Distribution kinetics of trophic single doses of methylmercury, tributyltin, and corresponding inorganic ions in the starfish Leptasterias polaris

Abstract

Whole-body autoradiography (WBARG) and a multicompartmental model wereused to describe, quantify, and compare the distribution kinetics over48 h of trophic single doses of methylmercury (MeHg), tributyltin(But(3)Sn), and the corresponding inorganic ions, Hg(II) and Sn(IV), instarfish Leptasterias polaris. The food consisted of homogenized musselflesh spiked with 2.5 nmol g(-1) of (HgCl2)-Hg-203, (CH3HgCl)-Hg-203,(SnCl4)-Sn-113 or (BUt)(3)(SnCl)-Sn-113. The model presented differsfrom conventional multicompartmental pharmacokinetic models ascompartment contents are related to the whole-body content rather thanthe concentration of metal species in a reference tissue. WBARGindicated that transfer of labelled compounds from the stomach(Compartment E) to pyloric caeca (Compartment C), and from pyloric caecato the rest of the starfish (coelomic fluid, gonads, body wall, podia;Compart ment R) proceeded mainly by transport via the pyloric ducts andby diffusion in the coelomic fluid, respectively, with a negligiblecontribution from the haemal system. Pyloric caeca were the main sitesof accumulation for inorganic Hg and Sn (61 to 63% of total content) atsteady-state while MeHg was more evenly distributed, each compartmentaccounting for one-third of the whole-body content. But(3)Sn content ofstarfish also tended to be more homogeneously distributed betweencompartments. Transfer of MeHg (rate constant alpha(1) = 0.208 h(-1))from the stomach to pyloric caeca proceeded at a rate similar toinorganic Hg(II) and Sn(IV) (alpha(1) = 0.196 and 0.178 h(-1),respectively) and was assumed to be mainly a passive process associatedwith food transport. However, MeHg was transfered at a faster rate(alpha(2) = 0.099 h(-1)) from the pyloric caeca to the rest of theindividual (coelomic fluid, gonads, body wall, podia) than inorganicHg(II) (alpha(2) = 0.061 h(-1)), this effect being associated to thefacility of MeHg to cross biological membranes. But(3)Sn distributionkinetic was the slowest (alpha(1) = 0.071 h(-1) and alpha(2) = 0.017h(-1)). This result may be related to the strong binding capacity ofBut(3)Sn towards biological ligands and its low water solubility,coupled to other physical properties like sterical hindrance. Althoughorgan distributions of MeHg and But(3)Sn at steady-state were rathersimilar, their kinetics were radically different, allowing a cleardistinction between the 2 organometals. This finding enhances thenecessity to consider the contaminant uptake problem from the point ofview of both thermodynamic and kinetic approaches. The model developedin this work allowed the distinction between 2 transfer modesdifferentiated by their own chemical mechanisms. Such a model could beused for further studies on the distribution kinetics of trace metals,organometals, and other substances (Like nutrients) in aquatic andterrestrial invertebrates.