EFFECTS OF TERATOGENIC EXPOSURES TO ZN2-MESSENGER-RNA CONTENTS OF XENOPUS EMBRYOS(, CD2+, NI2+, CO2+, AND CU2+ ON METALLOTHIONEIN AND METALLOTHIONEIN)

Xenopus laevis embryos were analyzed for metallo thionein by silver-sa turation assay and metallo thionein-mRNA by reverse transcriptase/poly merase chain reaction following exposures to the following metal chlor ides at levels that caused >95% malformations and <7% mortality: Zn2(300 mu M); Cd2+ (18 mu M); Ni2+ (56 mu M); Co2+ (1,800 mu M); and Cu2 + (5.6 mu M). At the beginning of the exposures (stages 8), metallothi onein-mRNA and metallothionein levels averaged 2.0x10(6) 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 copiesx10(6) /embryo, and metallothionein content averaged 6, 11, 15, and 18 pmol/embryo. In Zn2+-exposed embryos at the same stages, metal lothionein-mRNA content averaged 116, 11,400*, 3,210*, and 14 copiesx 10(6) /embryo and metallothionein content averaged 10, 18, 46*, and 9 0 pmol/embryo; in Cd2+-exposed embryos, metallothionein-mRNA content averaged 22, 7,170, 1,783*, and 240 copiesx10(6) /embryo and metallot hionein content averaged 8, 14, 33, and 56* pmol/embryo, respectively (P<0.05 versus controls). Exposure-response curves (Cd2+, 1-18 mu M; Zn2+, 3-300 mu M) indicated that Cd2+ was 3- to 5-times more potent t han Zn2+, based on metallothionein-mRNA response at stage 36 and metal lothionein response at stage 46. In Ni2+-, Co2+-, or Cu2+-exposed embr yos, 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 3 6 and declines to control levels by stage 46, while metallothionein co ntent increases progressively from stage 26 to 46; and (2) elevated me tallothionein-mRNA and metallothionein levels may contribute to malfor mations induced by Zn2+ or Cd2+, but are unlikely to account for the t eratogenic effects of Ni2+, Co2+, or Cu2+.