In contrast to glucocorticoid receptors, mineralocorticoid receptors are not autoregulated in rat distal colon epithelia.

Abstract

Glucocorticoids and mineralocorticoids exert profound effects on electrolyte balance in the rat distal colon by binding with high affinity and specificity to intracellular receptor proteins, termed glucocorticoid (GR) and mineralocorticoid (MR) receptors, respectively. Hormonal regulation of GR and MR expression represents an important mechanism for maintaining appropriate tissue sensitivity to these two classes of adrenocorticosteroids. In the present study the corticosteroid regulation of the expression of these two rat colonic receptors has been evaluated at the protein and mRNA levels. Preliminary experiments demonstrated that colonic MR and GR binding levels are significantly increased (approximately 60%) after adrenalectomy (14 days). Experiments were therefore performed to test the hypothesis that these increases in binding levels correlate with increased GR and MR mRNA and protein levels. Receptor mRNA levels were quantitated via ribonuclease protection assays using [32P]cRNA probes specific for either GR or MR mRNA, and GR and MR protein levels were quantitated via Western immunoblotting using the anti-GR BuGR2 and anti-MR hMRsN antibodies. Fourteen days after adrenalectomy, significant increases in GR mRNA (2.1-fold) and protein levels (1.6-fold) were detected. In contrast, neither MR mRNA nor protein levels were up-regulated by removal of endogenous corticosteroids. Additionally, GR and MR mRNA levels were measured after ip injection of adrenalectomized rats with pharmacological doses of either the pure glucocorticoid agonist RU 28362 or the mineralocorticoid agonist aldosterone in combination with the glucocorticoid antagonist RU 38486 (blocks aldosterone binding to the GR). Multiple ip injections of RU 28362 resulted in a significant decrease (80%) in GR mRNA levels without affecting MR mRNA levels. In contrast, multiple ip injections of aldosterone (plus RU 38486) had no effect on either MR or GR mRNA levels. A single ip injection of RU 28362 resulted in a detectable decrease (30%) in GR mRNA levels and again had no effect on MR mRNA levels. Although a single ip injection of aldosterone did not down-regulate MR mRNA levels, it did elicit a significant decrease (45%) in GR mRNA levels. Taken collectively, these data demonstrate that rat colonic GR and MR mRNA and protein levels are differentially up-regulated after removal of endogenous corticosteroids. Additionally, these data demonstrate that in response to its cognate ligand, the GR is capable of homologously down-regulating its own mRNA and protein levels. However, in response to its cognate ligand, the MR appears incapable of homologously down-regulating its own mRNA levels.(ABSTRACT TRUNCATED AT 400 WORDS)