[The importance of the ion-transport systems of the sarcolemma and sarcoplasmic reticulum in changing rat cardiac contractile function under a hypersodium medium].

: Following a reduced pressure in the left ventricle, elevated concentrations of sodium ions enhanced by half the contraction force of the rat isolated heart. This effect was shown to be independent of the Na-channels blockers or Na/H exchange of caffeine but quite susceptible to sodium channel blockers, caffeine, and the blocking agent for Na-Ca exchange Ni2+. A decrease in potassium concentration amplified, and elevation of K+ level attenuated the positive inotropic effect of the elevated concentration of sodium ions. The effect was preserved even after heart arrest induced by verapamil. The findings suggest that elevated concentration of sodium ions may affect the Na+/Ca2+ exchange and provoke Ca2+ release from sarcoplasmic reticulum by means of changing the sodium gradient. These data corroborate the Leblanc and Hume hypothesis of the sodium-induced calcium ions release from sarcoplasmic reticulum.