Effect of mercury on unidirectional sodium and calcium influx inAsellus aquaticus

Abstract

The effect of mercuric chloride (HgCl₂) and monomethyl mercury chloride (CH₃HgCl) on unidirectional²²Na and⁴⁵Ca influx were measured in the freshwater isopodAsellus aquaticus. Flux measurements involved short-term (20 min) exposure to²²Na or⁴⁵Ca following 2 h pre-exposure to Hg. Experiments were conducted at two different Na and Ca concentrations, 0.025 mmol L⁻¹ and 0.25 mmol L⁻¹.

HgCl₂ and CH₃HgCl inhibited Na influx at both Na concentrations although Na influx at 0.25 mmol L⁻ was always higher. This reflected the uptake kinetics of the Na pump which was determined to be saturable with a k_max values of 32 μmol Na g⁻¹ h⁻¹ and a Km value of 0.8 mmol L⁻¹. CH₃HgCl generally resulted in relatively greater inhibition of Na influx. At 0.025 mmol NaL⁻¹ all CH₃HgCl concentrations tested were inhibitory to Na influx, i.e., the lowest observed effect concentration (LOEC) was <0.04 μmol L⁻¹.

Ca influx was inhibited by HgCl₂ at all concentrations tested (LOEC <0.04). The degree of inhibition was unaffected by Ca concentration, which was seen as evidence for non-competitive inhibition. However, CH₃HgCl, which inhibited Ca influx from 0.025 mmol Ca L⁻¹ by >68% at molar Hg concentrations ≥1 μmol L⁻¹ showed no significant inhibitory affect at 0.25 mmol Ca L⁻¹. Elimination of CH₃HgCl inhibition at the higher Ca concentration suggests competition in this case, possibly at the level of access to the Ca pump.