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Departments of Pediatrics and Physiology, Steele Memorial Children's Research Center, University of Arizona Health Sciences Center, Tucson, Arizona 85724
Of the two known apical isoforms of the
Na+/H+ exchanger (NHE) family, only the NHE3
gene is regulated by glucocorticoids. The aim of these studies was to
investigate the mechanisms underlying the effects of methylprednisolone
(MP) on expression of NHE3 in the proximal and distal small intestine
of suckling and adult rats. Immunoblots showed that the glucocorticoid
responsiveness in the proximal small intestine was greatest in suckling
animals (NHE3/
-actin: 0.43 ± 0.09 control vs. 1.57 ± 0.15 MP;
P < 0.001), and responsiveness decreased with age with no
effect in adults (0.56 ± 0.14 vs. 0.64 ± 0.17). Distal small
intestine was responsive only in adult rats (0.49 ± 0.13 vs. 1.65 ± 0.09; P < 0.001). This pattern was confirmed at the mRNA
level and by 22Na+ uptake. Western blot and
[3H]dexamethasone mesylate binding showed that
the responsiveness of NHE3 to glucocorticoids is directly related to
the expression of glucocorticoid receptor (GR) in the small intestine.
These studies suggest that loss and gain of glucocorticoid
responsiveness in the proximal and distal small intestine,
respectively, are related to age- and segment-dependent expression of GR.
sodium/hydrogen exchange; methylprednisolone; rat development; intestinal epithelium
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