2,4-D butoxyethyl ester kinetics in embryos of Xenopus laevis: the role of the embryonic jelly coat in reducing chemical absorption.

The role of the jelly coat in providing a protective barrier to chemical absorption was studied using the embryos of the amphibian, Xenopus laevis. Embryos with or without a jelly coat were water exposed to the butoxyethyl ester of 2,4-dichlorophenoxyacetic acid (2,4-D BEE) and the rates of uptake, metabolism, distribution, and excretion were determined. The water uptake clearance rates were slower for embryos with a jelly coat (1.5-4.5 ml(water).g (embryo) (-1).h(-1) or 0.040-0.022 ml(water).h(-1) per embryo) in comparison to dejellied embryos (14-21 ml(water).g (embryo) (-1).h(-1) 0.0066-0.021 ml(water).h(-1) per embryo). This accounted for the much lower residues in embryos with a jelly coat than in dejellied embryos during 8 h of exposure. Despite quantitative differences in uptake, once 2,4-D BEE had entered the embryos, metabolism and distribution were similar between the two test groups. 2,4-D BEE was metabolized to 2,4-dichlorophenoxyacetic acid (2,4-D) with half-lives ranging from 35 to 42 minutes. The radioactive residues, as determined by whole body autoradiography, appeared throughout the embryo with a slight accumulation in the blastocoel. Furthermore, 35% of the radioactive residues were located in the jelly coat and 65% in the developing embryo. Based on a slower 2,4-D elimination in embryos with a jelly coat, the diffusive properties that decreased 2,4-D BEE uptake appeared to similarly decrease elimination of its metabolite. The common practice of removing jelly coats prior to embryonic amphibian toxicity studies, as in the widely used Frog Embryo Teratogenesis Assay-Xenopus (FETAX), is discouraged based on the kinetic differences observed in this study.