Silver ion (Ag+)-induced increases in cell membrane K+ and Na+ permeability in the renal proximal tubule: reversal by thiol reagents

Abstract

The initial mechanisms of injury to the proximal tubule following exposure to nephrotoxic heavy metals are not well established. We studied the immediate effects of silver (Ag+) on K+ transport and respiration with extracellular K+ and O2 electrodes in suspensions of renal cortical tubules. Addition of silver nitrate (AgNO3) to tubules suspended in bicarbonate Ringer's solution caused a rapid, dose-dependent net K+ efflux (Km = 10(-4) M, Vmax = 379 nmol K+/min/mg protein) which was not inhibited by furosemide, barium chloride, quinine, tetraethylammonium, or tolbutamide. An increase in the ouabain-sensitive oxygen consumption rate (QO2) (13.9 +/- 1.1 to 25.7 +/- 4.4 nmol O2/min/mg, P less than 0.001), was observed 19 sec after the K+ efflux induced by AgNO3 (10(-4) M), suggesting a delayed increase in Na+ entry into the cell. Ouabain-insensitive QO2, nystatin-stimulated QO2, and CCCP-uncoupled QO2 were not significantly affected, indicating preserved function of the Na+,K+-ATPase and mitochondria. External addition of the thiol reagents dithiothreitol (1 mM) and reduced glutathione (1 mM) prevented and/or immediately reversed the effects on K+ transport and QO2. We conclude that Ag+ causes early changes in the permeability of the cell membrane to K+ and then to Na+ at concentrations that do not limit Na+,K+-ATPase activity or mitochondrial function. These alterations are likely the result of a reversible interaction of Ag+ with sulfhydryl groups of cell membrane proteins and may represent initial cytotoxic effects common to other sulfhydryl-reactive heavy metals on the proximal tubule.