Presynaptic and postsynaptic effects of mercuric ions on guinea-pig ileum longitudinal muscle strip preparation.

The toxic effect of mercuric ions on intestinal cholinergic neurotransmission was investigated in vitro. Hg2+ inhibited the evoked release and enhanced the resting release of ACh. Smooth muscle contraction was irreversibly inhibited by Hg2+ in a concentration-dependent manner, and Na2EDTA did not antagonize this effect. We also investigated if Hg2+ enters the nerve terminal through Ca(2+)-channels, or Na(+)-channels, or both. The effects of mercuric ions obtained in our study were completely abolished by the combined administration of TTX and Co2+. It is suggested that the site of the action of mercuric ions is intracellular. We concluded that Hg2+ may interfere with cholinergic transmission by blocking [Ca2+]o-dependent release of ACh and by enhancing [Ca2+]o-independent resting release of ACh. The effect of Hg2+ was not only presynaptic since it also inhibited the effect of ACh on smooth muscle.