Lack of luminal or basolateral uptake and transepithelial transport of mercury in isolated perfused proximal tubules exposed to mercury-metallothionein

The aging kidney and the nephrotoxic effects of mercury

Owing to advances in modern medicine, life expectancies are lengthening and leading to an increase in the population of older individuals. The aging process leads to significant alterations in many organ systems, with the kidney being particularly susceptible to age-related changes. Within the kidney, aging leads to ultrastructural changes such as glomerular and tubular hypertrophy, glomerulosclerosis, and tubulointerstitial fibrosis, which may compromise renal plasma flow (RPF) and glomerular filtration rate (GFR). These alterations may reduce the functional reserve of the kidneys, making them more susceptible to pathological events when challenged or stressed, such as following exposure to nephrotoxicants. An important and prevalent environmental toxicant that induces nephrotoxic effects is mercury (Hg). Since exposure of normal kidneys to mercuric ions might induce glomerular and tubular injury, aged kidneys, which may not be functioning at full capacity, may be more sensitive to the effects of Hg than normal kidneys. Age-related renal changes and the effects of Hg in the kidney have been characterized separately. However, little is known regarding the influence of nephrotoxicants, such as Hg, on aged kidneys. The purpose of this review was to summarize known findings related to exposure of aged and diseased kidneys to the environmentally relevant nephrotoxicant Hg.

Molecular handling of cadmium in transporting epithelia

Cadmium (Cd) is an industrial and environmental pollutant that affects adversely a number of organs in humans and other mammals, including the kidneys, liver, lungs, pancreas, testis, and placenta. The liver and kidneys, which are the primary organs involved in the elimination of systemic Cd, are especially sensitive to the toxic effects of Cd. Because Cd ions possess a high affinity for sulfhydryl groups and thiolate anions, the cellular and molecular mechanisms involved in the handling and toxicity of Cd in target organs can be defined largely by the molecular interactions that occur between Cd ions and various sulfhydryl-containing molecules that are present in both the intracellular and extracellular compartments. A great deal of scientific data have been collected over the years to better define the toxic effects of Cd in the primary target organs. Notwithstanding all of the new developments made and information gathered, it is surprising that very little is known about the cellular and molecular mechanisms involved in the uptake, retention, and elimination of Cd in target epithelial cells. Therefore, the primary purpose of this review is to summarize and put into perspective some of the more salient current findings, assertions, and hypotheses pertaining to the transport and handling of Cd in the epithelial cells of target organs. Particular attention has been placed on the molecular mechanisms involved in the absorption, retention, and secretion of Cd in small intestinal enterocytes, hepatocytes, and tubular epithelial cells lining both proximal and distal portions of the nephron. The purpose of this review is not only to provide a summary of published findings but also to provide speculations and testable hypotheses based on contemporary findings made in other areas of research, with the hope that they may promote and serve as the impetus for future investigations designed to define more precisely the cellular mechanisms involved in the transport and handling of Cd within the body.

pH-dependent transport of cadmium in rat renal brush border membrane vesicles: cadmium efflux via H+-antiport

The effects of pH on the Cd efflux across the renal brush border membrane were investigated using isolated rat renal brush border membrane vesicles (BBMV). BBMV equilibrated at pH 5.5 or 7.5 (pHin) were mixed with an experimental buffer at pH 5.5 or 7.5 (pHout) containing CdCl2. After the incubation with CdCl2, BBMV were washed with a chelating agent, EGTA, to remove Cd bound to the outer surface of BBMV. The initial accumulation of Cd in BBMV incubated for 1, 3 or 5 min at pHin = 5.5 and pHout = 7.5 (outwardly directed H+-gradient) was significantly higher than that at pHin = pHout = 7.5, but the equilibrated Cd accumulation incubated for over 15 min was almost the same. FCCP, a protonophore, diminished the increasing effect of the H+-gradient on the initial Cd accumulation, whereas valinomycine, a K+ ionophore, did not. Kinetic analysis of the Cd accumulation incubated with different concentrations of CdCl2 suggests that Cd is taken up into BBMV via the unsaturable component and saturable component and the outwardly directed H+-gradient increases the Vmax value of the saturable component. The BBMV were incubated with CdCl2 at different temperatures under the outwardly directed H+-gradient, and the increase in Cd accumulation due to the gradient was analyzed by an Arrhenius plot. The plot of increased Cd accumulation was nonlinear, suggesting the involvement of a carrier-mediated process. These findings suggest that the outwardly directed H+-gradient increases the Cd efflux across the renal brush border membrane via H+-antiport system.

Understanding renal toxicity of heavy metals

The mechanisms by which metals induce renal injury are, in general, poorly understood. Characteristic features of metal nephrotoxicity are lesions that tend to predominate in specific regions of the nephron within specific cell types. This suggests that certain regions of the nephron are selectively sensitive to specific metals. Regional variability in sensitivity could result from the localization of molecular targets in certain cell populations and/or the localization of transport and binding ligands that deliver metals to targets within the nephron. Significant progress has been made in identifying various extracellular, membrane, and intracellular ligands that are important in the expression of the nephrotoxicity of metals. As an example, mercuric chloride induces a nephropathy that, at the lowest effective doses, is restricted primarily to the S3 segment of the proximal tubule, with involvement of the S2 and S1 segments at higher doses. This specificity appears to be derived, at least in part, from the distribution of enzymes and transport proteins important for the uptake of mercury into proximal tubule cells: apical gamma-glutamyltranspeptidase and the basolateral organic anion transport system. Regional distributions of transport mechanisms for binding proteins appear to be important in the expression of nephrotoxicity of metals. These and other new research developments are reviewed.