MEPS 124:143-158 (1995)  -  doi:10.3354/meps124143

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

Rouleau C, Pelletier E, Tjälve H

Whole-body autoradiography (WBARG) and a multicompartmental model were used to describe, quantify, and compare the distribution kinetics over 48 h of trophic single doses of methylmercury (MeHg), tributyltin (But3Sn), and the corresponding inorganic ions, Hg(II) and Sn(IV), in starfish Leptasterias polaris. The food consisted of homogenized mussel flesh spiked with 2.5 nmol g-1 of 203HgCl2, CH3203HgCl, 113SnCl4, or (But)3113SnCl. The model presented differs from conventional multicompartmental pharmacokinetic models as compartment contents are related to the whole-body content rather than the concentration of metal species in a reference tissue. WBARG indicated that transfer of labelled compounds from the stomach (Compartment E) to pyloric caeca (Compartment C), and from pyloric caeca to the rest of the starfish (coelomic fluid, gonads, body wall, podia; Compartment R) proceeded mainly by transport via the pyloric ducts and by diffusion in the coelomic fluid, respectively, with a negligible contribution from the haemal system. Pyloric caeca were the main sites of accumulation for inorganic Hg and Sn (61 to 63% of total content) at steady-state while MeHg was more evenly distributed, each compartment accounting for one-third of the whole-body content. But3Sn content of starfish also tended to be more homogeneously distributed between compartments. Transfer of MeHg (rate constant alpha1 = 0.208 h-1) from the stomach to pyloric caeca proceeded at a rate similar to inorganic Hg(II) and Sn(IV) (alpha1 = 0.196 and 0.178 h-1, respectively) and was assumed to be mainly a passive process associated with food transport. However, MeHg was transfered at a faster rate (alpha2 = 0.099 h-1) from the pyloric caeca to the rest of the individual (coelomic fluid, gonads, body wall, podia) than inorganic Hg(II) (alpha2 = 0.061 h-1), this effect being associated to the facility of MeHg to cross biological membranes. But3Sn distribution kinetic was the slowest (alpha1 = 0.071 h-1 and alpha2 = 0.017 h-1). This result may be related to the strong binding capacity of But3Sn towards biological ligands and its low water solubility, coupled to other physical properties like sterical hindrance. Although organ distributions of MeHg and But3Sn at steady-state were rather similar, their kinetics were radically different, allowing a clear distinction between the 2 organometals. This finding enhances the necessity to consider the contaminant uptake problem from the point of view of both thermodynamic and kinetic approaches. The model developed in this work allowed the distinction between 2 transfer modes differentiated by their own chemical mechanisms. Such a model could be used for further studies on the distribution kinetics of trace metals, organometals, and other substances (like nutrients) in aquatic and terrestrial invertebrates.

Mercury . Methylmercury . Tin . Tributyltin . Distribution . Kinetic . Food . Starfish . Leptasterias polaris

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