Effects of Hg2+ on Ca2+ dynamics in the scallop sarcoplasmic reticulum (Pecten jacobaeus): Protective role of glutathione

Scallop (Pecten jacobaeus) sarcoplasmic reticulum (SR) vesicles were functionally characterized and tested for Hg2+ effects. An SR-containing 20,000 g supernatant from adductor muscle homogenate was incubated with fluo-3, allowing free Ca2+ variations to be spectrofluorimetrically followed. Data showed an ATP-dependent thapsigargin-sensitive Ca2+ uptake, revealing the activity of the SR Ca2+ ATPase. Treatment with ryanodine elicited Ca2+ release, showing the presence of ryanodine sensitive Ca2+ channels, whereas InsP(3) caused negligible effects. Exposure to different concentrations of Hg2+ (1-50 mu M) produced a dose dependent Ca2+ release from SR vesicles, which was shown to depend on both Ca2+ ATPase inhibition and Ca2+ channel opening. Hg2+ binding to sulfhydryls was pointed out by incubation with Thiolyte, whereas an involvement of sulfhydryls in Ca2+ release was assessed by treatment with the sulfhydryl reagent N-ethylmaleimide (NEM). Yet, conversely to Hg2+, NEM seemed unable to open Ca2+ channels, suggesting that the latter effect occurs via some specific heavy metal interaction, possibly involving sulfhydryls not available to larger molecules or even components other than sulfhydryls. Pre incubation of SR with reduced glutathione (GSH) largely prevented Hg2+ effects, whereas a certain reduction of metal injury also occurred by adding GSH after Hg2+ exposure, thus confirming the role of GSH as a first line of defense against heavy metals. Copyright (C) 1997 Elsevier Science Inc.