Differential effects of corticosteroids on active electrolyte transport in the mammalian distal colon.
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Aldosterone↗
Biomedical subjects
H J Binder
Publications and source records attributed to H J Binder.
Electrolyte transport in distal colon of sodium-depleted rats: effect of sodium repletion.
Dietary sodium depletion increases plasma aldosterone level and, as a result, induces amiloride-sensitive electrogenic sodium absorption and electrogenic potassium secretion and stimulates Na+-K+-ATPase activity in rat distal colon, while inhibiting electroneutral sodium chloride absorption. To assess the events that occur as the aldosterone-stimulated colon reverts to normal, unidirectional 22Na and 36Cl fluxes were measured under voltage-clamp conditions across isolated distal colonic mucosa of rats that were initially dietary sodium depleted for 7 days and then sodium repleted for varying periods of time before the study. Within 8 h of dietary sodium repletion, plasma aldosterone level and Na+-K+-ATPase activity declined to normal, amiloride-sensitive electrogenic sodium absorption decreased by greater than 90%, and active electrogenic potassium secretion also decreased markedly. In contrast, electroneutral sodium chloride absorption did not completely return to levels seen in normal animals until approximately 64-88 h. These results demonstrate that maintenance of electrogenic sodium absorption and potassium secretion are directly dependent on elevated plasma aldosterone levels. The inhibition of electroneutral sodium absorption, although initiated by excess aldosterone, persists after normalization of the plasma aldosterone level, thereby implying that the inhibition is dependent on additional factor(s).
Aldosterone↗
Glucocorticoids have a different action than aldosterone on target tissue.
Studies were performed, using a synthetic glucocorticoid, RU 28362, that has negligible binding to aldosterone receptors, to examine whether the effect of glucocorticoids on the electrical properties of rat distal colon involves a specific mechanism or crossover binding to aldosterone receptor sites. Compared with control, RU 28362, in a dose of 70 micrograms.100 g body wt-1.day-1, increased transepithelial potential differences (VT) twofold and the equivalent short-circuit current (ISC) threefold; epithelial resistance (RT) was reduced 25%. These changes were qualitatively similar to, but less than, the changes induced by the same dose of aldosterone. The effects of dexamethasone in the same dose were indistinguishable from the changes caused by aldosterone. The results of cell-impalement studies corresponded to whole epithelial changes; RU 28362 induced a small increase in basolateral potential difference, from -43 +/- 2 to -45 +/- 1 mV, and a fall in apical potential difference, from 40 +/- 2 to 36 +/- 1 mV, but these effects on cell membrane voltage were less than caused by either aldosterone or dexamethasone. In contrast to the marked inhibitory action of amiloride to reduce VT in aldosterone- and dexamethasone-treated animals, this sodium channel blocker had no effect on VT in RU 28362-treated animals. Similarly, while spironolactone caused marked inhibition of the changes in VT, RT, and ISC induced by aldosterone, the inhibitory action on tissue from RU 28362-treated animals was slight or absent. These results suggest that glucocorticoids have a specific aldosterone receptor-independent action on the electrical properties of the large intestine.
Adrenalectomy↗
Use of laxatives in clinical medicine.
This presentation discusses the use of laxatives in clinical medicine. As laxatives are used frequently in patients with constipation, the causes of constipation are reviewed including endocrine, colonic, neurogenic and drug-induced etiologies. Laxatives are also used to eliminate fecal material prior to diagnostic studies. The problem of laxative abuse is also emphasized.
Cathartics↗
Aldosterone increases the maximal turnover rate of the sodium pump.
These experiments were performed to determine whether aldosterone-dependent effects in apical and basolateral membranes could be temporarily dissociated and whether aldosterone increases the maximal capacity, or maximum turnover rate of the sodium pump. Tissue from rat distal colon exposed to the action of high plasma levels of aldosterone for 4 h, 24 h and 7-10 days was compared to control tissue in a modified Ussing chamber, before and after addition of nystatin to the mucosal solution to remove the apical barrier to the cell entry of sodium. The maximum turnover rate of the sodium pump was represented by the equivalent short circuit current, Isc, after the addition of nystatin. After 4 h of aldosterone basal Isc increased 2.6-fold above control (43 +/- 34 microA.cm-2, p less than 0.05) and transepithelial PD, VT, increased 2-fold over control (-7.0 +/- 5.5 mV, lumen negative, p less than 0.05). Administration of aldosterone for 24 h caused further marked increases in Isc (11-fold) and a fall in RT to 50% of control. Similar changes were observed after 7-10 days on low sodium diet, and at all time intervals the changes were completely inhibited by amiloride (10(-4)M). Although aldosterone stimulated Isc within 4 h, there was no further increase in Isc in the presence of nystatin during the same time period compared to the control post-nystatin Isc of 419 +/- 79 microA.cm-2. However, after 24 h aldosterone caused approximately a 40% increase in the maximal turnover rate of the sodium pump, which persisted for 7-10 days.(ABSTRACT TRUNCATED AT 250 WORDS)
Aldosterone↗
Corticosteroids and intestinal ion transport.
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Animals↗
Mechanism of electroneutral sodium chloride absorption in distal colon of the rat.
This investigation was designed to establish the mechanism of sodium and chloride transport in the rat distal colon by determining ion fluxes across isolated mucosa under voltage clamp conditions. The net rates of sodium and chloride absorption in the distal colon of the rat were approximately equal (5.8 +/- 0.3 and 6.9 +/- 0.5 microEq/h X cm2, respectively) and significantly greater than the short circuit current (0.9 +/- 0.1 microEq/h X cm2). Net sodium absorption and net chloride absorption were markedly reduced by the removal of chloride and sodium, respectively, but were not affected by the absence of potassium from the mucosal bathing solution. Both net sodium absorption and net chloride absorption were also significantly inhibited by 1.0 mM amiloride and by 0.1 mM acetazolamide. In contrast, 0.1 mM amiloride and 1.0 mM furosemide did not inhibit either sodium or chloride absorption. These results confirm that electroneutral sodium chloride absorption is the predominant mechanism of sodium and chloride absorption and suggest that parallel ion (Na-H and Cl-HCO3) exchanges, rather than independent electrogenic sodium and chloride transport, coupled sodium-chloride cotransport, or coupled Na-K-2Cl cotransport, are most likely responsible for sodium chloride absorption in this epithelium.
Acetazolamide↗
Potassium/proton exchange in brush-border membrane of rat ileum.
These experiments were designed to determine whether proton-driven 86Rb uptake was present in apical membrane vesicles prepared from rat ileum. The uptake of 86Rb was approximately 300 to 350% greater in the presence of a 100-fold H+ gradient than in its absence and was greater at 1, 2 and 5 minutes ("overshoot") than that at 90 minutes. Proton-driven 86Rb uptake was decreased by 20% in TMA-nitrate compared to that in TMA-gluconate. 0.3 mM amiloride did not significantly inhibit proton-driven 86Rb uptake; in contrast, proton-driven 22Na uptake was significantly inhibited by 0.3 mM amiloride by 34%. Similarly, 25 mM KCl inhibited proton-driven 86Rb uptake more than that of 22Na, while the inhibition of proton-driven 22Na uptake by 25 mM NaCl was greater than that of 86Rb. In additional studies intravesicular acidification measured by acridine orange fluorescence was demonstrated in the presence of an outwardly directed K gradient. These studies demonstrate that a proton gradient stimulates 86Rb uptake and a K gradient induces intravesicular acidification; and that these fluxes are mediated by a K/H exchange distinct from Na/H exchange which is also present in this membrane. We conclude that a specific exchange process for K/H is located in ileal apical membrane vesicles.
Animals↗
Role of aldosterone in the regulation of sodium and chloride transport in the distal colon of sodium-depleted rats.
Dietary sodium depletion with elevated aldosterone levels induces electrogenic, amiloride-sensitive sodium absorption and inhibits electroneutral sodium chloride absorption in the rat distal colon. To assess the role of aldosterone in the production of these changes, unidirectional 22Na and 36Cl fluxes were performed under voltage clamp conditions across isolated distal colonic mucosa of rats given continuous aldosterone infusions for up to 12 days. Aldosterone infusion for 7-12 days produced identical changes in both electrogenic sodium absorption and electroneutral sodium chloride absorption compared with dietary sodium-depleted animals. In contrast, aldosterone at 24, 48, and 72 h produced varying changes in ion transport: electrogenic sodium absorption progressively increased, whereas electroneutral sodium chloride absorption was initially augmented but then inhibited. Aldosterone induced active potassium secretion, demonstrated by a reversed short-circuit current after addition of amiloride, in all experimental groups. These results demonstrate that the changes in ion transport observed in sodium-depleted animals are produced by aldosterone, and that aldosterone not only stimulates electrogenic sodium absorption and potassium secretion but in a time-dependent manner both stimulates and inhibits electroneutral sodium chloride absorption.
Aldosterone↗
Effect of glucocorticoids on rectal transport in normal subjects and patients with ulcerative colitis.
The acute effects of single pharmacological doses of glucocorticoid hormones on net electrolyte and water transport and electrical potential difference (pd) in the rectum was studied in control subjects and in patients with either active or inactive ulcerative colitis, using a dialysis technique. Compared with 17 control subjects, nine patients with active ulcerative colitis exhibited marked decreases in net sodium absorption and rectal pd, while these transport parameters were normal in six patients with inactive ulcerative colitis. Intravenous administration of hydrocortisone hemisuccinate (100 mg) resulted five hours later in significant and quantitatively similar increases in net sodium and water absorption and pd in nine control subjects, seven patients with active ulcerative colitis, and six patients with inactive ulcerative colitis. Intravenous administration of methylprednisolone phosphate (40 mg) to eight control subjects produced increases in net sodium and water absorption and pd five hours later, which did not differ significantly from those produced by hydrocortisone; methylprednisolone induced similar changes in two patients with active ulcerative colitis. These results indicate that single pharmacological doses of glucocorticoids stimulate acute increases in rectal sodium and water absorption in control subjects and in patients with acute ulcerative colitis. The ability of systemically administered glucocorticoids to reduce diarrhoea in ulcerative colitis may therefore be related to direct effects on distal colonic sodium and water transport, as well as to their better known anti-inflammatory action.
Acute Disease↗
Electrophysiology of the human colon: evidence of segmental heterogeneity.
The electrical properties of epithelial cell membranes in human descending and ascending colon were studied using microelectrodes and the Na channel blocker amiloride. Under control (pre-amiloride) conditions, the transepithelial electrical measurements in the two colonic segments were similar. The mucosal addition of 0.1 mM amiloride to descending colon totally abolished the transepithelial voltage (Vt) and short-circuit current (Isc), and significantly increased the total tissue resistance (Rt) by 19% (p less than 0.005). Intracellular recordings in descending colon obtained with microelectrodes revealed that the transepithelial effects of amiloride reflected hyperpolarization of the apical membrane and a significant increase in apical membrane resistance, changes which were consistent with amiloride-blockade of apical Na channels and complete inhibition of electrogenic Na transport. An appreciable amiloride-insensitive conductance was also present in the apical membrane of the descending colon. In contrast, the mucosal addition of 0.1 mM amiloride to ascending colon decreased Vt by only 30% (p less than 0.02) and Isc by 43% (p less than 0.05), but had no significant effect on Rt. These results indicate that electrogenic Na transport accounts totally for Isc in human descending colon in vitro, but only partly for Isc in human ascending colon, suggesting that Na transport in ascending colon is mediated primarily by electroneutral processes, and the apical membrane of human descending colon contains an amiloride-sensitive conductance for Na in parallel with an amiloride-insensitive conductance, which may transport K.
Adult↗
Sodium and chloride transport in the large intestine of potassium-loaded rats.
Increased dietary potassium ("potassium loading") induces several adaptive changes in colonic function, including increased potential-dependent potassium secretion, active potassium secretion, and Na-K-ATPase activity, but does not alter net sodium absorption in vivo. To establish whether potassium loading stimulates active sodium transport, unidirectional, net sodium, and chloride fluxes were determined under voltage-clamp conditions across isolated rat distal colonic mucosa. In normal animals net sodium flux (JNanet), net chloride flux (JClnet) and short-circuit current (Isc) were 6.1 +/- 1.1, 8.4 +/- 1.0, and 0.7 +/- 0.1 mu eq X h-1. cm-2, respectively; potassium loading significantly increased JNanet and Isc by 4.9 +/- 1.4 and 3.5 +/- 0.7 mu eq X h-1 X cm-2, respectively, without changing JClnet. Amiloride (0.1 mM) inhibited the increases in JNanet and Isc produced by potassium loading. In Cl-free Ringer solution in normal animals JNanet was reduced to 0.6 +/- 0.3 mu eq X h-1 X cm-2. Potassium loading produced identical increases in JNanet and Isc, which were also completely inhibited by 0.1 mM amiloride. These studies establish that potassium loading induces amiloride-sensitive electrogenic sodium absorption without affecting electroneutral sodium-chloride absorption.
Adaptation, Physiological↗
Sodium-proton exchange in colon brush-border membranes.
Apical membrane vesicles were prepared from proximal and distal segments of the large intestine of the rat by a method based on morphological criteria and were used to determine 22Na uptake. In both preparations an outwardly directed proton gradient stimulated 22Na uptake. In proximal colon a decrease in vesicular volume induced by an increased media osmolarity led to diminished 22Na uptake at 90 min; a significant (50-60%) portion of uptake represented binding. Initial uptake was linear for 10 s and extrapolated through zero, indicating minimal extravesicular binding. Initial uptake was a saturable function of medium Na concentration. In both preparations initial influx of 0.1 mM NaCl was inhibited by amiloride (0.1-1.0 mM), 15 mM NaCl, 15 mM LiCl, and 15 mM NH4Cl. From the characteristics of the initial 22Na influx we conclude that the apical membrane from colonocytes of proximal and distal segments contains a Na-H exchange with properties similar to those described in other epithelia.
Amiloride↗
Dietary potassium modulates active potassium absorption and secretion in rat distal colon.
To determine the effect of variations in body stores of potassium on the rate of active potassium transport in the large intestine, unidirectional 42K fluxes were performed under short-circuit conditions across isolated distal colonic mucosa of control, dietary potassium-depleted and dietary potassium-loaded rats. Potassium depletion stimulated net potassium absorption (JK net) (0.87 +/- 0.19 vs. 0.49 +/- 0.04 mu eq X h-1 X cm-2, P less than 0.025) due to a 40% increase in mucosal-to-serosal potassium transport (JK m----s). In sodium-free Ringer solution JK net in the potassium-depleted group was also significantly greater than in controls (1.93 +/- 0.26 vs. 1.01 +/- 0.11 mu eq X h-1 X cm-2, P less than 0.005). In contrast, in chloride-free Ringer solution JK net was identical in the control and potassium-depleted groups (0.39 +/- 0.05 vs. 0.46 +/- 0.07 mu eq X h-1 X cm-2, P = NS). Potassium loading reversed net potassium absorption to net potassium secretion (-0.76 +/- 0.08 mu eq X h-1 X cm-2, P less than 0.001) as the result of a decrease in JK m----s and an increase in serosal-to-mucosal potassium transport (JK s----m). Net potassium secretion was abolished in the absence of either sodium or chloride from the bathing solution but not by mucosal amiloride. In sodium-free Ringer solution JK net was similar in control and potassium-loaded groups, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)
Amiloride↗
Ion transport in proximal colon of the rat. Sodium depletion stimulates neutral sodium chloride absorption.
UNLABELLED: The model of sodium and chloride transport proposed for the colon is based on studies performed in the distal segment and tacitly assumes that ion transport is similar throughout the colon. In rat distal colon, neutral sodium-chloride absorption accounts for the major fraction of overall sodium absorption and aldosterone stimulates electrogenic, amiloride-sensitive sodium absorption. Since we have demonstrated qualitative differences in potassium transport in proximal and distal segments of rat colon, unidirectional 22Na and 36Cl fluxes were performed under short-circuit conditions across isolated proximal colon of control and sodium-depleted rats with secondary hyperaldosteronism. In the control group, net sodium absorption (JNanet) (7.4 +/- 0.5 mu eq/h . cm2) was greater than Isc (1.4 +/- 0.1 mu eq/h . cm2), and JClnet was 0 in Ringer solution. Residual flux (JR) was -5.2 +/- 0.5 mu eq/h . cm2 consistent with hydrogen ion secretion suggesting that neutral sodium absorption may represent sodium-hydrogen exchange. 1 mM mucosal amiloride, which inhibits sodium-hydrogen exchange in other epithelia, produced comparable decreases in JNanet and JR (4.1 +/- 0.6 and 3.2 +/- 0.6 mu eq/h . cm2, respectively) without a parallel fall in Isc. Sodium depletion stimulated JNanet, JClnet, and Isc by 7.0 +/- 1.4, 6.3 +/- 1.9, and 0.8 +/- 0.2 mu eq/h . cm2, respectively, and 1 mM amiloride markedly inhibited JNanet and JClnet by 6.0 +/- 1.1 and 4.0 +/- 1.6 mu eq/h . cm2, respectively, with only a minimal reduction in Isc. CONCLUSIONS: the predominant neutral sodium-absorptive mechanism in proximal colon is sodium-hydrogen exchange. Sodium depletion stimulates electroneutral chloride-dependent sodium absorption (most likely as a result of increasing sodium-hydrogen and chloride-bicarbonate exchanges), not electrogenic chloride-independent sodium transport. The model of ion transport in the proximal colon is distinct from that of the distal colon.
4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
Augmentation of neutral sodium chloride absorption by increased flow rate in rat ileum in vivo.
Studies in intact animals have shown that intestinal solute absorption is enhanced with increasing flow rates; the mechanism of this phenomenon has not been explored in detail. We used single pass perfusions of rat ileum to study the effect of higher flow rate on electrolyte absorption. Augmenting perfusion rate from 0.5 to 5.0 ml/min resulted in increased rates of sodium (11.0 +/- 0.9 vs. 23.5 +/- 2.7 mueq/min X g) and chloride (12.1 +/- 0.8 vs. 25.0 +/- 2.2 mueq/min X g) absorption, reduction in the estimated unstirred layer thickness (668 +/- 31 vs. 433 +/- 28 micron), minimal changes in intraluminal pressure and transmural potential difference, and a small, though significant, increase in intraluminal volume (19.4 +/- 8.4%). Removal of sodium from the perfusion medium abolished the effect of increased flow rate on chloride absorption as did removal of chloride on sodium absorption; addition of furosemide or acetazolamide to Ringer's solution also inhibited this effect. In separate experiments, stepwise increases in intraluminal volume were induced by elevating the outflow tubing; no effect on electrolyte transport was observed. These studies demonstrate that neutral sodium chloride absorption is enhanced in rat ileum at higher flow rates, perhaps as a result of a decrease in the thickness of unstirred layers.
Animals↗
Demonstration of specific high affinity receptors for aldosterone in cytosol of rat colon.
Since both aldosterone and glucocorticoids increase cation transport in rat distal colon, and a specific glucocorticoid high affinity cytosolic receptor has been identified in this tissue, it was possible that the action of aldosterone was dependent on interaction with the glucocorticoid receptor. Studies were, therefore, performed to determine whether a specific high affinity receptor for aldosterone was present in rat distal colon. At 4 C, aldosterone binding was saturable and exhibited a high affinity site with an apparent Kd of 6.2 +/- 0.9 X 10(-10) M and a calculated number of binding sites of 57.2 +/- 10.8 fmol/mg cytosol protein. Scatchard plot analysis also revealed a low affinity site with a Kd of 5.9 +/- 1.1 X 10(-8) M and 961 +/- 191 fmol/mg cytosol protein-binding sites. Competitive binding studies demonstrated that the high affinity binding protein was specific for aldosterone, compared to either dexamethasone or RU-28362. Since a specific high affinity receptor protein for aldosterone is present in rat distal colon, these data are consistent with a direct action of aldosterone that is independent of the glucocorticoid receptor system.
Aldosterone↗
The electrical basis for enhanced potassium secretion in rat distal colon during dietary potassium loading.
Previous studies in rat distal colon provide evidence for an active absorptive process for potassium under basal conditions, and for active potassium secretion during chronic dietary potassium loading. The present studies were performed with conventional and potassium-selective microelectrodes to determine the electrical basis for the increase in transcellular (active) potassium secretion observed during potassium loading. Compared to control tissues, potassium loading resulted in a 5-fold increase in transepithelial voltage (VT) and a 52% decrease in total resistance (RT) in the distal colon. The rise in VT was due to a decrease in apical membrane resistance and an increase in basolateral membrane voltage from -45 +/- 2 mV (cell interior negative) in control to -56 +/- 2 mV (p less than 0.001) in potassium loaded tissues. This difference in basolateral membrane voltage reflected in increase in intracellular potassium activity from 86 +/- 4 mM to 153 +/- 12 mM (P less than 0.001). In control tissues, the sequential mucosal addition of the sodium channel blocker amiloride (0.1 mM) and the potassium channel blocker tetraethylammonium chloride (TEA: 30 mM) produced no effect on the electrical measurements. However, in potassium loaded tissues, amiloride and TEA produced transepithelial changes consistent with inhibition of apical membrane conductances for sodium and potassium, respectively, reflected by increases in the resistance ratio, alpha (ratio of apical to basolateral membrane resistances). These data indicate that the decrease in apical membrane resistance during potassium loading was caused by an increase in apical membrane conductance for both potassium and sodium.
Amiloride↗