenosine Triphosphate; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D002118:Calcium