Increased free intrasynaptosomal Ca2+ by neurotoxic organometals: distinctive mechanisms

Effects of several alkylmetals on free intrasynaptosomal Ca2+ concentration, [Ca2+]i, were studied in vitro using the fluorescent Ca2+ indicator fura-2. Neurotoxic alkylmetals methylmercury (Met-Hg), triethyllead (TEL), triethyltin (TET), and trimethyltin (TMT) (at 2.5-30 microM) increased [Ca2+]i to different degrees. Met-Hg was the most potent, elevating [Ca2+]i 100-800 nM, dose dependently and significantly more than high K+ (150 nM) or veratridine (350 nM). The effect of Met-Hg could not be inhibited with a Ca2+ channel blocker, verapamil, nor with a Na+ channel blocker, tetrodotoxin. Inhibition of the mitochondrial Ca2+ uptake in situ with rotenone + oligomycin decreased the potency of Met-Hg to elevate [Ca2+]i but did not change the resting [Ca2+]i. Met-Hg also slightly decreased synaptosomal ATP. TEL and TET elevated [Ca2+]i by 100-200 nM. The effect of TEL, but not that of TET, could be blocked with verapamil (36%) and veratridine (67%). TEL was less efficient in the presence of ouabain. Neither TEL nor TET had significant mitochondrial effects in situ contributing to [Ca2+]i. TMT increased [Ca2+]i less than TET while dimethyltin and methyltin were inactive. These results indicate that neurotoxic derivatives of alkylmetals studied increase [Ca2+]i. This occurs mainly either by nonspecific increase (Met-Hg, TET) of Ca2+ leakage through the plasma membrane and/or specific interference with the mechanisms regulating Ca2+ fluxes through the plasma membrane (TEL).