METHYLMERCURY-INDUCED ELEVATIONS IN INTRASYNAPTOSOMAL ZINC CONCENTRATIONS - AN F-19-NMR STUDY

Methylmercury (MeHg) increases the concentration of intracellular Ca2 ([Ca2+](i)) and another endogenous polyvalent cation in both synaptos omes and NG108-15 cells. In synaptosomes, the elevation in [Ca2+](i) w as strictly dependent on extracellular Ca2+ (Ca-e(2+)); similarly, in NG108-15 cells, a component of the elevations in [Ca2+](i) was Ca-e(2) dependent. The MeHg-induced elevations in endogenous polyvalent cati on concentration were independent of Ca-e(2+) in synaptosomes and NG10 8-15 cells. The pattern of alterations in fura-2 fluorescence suggeste d the endogenous polyvalent cation may be Zn2+. Using F-19-NMR spectro scopy of rat cortical synaptosomes loaded with the fluorinated chelato r amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid (SF-BAPTA), we have determined unambiguously that MeHg increases the free intrasyn aptosomal Zn2+ concentration ([Zn2+](i)). In buffer containing 200 mu M EGTA to prevent the Ca-e(2+)-dependent elevations in [Ca2+](i), the [Zn2+](i) was 1.37 +/- 0.20 nM; following a 40-min exposure to MeHg-fr ee buffer [Zn2+](i) was 1.88 +/- 0.53 nM. Treatment of synaptosomes fo r 40 min with 125 mu M MeHg yielded [Zn2+](i) of 2.69 +/- 0.55 nM, whe reas 250 mu M MeHg significantly elevated [Zn2+](i) to 3.99 +/- 0.68 n M. No Zn2+ peak was observed in synaptosomes treated with the cell-per meant heavy metal chelator N,N,N',N'-tetrakis(2-pyridylmethyl)ethylene diamine (TPEN, 100 mu M) following 250 mu M MeHg exposure. [Ca2+](i) i n buffer containing 200 mu M EGTA was 338 +/- 26 nM and was 370 +/- 64 nM following an additional 40-min exposure to MeHg-free buffer. [Ca2](i) was 498 +/- 28 or 492 +/- 53 nM during a 40-min exposure to 125 o r 250 mu M MeHg, respectively. None of the values of [Ca2+](i) differe d significantly from either pretreatment levels or buffer-treated cont rols.