HgCl2-induced cell injury. Differential effects on membrane-located transport systems in unfertilized and fertilized sea urchin eggs

The inhibitory potency of HgCl2 on amino acid and Na+ transport and the mechanism of action of this heavy metal are studied in unfertilized and fertilized sea urchin eggs, in which amino acid transport systems comparable to that described in mammalian somatic cells have been characterized. These transport systems called "L" for leucine, "ASC" for alanine, serine, cysteine, and "A" for alanine are differentiated mainly by their Na+-dependency and by the amino acids transported. The carrier-mediated amino acid uptake is reduced in a dose- and time-dependent manner by HgCl2, with a 50% inhibitory concentration (IC50) in the micromolar range. The mechanisms of inhibition of the Na+-independent (L system) and the Na+-dependent (A or ASC system) transport components are different: in unfertilized eggs, HgCl2 directly interferes with the L amino acid carrier leading to a decrease of its affinity for amino-acids, whereas in fertilized eggs the inhibition of the Na+-dependent uptake of amino acids may result from an elevation of Na+ content induced both by an inhibition of the Na+ pump and by an increase in Na+ permeability. It is also shown that the action of HgCl2 on amino acid diffusion differs between unfertilized and fertilized eggs. Our findings are discussed in the context of the role of membrane in xenobiotic toxicity.