Inhibition of Na+-Glucose Cotransport in Kidney Cortical Cells by Cadmium and Copper: Protection by Zinc

Abstract

Properties of the inhibition of Na+-glucose cotransport by Cd2+ in mouse kidney cortical cells have been determined. In no case was any inhibition observed before 3 hr. The extent of inhibition was dependent upon both the concentration of Cd2+ and the length of exposure. Kinetic studies showed that metallothionein mRNA induction by Cd2+ was initiated within 1 hr after incubation with Cd2+ began and peaked by 3-6 hr. Metallothionein protein increased more slowly, beginning at 3 hr and continuing for at least 9 hr. The protein had both Cd2+ and Zn2+ bound to it throughout this period. Nevertheless, a pool of nonmetallothionein Cd2+ appeared after 3 hr, coinciding with the onset of inhibition of Na+-glucose cotransport, and increased over the next 9 hr. Pretreatment of cells with Zn2+ protected them from the effects of Cd2+ on Na+-glucose cotransport. It delayed the onset of inhibition of transport as well as the extent of inhibition. Detailed analysis of the distribution of Cd2+ and Zn2+ in the soluble fraction of these cells showed that the concentration of non-metallothionein bound Cd2+ was not suppressed by the presence of Zn-metallothionein after the onset of exposure to Cd2+ Incubation of cells with larger concentration of Zn2+ and Cu2+ also inhibited Na+-glucose cotransport.