| Abstrakt: |
Xenopus laevis embryos were analyzed for metallothionein by silver-saturation assay and metallothionein-mRNA by reverse transcriptase/polymerase chain reaction following exposures to the following metal chlorides at levels that caused >95% malformations and <7% mortality: Zn2+ (300 μM); Cd2+ (18 μM); Ni2+ (56 μM); Co2+ (1,800 μM); and Cu2+ (5.6 μM). At the beginning of the exposures (stages 8), metallothionein-mRNA and metallothionein levels averaged 2.0X106 copies/embryo and 19 pmol/embryo, respectively. In control embryos at stages 26, 36,42, and 46, metallothionein-mRNA content averaged 9, 37, 104, and 97 copies X106 /embryo, and metallothionein content averaged 6, 11, 15, and 18 pmol/embryo. In Zn2+-exposed embryos at the same stages, metallothionein-mRNA content averaged 116*, 11,400*, 3,210*, and 14 copiesX106/embryo and metallothionein content averaged 10, 18*, 46*, and 90* pmol/embryo; in Cd2+-exposed embryos, metallothionein-mRNA content averaged 22, 7, 170*, 1,783*, and 240 copies X106/embryo and metallothionein content averaged 8, 14, 33*, and 56* pmol/embryo, respectively (*plt;0.05 versus controls). Exposure-response curves (Cd2+, 1-18 μM; Zn2+, 3-300 μM) indicated that Cd2+ was 3- to 5-times more potent than Zn2+, based on metallothionein-mRNA response at stage 36 and metallothionein response at stage 46. In Ni2+-, Co2+-, or Cu2+-exposed embryos, metallothionein-mRNA and metallothionein contents did not differ significantly from controls. This study shows that (1) metallothionein-mRNA is greatly elevated in Zn2+- and Cd2+-exposed embryos at stage 36 and declines to control levels by stage 46, while metallothionein content increases progressively from stage 26 to 46; and (2) elevated metallothionein-mRNA and metallothionein levels may contribute to malformations induced by Zn2+ or Cd2+, but are unlikely to account for the teratogenic effects of Ni2+, Co2+, or Cu2+. [ABSTRACT FROM AUTHOR] |
Plný text