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 + /Ca 2+ antiport device in kidney tubular epithelial cells discussed in the literature to represent the predominant mechanistic device for Ca 2+ reabsorption in the kidney. (1) Inside-out oriented plasma membrane vesicles from tubular epithelial cells of guinea-pig kidney showed an ATP-driven Ca 2+ 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 Ca 2+ reabsorption. (2) The vesicle preparation contained high, digitalis-sensitive (Na + +K + -ATPase activities indicating its origin from the basolateral portion of plasma membrane. (3) The operation of Na + /Ca 2+ antiport device was excluded by the findings that steep Ca 2+ gradients formed by ATP-dependent Ca 2+ accumulation in the vesicles were not discharged by extravesicular Na + , and did not drive 45 Ca 2+ uptake into the vesicles via a Ca 2+ - 45 Ca 2+ exchange. (4) The ATP-dependent Ca 2+ uptake into the vesicles became increasingly depressed with time by extravesicular Na + . This was not due to an impairment of the Ca 2+ pump itself, but caused by Na + /Ca 2+ competition for binding sites on the intravesicular membrane surface shown to be important for high Ca 2+ accumulation in the vesicles. (5) Earlier observations on Na + -induced release of Ca 2+ from vesicles pre-equilibrated with Ca 2+ , seemingly favoring the existence of a Na + /Ca 2+ antiporter in the basolateral plasma membrane, were likewise explained by the occurrence of Na + /Ca 2+ competition for binding sites. The weight of our findings disfavors the transcellular pathway of Ca 2+ reabsorption through tubule epithelium essentially depending on the operation of a Na + /Ca 2+ antiport device.