Effect of Eu3+ on Calcium-Dependent Processes in Vertebrate Myocardium

The inotropic and chronotropic effects of Eu3+ on spontaneous atrial contraction of the frog Rana ridibunda myocardium at different temperatures were studied, as well as the effects of Ca2+ and Eu3+ on isolated rat heart mitochondria, energized with glutamate and malate (the Ist respiratory complex substrates). It turned out that Eu3+ in Ringer’s solution significantly reduced the amplitude of spontaneous contractions. However, the half-relaxation period of spontaneous contractions changed insignificantly. At the same time, Eu3+ also revealed a negative chronotropic effect, which is manifested in a decrease in the frequency of spontaneous contractions. The mitochondrial swelling study showed that Eu3+ increased the energy-dependent K+ transport into the matrix in the K-acetate medium. However, Eu3+ did not affect the inner membrane proton permeability since the mitochondria swelling in the medium with NH4NO3 did not depend on the presence of Eu3+. State 40 mitochondrial respiration (substrate in the medium) was slightly increased by Eu3+. At the same time, Eu3+ did not affect the energized mitochondria respiration in the presence of ADP (state 3) or 2,4-dinitrophenol (state 3UDNP). The respiration decreased in experiments with calcium load of mitochondria regardless of the medium presence of Eu3+. Eu3+ diminished the mitochondria Ca2+-induced swelling in the above media experiments. This result can be associated with the opening of the mitochondrial permeability transition pore in their inner membrane. Eu3+ did not affect the inner membrane potential (ΔΨmito). The data obtained allow us to understand better the Eu3+ action mechanisms on Ca2+-dependent processes in the vertebrate myocardium and mitochondria.