Unraveling the Relative Importance of Oral and Dermal Contaminant Exposure in Reptiles: Insights from Studies Using the Western Fence Lizard (<i>Sceloporus occidentalis</i>)

<div><p>Despite widespread recognition of significant data deficiencies, reptiles remain a relatively understudied taxon in ecotoxicology. To conduct ecological risk assessments on reptiles frequently requires using surrogate taxa such as birds, but recent research suggests that reptiles have significantly different exposure profiles and toxicant sensitivity. We exposed western fence lizards, <i>Sceloporus occidentalis</i>, to the same quantities of three model chemicals via oral (gavage) and dermal (ventral skin application) exposure for either 24 or 48 hours. Three phthalate esters (di-methyl phthalate [DMP], di-iso-butyl phthalate [DIBP], and di-n-octyl phthalate [DNOP]) were chosen as model chemicals because they represent a gradient of lipophilicity but are otherwise structurally similar. Overall, the more lipophilic phthalates (DIBP and DNOP) were found to have higher concentrations in tissues than the less lipophilic DMP. Significant differences in tissue concentrations between DIBP and DNOP were tissue-dependent, suggesting that delivery to a site of action following exposure is not only a simple function of lipophilicity. In dermal treatments, DMP usually had fewer detections (except in ventral skin samples), suggesting that lipophilicity (log K_ow>2) is a requirement for uptake across the skin. In general, tissue residues were greater in oral treatments than dermal treatments (significant in adipose and liver tissue), but differences were driven strongly by differences in DMP which did not appear to be absorbed well across skin. When differences in tissue residue concentrations between oral and dermal exposure did occur, the difference was not drastic. Taken together these results suggest that dermal exposure should be considered in risk assessments for reptilian receptors. Dermal exposure may be an especially important route for reptiles as their ectothermic physiology translates to lower energetic demands and dietary exposure compared to birds and mammals.</p></div

Phthalate concentrations in western fence lizard skin samples at 24 and 48 hour time points.

<p>Data for 24 hours and 48 hours are presented as log10 means (ng/g ± SE). Phthalate congeners are: di-methyl phthalate (DMP), di-iso-butyl phthalate (DIBP), and di-n-octyl phthalate (DNOP). Linear mixed effects models suggest significant effects of exposure route (χ² = 122.5, df = 2, p<0.001) and phthalate congener (χ² = 99.0, df = 2, p<0.001), and a significant effect of time (χ² = 9.13, df = 2, p = 0.002). See text for complete statistical information. Numbers above error bars indicate the number of detections out of 12 samples. At 48 hours, 2 oral treatment samples were lost during extraction and, therefore, detections listed are out of 10 samples.</p

Phthalate concentrations in western fence lizard blood samples at 24 and 48 hour time points.

<p>Data for 24 hours and 48 hours are presented as log10 means (ng/g ± SE). Phthalate congeners are: di-methyl phthalate (DMP), di-iso-butyl phthalate (DIBP), and di-n-octyl phthalate (DNOP). Linear mixed effects model suggests significant effects of exposure route (χ² = 31.71, df = 2, p<0.001) and phthalate congener (χ² = 140.3, df = 2, p<0.001), and a significant effect of time (χ² = 7.16, df = 2, p = 0.007). See text for complete statistical information. Numbers above error bars indicate the number of detections out of 12 samples. At 24 hours, 1 dermal treatment sample was lost during extraction and, therefore, detections listed are out of 11 samples.</p

Phthalate concentrations in western fence lizard adipose tissues at 24 and 48 hour time points.

<p>Data for 24 hours and 48 hours are presented as log10 means (ng/g ± SE). Phthalate congeners are: di-methyl phthalate (DMP), di-iso-butyl phthalate (DIBP), and di-n-octyl phthalate (DNOP). Linear mixed effects models suggest a significant effect of exposure route (χ² = 95.21, df = 2, p<0.001) and a significant difference between phthalate congeners (χ² = 70.96, df = 2, p<0.001), but no significant effect of time (χ² = 2.05, df = 2, p = 0.15). See text for complete statistical information. Numbers above error bars indicate the number of detections out of 12 samples.</p

Phthalate concentrations in western fence lizard liver samples at 24 and 48 hour time points.

<p>Data for 24 hours and 48 hours are presented as log10 means (ng/g ± SE). Phthalate congeners are: di-methyl phthalate (DMP), di-iso-butyl phthalate (DIBP), and di-n-octyl phthalate (DNOP). Linear mixed effects model suggests significant effects of exposure route (χ² = 31.18, df = 2, p<0.001) and phthalate congener (χ² = 63.41, df = 2, p<0.001), and a marginally significant effect of time (χ² = 3.58, df = 2, p = 0.059). See text for complete statistical information. Numbers above error bars indicate the number of detections out of 12 samples.</p