Metabolic alterations without metal accumulation in the ovary of adult Bufo arenarum females, observed after long-term exposure to Zn2+, followed by toxicity to embryos

Long-term exposure of aquatic organisms to metals, even those considered mi cronutrients, may affect their metabolism and produce sublethal effects. We evaluated the effects of long-term exposure of adult amphibian (Bufo arena rum) females to 4 mug/L of Zn2+ (ZnSO4 . H2O) in Ringer solution on the con centration of Zn and Fe, the activity of the key enzyme of the pentose phos phate pathway glucose 6-phosphate dehydrogenase, and glutathione content, b oth in the liver and ovary of these animals. We also performed early embryo nic development studies by in vitro insemination from control and treated f emales. Zn exposure rendered lower Zn concentrations in the ovaries than di d exposure of animals to Ringer solution without metal addition (97 +/- 50 versus 149 +/- 46 Zn mug/wet tissue g). Zn and Fe concentration correlation was positive and linear in the ovary, but was negative and nonlinear in th e liver of the studied females. The activity of the enzyme glucose 6-phosph ate dehydrogenase decreased (0.0599 +/- 0.0109 versus 0.0776 +/- 0.0263 mu mol of NADPH/min . mg of proteins) and the endogenous glutathione content i ncreased (0.027 +/- 0.005 versus 0.018 +/- 0.007 mg/10 mg of proteins) in t he ovary but remained unaltered in the liver as a consequence of Zn treatme nt. Our results suggest the existence of different mechanisms of regulation of Zn and Fe concentrations in the ovary and in the liver of adult B. aren arum females. Binding of Zn to low-molecular-weight proteins, as metallothi oneins, may occur in the liver, thus protecting this organ from toxic effec ts. In the ovary high-molecular-weight proteins, like glucose-6-phosphate d ehydrogenase, should be able to bind Zn, leading to oxidative stress respon sible for the observed increase in endogenous glutathione content. Inhibiti on of the pentose phosphate pathway in the ovary by Zn can be responsible f or the reproductive failure that we detected through embryos survival studi es during early life stages: 81.3 +/- 6.3% of embryos from control females survived versus 63.1 +/- 13.8% of embryos from Zn-treated females at the br anchial circulation stage of development.