Reversal of methylmercury-induced block of nerve-evoked release of acetylcholine at the neuromuscular junction.

Acute bath application of micromolar concentrations of methylmercury (MeHg) blocks the nerve-evoked release of acetylcholine at the neuromuscular junction by presynaptic effects. The goal of the present study was to try to reverse this block of stimulus-evoked release. Experiments were conducted using the phrenic nerve hemidiaphragm of the rat and conventional intracellular microelectrode techniques. Myofibers were cut ("cut muscle") to prevent contractions elicited by stimulation of the phrenic nerve. End-plate potentials (EPPs) were recorded before and during MeHg application and during subsequent reversal attempts. MeHg (100 microM) blocked the EPP within 8-9 min of application. The time to block did not differ if Sr2+ (2 mM) was substituted for Ca2+ prior to exposure to MeHg. Washing the preparation with MeHg-free physiological saline at the time the EPP was blocked failed to reverse the block of synaptic transmission even during protracted washing. Increasing the extracellular [Ca2+] from 2 to 4 mM, or application of 4-aminopyridine (50 or 100 microM) failed to reverse block of the EPP. D-Penicillamine was also ineffective at reversing transmission block when applied at 0.4 mM; however, when applied at 1 mM D-penicillamine caused a return of EPPs in three of eight experiments within 5-20 min of wash. Longer periods of washing with D-penicillamine or use of higher concentrations of D-penicillamine were not effective in reversing transmission block in the refractory preparations. Increasing the intensity or duration of stimulation at the time of EPP block was uniformly successful in reversing MeHg-induced block; in 9 of the 10 preparations tested, EPPs could again be elicited from MeHg-blocked preparations merely by increasing the intensity and/or duration of stimulation, despite the continued presence of MeHg. Following reversal of transmission block by MeHg, continued exposure to MeHg resulted in a subsequent block of the EPP within approximately 3 min. During this subsequent block of the EPP by MeHg, increasing extracellular [Ca2+] or adding 4-aminopyridine did restore synaptic transmission. These results indicate that a temporary reversal of MeHg-induced block of synaptic transmission can be produced and that this effect does not require extracellular Ca2+ during the initial stages, but does seem to require extracellular Ca2+ during later stages.