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 Na(2)EDTA did not antagonize this effect. We also investigated if Hg2+ enters the nerve terminal through Ca2+-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.