Alterations in renal Na,K-ATPase activity and protein expression in rat models of pressure and volume overload

To explore an unexamined mechanism of cardiorenal pathophysiology by assessing renal Na, K-ATPase kinetics in rat models of pressure overload, volume overload, and their combination.

Two rat models with differing renin-angiotensin-aldosterone system activity were used: control Hannover Sprague Dawley (HAN) rats and transgenic TGR(mREN2)27 rats, the later modeling pressure overload. Each model included sham and aortocaval fistula (ACF)-operated groups to induce volume overload. The kinetic parameters of Na,K-ATPase were determined: maximal velocity of enzyme reaction (V_max), and the Michaelis constant (K_m), representing the ATP concentration at half-maximal velocity and reflecting the enzyme's affinity for ATP.

Histological studies, along with assessment of selected markers of renal injury and remodeling, confirmed kidney tissue alterations in both TGR(mREN2)27 rats and animals subjected to ACF-surgery. Regarding Na,K-ATPase, V_max was higher in transgenic rats, as revealed by 2-way ANOVA (F _{(1, 80)} = 39.06, p < 0.0001). Following ACF, V_max remained unchanged in both control and transgenic rats. In contrary, ACF had opposing effects on K_m in the two rat models: it decreased in HAN rats , but increased in TGR(mREN2)27 rats after surgery.

With regard to functional properties of the Na, K-ATPase, an increased number of substrate molecules converted to products per active site per unit time (indicated by V_max) was detected in the kidney of TGR(mREN2)27 rats. Although the creation of ACF did not alter the V_max parameter, a notable impairment in the enzyme's ability to bind ATP substrate within physiologically relevant concentrations was observed in TGR(mREN2)27 rats, but not in HAN rats.