Nongenomic regulation by aldosterone of the epithelial NHE3 Na⁺/H⁺exchanger

Abstract

Ϫ absorption by regulating apical NHE3 activity. In microperfused rat MTALs, the addition of 1 nM aldosterone rapidly decreased HCO 3 Ϫ absorption by 30%. This inhibition was unaffected by three maneuvers that inhibit basolateral Na ϩ /H ϩ exchange and was preserved in MTALs from NHE1 knockout mice, ruling out the involvement of NHE1. In contrast, exposure to aldosterone for 15 min caused a 30% decrease in apical Na ϩ /H ϩ exchange activity over the intracellular pH range from 6.5 to 7.7, due to a decrease in Vmax. Inhibition of HCO 3 Ϫ absorption by aldosterone was not affected by 0.1 mM lumen Zn 2ϩ or 1 mM lumen DIDS, arguing against the involvement of an apical H ϩ conductance or apical K ϩ -HCO 3 Ϫ cotransport. These results demonstrate that aldosterone inhibits HCO 3 Ϫ absorption in the MTAL through inhibition of apical NHE3, and identify NHE3 as a target for nongenomic regulation by aldosterone. Aldosterone may influence a broad range of epithelial transport functions important for extracellular fluid volume and acid-base homeostasis through direct regulation of this exchanger.

Nongenomic Actions of Aldosterone on the Renal Tubule

“…35 Aldosterone inhibited NHE3 through a reduction in the exchanger's maximal velocity. 20 In OKP cells, a cell culture model of renal proximal tubule, glucocorticoids induced an acute increase in maximal velocity of NHE3 that was not blocked by cycloheximide, consistent with nongenomic regulation. 36 This effect was the result of glucocorticoid-induced exocytic insertion of NHE3 from intracellular vesicles to the plasma membrane.…”

“…Physiological concentrations (IC 50 ϭ0.6 nM) of aldosterone added to the basolateral solution induced a rapid (Ͻ5-minute) inhibition of HCO 3 Ϫ absorption that was the result of a decrease in apical NHE3 activity. 20,35 This inhibition was not blocked by inhibitors of transcription or translation or by spironolactone and was not reproduced by glucocorticoids, indicating a high degree of specificity for aldosterone. 35 Aldosterone inhibited NHE3 through a reduction in the exchanger's maximal velocity.…”

“…20,35 Although inhibition of NHE3 appears counterintuitive for the physiological function of aldosterone to promote renal Na ϩ retention, this effect in the MTAL may serve an important function for acid-base homeostasis. In the proximal tubule, NHE3 is the primary mediator of Na ϩ absorption, both as NaCl and NaHCO 3 .…”

“…Aldosterone has been shown recently to increase the apical abundance and activity of NHE3 in proximal tubules of adrenalectomized rats in vivo and in a human proximal tubule cell line 72,73 ; however, these effects are long term (3 to 5 days), and the possible role of nongenomic mechanisms was not reported. Nevertheless, when combined with the additional observations that NHE3 is regulated via a nongenomic pathway by glucocorticoids in a proximal tubule-derived cell line (OKP) 36 and that NHE3 is a target for nongenomic regulation by aldosterone in the MTAL, 20 there exists a strong basis for the hypothesis that aldosterone may influence proximal Na ϩ and fluid absorption through nongenomic regulation of NHE3.…”

ANG II, ANG-(1-7), ALDO and AVP biphasic effects on Na+/H+ transport: the role of cellular calcium

“…Several sites in the kidney, particularly cultured kidney cells, have been shown to be sensitive to non-genomic ALDO action [266], including principal cells that were freshly isolated from rabbits [267], the human distal colon [268], in vivo renal proximal tubules (S2 segment) [228], isolated renal proximal tubules (S3 segment) [229,233], medullary thick ascending limbs [269] and renal collecting duct cells [270]. Its non-genomic actions include effects on signal transduction pathways and ion transporters, such as the epithelial Na + channel [267], the Na + /H + exchanger [228,229,271,272] and the vacuolar H + -ATPase [230,233,258,259,273].…”