The effect of prenatal methylmercury administration on postnatal renal functional development

Methylmercury distribution in the pregnant rat and embryo during early midbrain organogenesis

BACKGROUND

The period of neurogenesis represents a window of susceptibility for in utero methylmercury (MeHg) exposure. This study examined the toxicokinetics of potentially neurotoxic doses of MeHg during neurogenesis in the developing rat to provide additional information in the areas of mercury speciation and inter-study variability.

METHODS

Pregnant Sprague-Dawley rats were dosed s.c. with 5-22 mg/kg MeHg on Day 11 of gestation to target rapidly dividing cells of the developing midbrain. Maternal liver, kidney, skin, blood, placenta, and the embryonic body and brain were evaluated for total and inorganic mercury content at 24, 48, and 72 hr after dosing. Tissue Hg partitioning ratios derived from our data were then compared to those derived from previous studies.

RESULTS

Mercury was present in all tissues examined by 24 hr after dosing, and levels remained relatively stable over the subsequent 2 days in most tissues. The exceptions were the maternal blood and kidney, in which total mercury decreased significantly over the three days after dosing. Inorganic mercury concentrations were similarly stable over time. At maternal MeHg doses above 12 mg/kg, non-linearities were observed in mercury accumulation in the embryo, placenta and maternal liver. The mercury tissue partitioning coefficients ranged from 0.09 for maternal blood:embryo to 1.97 for maternal blood:kidney.

CONCLUSIONS

Our observations at the 5 mg/kg dose were consistent with those of previous studies that involved evaluations at slightly later gestational times. The estimates of tissue partitioning coefficients we derived using multiple studies provide valuable insight into the effects of inter-study variability.

[Progress report. What can one expect from a "functional prenatal toxicology?"]

In the strategy of investigation applied and demanded in prenatal toxicology partial aspects have been overstressed until now, although here as well as in postnatal toxicology the problems should be viewed in its complexity (in this case, F1-generation in its several pre- and postnatal stages of development). Therefore, there was the urgent need to summarize the existing methods of functional investigation in the field of prenatal toxicology and to compare them with conventional morphological methods as to their meaningfulness and sensitivity. As a result, the routine introduction of clinical-functional investigations is demanded.

Effects of prenatal methylmercury exposure on urinary proximal tubular enzyme excretion in neonatal rats

The basal developmental pattern of excretion of 3 proximal tubular enzymes was determined in 8-h urinary specimens from neonatal rats. Gammaglutamyltransferase (GGT), alkaline phosphatase (ALP), and N-acetyl-beta-glucosaminidase (NAG) activities were measured at 3, 6, 9 and 12 days after birth. Subsequently, methylmercury chloride (CH3HgCl), known to induce foetotoxic changes in the proximal tubule was administered on days 8, 10 and 12 of gestation at 3 or 6 mg/kg and its effects on the enzyme activities were examined. Dose-related increases in the 3 enzyme activities occurred at dose levels that produced no maternal or postnatal toxicity, nor overt morphological malformation of the kidney. The peak of enzyme activities averaged about 200% and 130% of the control values for GGT, ALP, and NAG respectively, and occurred on days 3 and 6 in the treated groups. Urinary enzyme activities returned to the control levels from days 6 to 12. Our data point to the possibility of detecting CH3HgCl-induced prenatal effect on the kidney by measuring the 8-h urinary excretion of enzymes by rats in the early postnatal period.