The mechanisms of brush border Na+/H+ exchanger activation by corticosteroids.
BACKGROUND
Previously we showed that corticosterone and aldosterone increased proton fluxes in proximal tubule, by micropuncture and stationary microperfusion. Since the Na+/H+ exchanger is responsible for the main proximal proton secretion, we have now evaluated the effects aldosterone on Na+/H+ exchange activity in brush border vesicles. In order to evaluate the mechanism of action of glucocorticoids and mineralocorticoids, we studied the comparative effects of corticosterone and aldosterone on the abundance of NHE3 and NHE2 isoforms.


METHODS
We isolated renal brush border vesicles from rats by differential centrifugation in sham-operated, adrenalectomized, and adrenalectomized-aldosterone treated (ADX + aldosterone) animals. We measured the kinetics of H+ transport in response to increasing concentrations of Sodium Gluconate by fluorimetry using acridine orange. For Na+/H+ exchanger abundance we used Western blot analysis of brush border proteins in the above groups and in adrenalectomized-corticosterone treated rats.


RESULTS
The Vmax in adrenalectomized animals was 22,162+/-1828 fluorescence units/min; in sham animals, 37,020+/-2722; and in ADX + aldosterone, 42,344+/-3044 (p<0.01 adrenalectomized vs others). No differences in Km were observed. Adrenalectomy decreased NHE3 abundance over Sham by 32% without modifying NHE2. Corticosterone-replacement enhanced NHE3 abundance by 76% and failed to increase NHE2. Aldosterone enhanced NHE2 abundance by 75% and did not increase NHE3.


CONCLUSIONS
Mineralocorticoids enhance Na+/H+ exchange activity by increasing NHE2 abundance; glucocorticoids, by increasing NHE3 abundance.
Zallocchi
Marisa
M
Department of Biochemistry, FCEN, U.B.A, Buenos Aires, Argentina.
Igarreta
Pilar
P
Calvo
Juan Carlos
JC
Reboucas
Nancy Amaral
NA
Damasco
María Christina
MC
D000315:Adrenalectomy; D000450:Aldosterone / Q000494:pharmacology*; D000818:Animals; D000893:Anti-Inflammatory Agents / Q000494:pharmacology; D003345:Corticosterone / Q000494:pharmacology*; D005942:Gluconates / Q000378:metabolism; D007687:Kidney Tubules, Proximal / Q000166:cytology; D008297:Male; D008871:Microvilli / Q000187:drug effects* / Q000378:metabolism*; D020033:Protein Isoforms / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers / Q000378:metabolism*
D000893:Anti-Inflammatory Agents; D005942:Gluconates; D020033:Protein Isoforms; C513110:Slc9a2 protein, rat; C000613387:Slc9a3 protein, rat; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers; D000450:Aldosterone; C030691:gluconic acid; D003345:Corticosterone