Vascular tissue (Na,K)ATPase activity and aging in the F344 rat

Age-associated increase in salt sensitivity is accompanied by a shift in the atrial natriuretic peptide modulation of the effect of marinobufagenin on renal and vascular sodium pump

BACKGROUND

Marinobufagenin (MBG) promotes natriuresis via inhibition of renotubular Na/K-ATPase (NKA) and causes vasoconstriction via inhibition of vascular NKA. Atrial natriuretic peptide (ANP), via cyclic guanosine monophosphate (cGMP)/protein kinase G (PKG)-dependent mechanism, sensitizes renal NKA to MBG but reduces MBG-induced inhibition of vascular NKA. As aging is associated with a downregulation of cGMP/PKG signaling, we hypothesized that in older rats, ANP would not potentiate renal effects of MBG and would not oppose vascular effects of MBG.

METHODS

In younger (3-month-old) and older (12-month-old) Sprague-Dawley rats, we compared SBP, natriuresis, activity of NKA in aorta and renal medulla, and levels of MBG and α-ANP at baseline and following acute NaCl loading (20%, 2.5 ml/kg, intraperitoneally), and studied modulation of MBG-induced NKA inhibition by α-ANP in vitro.

RESULTS

As compared with younger rats, NaCl-loaded older rats exhibited a greater MBG response, greater SBP elevation (25 vs. 10 mmHg, P < 0.01) and greater inhibition of NKA in aorta (39 vs. 7%, P < 0.01), 30% less natriuresis, and less inhibition of renal NKA (25 vs. 42%, P < 0.05) in the presence of comparable responses of α-ANP and cGMP. In aorta and kidney of older rats, the levels of PKG were reduced, the levels of phosphodiesterase-5 were increased compared with that in young rats, and α-ANP failed to modulate MBG-induced NKA inhibition.

CONCLUSION

Age-associated downregulation of cGMP/PKG-dependent signaling impairs the ability of ANP to modulate the effects of MBG on the sodium pump, which contributes to salt sensitivity.

Restriction of energy intake, energy expenditure, and aging

Energy restriction (ER), without malnutrition, increases maximum life span and retards the development of a broad array of pathophysiological changes in laboratory rodents. The mechanism responsible for the retardation of aging by ER is, however, unknown. One proposed explanation is a reduction in energy expenditure (EE). Reduced EE may increase life span by decreasing the number of oxygen molecules interacting with mitochondria, thereby lowering reactive oxygen species (ROS) production. As a step toward testing this hypothesis, it is important to determine the effect of ER on EE. Several whole-body, organ, and cellular studies have measured the influence of ER on EE. In general, whole-body studies have reported an acute decrease in mass-adjusted EE that disappears with long-term ER. Organ-specific studies have shown that decreases in EE of liver and gastrointestinal tract are primarily responsible for initial reductions in EE with ER. These data, however, do not determine whether cellular EE is altered with ER. Three major processes contributing to resting EE at the cellular level are mitochondrial proton leak, Na(+)-K(+)-ATPase activity, and protein turnover. Studies suggest that proton leak and Na(+)-K(+)-ATPase activity are decreased with ER, whereas protein turnover is either unchanged or slightly increased with ER. Thus, two of the three major processes contributing to resting EE at the cellular level may be decreased with ER. Although additional cellular measurements are needed, the current results suggest that a lowering of EE could be a mechanism for the action of ER.

Lymphocyte Na,K-ATPase is reduced in aged people

Na,K-ATPase-dependent 86Rb uptake, maximum velocity (Vmax), Michaelis constant (Km) of the uptake, and [3H]-ouabain binding were investigated in the lymphocytes of 10 elderly subjects (age greater than 60 years), and in 10 middle-aged (41 to 60 years) and 10 young controls (age less than or equal to 40 years). 86Rb uptake was reduced in elderly versus both middle-aged and young subjects (20.14 +/- 3.30 v 35.60 +/- 2.67, P = .002, and v 36.53 +/- 4.49 nmol, P = .012), as was the number of [3H]-ouabain binding sites per cell (32,662 +/- 2,215 v 40,420 +/- 1,184, P = .011, and v 40,596 +/- 1,349, P = .014). Vmax was reduced in elderly v young subjects (1.20 +/- 0.10 v 1.64 +/- 0.13, P = .034), but not versus the middle-age group (1.20 +/- 0.10 v 1.54 +/- 0.12 nmol.min-1, NS). Km was no different among the three groups. No differences were found between middle-aged and young subjects. Significant correlations were observed between age and Na,K-ATPase-dependent 86Rb uptake (r = -.620, P = .00009), Vmax (r = -.439, P = .024), and [3H]-ouabain binding sites (r = -.648, P = .002). Moreover, the site number was positively correlated with both uptake (r = .635, P = .002) and Vmax (r = .554, P = .011). These differences were observed both in women and men. We conclude that there is an age-dependent reduction in lymphocyte Na,K-ATPase activity, which is fully manifested over 60 years, and that this alteration is probably due to the reduced number of functional units of Na,K-ATPase in advancing age.