Short-term NO synthase inhibition and the Na+-binding properties of cardiac Na,K-ATPase

It is known that hypertension is accompanied by increased [Na+](i). The functional properties of Na,K-ATPase, which transports the Na+ out and K+ into myocardial cells during the relaxation phase, were investigated in the left ventricle (LV), septum (SV) and the light ventricle (RV) of anesthetized dogs with moderate acute blood pressure elevation elicited by short-term (4-hour) NO synthase inhibition. The NO-insufficiency was induced by administration of an L-arginine analogue, the N-G-nitro-L-arginine methyl ester (L-NAME). Concerning the function of Na,K-ATPase under the conditions of lowered NO synthesis, we focused our attention to the binding of Na+ to the enzyme molecule. Activation of the enzyme by increasing Na+ concentrations revealed significant changes in both the maximal velocity (V-max) and the affinity for Na+ (K-Na) in all investigated heart sections. The V-max increased by 27 % in LV, by 87 % in SV and by 58 % in RV. The K-Na value increased by 86 % in LV, by 105 % in SV and by 93 % in RV, indicating an apparent decrease in the sensitivity of the Na+-binding site in the Na,K-ATPase molecule. This apparently decreased pump affinity for Na+ together with the increase of V-max suggest that, during the short-term inhibition of NO synthesis, the Na,K-ATPase is capable of extruding the excessive Na+ from the myocardial cells more effectively at higher [Na+](i) as compared to the Na,K-ATPase of control animals.