Comparative effects of divalent cations on the methylmercury-induced alterations of acetylcholine release

Bath application of methylmercury (MeHg) at the murine neuromuscular junction blocks synchronous evoked release of acetylcholine (ACh) and then increases spontaneous release of ACh effects observed electrophysiologically as cessation of EPPS, and increased MEPP frequency (MEPPf), respectively. The objectives of the present study were to test whether the effect of MeHg on spontaneous release was Ca++-specific by substituting Sr++ or Ba++ for Ca++, whether the time course of MeHg-induced block of synchronous evoked release was altered by varying Ca++ concentrations or substituting Sr++ and whether the processes involved in the decay of elevated MEPPf after repetitive stimulation (asynchronous evoked release) were altered by MeHg. MEPPf was recorded continuously from the rat hemidiaphragm using conventional methods during pretreatment with 2 mM Ca++, 2 mM Sr++ or 0.5 mM Ba++ and subsequently with the cation plus 100 microM MeHg. The time to peak MEPPf in MeHg was not different under any condition; however, peak MEPPf was lower in Sr++ solutions than in Ca++ or Ba++ solutions. EPPs were recorded from the rat hemidiaphragm cut muscle preparation during pretreatment with either 2, 4 or 8 mM Ca++ or 2 or 4 mM Sr++ and subsequently with the cation plus 100 microM MeHg. The latency to block of the EPP in 4 and 8 mM Ca++ was not significantly different from the latency in 2 mM Ca++. The latency to block in 2 or 4 mM Sr++ was also not different from that in Ca++. In addition, under all conditions EPP amplitude remained virtually unchanged from pretreatment values until block occurred after 8 to 9 min exposure to MeHg.(ABSTRACT TRUNCATED AT 250 WORDS)