Regulation of active sodium and potassium
transport in the distal colon of the rat. Role of
the aldosterone and glucocorticoid receptors.
S G Turnamian, H J Binder
J Clin Invest.
1989;
84(6)
:1924-1929.
https://doi.org/10.1172/JCI114380
.
To determine whether mineralocorticosteroids and glucocorticosteroids have specific effects
on colonic electrolyte transport, we compared the effect of aldosterone and RU 28362, a
glucocorticoid receptor-specific agonist that does not bind to the aldosterone receptor, on
unidirectional Na, Cl, and K fluxes across isolated mucosa of the rat distal colon.
Continuous infusion of aldosterone for 7 d produced changes in four specific transport
processes: induction of both active electrogenic, amiloride-sensitive sodium absorption and
active electrogenic potassium secretion, enhancement of active electroneutral potassium
absorption, and inhibition of electroneutral Na-Cl absorption, the predominant transport
process in this epithelium. In contrast, continuous infusion of RU 28362 for 1-11 d produced
a sustained increase in electroneutral Na-Cl absorption. This glucocorticoid
receptor-specific agonist did not induce electrogenic sodium absorption nor affect either potassium
absorption or secretion. These studies demonstrate that aldosterone (i.e., mineralocorticoid)
and glucocorticoid receptors modulate separate and specific changes in active sodium and
potassium transport. These results suggest that other glucocorticoids (e.g., dexamethasone,
methylprednisolone) are not glucocorticoid receptor-specific and that their effects on
electrogenic sodium absorption and potassium transport most likely represent the binding of
these agonists to the aldosterone receptor.
Research Article
Find the latest version:
Regulation of Active Sodium and Potassium Transport
in
the Distal Colon of the Rat
Role of the
Aldosterone
andGlucocorticoid
ReceptorsSteven G. Tumamian and HenryJ.Binder
Department ofInternal Medicine, Yale University, New Haven, Connecticut 06510
Abstract
Todeterminewhether mineralocorticosteroidsand glucocorti-costeroids have
specific
effectsoncolonic electrolyte transport, wecompared theeffect ofaldosterone and RU28362,a gluco-corticoidreceptor-specific agonist
that does not bind to the aldosterone receptor, on unidirectional Na, Cl, and K fluxes acrossisolatedmucosaof theratdistal colon. Continuous in-fusion of aldosterone for 7 dproduced changesinfourspecific
transport processes: induction of both active
electrogenic,
amiloride-sensitive
sodiumabsorption
andactive electrogenicpotassium
secretion, enhancement of active electroneutral po-tassiumabsorption,
and inhibition of electroneutral Na-Clab-sorption,
thepredominant
transport process inthisepithelium.
Incontrast, continuous infusion ofRU28362 for 1-11 d pro-ducedasustained increase in electroneutral Na-Cl
absorption.
This glucocorticoid
receptor-specific agonist
did not induce electrogenic sodiumabsorption
noraffect eitherpotassium
ab-sorption
orsecretion. These studiesdemonstratethat aldoste-rone (i.e., mineralocorticoid) andglucocorticoid
receptors modulate separateandspecific
changes in active sodium and potassium transport. These results suggest that othergluco-corticoids (e.g., dexamethasone,
methylprednisolone)
are notglucocorticoid
receptor-specific
and that their effects onelec-trogenic
sodiumabsorption
andpotassium
transport mostlikelyrepresent thebinding ofthese
agonists
tothealdosterone receptor.Introduction
Dietary sodium
depletion
results insignificant changes
in electrolytetransport in the distal colon of therat(1-4).
Theprimary effect
seen inthis
modelof secondary
hyperaldoste-ronism
is
aninduction
of active amiloride-sensitive sodium transport and activeelectrogenic potassium
secretion which arethechangesusually
observed in otheraldosterone-respon-sive
epithelia (1, 3). However, dietary
sodiumdepletion
also affects electroneutralsodium
andpotassium absorption
inthe rat distal colon:electroneutral
Na-Clabsorption
is inhibited andelectroneutralpotassium absorption
is stimulated (3, 5). Recentstudies with continuous infusion of aldosterone haveconfirmed
thatthechanges
insodium and chloridetransportAddressreprintrequeststoDr.Binder, Departmentof Internal
Medi-cine, YaleUniversitySchool ofMedicine, 333 Cedar Street, New Haven,CT06510.
Receivedfor publication20 December 1988 andinrevisedform8 June 1989.
observed in
dietary
sodium depletion are dueto aldosterone (6);similar studies
havenot yetbeenperformed forpotassium
transport.
Synthetic
glucocorticoids (e.g., dexamethasone, methyl-prednisolone)also produce substantial changes in both sodium andpotassiummovement in the mammalian distal colon (1, 3,4,7-12).Although both glucocorticoid and aldosterone (i.e., mineralocorticoid) receptors have been identified in the cyto-solofcolonic
epithelialcells(13-17),
there has been consider-ablecontroversywhetherseparate andspecific changes in ion transport are mediated by the activation of these glucocorti-coid andmineralocorticoidreceptors.Experiments with dexa-methasoneandmethylprednisolone reveal that many (but not all) of the changes inelectrolyte transport produced by these glucocorticoidsaresimilarto thoseobserved in dietary sodiumdepletion
andafter
aldosterone infusion(1, 3, 7, 18). However, in the absence ofcrossoverbindingof one corticosteroid to theopposite cystolic
receptor,it isnotknownwhetherglucocorti-coids
andaldosterone produce distinctanduniquechanges insodium
andpotassium
transport.Resolution of this questionhas beenlong hampered by the failureto
appreciate,
until recently,that most "pure" synthetic glucocorticoids (i.e., dexamethasone, methylprednisolone) alsobind
tothe aldosterone receptor atdoses that havebeenfrequently
employed in these experiments.The recent synthe-sis by RousselUCLAF,
aFrenchpharmaceuticalcompany,of afamily
of "ultrapure"glucocorticoids
hasprovidedanoppor-tunity
todelineate this problem in better detail (19). 11B,
1713-Dihydroxy-6-methyl-17a-(
1-propynyl)
androsta- 1,4,6-triene-3-one (RU28362)'
is suchanultrapure syntheticglucocorti-coid,
and hashigh affinity
only for theglucocorticoid (not thealdosterone)
receptor. In recentstudieswith this and relatedsynthetic
ultrapureglucocorticoids
in adrenalectomized rats, Bastl (20) has concluded that these glucocorticoidreceptor-specific agonists regulate
basal sodiumtransportwhich in the ratdistal
colon ispredominantly
electroneutral sodiumab-sorption.
Sincehigher
dosesof dexamethasone, butnotof
theglucocorticoid receptor-specific agonist,
inducedamiloride-sensitive, electrogenic
sodiumabsorption,
Bastl (20) specu-latedthat, wheneverglucocorticoids
appear tostimulateelec-trogenic
sodiumabsorption,
itmay beduetocrossover bind-ing ofthe glucocorticoid (i.e., dexamethasone, but not RU 28362) tothe aldosterone receptor. Bastlbased these conclu-sionson invivoluminalperfusion
studies thatwereperformed
1.Abbreviations usedinthispaper:G,conductance;I,,, short-circuitcurrentJmscl,mucosal-to-serosal chloride movement;
JmsNa,
mucosal-to-serosal sodiummovement;Jnetc',
netchlorideabsorption;JnftNa,
netsodium absorption;
JsmC',
serosal-to-mucosal chloride movement;JsmNa,serosal-to-mucosal sodium movement; PD, potential difference;
RU28362,
11#,
17#%dihydroxy-6-methyl-17a-(1-propynyl)
androsta-1,4,6-triene-3-one.
J.Clin.Invest.
© The AmericanSocietyforClinical
Investigation,
Inc.0021-9738/89/12/1924/06 $2.00
soonafteradrenalectomy. Thus,theseexperimentswere lim-ited by an inability to assess directly the effect of ultrapure glucocorticoids on active sodium and potassium transport processes, and bythe possible confoundingissue of endoge-nous steroid release after anesthesia and surgery (21). This previous studydid not address thequestionof the mechanism by which glucocorticoids affect electrolyte transport in the colon of animals with intact adrenalglands. Asaresult,these present studies were designed to evaluate and compare the effect of continuous infusion of aldosterone and RU 28362 for 1-1I1 d onunidirectional andnet
sodium, chloride,
and potas-sium fluxes thatwereperformed
acrossisolated distal colonic mucosa ofnonadrenalectomized
rats under voltage-clamp conditions. These results establish that aldosterone and gluco-corticoidreceptorsmediatespecific
and distinct effectsonac-tiveelectrogenicand electroneutral sodiumabsorptionaswell as on activepotassiumabsorptiveandsecretoryprocesses.
Methods
Nonfasting Sprague-Dawley ratsweighingbetween 250 and 300 gwere
usedin allexperiments. Allanimals receivedastandarddiet(Prolab
3000Formula, Agway,Syracuse, NY).The aldosteroneandRU28362 animalsreceived aldosteroneorRU28362,respectively, via subcuta-neouslyimplantedminipumps(AlzaCorp.,PaloAlto,CA)thatwere
filled withaconcentrationcalculatedtodeliver thespecificsteroidat
therateof 70
,gg
per 100 gbodywtperday. This dose of aldosterone was selected because in other studies it produced plasma aldosterone levelssimilar to those observed indietarysodiumdepletion (22).The colon was removedduringanesthesia, placedon aglass
pi-pette, andstrippedof itsserosaand part of the muscularis. Twopieces,
each measuring 2.3 cm in length, were obtained from each colon
beginning immediately cephaladto the lymph node located at the
pelvicbrim. Themucosawasmounted inLucite chambers with
inter-nal surface area of 1.13 cm2 and bathed in Ringer's solution that contained(inmillimolar):Na 140,K5.2,Ca 1.2, Mg 1.2, Cl 119.8,
HCO325, HPO45.2,glucose 10mM(pH7.4). Short-circuit current
(I,,),
potentialdifference(PD),and conductance (G)weredeterminedaspreviouslydescribed(3).Unidirectional22Na,36C1, and 42K fluxes
wereperformed under voltage-clamp conditions,asdescribed in prior studies(3),andnetfluxeswerecalculated by pairing tissuesonthebasis of G thatwerewithin 10%.
Inthe22Na and 36C] studies therewas a 15-min equilibration
pe-riod,whereas in the 42K experiments this periodwas55min.Positive values representnetabsorption, and negative ones,netsecretion. The resultsarepresented as the mean oftwo15-min periods; 10MM amilo-rideor1.0 mM ouabainwasaddedtothemucosa orserosal bathing
solution, respectively. Aftera 12-min equilibrium period,two addi-tional 15-min flux periodsweredetermined.
Aldosterone,amiloride, and ouabainwerepurchased from Sigma-Aldrich Co., St. Louis, MO; 42K and 36CI were purchased from New EnglandNuclear, Boston, MA; and22Nawaspurchased from
Amer-sham Corp., Arlington Heights, IL. RU 28362 was kindly provided by RousselUCLAF,Romainville, France.
Results
Sodium and chloride transport. Table I presents the results of unidirectionaland net sodium and chloride fluxes in both the control group and the 1-d and 74steroid-infusedgroups. In the control group, net sodium absorption was equal to net chloride absorption and wassignificantlygreater than the
I4C.
Since thediureticamiloride affects sodium absorption in nor-mal andhyperaldosteroneratsdifferently,theeffect of 10
JM
amiloride on ion transport was alsodetermined.Fig. 1 presents the results of 10 giM amiloride on JnaNa andI4,
inthe several experimental groups. As previously presented, amiloridedid notalter net Na absorption (Fig. 1 A), net Clabsorption(data notshown),orI4C (Fig. 1 B).Theseresults aresimilarto those previously reported in the distal colon of normal rats (23) and areconsistent withelectroneutral Na-Cl absorptionasthe pre-dominant sodium absorptive process in thisepithelium.The 4-d steroid-infusedgroupcomprisedanimals that re-ceived infusion of eitheraldosteroneor anequivalentamount
TableI. Unidirectionaland NetSodiumand ChlorideFluxesinControl,
J-d
Steroid-infusedand 7-dSteroid-infusedGroupsSodium fluxes Chloride fluxes
n J.m JamJ.n J.n Jam Jn G
peq/hWcm' Aeqlh*cm' Peq/h*cm' mS/cm
A. Control 6 9.7±0.7 4.9±0.7 4.9±0.5 12.9±0.4 8.4±0.7 4.5±0.7 0.9±0.2 8.0±0.8
B.
J-d
steroid infusionAldosterone 5 12.9±0.3 3.2±0.7 9.6±0.7 17.5±0.2 8.9±0.8 8.5±0.9 2.8±0.6 7.1±0.6 P* <0.02 NS <0.001 <0.001 NS <0.01 <0.02 NS RU 28362 6 15.2±1.1 3.7±0.3 11.5±1.0 19.9±1.6 8.7±0.7 11.2±1.6
0.6±0.1*
7.7±0.8 P* <0.005 NS <0.001 <0.005 NS <0.005 NS NSC. 7-d steroidinfusion
Aldosterone 5 11.5±0.3 4.8±0.8 6.7±0.7 9.5±0.5 8.3±1.2 1.2±1.2 6.8±1.2 9.3±1.0 P* NS NS NS <0.001 NS <0.05 <0.001 NS RU28362 7 16.8±0.5t 4.7±0.5t 12.1±0.8* 26.7±1.5t
13.4+1.
§ 13.3±1.4§ 0.7±0.2§ 5.6±0.7 P*<0.001
NS <0.001 <0.001<0.005
<0.001
NS <0.05 Values are mean±SE. All animals were fed regular diet. The aldosterone and the RU 28362 animals received continuous infusion of either al-dosteroneorRU 28362, respectively, at 70Mtg/100
g body wt per day for either 1 or 7 d. Unidirectional Na andCl
fluxeswere determined with 22Na and36C1under short circuit conditions. n,number of tissue pairs. *Compared with control group. tP <0.005 compared withControl
10 8 6 4
21
A
Aldosterone RU28362
1Day 7Days IDay 7Days
Fa a
V/
a
P/
Kica
P./;
NS p<0.02 p<O.to1 NS NS
88B
TJi~r~~+ts
1Day 7Days II 1Day 7Days7 -a
6-o
5
-4
-3-
Tb
~~
2-3 |
Atwa ¢=
NS p<0.02
LuControl
a+10gMAmilor[Oa
p< 0.005 NS
a p<0.005 ride b p<0.02
cp <0.05
NS
Figure 1. Effect of10,gMamilorideon(A)JneNaand(B)
I,,
innor-mal,1-and 7-d aldosterone and RU 28362-infusedgroups. The
numberof tissue pairs studied is provided inthe legendtoTableI.
Theopenand hatchedbarsrepresentthemean oftwo 15-minflux
periods beforeand after, respectively,theaddition of10
,uM
amilo-ridetothe mucosal bathing solution.Theprobability valuesshown
inthefigurecomparethepre-andpost-amilorideresults. The
sym-bols indicatecomparison with their respective control group: (a) P
<0.005compared withthe controlgroup;(b)P<0.02 compared
withthe controlgroup;(c) P<0.05 comparedwiththe controlgroup.
ofRU 28362. A significant increase in net sodium and net
chloride absorption, asaresultofastimulation of mucosalto
serosal fluxes,wasobservedin both -dsteroid-infusedgroups (Table I B). Therewasnosignificant change inJtmNaJsmClor in
either of thetwoexperimentalgroupscompared with the con-trolgroup, and conductancesweresimilarin the
experimental
and the controlgroups.TheIs,
in the l-daldosterone-infused group was significantly increased compared with the control group(2.8±0.6vs.0.9±0.2 peq/hcm2).
Incontrast,the 4SC inthe l-d RU 28362-infused group wasidentical tothat ofthe control group. The addition of 10
kLM
amiloride to the l-d aldosteronegroupresultedinaparallelreduction of bothJmsNaand JnetNa and
Is,
(Fig. 1). Inaddition,theamiloride-insensitivecomponent of
JnetNa
inthe l-d aldosterone group wassignifi-cantly greater than that in the control group (7.2±0.9 vs.
4.9±0.4
geq/h
cm2;P < 0.05). Incontrast, 10,gM
amiloride had no effect on any parameter of ion transport in the RU 28362group.TableI B alsopresentsthe resultsof iontransportin those
animals thathad received a7-d infusion ofeither aldosterone
orRU28362.Althoughnetsodium absorption inthe aldoste-rone group was slightly but not significantly increased
com-paredwith the controlgroup,the
I,,
in the aldosteronegroup, ashas been previouslyreported (3), wassubstantially andsig-nificantly increasedto6.8±0.9
geq/h
*cm2comparedwith thecontrolgroup. Incontrast, net chlorideabsorption inthe 7-d
aldosterone-infused group was significantly reduced compared with the control group as a result of a decrease in
Jmscl.
Also,10
,jiM
amiloride produced substantial changes in ion transport in the 7-d aldosterone group; amiloride reducedJnetNa
to zero and inhibitedI,,
(Fig.1).2
Infusion of RU 28362 for 7 d resulted in significantly higher ratesofnet sodium and chloride absorption (12.1±0.8 and 13.3±1.4 iieq/h
cm2,
respectively) compared with the control group. This stimulation was predominantly the result of substantial enhancement of bothJmsNa
andJmsCI.
IC
was similar in the 7-d RU 28362 and the control groups. The G was, however, significantly lower in the RU 28362 group (5.6±0.7mS/cm2)
than that of the control group (8.0±0.8mS/cm2).
Finally, amiloride had no effect on unidirectional or net Na absorption orI,,
in the 7-d RU 28362 group. As a result, theamiloride-insensitive
component ofJnetNa
was sig-nificantly greater in the RU 28362 group(11.7±0.7
meq/h.cm2)
than in both the aldosterone and the controlgroups. These results demonstrate that the long term effects of aldosterone and RU 28362 on sodium and chloride transport in the rat distal colon differ both qualitatively and quantita-tively.
The effect of RU 28362 on sodium and chloride transport with time is illustrated in Fig. 2. Infusion of RU 28362 for 1, 3, 7, and 11 d resulted in substantial stimulation of both
JnetNa
andJnetCI
comparedwith the control group; however, the rate ofnetsodiumand chloride absorption observed after 3, 7, and 11 d of RU 28362 infusion was not significantly greater than that observed after 1 d of steroid infusion. These results indi-cate that stimulation of electroneutral Na-Cl absorption by RU 28362 is rapid and is not time dependent.Potassium transport. Table II provides the results of unidi-rectional and net potassium fluxes in control, 7-d
aldosterone-infused, and 7-d RU 28362-infused groups. In the control group net potassium absorption was 0.41±0.8geq/h cm2)
which is similar to that previously reported by this laboratory (3, 5). Aldosterone infused for 7 d resulted in induction of active potassium secretion (-0.49±0.08jueq/h
.cm2)
as are-sult of a significant increase in
JsmK.
This effect of long-term aldosterone infusion on potassium transport in the distal colon of the rat is similar to that previously reported for prolonged dietary Na depletion (5). In contrast, a 7-d infusion of RU 28362 had no effect on unidirectional or net potassium fluxes and indicates that this glucocorticoid receptor-specific agonist did not alter potassium transport in the distal colon of the rat. Previous in vitro studies of potassium transport in the rat distal colon haveestablished that dietary sodium depletion not only induces active potassium secretion but also stimulates active K absorption (1, 5, 24). This stimulation of active K absorption can be demonstrated when the Ksecretory
process in inhibited by one or more experimental maneuvers (e.g., removal of Na from the serosal bathing solution, or the addi-tion of either ouabain or bumetanide to the serosal bathing solution [24]). To determine whether aldosterone or RU 28362 also induced active K absorption unidirectional42K
2. These results are virtually identical to previous
studies
of Na and Cl transport in both dietary Na-depletedanimals and those that hadre-ceived continuous infusion of aldosterone (3, 6) andhave been inter-preted to indicate that aldosterone both induces
amiloride-sensitive
electrogenic Naabsorptionandinhibits electroneutral Na-Cl
absorp-tion.
C
ca
a)
NM
E
0
0:
a)
14
-1
Aldosterone RU 28362
E v
a)
E
RU28362
Figure2.Net NaandCl transport in control group and the
experi-mental groupsinfused with RU 28362 for 1, 3, 7, and 11 d. The numberof tissue pairs studied in each group was 6, 6, 6, 7,and5,
re-spectively.
fluxeswereperformedin the presence of 1 mM ouabain in the serosal bathingmedia. Fig. 3 demonstrates that 1 mM ouabain did not alter
Jn,,K
in the control group (0.05 < P < 0.10) althoughthere was a small but statistical significant decrease in JsmK.In contrast, inthe7-daldosteronegroup,serosalouabain
not onlyinhibitednet K secretion but resulted in a substantial increase of the rate of net K absorption (1.16±0.14
Aeq/h
* cm2). njK in the aldosterone animals after the addition ofserosalouabainwasgreater than that observed in the control group(0.52±0.06Aeq/h
* cm2). In contrast, K transport in the RU28362groupafter ouabain addition was identical to that observedin the control group. In the 1-d aldosterone group, both activepotassium secretion(-0.72±0.06ueq/h* cm2) and, in the presence ofserosal ouabain, active K absorption (0.88±0.12Aeq/h
*cm2)were observed.The rates of these two potassiumtransport processes were not significantly different thanthat inthe 7-d aldosteronegroup.Discussion
Presentconcepts of the regulation of electrolytetransport by corticosteroids have evolved substantially over recent years (1).Itisnowwellaccepted that aldosterone stimulates active amiloride-sensitive sodium
absorption
and active potassium secretion in the distal colon ofratand rabbit(3-5).Glucocor-W Ringer m +Ouabain
Figure 3. Netpotassiumtransport incontrol, 1- and 7-d
aldosterone-infusedand 7-d RU28362-infused groups.Thenumber of tissue pairs is provided in the legendtoTable II.Openand hatched bars represent themeanoftwo15-min fluxperiodsbefore andafter,
re-spectively,theadditionof 1 mMouabaintotheserosalbathing solu-tion. Theprobabilityvaluesshown in thefigurecompare the pre-andpost-ouabainresults.Thesymbolsindicatecomparisonwith theirrespectivecontrol groups:(a)P<0.025comparedwith the control group;(b)P<0.001 comparedwith the control group;(c)
NS (notsignificant) compared with the control group.
ticosteroids havesimilar, but not identical effects on
electro-lytetransportin the mammalian colon(1,7-12),and consid-erable
uncertainty
exists regarding theexact role of aldoste-rone and glucocorticoids in the regulation of colonic ion transport. Thedemonstrationthat bothspecific mineralocor-ticoid andglucocorticoid
cytosolicreceptorsarepresent in the rat andrabbit colonis consistent with theinterpretation that eachcytosolic steroidreceptor mediatesspecificion transport processes(13-16).However, todatenostudyhasconvincingly
demonstrated which corticosteroid receptor isspecifically re-sponsible foreachof the severalpreviously identified
cortico-steroid-induced
changesinsodiumandpotassium transport. Recentlya series of compounds hasbeensynthesized by
the Frenchpharmaceuticalcompany, RousselUCLAF,which arewell
suited
toidentify
thespecific
role ofthetwo corticoste-roidreceptors(19).Binding
studies withglucocorticoid
recep-tor-specific agonists, designated
RU 26988 and RU 28362, havedemonstratedtheseagonists
bindwithhigh
affinitytotheglucocorticoid
receptor,donotdisplace
thebinding of radio-labeled aldosterone from the aldosterone receptor, and areTableII. UnidirectionalandNet PotassiumFluxesinControl, 7-dAldosterone-infused, and7-dRU28362-infusedGroups
n J., J., Jo,, I,, G
yeq/h* cm' mS/cm2
Control 6 0.83±0.07 0.42±0.04 0.41±0.08 1.1±0.2 8.0±0.7 Aldosterone 8 0.81±0.06 1.30±0.10 -0.49±0.08 5.0±0.3 13.1±1.2
P* NS
<0.001
<0.001
NS NSRU28362 7 0.97±0.12 0.61±0.08 0.36±0.08 0.9±0.2 7.9±0.9
P* NS NS NS NS NS
Values aremean±SE. Unidirectional potassium fluxesweredetermined with42K.Positive values representnetabsorption, andnegativeones,
netsecretion. See the legendtoTableIfor additional details. *Comparedwith control group.
CorticosteroidReceptors and ColonicIonTransport 1927
notdisplacedfromtheglucocorticoid receptor by aldosterone (16, 19). These compounds,therefore, permit investigation of the specific role of the glucocorticoid receptor in mediating changesincolonic iontransport.
Although four studies of colonic epithelial function have beenreported to date with theseagonists(16, 18,20, 25), none have identified the active electrolyte transport changes that occurwhen glucocorticoids are administered to nonadrenal-ectomized animals.Instudies of colonic electrolytemovement inadrenalectomizedratsin vivo,Bastl (20) has suggested that theglucocorticoid receptorregulated basal electroneutral so-diumabsorption. The major limitations of thisstudyarethat invivo studiesare notidealtoestablishthe presenceof active potassium secretion and that these synthetic glucocorticoid receptor-specific agonists were administeredas pulses during theinitial 26-h period after general anesthesia for surgical adre-nalectomy.Halevyet al.(25)reported that RU 28362infused intoratsfor24 hincreased amiloride-insensitive
Isc
whichmay represent either active chloride secretion or amiloride-resis-tant, electrogenic sodium transport. The former has been showntobestimulated
by dexamethasoneand methylpredni-solone inratdistal colon (26) andratileum (27), respectively, andthe latter wasrecentlydescribed in the rabbit cecum(28). Unfortunately,neitherionflux measurementsnorion replace-ment studies were performed to establish the basis for this increase inIsc.
Electrophysiologic studies comparedtheeffects ofRU 28362, dexamethasone, and secondary hyperaldoste-ronism in the rat distal colon and concluded that the two steroid receptors mediate changes in sodium and potassium transportthatdiffer bothqualitatively
andquantitatively
and thatdexamethasone
activates both types ofreceptors(18).
Two limitations of this study were the absence of
isotopic
sodiumand potassium fluxmeasurements and theuseofvery largeamountsof corticosteroids
(approximately
10times the estimatedKmof theglucorticoidreceptor and theamountused in the otherstudies [20,25]
andin the presentstudy).Continuous infusion ofaldosterone via
implanted
Alzet osmoticminipumps
produce identicalchangesinsodium and chloridetransport asdietary
sodiumdepletion
(6 and Table I). Long-term 7-d infusion of aldosterone resulted inbothstimu-lation ofelectrogenic amiloride-sensitive sodium absorption and
inhibition
of electroneutralNa-Clabsorption
which isthe primary sodium transport system in normal animals. These studiesfurtherdemonstrate thatinfusion of aldosterone pro-duced identical changes inpotassium
transport compared to those that are observed indietary sodium depleted
animals: both 1-and7-dinfusion ofaldosteroneresulted in active po-tassium secretion (TableII).
Whenaldosterone-stimulated po-tassium secretion wasinhibited bytheaddition of1 mM sero-salouabain,
electroneutral potassiumabsorption
wasalso en-hanced (Fig. 3). The rateofthepotassium absorptive
process in the presenceofouabainin thealdosterone-infused animals wassignificantlygreater than that observed in the control ani-malsand, therefore, indicates that aldosterone not only in-duced active potassium secretion but also stimulated active electroneutral'potassium absorption.Aldosterone infused for 1 d produced a modest (though
significant)
stimulation of electroneutral Na-Clabsorption,in additionto itsinduction ofelectrogenic
sodiumabsorption.
Wesuggest that this transientenhancement ofelectroneutral Na-Cl
absorption
isaresultof thebinding
ofaldosteronetotheglucocorticoid receptor since RU 28362 did not inhibit elec-troneutralNa-Clabsorptionatany time up to 11 d(Fig. 2) and experiments with both aldosterone (3) and sodium repletion (29)indicatethatinhibition ofelectroneutralNa-Cl absorption requiresexposure to aldosterone for at least 48-72 h. We spec-ulate that the stimulation of electroneutral Na-Clabsorption byaldosteroneat24 h issecondarytoitsbindingtothe gluco-corticoidreceptor3 but that at latter timepointsits inhibition whichmediated bythealdosteronereceptoris dominantover its enhancementbytheglucocorticoidreceptor.
Theglucocorticoid receptor-specificagonist RU 28362 re-sulted instimulation of electroneutralNa-Cl absorption which was observed asearly as 24 h after its infusion (Fig. 2). Al-though aldosteronebothinducedelectrogenic sodium absorp-tion andinhibitedelectroneutralNa-Cl absorption, there was noevidence ofeither induction ofelectrogenic sodium trans-port or inhibition of electroneutral Na-Cl absorption even after 11 d of infusionofRU 28362(Fig. 2). Studies of potas-siumtransport alsodidnotrevealanyeffect ofRU 28362 on eitherpotassium absorption or potassium secretion (Fig. 3). Theseresultsareinmarked contrast to the changes in sodium andpotassiumtransportproduced by aldosteroneand indicate that the twocorticosteroid receptors in the distal colon pro-duce
highly specific
changes in electrolytetransport. This dis-tinctseparation oftheaction ofRU28362andaldosteroneon ion transport in this epithelia indicates that RU 28362 is aspecific glucocorticoid
receptoragonist and,as a consequence, impliesthat thechangesinpotassium and electrogenicsodium transport mediated by aldosterone are mediated solely as a resultofaldosterone'sbindingtothemineralocorticoidrecep-tor.
Thus,
the mineralocorticoidreceptormediatesvery spe-cificchanges
in sodium andpotassium
transport in the rat distal colon whiletheglucocorticoid
receptor mediates other sodium (butnotpotassium)transport processes.The
efficacy
of corticosteroids in the therapy of inflamma-tory boweldisease has beenrecognized
for 40years.Originally, the usefulness ofcorticosteroids in these clinical conditionswas assumed
solely
the result of their anti-inflammatory ef-fect.Newerunderstanding
oftheroleof corticosteroidsin theregulation
of colonic electrolytetransport hasraisedthepossi-bility
thattheantidiarrhealaction ofthesesteroidsin inflam-matoryboweldiseasemayalso berelatedtotheir proabsorp-tiveeffectonelectrolyte
movement(1,
29).
The results of this presentstudy together with therecentdemonstrationthatso-dium and chloride are
actively
absorbedthroughout
thenor-mal human colon
predominantly
by electroneutral transport mechanisms(30-32)
may haveimportant therapeutic
impli-cations for
inflammatory
boweldisease. Anultrapure
gluco-corticoid
receptor-specific agonist
suchasRU28362 may pro-videgreaterefficacy
with fewer side effectsthanthepresently
used
"glucocorticoids"
which manifestsignificant
mineralo-corticoidagonistactivity.
3.Sincegeneralanesthesiaand abdominal surgerycanresult in
eleva-tion ofplasmacorticosteronelevels(21),itispossiblethat the increase
in electroneutralNa-Clabsorptionobservedafter24 hofaldosterone
infusion isaresult ofelevatedendogenouscorticosteroidlevels andnot
of aldosterone. Thispossibilityisunlikelysince in animalssubjectedto considerablygreaterstressthan theimplantation ofa subcutaneous
minipump, plasma corticosterone levels had returnedto normal
Acknowledgments
The authors acknowledgethe generosityofRoussel UCLAF
(Ro-mainville,France) fortheir kind gift ofRU28362,withoutwhich these
studies wouldnothave beenpossible.We alsoacknowledgethe
tech-nical assistance ofMs.Laurel P.Donahue.
This study wassupported in partby U.S. PublicHealth Service
researchgrants(DK-14669 and DK-18777) fromtheNational
Insti-tuteofDiabetes, DigestiveandKidneyDiseases. Dr. Turnamianwas a
recipient ofa Senior Research Fellowshipawardedbythe National
Foundationfor Ileitis and Colitis duringpartof these studies.
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