title: High net calcium uptake explains the hypersensitivity of the freshwater pulmonate snail, Lymnaea stagnalis, to chronic lead exposure.
abstract: Previous studies have shown that freshwater pulmonate snails of the genus Lymnaea are exceedingly sensitive to chronic Pb exposure. An EC20 of <4microgl(-1)Pb for juvenile snail growth has recently been determined for Lymnaea stagnalis, which is at or below the current USEPA water quality criterion for Pb. We characterized ionoregulation and acid-base balance in Pb-exposed L. stagnalis (young adults approximately 1g) to investigate the mechanisms underlying this hypersensitivity. After 21-day exposure to 18.9microgl(-1)Pb, Ca(2+) influx was significantly inhibited (39%) and corresponding net Ca(2+) flux was significantly reduced from 224 to -23nmolg(-1)h(-1). An 85% increase in Cl(-) influx was also observed, while Na(+) ion transport appeared unaffected. Finally, a marked alkalosis of extracellular fluid was observed with pH increasing from 8.35 in the control to 8.65 in the 18.9microgl(-1) Pb-exposed group. Results based on direct measurement of Ca(2+) influx in 1g snails gave an influx nearly an order of magnitude higher (750nmolg(-1)h(-1)) than in comparably sized fish in similar water chemistry. Under control conditions, specific growth rate in newly hatched snails was estimated at 16.7% per day over the first 38-day post-hatch and whole body Ca(2+) concentrations were relatively constant at approximately 1100nmolg(-1) over this period. Based on these data, it is estimated that newly hatched snails have net Ca(2+) uptake rates on the order of 7600nmolg(-1)h(-1). A model was developed integrating these data and measured inhibition of Ca(2+) influx rates of 13.4% and 38.7% in snails exposed to 2.7 and 18.9microgl(-1)Pb, respectively. The model estimates 45% and 83% reductions in newly hatched snail growth after 30-day exposure in these two Pb-exposed groups. These results compare well with previous direct measurements of 47% and 90% reductions in growth at similar Pb concentrations, indicating the high net Ca(2+) uptake is the controlling factor in observed Pb hypersensitivity.
authors.affiliation: RSMAS, University of Miami, FL 33145 USA. mgrosell@rsmas.miami.edu
authors.forename: Martin
authors.initials: M
authors.lastname: Grosell
authors.forename: Kevin V
authors.initials: KV
authors.lastname: Brix
mesh_terms: D000818:Animals; D002118:Calcium / Q000493:pharmacokinetics*; D002712:Chlorides / Q000493:pharmacokinetics; D004781:Environmental Exposure; D005618:Fresh Water; D006458:Hemolymph / Q000378:metabolism; D007854:Lead / Q000633:toxicity*; D008195:Lymnaea / Q000187:drug effects* / Q000254:growth & development / Q000378:metabolism*; D012964:Sodium / Q000493:pharmacokinetics; D014874:Water Pollutants, Chemical / Q000633:toxicity*
chemical_list: D002712:Chlorides; D014874:Water Pollutants, Chemical; D007854:Lead; D012964:Sodium; D002118:Calcium