The negative cumulative effects of lanthanides on the human body are well known; they are associated mainly with the toxic effects of rare earth metals (REE) on muscle tissue. However, the effects of low concentrations of these metals on muscle are less understood. In our work, we found out an unusual stimulating effect of low concentrations of europium (Eu3+) on spontaneous contractions of atria of a frog. The purpose of this study was to study the stimulating effect of Eu3+ on the contraction of atria, both normally and in the presence of the mitochondrial respiration inhibitor sodium azide (NaN3). The study was carried out using two experimental models: muscle preparations obtained from isolated atria of the heart of the frog Rana ridibunda and mitochondria isolated from the heart of male Wistar rats. As a result of the studies, it was established that Eu3+ in a concentration of 0.2 mM, at a temperature of 20°C, potentiated contractions of the frog atria in situ; both the amplitude and the maximum rate of increase of single spontaneous contractions increased. Spontaneous atrial contractions became more resistant to the effects of 1 mM NaN3. At the same time, Eu3+ did not affect the respiration of energized mitochondria (activated by ADP (state 3) or 2,4-dinitrophenol (state 3UDNP). The intensity of this respiration decreased after the calcium load of mitochondria, regardless of the presence of Eu3+ in the medium. Thus, Eu3+ ions at low concentrations (0.2 mM) stimulated atrial contraction and had a positive inotropic effect. The stimulating effect of low concentrations of Eu3+ on the heart can be explained by the synergism in the action of Ca2+ and Eu3+ on calcium channels, stimulation of Ca2+-dependent processes in cardiomyocytes and the absence of a negative effect on mitochondrial respiration.
