Mechanism of inhibition of renal amino acid reabsorption in rabbits by heavy metals

Acute cadmium uptake by rabbit kidneys: mechanism and effects

Trapping of an arterial bolus in the kidney by 40 sec aortic occlusion permitted demonstration of diffusion-limited and Zn-sensitive renal uptake of nonfiltered but diffusible Cd from plasma; mercaptoethanol (ME) or 2,3-dimercaptopropanol (BAL) had been added to the bolus to prevent sequestration of the metals by plasma protein. Cadmium taken up by the kidney under these conditions, whether from blood or glomerular filtrate, unlike Zn, did not return to blood over a period of 2-3 min. Nor was Cd thus accumulated removed by EDTA; it had presumably been transferred into cells. The diffusion dependence of this internalization shows it to be a relatively slow process, as it is in jejunum. In contrast, uptake of Cd tightly bound in a lipid-soluble complex with diethyldithiocarbamate (DDTC) is rapid and flow dependent. Slow uptake in the presence of ME and its inhibition by Zn are therefore not likely to involve movement of undissociated ME complex across the cell membranes. Instead, it is suggested that (1) Cd-binding sites on the membrane possess a relatively high affinity for Cd and can compete for it with ME, and (2) it is the resulting Cd-membrane interaction which, as in jejunum, is depressed by Zn. During transient occlusion kidneys could be loaded with up to 25 micrograms Cd/g cortex without evidence of immediate malfunction. Inhibition of amino acid transport, as previously described, is seen only after an initiation period of 1-2 days following CdME injection, although cortical Cd levels at that time have decreased. The finding of a slowly developing inhibition of amino acid carrier systems suggests an indirect action of Cd; by implication, unique threshold concentrations should not be defined for Cd in renal cortex following subchronic exposures without reference to the duration of exposure.

The selective action of nickel on tubule function in rabbit kidneys

Two days after intraperitoneal injection of NiCl2 (20 mumol/kg) into rabbits the same apparently uncompetitive inhibition of aspartate reabsorption is seen as was previously observed following acute exposure to other metals. This dose of Ni reduced the calculated maximum tubular transport rate for aspartate (Tm) and the apparent affinity constant (KM) by over 50%, but exerted no effect on either Tm or KM of cycloleucine or glucose reabsorption. The relative selectivity of the nephrotoxic action of Ni and other metals is reviewed; it raises the question whether the acute effects observed are appropriately described as a Fanconi-like syndrome.

Nature of inhibition of renal aspartate reabsorption in experimental Fanconi syndrome

Administration of heavy metals or maleic acid induces a condition resembling the Fanconi syndrome. In the case of metals, an apparently uncompetitive inhibition of reabsorption of amino acids was observed 2 days or longer after injection into rabbits (E. C. Foulkes and S. Blanck, Toxicol. Appl. Pharmacol. 64, 103-107, 1982). Transport of aspartate is now shown to be similarly inhibited by maleic acid and cephaloridin. Because maleic acid, as well as p-chloromercuribenzoate (PCMB) and Cd (in presence of mercaptoethanol), exerts an apparent uncompetitive effect within minutes of injection, the inhibition cannot simply reflect characteristics of regenerating epithelium. At low doses, PCMB inhibits aspartate reabsorption without altering transport of certain neutral amino acids, calcium, or p-aminohippurate. The apparent uncompetitive inhibition of aspartate reabsorption therefore does not connote general cytotoxicity. Metals cause relatively specific effects, manifested by functional lesions at the brush border. The mechanism of the aminoaciduric action of maleic acid remains unclear but cannot, under present conditions, involve back leakage of amino acids into tubular urine.