Abstract
Dramatic declines in amphibian populations have been described all over the world since the 1980s. The evidence that the sensitivity to environmental threats is greater in amphibians than in mammals has been generally linked to the observation that amphibians are characterized by a rather permeable skin. Nevertheless, a numerical comparison of data of percutaneous (through the skin) passage between amphibians and mammals is lacking. Therefore, in this investigation we have measured the percutaneous passage of two test molecules (mannitol and antipyrine) and three heavily used herbicides (atrazine, paraquat and glyphosate) in the skin of the frog Rana esculenta (amphibians) and of the pig ear (mammals), by using the same experimental protocol and a simple apparatus which minimizes the edge effect, occurring when the tissue is clamped in the usually used experimental device.The percutaneous passage (P) of each substance is much greater in frog than in pig. LogP is linearly related to logKow (logarithm of the octanol-water partition coefficient). The measured P value of atrazine was about 134 times larger than that of glyphosate in frog skin, but only 12 times in pig ear skin. The FoD value (Pfrog/Ppig) was 302 for atrazine, 120 for antipyrine, 66 for mannitol, 29 for paraquat, and 26 for glyphosate.The differences in structure and composition of the skin between amphibians and mammals are discussed.
Highlights
The evidence that the sensitivity to environmental threats is higher in amphibians than in mammals has been generally linked to the observation that amphibians are characterized by a rather permeable skin [1,2]
Data showed that the P values of two test molecules and three heavily used herbicides were much higher in frog skin than in pig skin
The following order of permeability values was found in frog: atrazine . antipyrine . mannitol . paraquat . glyphosate, while in pig it was: atrazine . antipyrine . paraquat . glyphosate and mannitol
Summary
The evidence that the sensitivity to environmental threats is higher in amphibians than in mammals has been generally linked to the observation that amphibians are characterized by a rather permeable skin [1,2]. In order to obtain quantitative information supporting the general belief that amphibians are more sensitive than mammals to contaminants, in this investigation we have measured the percutaneous (through the skin) passage of two test molecules (mannitol and antipyrine) and three heavily used herbicides (atrazine, paraquat and glyphosate) in frog (amphibians) and pig ear skin (the most appropriate model of human skin) [10], by using the same experimental protocol. The IUCN Red List Categories of Vulnerable, Endangered, or Critically Endangered species [12] includes 32.5% of the total number of amphibian species but only 12% and 23% of birds and mammals, respectively [13,14]. The absolute number of individuals is decreasing in 2468 amphibian species (43.2%), while the population is stable for 1552 species (27.2%) and it is increasing in only 28 cases (0.5%) [15]
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