Early effects of uranyl nitrate on respiration and K+ transport in rabbit proximal tubule

Abstract

The mechanisms by which uranyl nitrate (UN) is toxic to the proximal tubule are incompletely understood. To define these further we studied potassium (K+) transport and oxygen consumption (QO2) in rabbit proximal tubule suspensions in vitro immediately after exposure to UN using extracellular O2- and K+-sensitive electrodes. UN caused a cumulative dose-dependent inhibition of proximal tubule QO2, with a threshold concentration of 5 x 10-5 M. Kinetic analysis suggested two patterns of cell injury: a higher affinity inhibition of QO2 with a K(i) of 5 x 10-4 M, and a lower affinity inhibition of QO2 with a K(i) of 10 mM. QO2 was studied in detail in the presence of these K(i) concentrations of UN to define the initial cellular events. The results indicated that different cellular processes displayed different sensitivities to UN. At submillimolar concentrations UN caused progressive selective inhibition of ouabain-insensitive QO2 (15% inhibition at 2 minutes). Ouabain-sensitive QO2 and nystatin-stimulated QO2 were not affected, suggesting that Na+,K+-ATPase activity and its coupling to mitochondrial ATP synthesis were intact. Direct measurement of proximal tubule net K+ flux confirmed that Na+,K+-ATPase activity was unchanged. Similarly, UN did not inhibit basal (state 4) or ADP-stimulated (state 3) mitochondrial QO2 in digitonin-permeabilized tubules, confirming that the mitochondria were intact. In contrast, higher concentrations of UN (≥ 1 mM) caused rapid inhibition of QO2 and net K+ efflux, due to inhibition of Na+,K+-ATPase activity and mitochondrial injury. None of these effects was attenuated by the sulfhydryl-reducing agents dithiothreitol or reduced glutathione, distinguishing UN from other nephrotoxic heavy metals such as HgCl2 and AgNO3.