1 Institut für Klinische Physiologie and 2 Medizinische Klinik I Gastroenterologie und Infektiologie, Universitätsklinikum Benjamin Franklin, Freie Universität Berlin, D-12200 Berlin, Germany
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The potency of in vitro-added corticosteroids to
stimulate electrogenic Na+
absorption (JNa,
the Na+ absorptive short-circuit
current blockable by 104 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
µmol · 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.
short-circuit current; corticosterone; RU-28362; spironolactone; heterodimerization
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
THE WIDELY ACCEPTED MODEL of corticosteroid hormone
action on electrogenic Na+
absorption in rat distal colon implies an activation of
mineralocorticoid receptors (MRs) with a concomitant inhibition of
electroneutral NaCl absorption (11), whereas the activation of the
glucocorticoid receptor (GR) leads to an activation of electroneutral
Na+ absorption with concomitant
inhibition of electrogenic Na+
absorption (3). However, all mineralocorticoid (MC) and glucocorticoid (GC) hormones bind to both receptors, with MR and GR having different affinities. Higher concentrations of the synthetic
"glucocorticoid" hormone dexamethasone activate electrogenic
Na+ absorption as efficiently as
aldosterone by binding to the MR as well (5). The main GC hormone of
the rat, corticosterone, has a higher affinity for the MR than for the
GR (35). The MC specificity of a certain tissue is guaranteed by the
presence of the enzyme 11-hydroxysteroid dehydrogenase (11
-HSD),
which inactivates GCs by oxidation of the 11
-OH group (7, 9, 18).
However, the mechanisms responsible for the MR specificity in tissues
expressing both MR and GR are not completely understood and are just
beginning to emerge (14). This simple pattern of MC action was
challenged by a recent study of Lomax and Sandle (22) showing that the
GR-specific synthetic hormone RU-28362 induces electrogenic
Na+ absorption via the GR in rat
late distal colon. Because this observation is contrary to the existing
model of electrogenic Na+
absorption, we tried to find a general explanation for the MC action of
natural and synthetic corticosteroid hormones.
MRs and GRs are closely related members of the steroid receptor family that share a common mechanism of action (for review, see Ref. 40). Without ligand, the receptor is bound to a heteromeric complex that includes a 90-kDa heat shock protein and several smaller peptides. When steroid binding occurs, these associated proteins dissociate and the receptor undergoes a conformational change that exposes the DNA binding domain and a specific nuclear localization signal (NLS). Exposing the NLS is necessary for the transport of the receptor into the nucleus. Anti-MCs like spironolactone (2) do not induce a sufficient conformational change, which exposes the NLS and impairs nuclear translocation so that only a small portion of ligand-bound receptors is transferred to the nucleus (10, 28). In the nucleus, steroid-activated receptors bind as dimers to the glucocorticoid response element (GRE), which despite its name does not seem to differentiate between activated MR or GR (39, 40). The receptor dimerization and the binding to the GRE are highly cooperative (8). In tissues expressing both GR and MR, the occurrence of functional heterodimers with inhibitory or activating effects has been observed (20, 38), thereby enhancing the functional diversity of corticosteroid action (37). Because MR and GR are identical through the entire dimer interface region (21), there is no reason to assume that in colon heterodimerization of GR and MR does not take place. Therefore, a differentiated model of steroid binding with the occurrence of MR-MR, GR-GR, and MR-GR dimers has to be considered for rat colon as well.
The purpose of the present study was to determine the acute effects of
in vitro-added natural and synthetic glucocorticosteroids on
amiloride-sensitive colonic Na+
transport, as measured in standard short-circuit current
(Isc) experiments. This study presents the complete concentration dependency for a large set of steroids in in vitro assays in the same colonic segment under equal conditions. We investigated the action of the
steroids as they arrived at the steroid receptor within the target
tissue, making no attempts to block the activity of 11-HSD (7, 9,
18). It turned out that the kinetic appearance of the tested steroids
is very similar. They exhibit graded MC potency instead of a sharp
partition into exclusive groups of MC and non-MC. Experiments combining
aldosterone and RU-28362 corroborated the cooperative effects of GR and
MR in the induction of electrogenic
Na+ absorption. These results
suggest an activation of electrogenic Na+ absorption in rat distal colon
with participation of heterodimers between MR and GR.
![]() |
METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Tissue preparation. Untreated male Wistar rats (200-350 g) had free access to a standard rat diet (Altromin 1320) and tap water ad libitum. The animals were killed by inhalation of a saturated atmosphere of diethylether, according to the guidelines of the Society of Laboratory Animal Science. The colon was removed, rinsed with Ringer solution, and "totally" stripped of serosa and muscle layers, as described previously (13, 33). The segment used in this study was the late distal colon, which is defined as the distal 4-5 cm of the intestine, as measured from the anus. Two tissue pieces located just proximal and distal to the lymph node, situated at ~2.5 cm from the anus, were obtained from each late distal colon.
Electrophysiological measurements.
Epithelia were mounted into conventional Ussing chambers equipped with
water-jacketed gas lifts (32). The exposed area was 0.54 cm2; the circulating fluid was 10 ml on each side. The bathing fluid consisted of (in mM) 140.5 Na+, 5.4 K+, 1.2 Ca2+, 1.2 Mg2+, 123.8 Cl, 21 HCO
3, 2.4 HPO2
4, 0.6 H2PO
4, 10 D-(+)-glucose, 0.5
-OH-butyrate, 2.5 glutamine, and 10 D-(+)-mannose plus 50 mg/l
azlocillin. Azlocillin (Securopen, Bayer) did not affect
Isc at the
concentration used. The solution was gassed with a mixture of 95%
O2 and 5%
CO2 and had a pH of 7.4. All
experiments were performed at 37°C.
Isc
(µmol · h
1 · cm
2)
and tissue conductance (mS/cm2)
were recorded using automatic clamp devices (AC-microclamp, f+p
Datensysteme, Aachen, Germany). The bath resistance was measured before
each single experiment and was subtracted automatically.
Kinetics. For those steroids exhibiting complete concentration dependency patterns, kinetic parameters were evaluated. Michaelis-Menten constant (Km) and Hill coefficients were determined from Hill plots. Maximal velocity (Vmax) was determined by least-square best fit by varying Vmax for linearity of the plot. Due to the logarithmic y-axis, an inexact Vmax would only disturb the Hill curve at maximum substrate concentration but would not have much effect on the determination of Km.
Dimer formation in Fig. 5 combining aldosterone and RU-28362 was calculated using the software package Gepasi3 (24, 25). The forward and backward rate constants for binding of aldosterone to MR (K1 = 8.61 × 105 MStatistics. Results are given as means ± SE. The results were subjected to ANOVA. Significances were calculated using Student's unpaired t-test or the Mann-Whitney U test, as appropriate. For post hoc test for pairwise multiple comparisons between means, Tamhane's T2 test implemented in the program package SPSS was utilized. P < 0.05 was considered significant.
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Concentration dependencies. Most steroids tested produced a typical Isc time course similar to that of aldosterone (13). About 2 h after in vitro addition of the respective steroid, Isc started to increase and reached maximum values after 7-8 h. The amiloride-induced drop in Isc after 8 h (JNa) was determined in all experiments and served as a measure of MC efficiency. A drop in Isc to values close to zero indicates that most of the Isc is Na+ absorption.
In a previous study, a detailed concentration dependency for aldosterone was established (13). In this study, concentration dependencies of GC agonists, natural corticosteroids, and their metabolites were obtained (Fig. 1). In Fig. 1, each data point represents a group of independent 8-h experiments. A concentration of 10
|
Hill plot.
Kinetic results were obtained from Hill plots (Fig.
2). The slopes of these plots indicate the
Hill coefficients, which were similar for all steroids tested.
Numerical data are given in Table 1. It
turned out that
Vmax (range 9.1 ± 1.4 to 12.7 ± 1.3) as well as the Hill coefficients (range
1.6 to 2.1) showed no systematic variation. In contrast, regarding the
apparent Km,
three groups of substances can be defined (cf. Fig. 2). The first is
aldosterone with an apparent
Km of 1.2 nM. A
second group exhibiting apparent Km of 20-37
nM consists of dexamethasone, RU-28362, deoxycortisol, and
deoxycorticosterone. The third group is represented by corticosterone and cortisol, which produced apparent
Km values of 170 and 210 nM. An estimated
Km for
18-hydroxycorticosterone would be in the range of 10 µM, thus outside
the physiological range for corticosteroids.
|
|
Inhibition by mespirenone.
Mespirenone is a spirolactone with about three times higher anti-MC
effectiveness compared with spironolactone (23). To test whether the
anti-MC could block the effect of aldosterone and the six
glucocorticosteroids on
JNa in a similar
way, mespirenone was added at the beginning of the experiment. The
respective steroids were added at submaximal concentrations at the same
time (Table 2). Mespirenone prevented the
MC effect of aldosterone as well as that of the glucocorticosteroids.
Whereas the effect of 3 nM aldosterone on
JNa could be
completely inhibited by 1 µM mespirenone (data not shown), the effect
of GCs in concentrations leading to a submaximal
JNa were only
abolished at a concentration of 104 M, which is more than
four orders of magnitude higher than the Kd of mespirenone
for the MR (23). The best investigated member of the spirolactone
family, spironolactone itself, possesses a Kd value for the
GR of ~2.6 × 10
7 M
(27). Therefore, 10
4 M
mespirenone is most probably sufficient to block effectively not only
the MR but the GR as well. This suggests an involvement of the GR in
the MC response of GCs seen in these experiments.
|
Na+ conductance. The effect of amiloride on Isc is caused by a block of the apically located epithelial Na+ channel. Thus Isc changes are paralleled by changes in epithelial conductance [in this case Na+ conductance (GNa)]. Activation or inhibition of the Na+-K+-ATPase also influenced the epithelial GNa. When the Na+-K+-ATPase is activated in addition to Na+ channels being opened, a block of electrogenic Na+ absorption by amiloride would be accompanied by a larger change in GNa than when the ATPase is not activated.
With regards to the present study, a cumulative plot of all data of JNa vs. the corresponding partial ion GNa revealed a linear relationship for all steroids tested and at all concentrations of these steroids (Fig. 3). The intercept of +0.012 is obtained by subtracting the intercept obtained in our previous study on aldosterone (13) from the result of the linear regression of the GC data in this study. This means that there is principally no different action of all investigated corticosteroids regarding the opening of Na+ channels and the influence on the Na+-K+-ATPase in these in vitro measurements, even compared with aldosterone, suggesting a common mechanism of action.
|
Combination of aldosterone and RU-28362.
To show a joint action of MR and GR in activation of electrogenic
Na+ absorption, an MR-specific
concentration of aldosterone (0.1 nM) was combined with a GR-specific
concentration of RU-28362 (1 nM) in the same experiment
(n = 10). Whereas the single
application of aldosterone or RU-28362 alone at the respective
concentrations did not evoke any significant electrogenic
Na+ absorption after an 8-h
incubation, the combination of both steroids caused a electrogenic
Na+ absorption of 42 ± 6% of Vmax (Fig.
4). This resembles a
JNa evoked by ~0.5-1 nM aldosterone
alone. The result shows a cooperative action of both GR and MR in
the activation of
JNa.
|
Combination of spironolactone and RU-28362. Because the spirolactone mespirenone needed a high concentration not specific for MR to abolish the MC action of RU-28362, a possible positively synergistic mechanism of spirolactones with RU-28362 was investigated. For these experiments, the well-characterized spironolactone was used. A GR-specific concentration of RU-28362 (1 nM) was combined with an MR-specific concentration of spironolactone (10 nM). Although both steroids alone did not lead to a significant electrogenic Na+ absorption, the combination yielded 6.3 ± 1.5% of Vmax (n = 9) (Fig. 4). This means that an MR antagonist and a GR agonist can synergistically initiate a small but significant MC action.
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
In rat distal colon, Na+ absorption under unstimulated conditions is an electroneutral process (4). JNa can be induced by chronic hyperaldosteronism for 3 days, and this is accompanied by a complete inhibition of electroneutral NaCl absorption (4). Alternatively, JNa can be stimulated by addition of physiological concentrations of aldosterone in vitro, and in this case no influence on electroneutral NaCl absorption can be observed (13). GC-induced Na+ absorption has been investigated in many studies, but most of them used in vivo application of the hormones or Na+ deprivation of the animals. To avoid regulatory effects after hormone application, many in vivo studies make use of adrenalectomized rats. However, because there is still some risk of other systemic regulatory mechanisms after hormone application and adrenalectomy, in this study a complete in vitro assay was applied.
Using adrenalectomized rats is not necessary for in vitro measurements of JNa because 1) control rats did not show any amiloride-sensitive Na+ absorption and 2) a former study (13) demonstrated that the effect of aldosterone on JNa is easily reversible within a few hours after removal of aldosterone from the bathing solution.
MC action by natural corticosteroids. Deoxycorticosterone is an intermediary substance in the chain of steroid formation in between progesterone and corticosterone. In analogy, deoxycortisol is intermediary to 17-hydroxyprogesterone and cortisol. The concentration dependencies (Fig. 1, C and D) indicate that deoxycorticosterone was slightly more potent than deoxycortisol. Both steroids are about one order of magnitude more potent than the main natural GCs in rats or humans (corticosterone or cortisol) and therefore are potent MC hormones. The MC action of deoxycorticosterone has been shown before (26, 34). The physiological role in JNa is not clear yet, but their systemic concentration may be only transiently elevated during synthesis of other hormones.
Corticosterone is the main GC in rats and mice, whereas cortisol is predominant in most other mammals. In regards to apparent MC potency, corticosterone and cortisol form one group that exerts similar Km values, ~2 × 10Dexamethasone and RU-28362.
Dexamethasone is a well-investigated synthetic GC (6, 19, 29, 30) with
a relatively high affinity for the MR in rat (Kd of ~10 nM;
Ref. 35). Whereas high concentrations of dexamethasone applied in vivo
induce both electroneutral Na+
absorption and
JNa (5),
GR-specific concentrations of dexamethasone were reported to induce
only electroneutral Na+ absorption
(1) and even inhibit
JNa (3).
Therefore, all effects of dexamethasone regarding
JNa were
attributed to crossover binding of dexamethasone to MR, if the hormone
is used in higher concentrations. In the present study,
107 M dexamethasone induced
Na+ transport similarly to
aldosterone; however, the
Km value is shifted toward a 30-fold higher concentration of 37 nM.
Cooperative effects.
All six steroids for which complete concentration dependency curves
could be obtained (i.e., aldosterone, dexamethasone, RU-28362, deoxycorticosterone, cortisol, and corticosterone) showed Hill coefficients of ~2 (Table 1). This suggests a strong cooperative effect regarding the stimulation of electrogenic
Na+ absorption. The combination
experiments with MR-specific concentrations of aldosterone and
GR-specific concentrations of RU-28362 showed that this cooperative
effect includes both steroid receptor types. These results lead to a
model of MC action in rat distal colon that includes activating
heterodimers of MR and GR. In Fig. 5, we
calculated the number of respective homo- and heterodimers occurring
during the combination experiment with 0.1 nM aldosterone plus 1 nM
RU-28362, as shown in Fig. 4 (for details, see
METHODS). The
Kd for receptor
dimerization was arbitrarily chosen as 1 nM, which is supposed to
follow random statistics. RU-28362 is nearly exclusively bound to GR as
well as aldosterone to MR under these conditions. The amount of
activated MR does not allow formation of MR-MR homodimers within 8 h,
but the abundance of activated GR can account for
Na+ absorption after 2 h via
formation of MR-GR heterodimers. GR-GR homodimers are supposed to stay
without function regarding
JNa in this time
range. For RU-28362 alone, this means that a
Kd value for its
binding to MR in the range of 10 µM is sufficient to explain its
activating effect on
JNa via receptor
heterodimerization.
|
Role of GR homodimers. Within the model calculations, the MC action has been attributed to MR-MR and MR-GR dimers, leaving the GR-GR homodimers without function in this regard. The inhibitory function of GC on JNa observed by other groups (3) may need a longer time to be established, similar to the inhibitory action of aldosterone on electroneutral Na+ absorption (11), which was also not observed in our experiments. On the other hand, this inhibitory action of GC may only occur in an in vivo situation and needs other cofactors not provided in our experiments because the inhibitory effect of a GR-specific concentration of dexamethasone was reported to take place within 30 min (3), an effect never observed in the in vitro experiments.
In conclusion, we have shown that in rat distal colon aldosterone, natural corticosteroids, and synthetic GC induce electrogenic Na+ absorption of the same size and with similar kinetics, differing only in their Km values. This very homogeneous picture suggests a common mode of action. Inhibitory effects of GC on MC action reported by many investigators in in vivo studies could not be observed in these in vitro studies. Corticosterone may be a potent inductor of JNa under physiological conditions, e.g., in stress situations. Because of the strong cooperative effects of GR and MR in the induction of JNa, a model of MC action was proposed that was based on receptor heterodimerization between GR and MR. ![]() |
ACKNOWLEDGEMENTS |
---|
The superb technical assistance of Anja Fromm, Ursula Lempart, and Sieglinde Lüderitz is gratefully acknowledged.
![]() |
FOOTNOTES |
---|
This study was supported by the Sonnenfeld-Stiftung.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests: M. Fromm, Inst. f. Klinische Physiologie, UKBF der Freien Universität Berlin, D-12200 Berlin, Germany.
Received 14 August 1998; accepted in final form 20 October 1998.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Bastl, C. P.
Regulation of cation transport by low doses of glucocorticoids in in vivo adrenalectomized rat colon.
J. Clin. Invest.
80:
348-356,
1987[Medline].
2.
Bastl, C. P.
Effect of spironolactone on glucocorticoid-induced colonic cation transport.
Am. J. Physiol.
255 (Renal Fluid Electrolyte Physiol. 24):
F1235-F1242,
1988
3.
Bastl, C. P.,
G. Schulman,
and
E. J. Cragoe, Jr.
Glucocorticoids inhibit colonic aldosterone-induced conductive Na+ absorption in adrenalectomized rat.
Am. J. Physiol.
263 (Renal Fluid Electrolyte Physiol. 32):
F443-F452,
1992
4.
Binder, H. J.,
E. Foster,
E. Budinger,
and
J. P. Hayslett.
Mechanism of electroneutral sodium-chloride absorption in distal colon of the rat.
Gastroenterology
93:
449-455,
1987[Medline].
5.
Binder, H. J.,
F. McGlone,
and
G. I. Sandle.
Effects of corticosteroid hormones on the electrophysiology of rat distal colon: implications for Na+ and K+ transport.
J. Physiol. (Lond.)
410:
425-441,
1989[Abstract].
6.
Bridges, R. J.,
W. Rummel,
and
J. Schreiner.
In vitro effects of dexamethasone on sodium transport across rat colon.
J. Physiol. (Lond.)
383:
69-77,
1987[Abstract].
7.
Burton, A. F.,
and
F. H. Anderson.
Inactivation of corticosteroids in intestinal mucosa by 11-hydroxysteroid: NADP oxidoreductase (EC 1.1.1.146).
Am. J. Gastroenterol.
78:
627-631,
1983[Medline].
8.
Dahlman-Wright, K.,
H. Siltala-Roos,
J. Carlstedt-Duke,
and
J. A. Gustafsson.
Protein-protein interactions facilitate DNA binding by the glucocorticoid receptor DNA-binding domain.
J. Biol. Chem.
265:
14030-14035,
1990
9.
Epple, H. J.,
J. D. Schulzke,
H. Schmitz,
and
M. Fromm.
Enzyme- and mineralocorticoid receptor-controlled electrogenic Na+ absorption in human rectum in vitro.
Am. J. Physiol.
269 (Gastrointest. Liver Physiol. 32):
G42-G48,
1995
10.
Fejes-Tóth, G.,
D. Pearce,
and
A. Náray-Fejes-Tóth.
Subcellular localization of mineralocorticoid receptors in living cells: effects of receptor agonists and antagonists.
Proc. Natl. Acad. Sci. USA
95:
2973-2978,
1998
11.
Foster, E. S.,
T. W. Zimmermann,
J. P. Hayslett,
and
H. J. Binder.
Corticosteroid alteration of active electrolyte transport in rat distal colon.
Am. J. Physiol.
245 (Gastrointest. Liver Physiol. 8):
G668-G675,
1983
12.
Fromm, M.,
W. Oelkers,
and
U. Hegel.
Time course of aldosterone and corticosterone plasma levels in rats during general anesthesia and abdominal surgery.
Pflügers Arch.
399:
249-254,
1983[Medline].
13.
Fromm, M.,
J. D. Schulzke,
and
U. Hegel.
Control of electrogenic Na+ absorption in rat late distal colon by nanomolar aldosterone added in vitro.
Am. J. Physiol.
264 (Endocrinol. Metab. 27):
E68-E73,
1993
14.
Funder, J.,
and
K. Myles.
Exclusion of corticosterone from epithelial mineralocorticoid receptors is insufficient for selectivity of aldosterone action: in vivo binding studies.
Endocrinology
137:
5264-5268,
1996[Abstract].
15.
Garty, H.,
K. Peterson-Yantorno,
C. Asher,
and
M. M. Civan.
Effects of corticoid agonists and antagonists on apical Na+ permeability of toad urinary bladder.
Am. J. Physiol.
266 (Renal Fluid Electrolyte Physiol. 35):
F108-F116,
1994
16.
Grubb, B. R.,
and
P. J. Bentley.
Effects of corticosteroids on short-circuit current across the cecum of the domestic fowl, Gallus domesticus.
J. Comp. Physiol. [B]
162:
690-695,
1992[Medline].
17.
Halevy, J.,
E. L. Boulpaep,
J. Budinger,
H. J. Binder,
and
J. P. Hayslett.
Glucocorticoids have a different action than aldosterone on target tissue.
Am. J. Physiol.
254 (Renal Fluid Electrolyte Physiol. 23):
F153-F158,
1988
18.
Hierholzer, K.,
H. Siebe,
and
M. Fromm.
Inhibition of 11-hydroxysteroid dehydrogenase and its effect on epithelial sodium transport.
Kidney Int.
38:
673-678,
1990[Medline].
19.
Jorkasky, D.,
M. Cox,
and
G. M. Feldman.
Differential effects of corticosteroids on Na+ transport in rat distal colon in vitro.
Am. J. Physiol.
248 (Gastrointest. Liver Physiol. 11):
G424-G431,
1985[Medline].
20.
Liu, W.,
J. Wang,
N. S. Sauter,
and
D. Pearce.
Steroid receptor heterodimerization demonstrated in vitro and in vivo.
Proc. Natl. Acad. Sci. USA
92:
12480-12484,
1995[Abstract].
21.
Liu, W.,
J. Wang,
G. Yu,
and
D. Pearce.
Steroid receptor transcriptional synergy is potentiated by disruption of the DNA-binding domain dimer interface.
Mol. Endocrinol.
10:
1399-1406,
1996[Abstract].
22.
Lomax, R. B.,
and
G. I. Sandle.
Comparison of aldosterone- and RU 28362-induced apical Na+ and K+ conductances in rat distal colon.
Am. J. Physiol.
267 (Gastrointest. Liver Physiol. 30):
G485-G493,
1994
23.
Losert, W.,
D. Bittler,
M. Buse,
J. Casals-Stenzel,
M. Haberey,
H. Laurent,
K. Nickisch,
E. Schillinger,
and
R. Wiechert.
Mespirenone and other 15,16-methylene-17-spirolactones, a new type of steroidal aldosterone antagonists.
Drug Res.
36:
1583-1600,
1986[Medline].
24.
Mendes, P.
GEPASI: a software package for modelling the dynamics, steady states and control of biochemical and other systems.
Comput. Appl. Biosci.
9:
563-571,
1993[Abstract].
25.
Mendes, P.
Biochemistry by numbers: simulation of biochemical pathways with Gepasi 3.
Trends Biochem. Sci.
22:
361-363,
1997[Medline].
26.
Pácha, J.,
M. Popp,
and
K. Capek.
Effects of deoxycorticosterone acetate on the electrical properties of rat large intestine: segmental differences.
Physiol. Bohemoslov.
36:
141-147,
1987[Medline].
27.
Reul, J. M.,
E. R. de Kloet,
F. J. van Sluijs,
A. Rijnberk,
and
J. Rothuizen.
Binding characterisistics of mineralocorticoid and glucocorticoid receptors in dog brain and pituitary.
Endocrinology
127:
907-915,
1990[Abstract].
28.
Rossier, B. C.,
M. Claire,
M. E. Rafestin-Oblin,
K. Geering,
H. P. Gaeggeler,
and
P. Corvol.
Binding and antimineralocorticoid activities of spirolactones in toad bladder.
Am. J. Physiol.
244 (Cell Physiol. 13):
C24-C31,
1983
29.
Sandle, G. I.
Segmental variability of glucocorticoid induced electrolyte transport in rat colon.
Gut
32:
936-940,
1991[Abstract].
30.
Sandle, G. I.,
and
F. McGlone.
Acute effects of dexamethasone on cation transport in colonic epithelium.
Gut
28:
701-706,
1987[Abstract].
31.
Schulman, G.,
N. M. Robertson,
I. B. Elfenbein,
D. Eneanya,
G. Litwack,
and
C. P. Bastl.
Mineralocorticoid and glucocorticoid receptor steroid binding and localization in colonic cells.
Am. J. Physiol.
266 (Cell Physiol. 35):
C729-C740,
1994
32.
Schultz, S. G.,
and
R. Zalusky.
Ion transport in isolated rabbit ileum. I. Short-circuit current and Na+ fluxes.
J. Gen. Physiol.
47:
567-584,
1964
33.
Schulzke, J. D.,
M. Fromm,
and
U. Hegel.
Epithelial and subepithelial resistance of rat large intestine: segmental differences, effect of stripping, time course, and action of aldosterone.
Pflügers Arch.
407:
632-637,
1986[Medline].
34.
Sellin, J. H.,
and
R. C. DeSoignie.
Steroids alter ion transport and absorptive capacity in proximal and distal colon.
Am. J. Physiol.
249 (Gastrointest. Liver Physiol. 12):
G113-G119,
1985[Medline].
35.
Sutanto, W.,
and
E. R. de Kloet.
Species-specificity of corticosteroid receptors in hamster and rat brains.
Endocrinology
121:
1405-1411,
1987[Abstract].
36.
Teutsch, G.,
G. Costerousse,
R. Deraedt,
J. Benzni,
M. Fortin,
and
D. Philibert.
17-Alkynyl-11
,17-dihydroxyandrostane derivatives: a new class of potent glucocorticoids.
Steroids
38:
651-665,
1981[Medline].
37.
Trapp, T.,
and
F. Holsboer.
Heterodimerization between mineralocorticoid and glucocorticoid receptors increases the functional diversity of corticosteroid action.
Trends Pharmacol. Sci.
17:
145-149,
1996[Medline].
38.
Trapp, T.,
R. Rupprecht,
M. Castren,
J. M. Reul,
and
F. Holsboer.
Heterodimerization between mineralocorticoid and glucocorticoid receptor: a new principle of glucocorticoid action in the CNS.
Neuron
13:
1457-1462,
1994[Medline].
39.
Tsai, S. Y.,
J. Carlstedt-Duke,
N. L. Weigel,
K. Dahlman,
J. A. Gustafsson,
M. J. Tsai,
and
B. W. O'Malley.
Molecular interactions of steroid hormone receptor with its enhancer element: evidence for receptor dimer formation.
Cell
55:
361-369,
1988[Medline].
40.
Tsai, S. Y.,
and
B. W. O'Malley.
Molecular mechanisms of action of steroid/thyroid receptor superfamily members.
Annu. Rev. Biochem.
63:
451-486,
1994[Medline].
41.
Turnamian, S. G.,
and
H. J. Binder.
Regulation of active sodium and potassium transport in the distal colon of the rat. Role of the aldosterone and glucocorticoid receptors.
J. Clin. Invest.
84:
1924-1929,
1989[Medline].
42.
Will, P. C.,
R. N. Cortright,
R. C. DeLisle,
R. C. Douglas,
and
U. Hopfer.
Regulation of amiloride-sensitive electrogenic sodium transport in the rat colon by steroid hormones.
Am. J. Physiol.
248 (Gastrointest. Liver Physiol. 11):
G124-G132,
1985