Plasma sex steroid concentrations and gonadal aromatase activities in African clawed frogs (Xenopus laevis) from South Africa

Abstract

Adult African clawed frogs (Xenopus laevis) were collected from a corn-growing region (CGR) and a non-corn-growing region (NCGR) with different exposure profiles for atrazine and related triazines. Physical, chemical, and biological parameters from the catchment areas were also measured. Frogs were surveyed for possible effects of exposure to triazine herbicides on plasma testosterone (T) and estradiol (E2) titers, gonadal aromatase activity, and gonad growth (GSI). Concentrations of both T and E2 varied among locations and were correlated to some accessory factors, such as pH, several ions, and metals. Greatest median plasma T concentrations (males: 19 ng/ml; females: 16 ng/ml) occurred in frogs inhabiting NCGR as compared to those from the CGR (males: 4 ng/ml; females: 1 ng/ml). Median E2 concentrations were also greater in frogs collected from the NCGR (males: 3 ng/ml; females: 28 ng/ml) than those in frogs from the CGR (males: 2 ng/ml; females: 5 ng/ml). Because some exposure to agricultural chemicals at both regions occurred, as did simultaneous exposures to multiple chemicals, a regression analysis was employed. Negative correlations were observed between plasma T concentrations and concentrations of atrazine, deisopropylatrazine, deethylatrazine, and tertbuthylazine in females and between T and diaminochlorotriazine in males. Estradiol in females exhibited a significant negative correlation with atrazine and deethylatrazine. No correlations were observed between gonadal aromatase activity or GSI and any of the agricultural chemicals measured. Median aromatase activities in ovaries varied among sampling sites ranging from 7 to >3000 times greater than those in males when measurable. Testicular aromatase activity was below the detection limit of the assay in male frogs at most of the sites. Although exposure to agricultural inputs did not affect aromatase activities, effects of atrazine or coapplied pesticides on sex steroid homeostasis cannot be excluded at this point.

Aromatase expression in Xenopus oocytes: a three cell-type model for the ovarian estradiol synthesis

“…(Mulner et al 1978, Fortune 1983, Gohin et al 2010. Moreover, Xenopus intra-oocyte E 2 concentrations are in the range of E 2 content previously measured in the whole follicles (Fortune 1983) and in plasma varying from 1 to 28 ng/ml corresponding to a maximum of 100 nM (Pickford & Morris 1999, Hecker et al 2004, Urbatzka et al 2007). Moreover, E 2 appeared to be maternally inherited as E 2 is still detected in Xenopus unfertilized eggs (Kloas 2002).…”

Response to Comment on “Gonadal Development of Larval Male Xenopus laevis Exposed to Atrazine in Outdoor Microcosms”

“…Last, Hayes' comments do not address our observation that, regardless of temperature, no concentration response was observed with atrazine in terms of either gonadal anomalies or testicular oocytes, an observation consistent with reports on the lack of response of other measurement endpoints such as steroid concentrations and aromatase activity (12)(13)(14). …”

Amphibians as Models for Studying Environmental Change

“…Extraordinary findings are often accompanied by controversy, which was certainly the case surrounding evidence of hermaphroditism in amphibians exposed to a common herbicide (Hayes et al 2002(Hayes et al , 2006Hecker et al 2004Hecker et al , 2005. However, this and other controversies in amphibian ecotoxicology (Monsanto 2005;Relyea 2005) have given way to new debates regarding how chemicals are approved for use and regulated, as well as questions about the universality of dose-response paradigms in toxicology.…”