Mechanisms of Absorption of Inorganic Mercury from Rat Small Intestine. II. Composite Effects of pH and Halide Ions on Transport of Mercuric Chloride into Isolated Brush Border Membrane Vesicles in Rats

Abstract

Composite effects of pH and halide ions on the transport of HgCl2 into brush border membrane vesicles (BBMV) were investigated in rats. BBMV were incubated for 10 min. in buffer solution at different pH containing 10(-4) M HgCl2. The increase in pH increased the uptake of Hg by BBMV as a result of the increase in transport of Hg into intravesicular space and decrease in binding of Hg to BBMV. The isotonic displacement of NaCl in the buffer solution by LiCl or KCl did not change the uptake of Hg at each pH. The displacement of NaCl by mannitol increased the uptake of Hg at each pH, while the displacement by NaBr or NaI decreased the uptake and diminished the increasing effect of pH on the uptake of Hg. These changes in uptake of Hg due to the displacement were mainly ascribed to the changes in transport of Hg. These results suggest that the increase in pH mainly increases the transport of HgCl2 as a result of the conversion to hydroxide forms of Hg such as Hg(OH)Cl and Hg(OH)2, and Cl- Br- and I- act as the competing ions with OH- and decrease the transport of Hg.

Intestinal Absorption of Inorganic Mercury in Rat

“…A simple process of diffusion, proportional to the concen-tration of mercury in the lumen of the intestines seems more likely. This statement conforms with data of others obtained for mercury both in vivo and in vitro, indicating that the transport of mercury across the intestinal wall is linked with water transport (Cikrt 1971;Endo et al 1988a) and depends on the chemical form and dimensions of the molecule (Cikrt 1971;Endo et al 1984Endo et al , 1988bEndo et al & 1990.…”

Experimental localization of intestinal uptake sites for metals (Cd, Hg, Zn, Se) in vivo in mice.

“…The mercury deposition in the proximal part of jejunum with a peak at segment 7 and maximum labeling at 3 hr postdosage probably represents internalized mercury. The preferential labeling of the proximal part of jejunum, posterior to the outlets of the bile and pancreatic ducts agrees with results of previous investigations, which demonstrated enhanced absorption of mercury at increasing pH (7,13,14). Thus, the present study using the undisturbed intestinal system filled with digesta supports the studies using ligated perfused intestinal segments in vitro or in situ, indicating proximal jejunum as the major site for uptake of inorganic mercury; however, the absorption seems to occur in a larger part of jejunum than previously reported.…”

Mechanisms of Absorption of Inorganic Mercury from Rat Small Intestine. IV: Effect of Chelating Agents and Cysteine on Absorption of Mercuric Chloride in Situ and in Vitro

“…Therefore, we forwarded the hypothesis that EDTA and Cys decrease the absorption of HgCl, through the pores by the solvent drag as a result of the increase in molecular size of HgCl,. On the other hand, our experiments in vitro (Endo et al 1986(Endo et al , 1988b(Endo et al & 1990 showed that the rise in intraluminal pH increased the transports of HgC12 through the intestinal wall and the intestinal brush border membrane, and decreased the HgC12 binding as a result of the conversion of HgC12 into Hg(0H)Cl and Hg(OH)2. The dissociations of carboxyl and sulfhydryl groups are considered to be a prerequisite for the chelation of EDTA with HgCl, and the binding of Cys to HgCl,, respectively.…”

“…Intestinal brush border membrane vesicles (BBMV) were prepared according to the previous report (Endo et al 1988b). BBMV suspended in 300 rnM mannitol were added to an equal volume of PBM at pH 6.4 containing HgCI, and an squimolar Cit, Tar, Cys, Pen or EDTA, and BBMV were incubated for 10 min.…”

“…The final concentrations of HgCI, and agents in the incubation medium were M, respectively, and the final concentration of BBMV was about 0.65 mg protein/ml. After incubation, the Hg uptake by BBMV was measured by rapid filtration using the cellulose nitrate filter (Endo et a/. 1988b).…”