MUSSEL MICROSOMAL NA-MG2+-ATPASE SENSITIVITY TO WATERBORNE MERCURY, ZINC AND AMMONIA()

1. In Mytilus galloprovincialis Lam. both the Na+ + K+)- and the Na+-A TPases show a tissue- and dose dependent response to mercury (0.1 and 0.25 mg . L(-1) Hg2+), zinc (0.5 and 1.0 mg . L(-1) Zn2+) and ammonia (0.4 and 1.0 mg . L(-1) ammonia-N). 2. The two ATPases were generally more susceptive to the toxicants in the gills than in the mantle; in t he case of heavy metals this finding complies with the higher metal in corporation in the gills with respect to the mantle. 3. In the gills t he two ATPase inhibition by mercury was enhanced by the co-presence of environmental 0.4 mg . L(-1) ammonia-N; zinc removed the (Na+ + K+)-A TPase inhibition by 0.1 mg . L(-1) Hg2+ as well as the enzyme activati on promoted by 0.4 mg . L(-1) ammonia-N; both ATPases were inhibited b y the 1.0 mg . L(-1) ammonia-N treatment. 4. Mantle ATPases were appar ently refractory to mercury but not to zinc: the (Na+ + K+)-ATPase was similarly stimulated by the exposure to 1.0 mg . L(-1) Zn2+ and to th e mixture Zn2+ + Hg2+ and unaffected by 0.5 mg . L(-1) Zn2+ alone; the Na+-ATPase was depressed by the exposures to 1.0 mg . L(-1) ammonia-N and to the mixture Zn2+ + ammonia-N and conversely stimulated by Hg2 + ammonia-N, whereas all other treatments were uneffective. 5. In vit ro approaches pointed out a susceptivity to the toxicants only for the (Na+ + K+)-ATPase. The enzyme response in vitro often differed from t hat in vivo.