Relationship of vascular sodium-potassium pump activity to intracellular sodium in hypertensive rats

The activity of the sodium-potassium (Na+-K+) pump in arterial tissue from rats with chronic DOCA-salt or one-kidney Grollman renal hypertension was assayed in vitro as the rate of ouabain-sensitive 86Rb+ uptake. Estimates of total cell Na+ and tissue K+ were made by a lithium-substitution method on the same segment of arterial tissue. In both freshly excised tail arteries and aortas, and in aortas after overnight cold storage at 4 degrees C and a 3-hour equilibration at 37 degrees in Krebs-Henseleit, cell Na+ content did not differ significantly between control and hypertensive groups. However, ouabain-sensitive 86Rb+ uptake was increased in arteries from DOCA-salt and renal hypertensive rats as compared to controls. In overnight-stored and 3-hour equilibrated aortic tissue, we then used low Na+ medium to reduce, or monensin, a Na+ ionophore, to increase total cell Na+, to study the relationship between cell Na+ and Na+-K+ pump activity. We observed a sigmoid relationship between total cell Na+ and ouabain-sensitive 86Rb+ uptake in tissue from all groups of rats. However, in tissue from DOCA-salt and renal hypertensive rats, the relationship between cell Na+ and pump activity was shifted, indicating a greater pump activity for each level of total cell Na+ and greater maximal pumping. These data suggest that increases observed in pump activity in in vitro arterial tissue from hypertensive rats may not be solely attributable to elevated cell Na+ content and may also involve increases in number of active sarcolemmal pump molecules per unit tissue weight or in their turnover rate.