EFFECTS OF MERCURIC CHLORIDE AND SODIUM SELENITE ON SOME IMMUNOLOGICAL AND BIOCHEMICAL RESPONSES IN FISH

Abstract

The immunomodulatory and biochemical effects of mercuric chloride and sodium selenite on tilapia, Oreochromis aureus and blue gourami, Trichogaster trichopterus were studied. Tilapia was exposed to a higher dosage of mercury and selenium. Some parameters of the immune responses which were examined included serum or plasma and kidney lysozyme activity; chemiluminescent response (CL) of kidney cells; serum natural hemolytic and antibacterial activities; serum or plasma agglutinating antibody titre, and concanovalin A ( con A)-induced lymphocyte proliferation test. In addition, a challenge test with a virulent Aeromonas hydrophila PPD 134/91 is employed to evaluate the disease resisting capability of chemical-exposed fish. At the biochemical level, total serum proteins, tissue glutathione (GSH) contents, glutathione peroxidase (GSH-Px) activity and mercury levels of blue gourami were determined. The results are summarized in the following • In the vivo studies, inorganic mercury had no effect on serum or plasma lysozyme activity in both fish species, but it enhanced the kidney lysozyme activity. The CL response of kidney cells also increased slightly. However, no statistical differences between treatment and control were found. Serum natural hemolytic and antimicrobial activities were not affected In contrast, selenium had no effect on most of the parameters tested. Nevertheless, simultaneous addition of equiconcentrations of selenium and mercury reduced the kidney lysozyme activity of blue gourami to that observed in controls. • Although serum or plasma agglutinating antibody titre was not affected, it was always lower than the control. This result suggests that mercury may have some effects on primary and secondary immune responses. Con A-induced lymphoblastogenesis varied among separate experiments, but the results obtained from a month long exposure and those from immunised fish showed that Hg2+ exerted a stimulatory effect on con A-induced mitogenesis. • In the in vitro studies, mercuric mercury suppressed CL response significantly, and a cytotoxic effect was observed in tilapia head-kidney cells which have been treated with 0.5 ppm Hg2+ Selenium showed a dose-dependent stimulatory effect on CL response, but the simultaneous addition of selenium and mercmy to culture medium did not show any reduction of mercury toxicity to the isolated kidney cells from the fish. Incubation of kidney cells from blue gourami with mercury for 48 hours or pretreatment with mercury for 4 hours reduced lymphocyte mitotic rate at 0.09 ppm and 0.18 ppm Hg2+ Kidney cells treated with selenium by either method showed a marked decrease in lymphocyte proliferation capability. • The challenge test on the mercury-treated blue gourami showed a decrease in LD50, but the result was not conclusive. • High concentrations of mercury resulted in a higher serum protein level in tilapia after 12 hours or exposure. However, at 10 ppm of SeO32- , no effect was observed. Subchronic exposure shows that both mercury and selenium cause hemodilution with lower serum protein level after 21 days of exposure, and simultaneous treatment with mercury and selenium did not alter the serum protein level. • Inorganic mercury affected kidney and liver GSH contents in a dose-dependent manner, while brain GSH content was not affected. Coadministration of selenium reduced kidney and brain GSH contents to control levels. Liver GSH content is slightly lower, but there was no significant difference compared to the group treated with mercury alone. GSH-Px activity was not affected by either mercury or selenium. • Selenium seems to retard uptake of mercury as revealed by the lower mercury contents found in kidney, liver and brain. In contrast, it retained tissue mercury during the depuration period. The findings imply that selenium may protect an organism through the formation of Hg-Se complexes, which subsequently reduce the availability of mercury to other important macromolecules. In conclusion, the present study revealed that inorganic mercury may cause some adverse effects on the defence mechanisms of fish. Moreover, it may induce severe cellular damages by enhancing the production of free radicals as shown in CL response. Inorganic mercury appeared to increase also the amount of tissue GSH On the other hand, selenium alone does not exert much negative effect on the parameters examined. Neither does it prevent cells from mercury intoxication in vitro. Nevertheless, selenium may protect an organism by retarding the uptake of mercury from the surrounding and through the formation of Hg-Se complexes.