Comparative Study of Effect of Pr<sup>3</sup>+ and La<sup>3</sup>+ Ions on Calcium-Dependent Processes in Frog Cardiac Muscle and Rat Heart Mitochondria
The inotropic effect of Pr<sup>3+</sup> and La<sup>3</sup>+ ions on heart muscle of frog Rana ridibunda as well as the ions influence on respiration, swelling and potential (A¥<sub>m</sub>i<sub>to</sub>) on the inner membrane of Ca<sup>2</sup>+-loaded rat heart mitochondria, energized by glutamate and malate or succinate in the presence of rotenone were studied. It was found that 2 mM Pr<sup>3</sup>+, injected in Ringer solution, reduces the force of spontaneous and induced by electrical stimulation contractions in the heart and also has a negative chronotropic effect decreasing the frequency of spontaneous contractions. Pr<sup>3</sup>+ and La<sup>3</sup>+ prevented a decrease in 2,4-dinitrophenol (DNP)-uncoupled respiration rate of energized rat heart mitochondria, swelling of these organelles in salt media and a reduction in A¥<sub>m</sub>i<sub>to</sub> on the inner mitochondrial membrane which were induced by Ca<sup>2</sup>+ ions. Retardation by Pr<sup>3</sup>+ and La<sup>3</sup>+ ions of these calcium-induced effects may suggest that these metals inhibit in the inner mitochondrial membrane the opening of a calcium-dependent nonspecific channel caused by Ca<sup>2</sup>+ overload of mitochondria. Data obtained are important for a better understanding of mechanisms in the damaging action of rare earth elements on Ca<sup>2</sup>+-dependent processes in vertebrate myocardium.