FOURNIER, F.*; KARASOV, W.H.; MEYER, M.W.: The effect of feather development on the pharmacokinetics and oral bioavailability of methylmercury in common loon chicks
Mercury is known to preferentially bind to structures containing sulfhydryl groups (e.g. keratin in growing feathers), and the latter affinity may influence the accumulation and excretion of ingested mercury in growing birds. We therefore determined the effect of feather growth on the pharmacokinetics and oral bioavailability of methylmercury in common loon chicks (Gavia immer) as part of an ongoing research project aimed at elucidating the ecological risk of mercury to loons. Oral bioavailability was determined by comparing the blood concentration through time after oral (500 MICROg/kg body mass) and intravascular (IV; 200 MICROg/kg body mass) administration at two ages (35 and 84 days) during postnatal development (during and after feather growth). The area under the blood concentration-time curves extrapolated to infinity (AUC) were calculated from the orally and IV-dosed chicks using the trapezoidal method. Bioavailability was estimated as the ratio of the dose-corrected oral AUC to the IV AUC. Bioavailability differed between ages and it was 77% at age 35 days and 92% for 84 day old chicks. We determined pharmacokinetic parameters by fitting a two-compartment non-linear regression model to both concentration-time data sets, and this resulted in models that accurately predicted the initial rapid distribution phase and the linear excretion phase for both data sets (r2 > 0.9 in all cases). The excretion rate constant differed between ages and was found to be 0.006 and 0.048 day-1 for chicks aged 35 and 84 days, respectively. The mean elimination half-life of methylmercury in loon chicks was determined as 14 and 116 days for chicks aged 35 and 84 days, respectively. These results suggest that growing feathers are an important excretory route for ingested mercury.