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 Na-22 and Ca-45 influx were measured in the freshwater isopod Asellus aquaticus. Flux measurements involved short-term (20 min) exposure to Na-22 or Ca-45 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 k(max) values of 32-mu-mol 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 < 0.04-mu-mol 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 greater-than-or-equal-to 1-mu-mol 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.