Multidrug resistance proteins and the renal elimination of inorganic mercury mediated by 2,3-dimercaptopropane-1-sulfonic acid and meso-2,3-dimercaptosuccinic acid

Abstract

Current therapies for inorganic mercury (Hg(2+)) intoxication include administration of a metal chelator, either 2,3-dimercaptopropane-1-sulfonic acid (DMPS) or meso-2,3-dimercaptosuccinic acid (DMSA). After exposure to either chelator, Hg(2+) is rapidly eliminated from the kidneys and excreted in the urine, presumably as an S-conjugate of DMPS or DMSA. The multidrug resistance protein 2 (Mrp2) has been implicated in this process. We hypothesize that Mrp2 mediates the secretion of DMPS- or DMSA-S-conjugates of Hg(2+) from proximal tubular cells. To test this hypothesis, the disposition of Hg(2+) was examined in control and Mrp2-deficient TR(-) rats. Rats were injected i.v. with 0.5 mumol/kg HgCl(2) containing (203)Hg(2+). Twenty-four and 28 h later, rats were injected with saline, DMPS, or DMSA. Tissues were harvested 48 h after HgCl(2) exposure. The renal and hepatic burden of Hg(2+) in the saline-injected TR(-) rats was greater than that of controls. In contrast, the amount of Hg(2+) excreted in urine and feces of TR(-) rats was less than that of controls. DMPS, but not DMSA, significantly reduced the renal and hepatic content of Hg(2+) in both groups of rats, with the greatest reduction in controls. A significant increase in urinary and fecal excretion of Hg(2+), which was greater in the controls, was also observed following DMPS treatment. Experiments utilizing inside-out membrane vesicles expressing MRP2 support these observations by demonstrating that DMPS- and DMSA-S-conjugates of Hg(2+) are transportable substrates of MRP2. Collectively, these data support a role for Mrp2 in the DMPS- and DMSA-mediated elimination of Hg(2+) from the kidney.

Fig. 1

Content of Hg²⁺ in the left kidney and total renal mass (percentage of administered dose) of control and TR⁻ rats injected i.v. with 0.5 μmol/kg HgCl₂. Rats were injected i.p. with 100 mg/kg DMPS or 2 ml/kg saline 24 and 28 h after injection of HgCl₂. Kidneys were harvested for determination of Hg²⁺ content after 48-h exposure to HgCl₂. Data represent mean ± S.E. of four rats. *, significantly different (p < 0.05) from the corresponding mean for control rats treated in the same manner.

Fig. 2

Concentration of Hg²⁺ (percentage of administered dose per gram of tissue) in the various zones of kidney of control and TR⁻ rats injected i.v. with 0.5 μmol/kg HgCl₂. Rats were injected i.p. with 100 mg/kg DMPS or 2 ml/kg saline 24 and 28 h after injection of HgCl₂. Kidneys were harvested for determination of Hg²⁺ content after 48-h exposure to HgCl₂. Data represent mean ± S.E. of four rats. *, significantly different (p < 0.05) from the corresponding mean for control rats treated in the same manner. +, significantly different (p < 0.05) from the corresponding mean for the same strain of rats treated with saline.

Fig. 3

Content of Hg²⁺ in liver and blood (percentage of administered dose) of control and TR⁻ rats injected i.v. with 0.5 μmol/kg HgCl₂ followed 24 and 28 h later by treatment i.p. with 100 mg/kg DMPS or 2 ml/kg saline. Livers and blood were harvested 48 h after exposure to HgCl₂. Data represent mean ± S.E. of four rats. *, significantly different (p < 0.05) from the corresponding mean for control rats treated in the same manner. +, significantly different (p < 0.05) from the corresponding mean for the same strain of rats treated with saline.

Fig. 4

Amount of Hg²⁺ excreted in urine and feces (percentage of administered dose) of control and TR⁻ rats injected i.v. with 0.5 μmol/kg HgCl₂, followed 24 and 28 h later by injection with 100 mg/kg DMPS or saline (2 ml/kg). Data represent the amount of Hg²⁺ in urine and feces collected during the 24 h following treatment with DMPS. Data represent mean ± S.E. of four rats. *, significantly different (p < 0.05) from the corresponding mean for control rats treated in the same manner.

Fig. 5

Content of Hg²⁺ (percentage of administered dose) in the left kidney and total renal mass of control and TR⁻ rats injected i.v. with 0.5 μmol/kg HgCl₂. Rats were injected i.p. with 100 mg/kg DMSA or 2 ml/kg saline 24 and 28 h after injection of HgCl₂. Kidneys were harvested for determination of Hg²⁺ content after 48-h exposure to HgCl₂. *, significantly different (p < 0.05) from the corresponding mean for control rats treated in the same manner. +, significantly different (p < 0.05) from the corresponding mean for the same strain of rats treated with saline.

Fig. 6

Concentration of Hg²⁺ (percent administered dose per gram of tissue) in the various zones of kidneys removed from the same animals. Rats were injected i.p. with 100 mg/kg DMSA or 2 ml/kg saline 24 and 28 h after injection of HgCl₂. Kidneys were harvested for determination of Hg²⁺ content after 48-h exposure to HgCl₂. *, significantly different (p < 0.05) from the corresponding mean for control rats treated in the same manner. +, significantly different (p < 0.05) from the corresponding mean for the same strain of rats treated with saline.

Fig. 7

Content of Hg²⁺ in the liver and blood (percentage of administered dose) of control and TR⁻ rats injected i.v. with 0.5 μmol/kg HgCl₂ followed 24 and 28 h later by injection i.p. with 100 mg/kg DMSA. Livers and blood were harvested for determination of Hg²⁺ content 48 h after injection with HgCl₂. Data represent mean ± S.E. of four rats. *, significantly different (p < 0.05) from the corresponding mean for control rats treated in the same manner. +, significantly different (p < 0.05) from the corresponding mean for the same strain of rats treated with saline.

Fig. 8

Content of Hg²⁺ excreted in urine (A) and feces (B) (percentage of administered dose) from control and TR⁻ rats injected i.v. with 0.5 μmol/kg HgCl₂ followed 24 and 28 h later by injection i.p. with 100 mg/kg DMSA. Data represent the amount of Hg²⁺ in urine and feces collected during the 24 h following treatment with DMSA. Data represent mean ± S.E. of four rats. *, significantly different (p < 0.05) from the corresponding mean for control rats treated in the same manner. +, significantly different (p < 0.05) from the corresponding mean for the same strain of rats treated with saline.

Fig. 9

Transport of Hg²⁺ into inside-out membrane vesicles expressing MRP2. The transport of 5 μM Hg²⁺, conjugated to either 12.5 μM DMPS or DMSA, was measured at 37°C for 15 s in control and MRP2-expressing membrane vesicles. Data represent mean ± S.E. of two experiments, performed in triplicate. *, significantly different (p < 0.05) from the corresponding mean for control vesicles treated with the same mercuric conjugate.