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 Pr3+ and La3+ ions on heart muscle of frog Rana ridibunda as well as the ions influence on respiration, swelling and potential (A¥mito) on the inner membrane of Ca2+-loaded rat heart mitochondria, energized by glutamate and malate or succinate in the presence of rotenone were studied. It was found that 2 mM Pr3+, 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. Pr3+ and La3+ 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¥mito on the inner mitochondrial membrane which were induced by Ca2+ ions. Retardation by Pr3+ and La3+ 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 Ca2+ overload of mitochondria. Data obtained are important for a better understanding of mechanisms in the damaging action of rare earth elements on Ca2+-dependent processes in vertebrate myocardium.