The mechanisms of the effect of a high extracellular sodium concentration on the contractile activity of the isolated rat heart

: Increase in the extracellular concentration of sodium (from 140 mM to 200 mM) causes an initial increase in the developing left-ventricular pressure which is replaced by increase of the amplitude of contractions by 40-50% in comparison with the initial condition. Lithium chloride (60 mM) did not cause similar changes. The initial diminution of myocardial contractility under the effect of a hypersodium medium was not sensitive to verapamil, ethmosin, lidocaine, caffeine or the Na-H exchange blocker hexamethylenamyloride (HMA). Verapamil (0.1 microM) or HMA (1 microM) did not influence the increase in the developing pressure induced by the hypersodium medium. The favorable inotropic effect of the hypersodium medium was manifested after arrest of the heart with verapamil (2 microM). Caffeine (2 microM), ethmosin (1 microM) or lidocaine (100 microM) weakened the favorable effect of the hypernatrium medium.