Methylmercury-induced elevations in intrasynaptosomal zinc concentrations: an 19F-NMR study

Methylmercury (MeHg) increases the concentration of intracellular Ca2+ ([Ca2+]i) and another endogenous polyvalent cation in both synaptosomes and NG108-15 cells. In synaptosomes, the elevation in [Ca2+]i was strictly dependent on extracellular Ca2+ (Ca2+e); similarly, in NG108-15 cells, a component of the elevations in [Ca2+]i was Ca2+e dependent. The MeHg-induced elevations in endogenous polyvalent cation concentration were independent of Ca2+e in synaptosomes and NG108-15 cells. The pattern of alterations in fura-2 fluorescence suggested the endogenous polyvalent cation may be Zn2+. Using 19F-NMR spectroscopy of rat cortical synaptosomes loaded with the fluorinated chelator 1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'- tetraacetic acid (5F-BAPTA), we have determined unambiguously that MeHg increases the free intrasynaptosomal Zn2+ concentration ([Zn2+]i). In buffer containing 200 microM EGTA to prevent the Ca2+e-dependent elevations in [Ca2+]i, the [Zn2+]i was 1.37 +/- 0.20 nM; following a 40-min exposure to MeHg-free buffer [Zn2+]i was 1.88 +/- 0.53 nM. Treatment of synaptosomes for 40 min with 125 microM MeHg yielded [Zn2+]i of 2.69 +/- 0.55 nM, whereas 250 microM MeHg significantly elevated [Zn2+]i to 3.99 +/- 0.68 nM. No Zn2+ peak was observed in synaptosomes treated with the cell-permeant heavy metal chelator N,N,N',N'-tetrakis(2- pyridylmethyl)ethylenediamine (TPEN, 100 microM) following 250 microM MeHg exposure. [Ca2+]i in buffer containing 200 microM 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 or 250 microM MeHg, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)