Characterization of 3-nitropropionic acid-induced bradycardia in isolated atria

3-Nitroprospionic acid (3-NPA) was identified as the toxic component of several plants and fungi. Recently, it was described that 3-NPA produced hypotension, accompanied by a paradoxical bradycardia. In this study, we identified a possible mechanism of action, explaining the effect of 3-NPA on cardiac tissue. We used isolated, spontaneously beating atria and heart mitochondria to measure electrophysiological properties and mitochondrial oxygen consumption. We also measured Na⁺/K⁺ ATPase activity and intracellular ATP levels. In isolated spontaneously beating atria, 3-NPA (10^-4 M) decreased heart rate by 62 ± 3%. This agent did not affect the amplitude or duration of action potentials. The duration of intervals between two action potentials, however, was prolonged from 530 ± 285 to 1400 ± 600 ms after 3-NPA exposure (10^-2 M). Oxygen consumption by heart mitochondria was inhibited by 3-NPA when either malate/glutamate or succinate were used as metabolism substrates. Cytochrome C oxidase activity was not affected by 3-NPA. Finally, atrial ATP content decreased 65 ± 3% after 3-NPA treatment. In conclusion, we show that 3-NPA decreases atrial rate by increasing the action potential phase 4, probably by inhibition of mitochondrial respiration, thereby decreasing cardiac ATP content. This suggests that 3-NPA-induced bradycardia may be related to intracellular ATP depletion.