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-chloromercuriphenyl-sulfonate (PCMBS-) (10-1000 microM) 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 microM of Hg2+, MeHg+, and EtHg+ reduced basal influx of 45Ca2+. PCMB reduced basal influx at concentrations of 10, 50, and 100 microM, but increased influx at 1000 microM. 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 (microM) 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 (microM) Hg2+, 49 (43, 55); MeHg+, 72 (67, 77); and EtHg+, 147 (142, 152). In contrast, Me2Hg, PCMB, and PCMBS- (10-1000 microM) 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 microM) on either phase of 45Ca2+ uptake into synaptosomes. Likewise, increasing [Ca2+]e failed to overcome block of the slow component by Hg2+ (100 microM). Increasing [Ca2+]e was able to overcome, in part, block of the fast phase induced by Hg2+ (100 microM) 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.(ABSTRACT TRUNCATED AT 400 WORDS)