Role of glucocorticoids in the maturation of renal cortical Na+/H+ exchanger activity during fetal life in sheep

Abstract

We have studied the role of glucocorticoids in inducing the maturation in activity of the proximal tubule Na+/H+ exchanger that follows birth. Renal cortical microvillus membrane vesicles were prepared from 132-day gestation sheep fetuses (n = 8) that had received intraperitoneal cortisol (13 micrograms.kg-1.h-1) for the previous 48 h. Membrane vesicles were also obtained from sham-operated twin controls (n = 8). Amiloride-sensitive uptake of 22Na+ by these vesicles was measured, and Woolf-Augustinsson-Hofstee plots were used to determine the Michaelis constant (Km) and maximal velocity (Vmax). There was no significant difference in Km; however, the Vmax was 61% higher in cortisol-treated fetuses. Posttreatment circulating cortisol levels were significantly higher in the treated fetuses. Total RNA was collected from renal cortex of the eight pairs of twins when killed. Renal cortex Na+/H+ exchanger 3 (NHE3) mRNA levels were approximately fourfold higher in cortisol-treated than in control fetuses. Although proximal tubule Na+/H+ exchanger activity and renal cortex NHE3 mRNA levels increased significantly in cortisol-treated fetuses, cortisol infusion did not stimulate renal sodium reabsorption in the fetus but rather produced a natriuresis. These results demonstrate that glucocorticoids can induce an increase in both Na+/H+ exchanger activity and NHE3 mRNA levels during the last trimester of gestation in sheep. However, these changes are not associated with an increased ability of the fetal kidney to reabsorb sodium.

Genomic Organization and Glucocorticoid Transcriptional Activation of the Rat Na+/H+ Exchanger Nhe3 Gene

“…As mentioned previously, glucocorticoids elevate Na ϩ /H ϩ exchanger NHE3 activity and/or mRNA abundance in ileum (29), renal proximal tubules (27,28), and OK cells (56). The studies described in this report extend these observations by demonstrating that the transcriptional activity of the 5Ј-flanking region of the rat Nhe3 gene fused to the luciferase reporter gene is activated by glucocorticoids in transiently transfected OK and LLC-PK 1 cells.…”

“…Gene Promoter-Glucocorticoids have been reported to elevate NHE3 activity and mRNA abundance in epithelial cells from the ileum (29), renal proximal tubules (27,28), and cultured renal OK cells (56) which retain certain phenotypic characteristics of proximal tubule cells. In view of the observed putative GRE half-sites in the Nhe3 promoter, it was of interest to assess whether glucocorticoid induction of NHE3 expression could be accounted for by alterations in gene transcription.…”

“…Since the mRNA and/or activity of Na ϩ /H ϩ exchangers in renal or intestinal epithelial cells is elevated following prolonged exposure to various stimuli, including phorbol esters (67), cAMP (68), acidic medium (22,23,25,26,69,70), triiodothyronine (71), and glucocorticoids (27)(28)(29)56), we examined the 5Ј-flanking region of the Nhe3 gene for potential cis-acting DNA response elements that may be involved in some of these responses.…”

“…Aside from acidosis, chronic incubation of renal IMCD cells in hyperosmotic media also stimulates Na ϩ /H ϩ exchanger activity, which is associated with increased NHE2 and reduced NHE1 mRNA abundances (24). Last, long-term administration of glucocorticoids to rabbits or sheep selectively elevates NHE3 activity and mRNA in renal proximal convoluted tubules (27,28) and in ileum (29), but has no effect on NHE1 or NHE2. At present, the underlying mechanisms for acid, hyperosmolarity, and glucocorticoid regulation of specific isoform mRNA abundances have yet to be resolved in detail, but are likely to involve altered rates of gene transcription and/or mRNA stability.…”

Influence of Dexamethasone on Na⁺/H⁺ Exchanger Activity in Dendritic Cells

“…Dexamethasone treatment is followed by a significant increase of the mRNA encoding the NHE isoforms NHE2 and NHE3. Previously, NHE3 has similarly been shown to be genomically upregulated by glucocorticoids in other cell types [24,27,[47][48][49]. However, even after dexamethasone treatment, the transcript levels of NHE2 and NHE3 were still one order of magnitude lower than those of the isoforms NHE1, NHE6 and NHE9, which according to the present study are the predominant isoforms in bone marrow derived murine dendritic cells.…”