Effects of teratogenic concentrations of Zn2+, Cd2+, Ni2+, Co2+, and Cu2+ in FETAX assays on metallothionein (MT) and MT-mRNA contents of Xenopus laevis embryos
Sunderman FW Jr; Plowman, MC; Slaisova, O; Grbac-Ivankovic, S; Foglia, L; Crivello, JF
Xenopus laevis embryos were exposed to metals from 5 to 101 hours post-fertilization according to the FETAX (frog embryo teratogenesis assay: Xenopus) protocol. Control and metal-exposed embryos were assayed for metallothionein (MT) and MT-mRNA after each 24-hour interval. Metals were added to FETAX medium at concentrations (umol/L) that caused malformations in greater than 95% and mortality in less than 7% of embryos: Zn2+ (300); Cd2+ (18); Ni2+ (56); Co2+ (1,800); and Cu2+ (5.6). Embryos were analyzed for MT by silver-saturation and MT-mRNA by reverse transcriptase/polymerase chain reaction methods. In controls at 29, 53, 77, and 101 hours post-fertilization, median MT-mRNA contents were 7, 19, 33, and 34 copies x 10(6)/embryo, and median MT contents were 6, 11, 17, and 20 pmol/embryo, respectively. In Zn2+-exposed embryos at the same four intervals, median MT-mRNA contents were 10, 21400,* 158, and 1* copies x 10(6)/embryo, and median MT contents were 7, 19, 46,* and 78* pmol/embryo. In Cd2+-exposed embryos at the same four intervals, median MT-mRNA contents were 17, 4500,* 2020,* and 239 copies x 10(6)/embryo, and median MT contents were 6, 15, 30,* and 56* pmol/embryo (* p less than 0.05 vs controls). MT-mRNA and MT contents were unaffected in Ni2+-, Co2+-, or Cu2+-exposed embryos. This study shows that increased MT-mRNA and MT levels may be involved in Zn2+- and Cd2+-teratogenesis, but are unlikely to contribute to teratogenicity of Ni2+, Co2+, or Cu2+ in Xenopus embryos.