Changes of sodium and ATP affinities of the cardiac (Na,K)-ATPase during and after nitric oxide deficient hypertension.

In the cardiovascular system, NO is involved in the regulation of a variety of functions. Inhibition of NO synthesis induces sustained hypertension. In several models of hypertension, elevation of intracellular sodium level was documented in cardiac tissue. To assess the molecular basis of disturbances in transmembraneous transport of Na+, we studied the response of cardiac (Na,K)-ATPase to NO-deficient hypertension induced in rats by NO-synthase inhibition with 40 mg/kg/day N(G)-nitro-L-arginine methyl ester (L-NAME) for 4 four weeks. After 4-week administration of L-NAME, the systolic blood pressure (SBP) increased by 36%. Two weeks after terminating the treatment, the SBP recovered to control value. When activating the (Na,K)-ATPase with its substrate ATP, no changes in Km and Vmax values were observed in NO-deficient rats. During activation with Na+, the Vmax remained unchanged, however the K(Na) increased by 50%, indicating a profound decrease in the affinity of the Na+-binding site in NO-deficient rats. After recovery from hypertension, the activity of (Na,K)-ATPase increased, due to higher affinity of the ATP-binding site, as revealed from the lowered Km value for ATP. The K(Na) value for Na+ returned to control value. Inhibition of NO-synthase induced a reversible hypertension accompanied by depressed Na+-extrusion from cardiac cells as a consequence of deteriorated Na+-binding properties of the (Na,K)-ATPase. After recovery of blood pressure to control values, the extrusion of Na+ from cardiac cells was normalized, as revealed by restoration of the (Na,K)-ATPase activity.