Modifications of Ca²⁺ signaling by inorganic mercury in PC12 cells

The effects of different levels of inorganic mercury (Hg²⁺) on depolarization- or agonist-stimulated Ca²⁺ signals were studied in PC12 cells. Exposure to 50-300 nM Hg²⁺ did not alter the resting cytosolic free Ca²⁺ concentration ([Ca²⁺]_i), but enhanced the Ca²⁺ response to KCl-induced depolarization. Patch-clamp experiments revealed that these Hg²⁺ concentrations increased the voltage-dependent Ca²⁺ current through L-type channels. Also, Hg²⁺ treatment amplified the intracellular [Ca²⁺]_i transients elicited by extracellular ATP. In contrast, the Ca²⁺ increase stimulated by bradykinin was unaffected. At slightly higher concentrations (1 to 2 μM), Hg²⁺ caused a sustained rise of the resting [Ca²⁺]_i. This increase did not occur in Ca²⁺-free medium and was prevented by pretreatment with NiCl₂ or with the L-type Ca²⁺ channel blockers, verapamil and nifedipine. Hg²⁺ did not mobilize Ca²⁺ from intracellular stores sensitive to thapsigargin, 2,5-di-(tert-butyl)-benzohydroquinone, or caffeine. At 2 μM, Hg²⁺ inhibited the [Ca²⁺]_i transients elicited by bradykinin, ATP, or KCl-induced depolarization. The loss of the intracellular Ca²⁺ response to bradykinin was independent from the Ca²⁺ overload elicited by Hg²⁺; instead, it was associated with inhibition of polyphosphoinositide generation. Exposure to the lower Hg²⁺ concentrations (0.3-0.5 μM greatly potentiated NGF-induced PC12 cell differentiation. Conversely, treatment with 2 μM Hg²⁺ caused cell death. Our results show that inorganic mercury has selective and different effects on Ca²⁺ signaling in PC12 cells depending on the concentration, within a narrow range.