M. G. Yang

Deposition of Mercury in Fetal and Maternal Brain

Summary Ten 16-day pregnant rats were force-fed a tracer-dose of 203Hg as methyl mercury chloride. The rat was then killed at 1, 2, 4 or 5 days after force-feeding and the 10 maternal brains as well as the 114 fetal brains were each separated into several brain parts. The uptake of mercury in the maternal brain was greatest in the cerebrum followed in decreasing order by cerebellum, pons plus midbrain, medulla, and hypothalamus. The uptake expressed as a percentage of the dose remained essentially the same throughout the 5-day period after force-feeding. In the fetal brain the uptake however increased more than 3-fold from day 17 to day 21 of fetal life. Expressed on the basis of % of dose per g tissue, the fetal cerebellum contained the highest concentration of mercury when compared to either fetal cerebrum or pons, midbrain plus medulla, or to any maternal brain parts. The total fetal brain mass from an average litter accumulated, during the first 2 days after force-feeding, approximately the same percentage of the dose as in the mothers brain. However, during the following 3 days, the total brain mass of each litter had 2.5 to 3.5 times the uptake level in the mothers brain.

Translocation and fluxes of mercury in neonatal and maternal rats treated with methyl mercuric chloride during gestation.

Summary Pregnant rats, during the 16th day of gestation, were force-fed a tracer dose of 203Hg as methyl mercuric chloride. At parturition, 7, 14,21, and 28 days after parturition, the 203Hg radioactivities in the different brain parts and organs of maternal and neonatal rats were determined. The concentration of 203Hg in the brain, brain parts, and organs of mothers and pups decreased with time after force-feeding. At all time periods, cerebrum contained the greatest quantity of 203Hg and pituitary the lowest in both mothers and pups due primarily to the size of the tissue. The blood concentration of 203Hg in the pups at birth was 27% higher than that of the darns. Similarly, at birth, the concentration of 203Hg in the different brain parts and organs of the pups, except in the kidney, was higher than corresponding tissues in the dams. Mercury pool size in the brain and brain parts of maternal and neonatal rats also primarily reflected the size of the tissues studied. In general, a faster turnover of mercury in the pups was observed. The half-life of mercury in the maternal brain, kidneys, liver, and gastrocnemius were 10.79, 27.61,9.38, and 6.73 days, respectively. In the pups, corresponding half-life values were 10.00, 20.20, 13.50, and 12.86 days. In another study, foster rearing of pups originally contaminated in utero with the radioactive mercury demonstrated that mercury excreted by pups recirculated to the foster dams. The uptake of 203Hg in the brain, brain parts and organs of foster dams ranged from 0.0001 to 0.067% of the original dose administered to the mothers of the contaminated pups.

Carbon-Mercury Bond Cleavage in Blood of Rats Fed Methyl Mercuric Chloride

Summary Methyl mercuric chloride labeled with 14C or 203Hg or both were forcefed to rats in order to quantitatively determine breakage of the carbon–mercury bond in different fractions of blood. Based on the hypothesis that a change in the ratio of 14C to 203Hg radioactivities in the tissue from that in the compound fed to rats would indicate breakage, values ranging from 5.1% to 10.6% were observed depending upon the blood fraction studied. Since the site of deposition of mercurial compounds is important in determining toxicities, the concentrations of either 14C or 203Hg radioactivities in blood fractions were determined after force-feeding the single or double-labeled methyl mercuric chloride. One day after force-feeding, the highest concentration in the erythrocytes was in the hemoglobin. In the plasma, fat contained the highest concentration. However, plasma fat radioactivities constitute only 0.1% of the whole blood radioactivities whereas plasma protein radioactivities constitute 0.7%. Nearly 99% of blood radioactivities was in the hemoglobin.

Behavioral Changes of Young Rats Force-Fed Methyl Mercury Chloride

Summary A single dose of methyl mercury chloride was force-fed at 2.0 mg/100 g body weight to 15- and 21-day-old and at 2.5 mg/100 g to 60-day-old male Sprague-Dawley rats. Control rats received the carrier for the mercury compound. T-maze and open-field performances of the rats treated with the 2.0 mg/100 g body weight were not altered when compared to control rats. However, statistically significant differences were found in the T-maze test in number of days to reach criterion between the two groups of 60-day-old rats. Furthermore, statistical differences were found between the two groups in several parameters measured in the open field which indicate alteration in the emotionality of the treated rats. Histological examination of the brain at the end of the behavioral tests revealed no lesions attributable to the mercury treatment. Thus, our data indicated that certain subtle changes due to mercury poisoning can more readily be detected with behavioral tests than by histopathological examination.

The influence of high-fat diets on estrous cycles, sperm production and fertility of rats.

Summary The estrous cycle lengths of rats fed high-fat diets ad libitum or restricted, a grain diet or a low-fat diet were determined in two long-term studies. In the first experiment, with a 40% fat diet, the rats had prolonged estrous cycles beginning at 37–40 weeks of age caused primarily by a prolongation of diestrus. During the early stages of feeding the high-fat diet did not prolong the estrous cycles. In the second experiment, the diet contained 60% fat and the prolongation of diestrus occurred at 23–27 weeks of age. In male rats, however, high fat diets from weanling to 11 weeks of age did not affect fertility or sperm production.

Oral Contraceptives, Norethynodrel and Mestranol: Effects on Glucose Tolerance, Tissue Uptake of Glucose-U-14C and Insulin Sensitivity

Summary Mestranol and norethynodrel were fed singly or in combination to 11-week-old female Sprague-Dawley rats to determine their effects on glucose tolerance, in vivo tissue uptake and utilization of glucose-U-14C and in vitro insulin sensitivity. The results indicate that norethynodrel impaired oral glucose tolerance, and reduced the levels of radioactivity in adipose tissues at various time intervals after an oral glucose-U-14C load. Mestranol slightly depressed gastric emptying and intestinal absorption of the radioactive glucose load. In vitro incubation of diaphragm muscle and adipose tissue revealed that the impairment of glucose tolerance may have resulted from reduced insulin sensitivity in these tissues.

Carbon–Mercury Bond Breakage in Milk, Cerebrum, Liver, and Kidney of Rats Fed Methyl Mercuric Chloride

Summary A double-label technique was used to determine the carbon–mercury bond breakages in milk and milk fractions, cerebrum, liver and kidneys of rats one day after force-feeding double-labeled methyl mercuric chloride. Breakages of the carbon–mercury bond occurred mainly in the fat and casein fractions and not in the whey of milk. In the cerebrum, liver and kidney, the breakages of the bond were respectively 6.2%, 6.2%, and 8.0%. The 14C and 203Hg concentrations (percent of dose/g) were high in fat and casein fractions and low in whey fraction of milk. The 14C radioactivities in milk were distributed 46.2% in fat, 32.3% in casein and 21.5% in whey. For 203Hg radioactivities in milk, distribution was 55.1% in whey, 41.0% in casein and 3.9% in fat. Of the three organs studied, the kidney contained the highest percent of dose/gram tissue whereas the cerebrum contained the lowest.

Utilization of 14C-labeled E. coli by the Rat Cecum and After Force-feeding

Summary Radioactive E. coli produced by means of adding glucose-U-14C into the culture medium was placed into the cecum of six adult male rats. The rats were then placed in metabolic chambers for the collection of respiratory CO2. At the end of 4 hr, almost 5% of the dose appeared as CO2 indicating that the E. coli were digested in and absorbed from the cecum. Further evidence that this occurred came from the finding that 30.5% of the dose appeared in the carcass of the rats killed at the end of the metabolic CO2 collection period. About 70% of the dose remained in the cecum and large intestine at this time which is the portion not absorbed. Another six comparable rats were gavaged with the radioactive E. coli. Carcass and fecal analyses and measurements of activities in the gastrointestinal contents indicate that 24 hr after gavage, the rat retained 43.5% of the gavaged dose. They did not digest 7.3% (fecal activity) plus 4.2% (remaining in the gastrointestinal tract) for a total of 11.5%. Thus, the digestion coefficient for E. coli is 88.5% and net utilization is 83.8% since 4.7% of the gavaged dose appeared in the urine in 24 hr. Fractionation of carcass samples revealed that the rat utilized the products obtained from the digestion or degradation of the radioactive E. coli force-fed or placed in the cecum for the formation of lipids, protein and other carbon-containing materials.

Carcinogenicity of Long-Term Feeding of Cycad Husk to Rats

Summary Guamanians eat the fleshy husk of the seeds of the cycad, Cycas circinalis, which is indigenous to Guam. The fresh husk is eaten to relieve thirst. The dried husk is eaten as candy. The kernel of this seed is soaked in water, sun dried, and ground for preparation as an ingredient in foods. This processed kernel was not toxic or carcinogenic in contrast to unprocessed kernels. Since the husk is consumed without cooking or processing other than drying, and since the husk is reported to be eaten primarily by children, it is imperative to evaluate the safety of this food. The acute toxic effects have already been studied whereas the carcinogenic properties of the husk have not. For this reason rats were fed diets containing 0.5-2.0% of the dry husk for prolonged periods. Malignant tumors were found in the liver and kidney of experimental animals. Metastatic tumors were found in the pancreas, spleen, and lung.

Oral contraceptive steroids: effects on various nutrient balances and body composition in adult female rats.

Summary Norethynodrel and mestranol fed to 11-week-old female rats on a physiological level reduced food consumption and body weight gain. The withdrawal of the oral steroids for 42 days did not accelerate the growth rates of the treated rats when they had been previously fed the steroids for 28 or 180 days. Treatment with steroids for a short or long period did not affect digestibility of protein, fat, sodium, or potassium. However, retention of dietary nitrogen was higher for control rats than for treated rats after 22 days of steroid treatment (p <0.05). At this time the treated rats retained significantly more dietary sodium than the control rats (p <0.05). After feeding the oral steroids for 178 days, the treated rats retained more dietary nitrogen than did the control rats (p <0.01). No statistically significant effect on sodium retention was observed at this time. Thus, the effects caused by the oral steroids on the retention of this mineral as well as nitrogen was temporary in nature. The oral steroids did not significantly alter the retention of dietary potassium whether the treatment was for short or long time. Concomitant with the increased retention of sodium measured after 22 days of treatment was an increased proportion of water in the carcasses of treated rats compared to the control rats. Again, this effect disappeared after 178 days of treatment or after refeeding of control diet subsequent to 22 or 180 days of treatment. Furthermore, there was no difference in body moisture concentration when it was expressed as a percentage of lean body masses.