Branchial ion fluxes and toxicant extraction efficiency in lamprey ( Petromyzon marinus) exposed to methylmercury

Abstract

This study used tubes to hold eel-like freshwater fish, larval lampreys ( Petromyzon marinus). This apparatus allowed convenient measurement of: (1) how toxicants affect flux rates of Na + and Cl − across the pharynx; and (2) the efficiency with which the pharynx extracts toxicant from solution. Methylmercury was presented to lampreys for 2–4 h at 13°C in an aqueous medium (0.2 mM NaCl and KHCO 3) at a concentration of 180 μg Hg/l (about a 40-h LC 50). Compared to control lampreys, animals exposed to methylmercury exhibited a significantly lower influx of Cl − (by 30%: 0.15 vs. 0.21 μM/g per h), a greater efflux of Na + (by 30%: 0.34 vs. 0.26 μM/g per h), and a greater efflux of Cl − (by 22%: 0.36 vs. 0.3 μM/g per h). Influx of Na + (0.23 μM/g per h) was not affected by methylmercury. The elevated efflux of Na + and Cl − from the gills is consistent with established hypotheses that methylmercury increases the permeability of cell membranes to cations and other small molecules. Our data do not, however, support an oftenstated hypothesis that methylmercury inhibits the enzyme responsible for Na + uptake into cells, Na +K +ATPase. Lampreys extracted methylmercury from solution with efficiencies of 41–66%, higher percentages than are recorded for other fish. Such exceptional extraction efficiencies may relate to the soft water used in this study.