Effect of mercury on unidirectional sodium and calcium influx in Asellus aquaticus

The effect of mercuric chloride (HgCl2) and monomethyl mercury chloride (CH3HgCl) on unidirectional 22Na and 45Ca influx were measured in the freshwater isopod Asellus aquaticus. Flux measurements involved short-term (20 min) exposure to 22Na or 45Ca 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-1 was always higher. This reflected the uptake kinetics of the Na pump which was determined to be saturable with a kmax values of 32 mumol 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 Na L-1 all CH3HgCl concentrations tested were inhibitory to Na influx, i.e., the lowest observed effect concentration (LOEC) was less than 0.04 mumol L-1. Ca influx was inhibited by HgCl2 at all concentrations tested (LOEC less than 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 greater than 68% at molar Hg concentrations greater than or equal to 1 mumol 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.