Effects of corticosteroids on short-circuit current across the cecum of the domestic fowl, Gallus domesticus

Short-term dehydration influences baseline but not stress-induced corticosterone levels in the house sparrow (Passer domesticus)

Future environmental variations linked to climate change are expected to influence precipitation regimes and thus drinking water availability. Dehydration can be a particularly challenging physiological state for most organisms, yet no study has examined the effect of dehydration on the functioning of the hypothalamic-pituitary-adrenal axis in wild endothermic animals, despite its central role in maintaining homeostasis. In this study, we experimentally imposed a temporary water shortage (∼20 h) on captive house sparrows in order to investigate the consequences of short-term dehydration on baseline and stress-induced corticosterone levels. As expected, water-deprived birds displayed higher plasma osmolality and haematocrit. Additionally, water-deprived birds had lower defecation rates, suggesting that the mechanisms allowing caecal water absorption may be triggered very rapidly during water deprivation. Baseline but not stress-induced corticosterone levels were higher in water-deprived birds. Taken together, these results suggest that water restriction may have critical consequences on several corticosterone-related traits such as energy budget (protein catabolism and possibly feeding reduction), enhanced mobility (to promote water acquisition) and potential responses to predators (thirst threshold overriding the acute stress response). Owing to the possible fitness consequences of such components of the day-to-day life of birds, further studies should aim at investigating the influence of future changes in precipitation regimes and drinking water availability on bird populations.

11beta-hydroxysteroid dehydrogenase type 2 in zebra finch brain and peripheral tissues

The enzyme 11betaHSD2 inactivates glucocorticoids by synthesizing metabolites that bind poorly to mineralocorticoid and glucocorticoid receptors. Oscine songbirds (Passeriformes) are important models for investigating stress hormone effects on brain and behavior but nothing is known about 11betaHSD2 activity in the songbird brain. We measured 11betaHSD2 mRNA expression and enzymatic activity in brain of adult and developing male and female zebra finches. Since 11betaHSD2 plays an important role in GC metabolism in some peripheral organs we measured mRNA and catalytic activity also in the adult liver, kidney colon and gonads. 11betaHSD2 mRNA was detected in all brain regions examined with expression in the cerebellum and hypothalamus greater in females than in males; expression in ovaries was greater than in testes. No differences were detected in the other peripheral tissues. Catalytic activity of 11betaHSD2 could be measured in brain, but at low levels and no sex differences were measured in any region tested. Because 11betaHSD2 protects mineralocorticoid sensitive tissues from inappropriate CORT action, we also measured mineralocorticoid receptor (MR) expression in adult brain kidney and liver. MR mRNA was detected in all tissues with similar levels of expression in neural and peripheral tissues. The wide distribution of 11betaHSD2 and MR throughout the songbird brain suggests that concentrations of glucocorticoids may be locally regulated in brain to modulate their actions on MR and possibly also glucocorticoid receptors (GR). Notable differences between mRNA expression and activity point to post-transcriptional regulation of the 11betaHSD2 enzyme.

Chicken 11beta-hydroxysteroid dehydrogenase type 2: partial cloning and tissue distribution

NAD(+)-dependent 11beta-hydroxysteroid dehydrogenase (11HSD2) converts glucocorticoids to 11-oxo derivatives and thus decreases their local concentration and prevents them from activating corticosteroid receptors. In this paper we report the partial cloning, characterization and tissue distribution of chicken 11HSD2. A cDNA of 991bp was cloned from kidney mRNA by reverse transcription and polymerase chain reaction. At the amino acid level, the sequence of PCR product had 56-59% homology with mammalian and 46-48% with fish 11HSD2. The consensus sequences of the short-chain dehydrogenase/reductase superfamily such as the catalytic activity motif Tyr-X-X-X-Lys and cosubstrate-binding motif Gly-X-X-X-Gly-X-Gly, were found in the cloned cDNA. Analysis of the tissue expression of chicken 11HSD2 mRNA and NAD(+)-dependent 11beta-oxidase activity showed a similar tissue distribution pattern in the majority of tissues. High levels of expression and activity were found in kidney, small intestine, colon and oviduct; low in ovary and almost zero in brain, liver and testis.

Glucocorticoid metabolism and Na+transport in chicken intestine

The role of aldosterone in regulation of electrogenic Na+ transport is well established, though mineralocorticoid receptors bind glucocorticoids with similar binding affinity as aldosterone and plasma concentration of aldosterone is much lower than glucocorticoids. In mammals, the aldosterone specificity is conferred on the low-selective mineralocorticoid receptors by glucocorticoid inactivating enzyme 11beta-hydroxysteroid dehydrogenase (11HSD) that converts cortisol or corticosterone into metabolites (cortisone, 11-dehydrocorticosterone) with lower affinity for these receptors. The present study examined the chicken intestine, whether changes in 11HSD activity are able to modulate the effect of corticosterone on Na+ transport, and how the metabolism of this hormone is distributed within the intestinal wall. This study shows that not only aldosterone, but also corticosterone (B), was able to increase the electrogenic Na+ transport in chicken caecum in vitro. The effect of corticosterone was higher in the presence of carbenoxolone, an inhibitor of steroid dehydrogenases, and was comparable to the effect of aldosterone. The metabolism of B in the intestine was studied; results showed oxidation of this steroid to 11-dehydrocorticosterone (A) and reduction to 11-dehydro-20beta-dihydrocorticosterone (20diA) as the main metabolic products at low nanomolar concentration of the substrate. In contrast, 20beta-dihydrocorticosterone and 20diA were the major products at micromolar concentration of B. Progesterone was converted to 20beta-dihydroprogesterone. The metabolism of corticosterone was localized predominantly in the intestinal mucosa (enterocytes). In conclusion, the oxidation at position C11 and reduction at position C20 suggest that both 11HSD and 20beta-hydroxysteroid dehydrogenase (20HSD) operate in the chicken intestine and that the mucosa of avian intestine possesses a partly different system of modulation of corticosteroid signals than mammals. This system seems to protect the aldosterone target tissue against excessive concentration of corticosterone and progesterone.

Electrogenic Na+ transport in rat late distal colon by natural and synthetic glucocorticosteroids

The potency of in vitro-added corticosteroids to stimulate electrogenic Na+ absorption (JNa, the Na+ absorptive short-circuit current blockable by 10(-4) M amiloride) was determined in rat late distal colon. JNa was determined 8 h after steroid addition from the drop in short-circuit current caused by 10(-4) M amiloride. The concentration dependency of JNa was obtained for seven corticosteroids and compared with that established for aldosterone. Apparent mineralocorticoid potencies as determined from apparent Michaelis-Menten constant (Km) values were as follows: aldosterone 1. 2 nM >> RU-28362 20 nM = deoxycorticosterone 20 nM > deoxycortisol 36 nM >/= dexamethasone 37 nM >> corticosterone 170 nM > cortisol 210 nM. These steroids exhibited Vmax values of 9-13 micromol. h-1. cm-2 and similar concentration dependencies. Hill coefficients were between 1.6 and 2.1, suggesting cooperative effects between activated receptors. We conclude that corticosteroids exhibit graded mineralocorticoid potency instead of a sharp partition into exclusive groups of mineralocorticoid and nonmineralocorticoid hormones. The low apparent Km value of RU-28362 for mineralocorticoid action and the need for high concentrations of the mineralocorticoid antagonist mespirenone to block this response indicated that JNa in a native mammalian epithelium can be mediated by the glucocorticoid receptor. Glucocorticoid receptor-specific amounts of RU-28362 in combination with mineralocorticoid receptor-specific amounts of aldosterone or of the mineralocorticoid antagonist spironolactone showed cooperative action, suggesting a heterodimeric activation of JNa by the glucocorticoid receptor and mineralocorticoid receptor.

Metabolism of corticosterone in mammalian and avian intestine

11beta-hydroxysteroid dehydrogenase (11betaHSD) catalyzes the conversion of the glucocorticoids, corticosterone and cortisol, to the respective derivatives 11-dehydrocorticosterone and cortisone. The recent findings underline the importance of this enzyme in excluding glucocorticoids from mineralocorticoid receptors. In the present study, 11betaHSD activity was compared in the intestine of herbivorous (guinea pig), omnivorous (rat), and granivorous (hen) animals, i.e., in animals in which the Na+ transport either is or is not regulated by aldosterone under normal conditions and in which the plasma levels of individual glucocorticoids are different. Slices of various intestinal segments were incubated in the presence of corticosterone or 11-dehydrocorticosterone, and the steroids were extracted and analyzed by HPLC. In the mammalian intestine, the activity of 11betaHSD was very low (approaching zero) in aldosterone-insensitive segments (duodenum, jejunum) but significant activity was revealed in aldosterone-sensitive segments (ileum, cecum, and proximal and distal colon). In comparison with the rat, the guinea pig large intestine exhibited significantly higher activity of 11betaHSD. There was no detectable reductase activity (conversion of 11-dehydrocorticosterone to corticosterone) in any intestinal segments of either species. Unexpectedly, no 11betaHSD activity was observed in the avian intestine. It was found that, in contrast to the mammalian intestine, corticosterone was metabolized to 20-dihydrocorticosterone while 11-dehydrocorticosterone was converted to 11-dehydro-20-dihydrocorticosterone. The distribution of 20-hydroxysteroid dehydrogenase (20HSD) activity in the avian intestine was homogenous along the intestine and did not correlate with the mineralocorticoid sensitivity of intestinal segments. To trace different cosubstrate dependence of 11betaHSD and 20HSD, homogenates of ileum and distal colon were incubated with NAD+/NADH or NADP+/NADPH, respectively. In accordance with slice experiments mammalian intestine displayed only oxidation of corticosterone to 11-dehydrocorticosterone and NAD+ preference. In avian intestine, the metabolite formed from corticosterone was 11-dehydrocorticosterone in the presence of NAD+ or NADP+ whereas in the presence of NADPH 11-dehydro-20-dihydrocorticosterone and 20-dihydrocorticosterone were formed. Given the wide similarity between mineralocorticoid regulation of epithelial transport in mammals and birds, the unexpected finding of differences in the metabolism of corticosterone suggests that role of 20HSD is to allow aldosterone occupancy of mineralocorticoid receptors.

11 beta-hydroxysteroid dehydrogenase isoforms and their implications for blood pressure regulation

11 beta-hydroxysteroid dehydrogenase (11 beta-HSD) catalyzes the reversible conversion of physiological glucocorticoids (cortisol, corticosterone) to inactive products. The enzyme thus protects non-selective renal mineralocorticoid receptors from circulating glucocorticoids (ensuring aldosterone-selectivity in vivo), excludes maternal glucocorticoids from the foetal circulation and modulates glucocorticoid access to glucocorticoid receptors in other tissues. 11 beta-HSD has been purified from rat liver, antisera raised, a cDNA isolated and its human homologue cloned. However, it is difficult to reconcile all of the actions of 11 beta-HSD with a single enzyme. Here data are reviewed that demonstrate not only molecular heterogeneity of the 'liver-type' 11 beta-HSD, but also the existence of a novel high affinity isoform in the placenta and perhaps distal nephron. These data are discussed in the light of their potential physiological and pathological importance, with particular reference to the pathogenesis of hypertension.