[The effect of Mn++ on the electrotropic and inotropic vagal effect]

An isolated rabbit atrium, innervated by vagal nerve, was tested. Increased Mn++ concentrations decrease the areas of the action potentials and the amplitudes of contractions. At 2 mM Mn++ the contraction disappears and an extremely sharp action potential is recorded. By variation of both Ca++ and Mn++ concentrations one can observe parallely shifted concentration-response curves for the amplitudes of the contractions. This result is described by the model of a competitive-antagonistic Mn++-Ca++ action. At 1 mM Mn++ the inotropic and electrotropic effects disappear. Small concentrations of Mn++ (0.15 mM) increase the electrotropic vagal effects. At 0.5 mM Mn++ both vagal effects are diminished. The discussion of the results is based on the action of acetylcholine on the slow inward current of calcium and sodium ions.