ATP in equilibrium with 32Pi exchange catalyzed by plasma membrane Ca(2+)-ATPase from kidney proximal tubules.

The Ca(2+)-stimulated adenosine 5'-triphosphate-orthophosphate (ATP in equilibrium with 32Pi) exchange reaction was studied using a vesicular preparation derived from plasma membrane of kidney proximal tubules. With native inside-out vesicles, ATP in equilibrium with 32Pi was stimulated by micromolar Ca2+ concentrations. Treatment of the vesicles with the Ca2+ ionophore A23187 that abolished Ca2+ accumulation, strongly inhibited ATP in equilibrium with 32Pi. When Ca(2+)-ATPase was solubilized with the nonionic detergent octaethylene glycol mono n-dodecyl ether, maximal activation of ATP in equilibrium with 32Pi required millimolar Ca2+ concentrations. These Ca2+ concentrations inhibited ATP hydrolysis. ATP in equilibrium with 32Pi exhibited a Michaelian dependence on Pi and Mg2+, was stimulated by ATP, and depended on the ATP/ADP ratio. ATP in equilibrium with 32Pi was modified by the osmolytes urea, trimethylamine-N-oxide, and sucrose, which are representative of the methylamines and polyols that normally accumulate in renal tissue. These compounds did not modify the apparent affinity for Pi; they affected the response to ADP in the same fashion as the overall rate of ATP in equilibrium 32Pi, and their effects depended on medium pH. These data show that the Ca(2+)-ATPase from plasma membrane kidney proximal tubules can operate simultaneously in forward and backward directions. They also show that ATP in equilibrium with 32Pi is modulated by the ligands Ca2+, ATP, ADP, Pi, Mg2+, and H+, and by organic solutes found in renal tissue.