Effect of chronic salt loading on renal Na-K-ATPase activity in the rat.

The effect of chronic salt loading in rats fed regular chow diet on renal Na-K-ATPase was studied. The high salt intake was associated with increased filtered load of sodium (control: 126 +/- 3.9 mueq/min, salt loaded: 146 +/- 2.5, mueq/min, P less than 0.001), increased net reabsorption of sodium (control: 125.3 +/- 3.9 mueq/min, salt load: 134.8 +/- 2.4 mueq/min, P less than 0.05), increased urinary excretion of potassium (control: 2.4 +/- 0.09 mueq/min/min; salt loaded: 3.0 +/- 0.1 mueq/min, P less than 0.001) and increase in single kidney weight (control: 0.798 +/- 0.010 g, salt loaded: 0.937 +/- 0.015 g, P less than 0.001). The above mentioned changes were associated with significant increase in renal microsomal and whole homogenate medullary Na-K-ATPase activity in the salt loaded group (microsomes: control 74.1 +/- 4.9 mumole Pi/mg prot/hr, salt loaded 112.7 +/- 6.0 mumole Pi/mg prot/hr, P less than 0.001; whole homogenate: control 22.7 +/- 1.0 mumole Pi/mg prot/hr, salt load 29.4 +/- 1.6 mumole Pi/mg prot/hr, P less than 0.005), while cortical and papillary Na-K-ATPase activity remained unchanged. Taken together, these results show that increased filtered and reabsorbed load of sodium, which follows high salt intake, is associated with an increased renal Na-K-ATPase activity. The preferential rise in medullary enzymatic activity may be interpreted as suggesting that these changes may stem from increased delivery and reabsorption of sodium in the ascending limb of Henle's loop.