Factors affecting the absorption of phenolics and carboxylic acids in the guppy (Poecilia reticulata)

The rate of absorption of 17 phenols, anisoles, and carboxylic acids in the guppy (Poecilia reticulata Peters) was measured at pH levels from 3 to 9. The rate was directly proportional to the concentration, and the compounds did not interfere with each other's absorption. The primary route of uptake was across the gill epithelium, but 25-40% of the total amount penetrated across the skin. When the fish were exposed at a pH low enough to prevent ionization of acidic compounds, the initial rate of uptake increased with lipophilicity up to a partition coefficient of about 10(4). Above this point, the absorption rate did not correlate with lipophilicity indicating that the uptake rate of the most lipophilic compounds was limited by the unstirred water layers. The effect of lipophilicity on absorption accounts for about half of the effect found in toxicity and bioconcentration. The absorption rate of acids declined with pH rising, but did not follow changes in the concentration of nonionized acid. The slopes of the absorption rate vs pH curves were distinctly less steep than the slope of the dissociation curve. In addition, the curves of the most lipophilic acids showed a shift to the right, which can be ascribed to the unstirred layers on the surface of the epithelia.