Sodium-pump gene-expression, protein abundance and enzyme activity in isolated nephron segments of the aging rat kidney

Abstract

Aging is associated with alteration in renal tubular functions, including sodium handling and concentrating ability. Na-K-ATPase plays a key role in driving tubular transport, and we hypothesized that decreased concentrating ability of the aging kidney is due in part to downregulation of Na-K-ATPase. In this study, we evaluated Na and K balance, aldosterone levels, and Na-K-ATPase gene expression, protein abundance, and activity in aging rat kidney. Na-K-ATPase activity (assayed microfluorometrically), mRNA (RT-PCR), and protein abundance (immunoblotting) were quantitated in the following isolated nephron segments: PCT, PST, MTAL, DCT, and CCD from 2, 8, 15, and 24 month-old-rats. In the course of aging, creatinine clearance decreased from 0.48 ± 0.02 mL/min/100 g BW to 0.28 ± 0.06 (P < 0.001) and aldosterone decreased from 23.6 ± 0.8 ng/dL to 13.2 ± 0.6 (P < 0.001). Serum Na+ and K+ increased by 4.0% and 22.5%, respectively. Na-K-ATPase activity, mRNA, and protein abundance of the α1 subunit displayed similar trends in all assayed segments; increasing in PCT and PST; decreasing in MTAL and DCT; increasing in CCD: in PCT they increased by 40%, 75%, and 250%, respectively; while in PST they increased by 80%, 50%, and 100%, respectively (P < 0.001). In MTAL they declined by 36%, 24%, and 34%, respectively, and in DCT by 38%, 59%, and 60%, respectively (P < 0.001). They were higher in CCD by 110%, 115%, and 246%, respectively (P < 0.001). Rats maintained Na/K balance; however with a steady state elevated serum K+. These results reveal quantitative changes in axial distribution of Na-K-ATPase at the level of gene expression, protein abundance, and activity in the nephrons of aging animals and may explain, in part, the pathophysiology of the senescent kidney.

Mechanisms of altered renal sodium handling in age-related hypertension

“…A recent study demonstrated that aged FBN rats exhibit decreased basal NKCC2 protein expression and fail to increase NKCC2 expression in response to water restriction (71). These findings are consistent with the observation that basolateral Na ϩ -K ϩ -ATPase activity is reduced in the medullary thick ascending limb of aged Sabra rats (63). Although further studies investigating NKCC2 activity are required, these reports suggest that aging may be associated with a decline in NKCC2 activity and responsiveness that could have contributed to a delayed natriuretic response to furosemide observed previously in a small group of elderly patients (9).…”

“…Furthermore, aged FBN rats fail to increase ENaC expression in response to water restriction (71). Interestingly, aging was associated with increased cortical collecting duct Na ϩ -K ϩ -ATPase activity in Sabra rats (63).…”

“…The sodium hydrogen exchanger and various organic ion cotransporters in the proximal tubule reabsorb~65-70% of filtered sodium. A recent ex vivo study demonstrated that the F2 activity of the basolateral Na ϩ -K ϩ -ATPase, which creates the gradient driving apical sodium reabsorption, is increased in proximal convoluted tubule and proximal straight tubule segments isolated from aged Sabra rats (63). Similarly, ANG II-evoked Na ϩ -K ϩ -ATPase activity is enhanced in renal proximal tubules isolated from aging FBN rats (13), which develop age-related HTN.…”

“…In FBN rats, which develop age-related HTN (12,13,58), aging was associated with an increase in basal NCC expression and a failure to increase NCC expression in response to water restriction (71). In contrast, diminished Na ϩ -K ϩ -ATPase activity was observed in distal convoluted tubule segments isolated from aged Sabra rats (63), suggesting that the driving force for sodium reabsorption through the NCC is reduced. Furthermore, aging in C57Bl6/ CBA/129 mice was associated with decreased basal activity of the NCC (72).…”

Mineralocorticoids modulate the expression of the β-3 subunit of the Na+, K+-ATPase in the renal collecting duct

“…3,6 It has been described that the renal tubule expresses the b 1 subunit, and most of the published studies on sodium pump of renal tissue have focused only on the variability of a 1 and b 1 . [2][3][4]7 However, Northern blot and Western blot results obtained at the time of cloning of the b 3 subunit suggest that the kidney may express this isoform in the nephron and/or in interstitial cells. [8][9][10][11] Changes in the expression of b subunits affect the sodium pump function.…”