Effects of charge and lipophilicity on mercurial-induced reduction of ⁴⁵Ca²⁺ uptake in isolated nerve terminals of the rat

The goal of this study was to compare the ability of neurotoxic mercurials which differ in ionic charge and/or lipophilicity to block nerve-terminal calcium channels. To do so, we examined the acute effects of methyl mercury (MeHg⁺), ethyl mercury (EtHg⁺), inorganic mercury (Hg²⁺), dimethyl mercury (Me₂Hg), p-chloromercuribenzoate (PCMB), and p-chloromercuriphenylsulfonate (PCMBS^-) (10-1000 μm) on ⁴⁵Ca²⁺ flux into rat forebrain synaptosomes at rest and during depolarization. Basal (depolarization-independent) entry of ⁴⁵Ca²⁺ was measured during 10-sec exposure to mercurials in solutions containing 5 mm KCl. Concentrations of 50, 100, 250, 500, and 1000 μm of Hg²⁺, MeHg⁺, and EtHg⁺ reduced basal influx of ⁴⁵Ca²⁺. PCMB reduced basal influx at concentrations of 10, 50, and 100 μm, but increased influx at 1000 μm. PCMBS^- and (Me)₂Hg had no effect on basal flux at any concentration tested. Uptake of ⁴⁵Ca²⁺ was measured after 1 sec of K⁺-induced depolarization (41.25 mm) to determine influx through voltage-dependent Ca²⁺ channels ("fast" phase) or during the last 10 sec of a 20-sec period of depolarization for uptake associated with a reversed Na⁺ Ca²⁺ exchanger and a residual noninactivating Ca²⁺ channel component ("slow" phase). Fast and slow components of ⁴⁵Ca²⁺ uptake into synaptosomes were blocked in a concentration-dependent manner by MeHg⁺, EtHg⁺, and Hg²⁺. For block of the fast component, the calculated IC₅₀'s and confidence intervals were (μm) EtHg⁺, 92 (82, 102); Hg²⁺, 155 (149, 161); and MeHg⁺, 196 (120, 272). IC₅₀'s and the confidence intervals for the slow component of uptake were (μm) Hg²⁺, 49 (43, 55); MeHg⁺, 72 (67, 77); and EtHg⁺, 147 (142, 152). In contrast, Me₂Hg, PCMB, and PCMBS^- (10-1000 μm) caused no appreciable reduction in either phase of ⁴⁵Ca²⁺ uptake. Increasing [Ca²⁺]_e was unable to overcome the block induced by MeHg⁺ and EtHg⁺ (100 μm) on either phase of ⁴⁵Ca²⁺ uptake into synaptosomes. Likewise, increasing [Ca²⁺]_e failed to overcome block of the slow component by Hg²⁺ (100 μm). Increasing [Ca²⁺]_e was able to overcome, in part, block of the fast phase induced by Hg²⁺ (100 μm) although the percentage of reversal was not statistically significant. The magnitude of block of ⁴⁵Ca²⁺ uptake increased as a function of increasing [K⁺]_e for MeHg⁺ and EtHg⁺, suggesting the block to be voltage-dependent. Thus, mercurials of dissimilar charge and lipophilicity affect synaptosomal Ca²⁺ uptake differentially. Specifically, results of this study suggest that: (1) high concentrations of certain mercurials affect resting Ca²⁺ permeability; (2) a positive charge is important for interaction of mercurials with Ca²⁺ channels during depolarization; (3) organic, monovalent mercurials block influx of ⁴⁵Ca²⁺ into synaptosomes in a noncompetitive, voltage-dependent manner; and (4) divalent mercury blocks influx of ⁴⁵Ca²⁺ into synaptosomes in a competitive, voltage-independent manner.