Cadmium inhibits glucose uptake in primary cultures of mouse cortical tubule cells

Abstract

We studied the effect of cadmium (Cd2+) on transport of alpha-methylglucoside in primary cultures of mouse kidney cortical tubule cells grown in defined medium. When cultured cells were exposed to Cd2+ concentrations from 0 to 6 microM for 24 h, uptake of alpha-methylglucoside was inhibited in a dose-dependent manner by up to 50%. By contrast, acute exposure of the cells to 7 microM Cd2+ for 60 min did not inhibit alpha-methylglucoside uptake. Increasing Cd2+ concentrations progressively decreased the Vmax of Na(+)-dependent glucose cotransport but not the Km for glucose. Cell ATP/ADP ratios of unexposed monolayers and of cells exposed to 4.5 microM Cd2+ for 24 h were 5.0 and 4.9, respectively (n = 3). Intracellular volume, lactate dehydrogenase activity, and cell Na+ and K+ concentrations were unaltered even after 24 h of exposure to 7 microM Cd2+. Untreated and Cd2+-treated monolayers preloaded with alpha-methylglucoside released the sugar analogue into the medium at nearly identical rates, indicating that Cd2+ did not alter cell permeability to glucose. Uptake of the amino acid analogue alpha-(methylamino)isobutyric acid was not affected by prior Cd2+ exposure. Whereas cell DNA content declined in Cd2(+)-exposed plates, both Na(+)-glucose and Na(+)-amino acid cotransport were enhanced at lower cell densities. Protein and DNA synthesis, estimated, respectively, by incorporation of [3H]leucine and [3H]thymidine into acid-insoluble material, were not significantly affected at 6 microM Cd2+. We conclude that after a lag time Cd2+ selectively inhibits renal Na(+)-dependent glucose transport despite an unchanged gradient for Na+ across the cell membrane.

Renal type II Na/Pi-cotransporter is strongly impaired whereas the Na/sulphate-cotransporter and aquaporin 1 are unchanged in cadmium-treated rats

“…The Na+-dependent transports decreased in Vma x without changing the Km value for substrates, thus suggesting that the Cd treatment causes either a decreased number of active transporters or a decreased turnover rate of an unchanged number of transporters, or both, while the affinity for the substrate remains unaffected. Similar conclusions were drawn from the glucose transport data obtained in vitro, in primary cultures of mouse cortical tubule cells treated with Cd [7]. The Na+-independent transport systems in renal cortical basolateral membranes were not affected by Cd treatment [17].…”

“…Based on transport and kinetic experiments, previous studies have indicated that glucosuria, aminoaciduria and phosphaturia in Cd-intoxicated rats may result from reabsorptive defects due to a decreased Vmax of the specific carriers in the proximal tubule brushborder membrane [2,7,17,19,20]. This impaired transport may be due to: (1) a loss of carriers in the brush-border membrane [2,7,17,19,20], (2) a direct inhibiton of carriers by Cd, possibly by binding to functionally important sulphydryl groups in membrane proteins [2,5,17,20,30], (3) changes in membrane lipid composition similar to that in experimental ischaemia [22], or (4) direct interaction with membrane phospholipids [30], which may affect the binding capacity of membrane transporters.…”

“…This impaired transport may be due to: (1) a loss of carriers in the brush-border membrane [2,7,17,19,20], (2) a direct inhibiton of carriers by Cd, possibly by binding to functionally important sulphydryl groups in membrane proteins [2,5,17,20,30], (3) changes in membrane lipid composition similar to that in experimental ischaemia [22], or (4) direct interaction with membrane phospholipids [30], which may affect the binding capacity of membrane transporters. Not all these possibilities have been investigated in detail.…”

“…In isolated renal BBMV in vitro, the Cd ion directly induces an acute inhibition of glucose, amino acid, and P~ transport [5,17,20]. However, from the experiments in vivo [14], or in tissue culture [7], it is clear that if Cd is applied as CdC12 or Cd-metallothionein a lag period of 1-2 days for the nephrotoxic effect to occur is needed. The decreased Na+-dependent transports in the brush-border membrane cannot be explained by the initial decrease of the cellular Na+/K+-ATPase activity with a consequential loss of the transmembrane Na + gradient; after 4 days of Cd treatment the enzyme activity was still not different from controls [18].…”

Cadmium and transport of ions and substances across cell membranes and epithelia

“…in doses ranging from 2.5 to 7.5 lM resulted in a dose-dependent decrease of the V max of Na ? -glucose cotransport, without affecting the affinity of the transporters (Blumenthal et al 1990). Kinne et al demonstrated that the inhibition of L-glutamate transport in cortical BBMV isolated from male New Zealand white rabbits, preincubated for 30 min with 1 lM to 1 mM CdCl 2, could be attributed to a marked decrease in the V max of the transport system (half maximal inhibition at 50 lM CdCl 2 ), whereas the affinity for glutamate seemed to increase if it changed at all (Kinne et al 1995).…”