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Biomarkers of effect in toads and frogs.

Amphibians are good bioindicators of environmental pollution due to their susceptibility to chemicals during their freshwater cycles. The effects of environmental pollution, together with changes in human activity and climate, have contributed to the reduction in the amphibian population over recent decades. However, toxicological research on amphibians has been rather scarce compared with that on other vertebrates. In this article we review the biochemical alterations underlying xenobiotic action and/or the detoxifying responses described for anuran species, with the aim of establishing possible biomarkers of effect. During the embryonic development of anurans, morphological and behavioural alterations are the effects most frequently cited in connection with chemical exposures. However, such biomarkers have a low sensitivity and are unspecific compared with biochemical alterations. Some primary pesticide targets, in particular cholinesterases for organophosphates and carbamates, have been evaluated. Esterases change seasonally and with the stage of development, and their sensitivity to anticholinesterase agents varies between species. Thus their use as biomarkers in anurans must be carefully analysed. Enzymes and endogenous compounds related to oxidative metabolism may also be used as biomarkers of effect. Glutathione pool, glutathione-S-transferases and metallothioneins respond in different ways to pesticides and heavy metals in anuran embryos and tadpoles. Mixed-function oxidases, in turn, are less developed in amphibians, and show a reduced induction in response to pesticide exposures. Endogenous polyamine levels are also proposed as good age-related biomarkers of damage. Finally, molecular biomarkers related to receptor binding, signal transduction and genetic response have gained increasing relevance, as they have been implicated in the fertilisation process and the earliest events in anuran development. The identification of transcription factors associated with the exposure of amphibians to xenobiotics as well as other alterations in hormone signalling appears highly promising. However, these techniques are likely to complement other methods. In conclusion, the use of several biomarkers with multiple endpoints is needed to link exposure to response and to provide better predictive tools for the environmental protection of endangered anuran species.

Animals↗
PubMed1974

Intracellular sodium and potassium concentrations in toad and frog oocytes during development.

1. During growth of oocytes of Bufo bufo from approximately 700-2000 mum in diameter, intracellular Na rose from 20 to 120 m-mole/l. cell water while K fell from 120 to 15 m-mole/l. cell water.2. In Rana temporaria Na similarly rose from 9 to 190 m-mole/l. cell water and K fell from 126 to 28 m-mole/l. cell water.3. At the same time in both Bufo and Rana the ouabain-sensitive Na efflux, the ouabain-insensitive Na efflux, and the total Na influx all increased.4. It is concluded that the predominant process is an increase of passive Na permeability. The increase of apparently active Na efflux may be secondary to the consequent rise in intracellular Na.

Animals↗
PubMed1981

Active urea transport through isolated skins of frog and toad.

Urea influxes (Ji) and effluxes (Je) were studied across the isolated skins of Rana esculenta, Bufo bufo, and B. viridis. Two symmetrical pieces of the same skin, bathed in Ringer + 2 mM urea, were used for the two fluxes. In R. esculenta the urea fluxes are passive when the animals are kept in running water but become active after dehydration in air or preadaptation in saline solutions. The ratio Ji/Je can vary between 3 and 27 and the Ji between 2 and 22 nmol . h-1 . cm-2 according to preadaptation. Only the active fluxes obey saturation kinetics. Urea absorption is always independent of sodium transport. In toads, active urea transport occurs even when hydrated. It is markedly stimulated by saline preadaptation. A correlation between the degree of active urea transport across the skin and the capacity of the species to endure dehydrating conditions would appear to exist. The physiological significance of this transport mechanism is discussed.

Animals↗