Animal model to study the effect of adrenal hormones on epithelial function

Dietary Na+ depletion up-regulates NKCC1 expression and enhances electrogenic Cl- secretion in rat proximal colon

The corticosteroid hormone, aldosterone, markedly enhances K+ secretion throughout the colon, a mechanism critical to its role in maintaining overall K+ balance. Previous studies demonstrated that basolateral NKCC1 was up-regulated by aldosterone in the distal colon specifically to support K+ secretion-which is distinct from the more well-established role of NKCC1 in supporting luminal Cl- secretion. However, considerable segmental variability exists between proximal and distal colonic ion transport processes, especially concerning their regulation by aldosterone. Furthermore, delineating such region-specific effects has important implications for the management of various gastrointestinal pathologies. Experiments were therefore designed to determine whether aldosterone similarly up-regulates NKCC1 in the proximal colon to support K+ secretion. Using dietary Na+ depletion as a model of secondary hyperaldosteronism in rats, we found that proximal colon NKCC1 expression was indeed enhanced in Na+-depleted (i.e., hyperaldosteronemic) rats. Surprisingly, electrogenic K+ secretion was not detectable by short-circuit current (ISC) measurements in response to either basolateral bumetanide (NKCC1 inhibitor) or luminal Ba2+ (non-selective K+ channel blocker), despite enhanced K+ secretion in Na+-depleted rats, as measured by 86Rb+ fluxes. Expression of BK and IK channels was also found to be unaltered by dietary Na+ depletion. However, bumetanide-sensitive basal and agonist-stimulated Cl- secretion (ISC) were significantly enhanced by Na+ depletion, as was CFTR Cl- channel expression. These data suggest that NKCC1-dependent secretory pathways are differentially regulated by aldosterone in proximal and distal colon. Development of therapeutic strategies in treating pathologies related to aberrant colonic K+/Cl- transport-such as pseudo-obstruction or ulcerative colitis-may benefit from these findings.

Flupirtine enhances NHE-3-mediated Na+ absorption in rat colon via an ENS-dependent mechanism

Recent studies in our lab have shown that the KV7 channel activator, flupirtine, inhibits colonic epithelial Cl- secretion through effects on submucosal neurons of the enteric nervous system (ENS). We hypothesized that flupirtine would also stimulate Na+ absorption as a result of reduced secretory ENS input to the epithelium. To test this hypothesis, unidirectional 22Na+ fluxes were measured under voltage-clamped conditions. Pharmacological approaches using an Ussing-style recording chamber combined with immunofluorescence microscopy techniques were used to determine the effect of flupirtine on active Na+ transport in the rat colon. Flupirtine stimulated electroneutral Na+ absorption in partially seromuscular-stripped colonic tissues, while simultaneously inhibiting short-circuit current (ISC; i.e., Cl- secretion). Both of these effects were attenuated by pretreatment with the ENS inhibitor, tetrodotoxin. The Na+/H+ exchanger isoform 3 (NHE-3)-selective inhibitor, S3226, significantly inhibited flupirtine-stimulated Na+ absorption, whereas the NHE-2-selective inhibitor HOE-694 did not. NHE-3 localization near the apical membranes of surface epithelial cells was also more apparent in flupirtine-treated colon versus control. Flupirtine did not alter epithelial Na+ channel (ENaC)-mediated Na+ absorption in distal colonic tissues obtained from hyperaldosteronaemic rats and had no effect in the normal ileum but did stimulate Na+ absorption in the proximal colon. Finally, the parallel effects of flupirtine on ISC (Cl- secretion) and Na+ absorption were significantly correlated with each other. Together, these data indicate that flupirtine stimulates NHE-3-dependent Na+ absorption, likely as a result of reduced stimulatory input to the colonic epithelium by submucosal ENS neurons.NEW & NOTEWORTHY We present a novel mechanism regarding regulation of epithelial ion transport by enteric neurons. Activation of neuronal KV7 K+ channels markedly stimulates Na+ absorption and inhibits Cl- secretion across the colonic epithelium. This may be useful in developing new treatments for diarrheal disorders, such as irritable bowel syndrome with diarrhea (IBS-D).

Aldosterone up-regulates basolateral Na+ -K+ -2Cl- cotransporter-1 to support enhanced large-conductance K+ channel-mediated K+ secretion in rat distal colon

Na+ -K+ -2Cl- cotransporter-1 (NKCC1) facilitates basolateral K+ and Cl- uptake, supporting their efflux across mucosal membranes of colonic epithelial cells. NKCC1 activity has also been shown to be critical for electrogenic K+ secretion induced by aldosterone, which is known to stimulate large-conductance K+ (BK) channel expression in mucosal membranes. This study was aimed to (1) identify whether aldosterone enhances NKCC1 expression specifically to support BK-mediated K+ secretion and (2) to determine whether increased NKCC1 supports electrogenic Cl- secretion in parallel to K+ secretion. Dietary Na+ depletion was used to induce secondary hyperaldosteronism in rats, or aldosterone was administered ex vivo to rat distal colonic mucosae. NKCC1-dependent electrogenic K+ or Cl- secretion was measured as a function of short circuit current (ISC ). qRT-PCR, western blot, and immunofluorescence analyses were performed using standard techniques. Aldosterone enhanced NKCC1 and BKα expression and electrogenic K+ secretion in the distal colon, which was inhibited by either serosal bumetanide (NKCC1 inhibitor) or mucosal iberiotoxin (IbTX; BK channel blocker), but not TRAM-34 (IK channel blocker). Expression of NKCC1 and BKα proteins was enhanced in crypt cells of hyper-aldosterone rats. However, neither NKCC1-dependent Cl- secretion nor CFTR (apical Cl- channel) expression was enhanced by aldosterone. We conclude that aldosterone enhances NKCC1 to support BK-mediated K+ secretion independently of Cl- secretion in the distal colon. The regulation of NKCC1 expression/K+ secretion by aldosterone may be a therapeutic target in treating gastrointestinal disorders associated with alterations in colonic K+ transport, such as colonic pseudo-obstruction, and hyperkalemia associated with renal disease.

Segmental differences in upregulated apical potassium channels in mammalian colon during potassium adaptation

Rat proximal and distal colon are net K⁺ secretory and net K⁺ absorptive epithelia, respectively. Chronic dietary K⁺ loading increases net K⁺ secretion in the proximal colon and transforms net K⁺ absorption to net K⁺ secretion in the distal colon, but changes in apical K⁺ channel expression are unclear. We evaluated expression/activity of apical K⁺ (BK) channels in surface colonocytes in proximal and distal colon of control and K⁺-loaded animals using patch-clamp recording, immunohistochemistry, and Western blot analyses. In controls, BK channels were more abundant in surface colonocytes from K⁺ secretory proximal colon (39% of patches) than in those from K⁺-absorptive distal colon (12% of patches). Immunostaining demonstrated more pronounced BK channel α-subunit protein expression in surface cells and cells in the upper 25% of crypts in proximal colon, compared with distal colon. Dietary K⁺ loading had no clear-cut effects on the abundance, immunolocalization, or expression of BK channels in proximal colon. By contrast, in distal colon, K⁺ loading 1) increased BK channel abundance in patches from 12 to 41%; 2) increased density of immunostaining in surface cells, which extended along the upper 50% of crypts; and 3) increased expression of BK channel α-subunit protein when assessed by Western blotting (P < 0.001). Thus apical BK channels are normally more abundant in K⁺ secretory proximal colon than in K⁺ absorptive distal colon, and apical BK channel expression in distal (but not proximal) colon is greatly stimulated as part of the enhanced K⁺ secretory response to dietary K⁺ loading.

Multiple mineralocorticoid response elements localized in different introns regulate intermediate conductance K+ (Kcnn4) channel expression in the rat distal colon

An elevated plasma aldosterone and an increased expression of the intermediate conductance K⁺ (IK/Kcnn4) channels are linked in colon. This observation suggests that the expression of Kcnn4 gene is controlled through the action of aldosterone on its cognate receptor (i.e., mineralocorticoid receptor; MR). In order to establish this, we performed chromatin immunoprecipitation (ChIP) assay to identify the MR response elements (MREs) in a region that spanned 20 kb upstream and 10 kb downstream of the presumed transcription start site (TSS) using chromatin from the colonic epithelial cells of normal and aldosterone-treated rats. MREs were immunoprecipitated in an approximately 5 kb region that spanned the first and second introns in the aldosterone rats. These regions were individually cloned in luciferase-expression vector lacking enhancer activity. These clones were tested for enhancer activity in vitro by transfecting in HEK293T and CaCo2 cells with MR and aldosterone treatment. At least four regions were found to be responsive to the MR and aldosterone. Two regions were identified to contain MREs using bioinformatics tools. These clones lost their enhancer activity after mutation of the presumptive MREs, and thus, established the functionality of the MREs. The third and fourth clones did not contain any bioinformatically obvious MREs. Further, they lost their activity upon additional sub-cloning, which suggest cooperativity between the regions that were separated upon sub-cloning. These results demonstrate the presence of intronic MREs in Kcnn4 and suggest a highly cooperative interaction between multiple intronic response elements.

Enhanced K(+) secretion in dextran sulfate-induced colitis reflects upregulation of large conductance apical K(+) channels (BK; Kcnma1)

Defective colonic Na(+) and Cl(-) absorption is a feature of active ulcerative colitis (UC), but little is known about changes in colonic K(+) transport. We therefore investigated colonic K(+) transport in a rat model of dextran sulfate-induced colitis. Colitis was induced in rat distal colon using 5% dextran sulfate sodium (DSS). Short-circuit current (Isc, indicating electrogenic ion transport) and (86)Rb (K(+) surrogate) fluxes were measured in colonic mucosa mounted in Ussing chambers under voltage-clamp conditions in the presence of mucosal orthovanadate (a P-type ATPase inhibitor). Serum aldosterone was measured by immunoassay. Control animals exhibited zero net K(+) flux. By contrast, DSS-treated animals exhibited active K(+) secretion, which was inhibited by 98, 76, and 22% by Ba(2+) (nonspecific K(+) channel blocker), iberiotoxin (IbTX; BK channel blocker), and TRAM-34 (IK channel blocker), respectively. Apical BK channel α-subunit mRNA abundance and protein expression, and serum aldosterone levels in DSS-treated animals, were enhanced 6-, 3-, and 6-fold respectively, compared with controls. Increasing intracellular Ca(2+) with carbachol (CCH), or intracellular cAMP with forskolin (FSK), stimulated both active Cl(-) secretion and active K(+) secretion in controls but had no or little effect in DSS-treated animals. In DSS-induced colitis, active K(+) secretion involves upregulation of apical BK channel expression, which may be aldosterone-dependent, whereas Cl(-) secretion is diminished. Since similar ion transport abnormalities occur in patients with UC, diarrhea in this disease may reflect increased colonic K(+) secretion (rather than increased Cl(-) secretion), as well as defective Na(+) and Cl(-) absorption.

Aldosterone induces active K⁺ secretion by enhancing mucosal expression of Kcnn4c and Kcnma1 channels in rat distal colon

Although both Kcnn4c and Kcnma1 channels are present on colonic mucosal membranes, only Kcnma1 has been suggested to mediate K(+) secretion in the colon. Therefore, studies were initiated to investigate the relative roles of Kcnn4c and Kcnma1 in mediating aldosterone (Na-free diet)-induced K(+) secretion. Mucosal to serosal (m-s), serosal to mucosal (s-m), and net (86)Rb(+) (K(+) surrogate) fluxes as well as short circuit currents (I(sc); measure of net ion movement) were measured under voltage clamp condition in rat distal colon. Active K(+) absorption, but not K(+) secretion, is present in normal, while aldosterone induces active K(+) secretion (1.04 ± 0.26 vs. -1.21 ± 0.15 μeq·h(-1)·cm(-2); P < 0.001) in rat distal colon. Mucosal VO(4) (a P-type ATPase inhibitor) inhibited the net K(+) absorption in normal, while it significantly enhanced net K(+) secretion in aldosterone animals. The aldosterone-induced K(+) secretion was inhibited by the mucosal addition of 1) either Ba(2+) (a nonspecific K(+) channel blocker) or charybdotoxin (CTX; a common Kcnn4 and Kcnma1 channel blocker) by 89%; 2) tetraethyl ammonium (TEA) or iberiotoxin (IbTX; a Kcnma1 channel blocker) by 64%; and 3) TRAM-34 (a Kcnn4 channel blocker) by 29%. TRAM-34, but not TEA, in the presence of IbTX further significantly inhibited the aldosterone-induced K(+) secretion. Thus the aldosterone-induced Ba(2+)/CTX-sensitive K(+) secretion consists of IbTX/TEA-sensitive (Kcnma1) and IbTX/TEA-insensitive fractions. TRAM-34 inhibition of the IbTX-insensitive fraction is consistent with the aldosterone-induced K(+) secretion being mediated partially via Kcnn4c. Western and quantitative PCR analyses indicated that aldosterone enhanced both Kcnn4c and Kcnma1α protein expression and mRNA abundance. In vitro exposure of isolated normal colonic mucosa to aldosterone also enhanced Kcnn4c and Kcnma1α mRNA levels, and this was prevented by exposure to actinomycin D (an RNA synthesis inhibitor). These observations indicate that aldosterone induces active K(+) secretion by enhancing mucosal Kcnn4c and Kcnma1 expression at the transcriptional level.

Glucocorticoids stimulate cation absorption by semicircular canal duct epithelium via epithelial sodium channel

The semicircular canal duct (SCCD) epithelium is a vestibular epithelial domain that was recently shown to actively contribute to endolymph homeostasis by Cl(-) secretion under control of beta(2)-adrenergic stimulation. By analogy to other Cl(-) secretory epithelia, we hypothesized that SCCD also provides an active absorptive pathway for Na(+) under corticosteroid control. Measurements of short-circuit current (I(sc)) demonstrated stimulation (7-24 h) by the glucocorticoids hydrocortisone (EC(50) 13 nM), corticosterone (33 nM), prednisolone (70 nM), and dexamethasone (13 nM) over physiologically and therapeutically relevant concentrations and its block by amiloride (IC(50) 470 nM) and benzamil (57 nM), inhibitors of the epithelial sodium channel (ENaC). I(sc) was also partially inhibited by basolateral ouabain and Ba(2+), indicating the participation of Na(+)-K(+)-ATPase and a K(+) channel in Na(+) transport. By contrast, aldosterone stimulated I(sc) only at unphysiologically high concentrations (EC(50) 102 nM). The action of all steroids was blocked by mifepristone (RU-486; K(d) approximately 0.3 nM) but not by spironolactone (K(d) approximately 0.7 microM). Expression of mRNA for the alpha-, beta-, and gamma-subunits of ENaC was demonstrated in the presence and absence of glucocorticoids. These findings are the first to identify SCCD in the vestibular labyrinth as a site of physiologically significant ENaC-mediated Na(+) absorption and osmotically coupled water flux. They further demonstrate regulation of Na(+) transport by natural and therapeutic glucocorticoids. The results provide for the first time an understanding of the therapeutic benefit of glucocorticoids in the treatment of Meniere's disease, a condition that is associated with increased luminal fluid volume.

Regulation of collecting duct AQP3 expression: response to mineralocorticoid

Adrenocortical steroid hormones are importantly involved in the regulation of extracellular fluid volume. The present study was aimed at examining whether aldosterone and/or glucocorticoid regulates the abundance of aquaporin-3 (AQP3), -2, and -1 in rat kidney. In protocol 1, rats were adrenalectomized, followed by aldosterone replacement, dexamethasone replacement, or combined aldosterone and dexamethasone replacement (rats had free access to water but received a fixed amount of food). Protocol 2 was identical to protocol 1, except that all groups received fixed daily food and water intake. In both protocols 1 and 2, aldosterone deficiency was associated with increased fractional Na excretion and severe hyperkalemia. Semiquantitative immunoblotting revealed that aldosterone deficiency was associated with a dramatic downregulation of AQP3 abundance. Consistent with this, immunocytochemistry and immunoelectron microscopy revealed a marked decrease in AQP3 labeling in the basolateral plasma membranes of collecting duct principal cells. In contrast, AQP1 and AQP2 abundance and distribution were unchanged. Glucocorticoid deficiency revealed no changes in AQP3, -2, or -1 abundance. In protocol 3, Na restriction (to increase endogenous aldosterone levels) or exogenous aldosterone infusion in either normal rats or vasopressin-deficient Brattleboro rats was associated with a major increase in AQP3 abundance. In protocol 4, aldosterone levels were clamped by infusion of aldosterone, while Na intake was altered from a low to a high level. Under these circumstances, there were no changes in AQP3 or AQP2 abundance, although the level of the thiazide-sensitive Na-Cl cotransporter was decreased. In conclusion, the results uniformly demonstrate that aldosterone regulates AQP3 abundance independently of Na intake. In contrast, changes in glucocorticoid levels in these models do not influence AQP3 or AQP2 abundance. Therefore, in the collecting duct aldosterone may regulate, at least in part, AQP3 expression in addition to regulating Na and K transport.

Sodium transporter abundance profiling in kidney: effect of spironolactone

Renal tubule profiling studies were carried out to investigate the long-term effects of administration of spironolactone, a mineralocorticoid receptor antagonist, on abundances of the major Na transporter and Na channel proteins along the rat renal tubule. Oral administration of spironolactone for 7 days to NaCl-restricted rats did not significantly alter abundances of Na transporters expressed proximal to the macula densa, while substantially decreasing the abundances of the thiazide-sensitive Na-Cl cotransporter (NCC), the alpha-subunit of the amiloride-sensitive epithelial Na channel (ENaC), and the 70-kDa form of the gamma-subunit of ENaC. A dependency of NCC expression on aldosterone was confirmed by showing increased NCC expression in response to aldosterone infusion in adrenalectomized rats. Immunoperoxidase labeling of ENaC in renal cortex confirmed that dietary NaCl restriction causes a redistribution of ENaC to the apical domain of connecting tubule cells and showed that high-dose spironolactone administration does not block this apical redistribution. In contrast, spironolactone completely blocked the increase in alpha-ENaC abundance in response to dietary NaCl restriction. We conclude that the protein abundances of NCC, alpha-ENaC, and the 70-kDa form of gamma-ENaC are regulated via the classical mineralocorticoid receptor, but the subcellular redistribution of ENaC in response to dietary NaCl restriction is not prevented by blockade of the mineralocorticoid receptor.

Distal tubular electrolyte transport during inhibition of renal 11beta-hydroxysteroid dehydrogenase

To test the proposal that the enzyme 11beta-hydroxysteroid dehydrogenase (11beta-HSD) confers aldosterone specificity on mineralocorticoid receptors in the distal nephron by inactivating glucocorticoids, we performed a free-flow micropuncture study of distal tubular function in adrenalectomized rats infused with high-dose corticosterone. One-half of the rats were additionally given intravenous carbenoxolone (CBX; 6 mg/h) to inhibit renal 11beta-HSD activity. Although this maneuver lowered fractional Na(+) excretion (1.1 +/- 0.2 vs. 1.9 +/- 0.2%, P < 0.01), Na(+) reabsorption within the accessible distal tubule was found to be similar in the two groups of animals. In contrast, distal tubular K(+) secretion was enhanced in CBX-treated rats: fractional K(+) deliveries to the early and late distal collection sites in the corticosterone-alone group were 13 +/- 1 and 20 +/- 3%, respectively (not significant), whereas corresponding data in the CBX-treated group were 9 +/- 1 and 24 +/- 2% (P < 0.01). This stimulation of distal K(+) secretion provides the first direct in vivo evidence that 11beta-HSD normally prevents corticosterone from exerting a mineralocorticoid-like effect in the distal tubule. The reduction in fractional Na(+) excretion during inhibition of 11beta-HSD, in the absence of a change in end-distal Na(+) delivery, suggests enhanced Na(+) reabsorption in the collecting ducts.

Glucocorticoid modulates Na+/H+ exchange activity in vascular smooth muscle cells by nongenomic and genomic mechanisms

BACKGROUND

In vascular smooth muscle cells (VSMCs), Na+/H+ exchange (NHE) plays an important role in intracellular pH (pHi) regulation. The genomic effect of glucocorticoid (GC) on NHE activity has been suggested in VSMCs. However, the nongenomic and genomic effects of GC on NHE activity and the underlying intracellular signaling mechanisms have not yet been demonstrated in VSMCs. Also, it is not known whether there are specific surface-binding sites of GC to the plasma membrane of VSMCs.

METHODS

The effects of short (3 h)- and long (24 h)-term exposure to corticosterone (CORTI) on NHE activity were studied in cultured rat aortic VSMCs by using pHi measurement with the pH-sensitive fluorescent dye 2'7'-bis(carboxyethyl)-5(6)-carboxyfluorescein. The NHE activity was calculated from the initial rate of Na+-dependent pHi recovery after the acid load.

RESULTS

Short-term exposure of VSMCs to CORTI (10-6 mol/L) increased NHE activity, whereas long-term exposure to CORTI decreased it. The inhibitors of gene transcription (actinomycin D) and of protein synthesis (cycloheximide) did not affect the short-term effect of CORTI on NHE activity, but inhibited the long-term effect of CORTI on NHE activity. The cytosolic GC receptor (GR) antagonist (RU38486) inhibited both the short- and long-term effects of CORTI on NHE activity, but the cytosolic mineralocorticoid receptor antagonist (spironolactone) did not influence either the short- or long-term CORTI effects. Two protein kinase C (PKC) inhibitors (staurosporine A and calphostin C) and PKC down-regulation [24-h pre-exposure to phorbol 12-myristate 13-acetate (PMA)] inhibited both short- and long-term CORTI effects. Exposure to PMA for three hours mimicked the short-term CORTI effect. The short-term CORTI effect was inhibited by the disruptor of microtubule (colchicine), but not by the disruptor of filamentous-actin (cytochalasin B). The long-term exposure to CORTI decreased NHE (NHE-1) mRNA levels to 0.65 times the control level, whereas the short-term exposure to CORTI caused no effect. Scatchard analysis of [3H]CORTI surface binding to VSMCs showed a single class of CORTI binding sites with a Bmax of 876.2 fmol per mg of cell protein and a Kd of 12.2 nmol/L. RU38486 also inhibited [3H]CORTI surface binding to VSMCs.

CONCLUSIONS

In VSMCs, NHE activity is stimulated by short-term exposure to CORTI, but is inhibited by long-term exposure to CORTI. The short-term stimulatory effect of CORTI on NHE activity is independent of gene transcription and protein synthesis, is mediated through the CORTI surface receptor, and occurs through a microtubule-dependent process. The long-term inhibitory effect of CORTI on NHE activity requires gene transcription and protein synthesis and occurs only through the cytosolic GR. The short- and long-term effects of CORTI on NHE activity occur via PKC activation. Therefore, CORTI differentially modulates NHE activity in VSMCs by nongenomic and genomic mechanisms.

Aldosterone activates Na+/H+ exchange in vascular smooth muscle cells by nongenomic and genomic mechanisms

BACKGROUND

In vascular smooth muscle cells (VSMCs), Na+/H+ exchange (NHE) plays an important role in intracellular pH (pHi) regulation. Recently, nongenomic effect of aldosterone (ALDO) on NHE activity has been suggested in VSMCs. However, the nongenomic and genomic effects of ALDO on NHE and the intracellular signaling mechanisms for these effects have not fully been determined in VSMCs.

METHODS

The effects of short- (3 hr) and long- (24 hr) term exposure to ALDO on NHE activity were examined in cultured VSMCs from rat thoracic aortae by using single-cell pHi measurement with the pH-sensitive dye 2'7'-bis(carboxyethyl)-5(6)-carboxyfluorescein. The NHE activity was calculated from the initial rate of Na+-dependent pHi recovery after acid load.

RESULTS

The NHE activity significantly increased after short- and long-term exposure of VSMCs to ALDO (10(-6) M). The inhibitors of gene transcription (actinomycin D) and of protein synthesis (cycloheximide) had no effect on the short-term ALDO effect, but inhibited the long-term ALDO effect. The antagonists of the mineralocorticoid receptor (MR) (spironolactone) and of the glucocorticoid receptor (GR) (RU38486) caused no effect on the short-term ALDO effect, but inhibited the long-term ALDO effect. Two protein kinase C (PKC) inhibitors (staurosporine A and calphostin C) and PKC down-regulation (24 hr pre-exposure to phobol 12-myristate 13-acetate, PMA) inhibited both the short- and long-term ALDO effects. Exposure of VSMCs to PMA for 3 hours mimicked the short-term effect of ALDO on NHE activity. ALDO significantly increased PKC activity in VSMCs. The short-term ALDO effect was inhibited by disruptors of microtubule (colchicine) and of filamentous-actin (cytochalasin B). Long-term exposure of ALDO caused a threefold increase in NHE (NHE-1) mRNA levels.

CONCLUSIONS

The short-term effect of ALDO on NHE activity is not mediated through either MR or GR, occurs independent of gene transcription and protein synthesis, and occurs through a mechanism involving the structural elements of cytoskeleton. The long-term effect of ALDO on NHE activity occurs through both MR and GR and requires gene transcription and protein synthesis. Both short- and long-term effects of ALDO are mediated through PKC activation. Therefore, ALDO activates NHE by nongenomic and genomic mechanisms in VSMCs.

Corticosterone and 11-dehydrocorticosterone stimulate Na,K-ATPase gene expression in vascular smooth muscle cells

BACKGROUND

In mineralocorticoid target tissues such as kidney and colon, the enzyme 11 beta-hydroxysteroid dehydrogenase (11 beta OHSD) catalizes the reversible conversion of corticosterone (CS) to inactive 11-dehydrocorticosterone (DHCS) in rats, and cortisol to inactive cortisone in humans. This enzyme is also expressed in vascular smooth muscle cells (VSMC).

METHODS

In cultured VSMC from rat thoracic aortae, we examined the effects of CS and DHCS on Na,K-ATPase alpha 1- and beta 1-mRNA accumulation by Northern blot analysis, on alpha 1- and beta 1-subunit protein accumulation by Western blot analysis, and on Na,K-ATPase activity by the coupled assay method.

RESULTS

In VSMC, CS and DHCS (10(-6) M) increased alpha 1-mRNA level 2.6- and 2.5-fold at 48 hours and beta 1-mRNA level 9.2- and 9.1-fold at 12 hours, respectively. The RNA transcription inhibitor (actinomycin D) abolished both CS- and DHCS-mediated alpha 1- and beta 1-mRNA induction. The glucocorticoid receptor antagonist (RU38486) and the mineralocorticoid receptor antagonists (ZK91587) inhibited both CS- and DHCS-mediated alpha 1- and beta 1-mRNA induction. The 11 beta OHSD inhibitor (carbenoxolone) inhibited DHCS-mediated alpha 1- and beta 1-mRNA induction, whereas it caused no effect on CS-mediated alpha 1- or beta 1-mRNA induction. The addition of CS or DHCS to VSMC significantly increased alpha 1- and beta 1-subunit protein levels and Na,K-ATPase activity. When adrenalectomized rats were treated with CS or DHCS for 12 hours, aorta alpha 1- and beta 1-mRNA levels increased 3.0- and 8.7-fold or 3.4- and 8.4-fold, respectively.

CONCLUSIONS

In VSMC, both CS and DHCS stimulate Na,K-ATPase alpha 1- and beta 1-mRNA accumulation, alpha 1- and beta 1-subunit protein accumulation, and Na,K-ATPase activity. The CS-mediated alpha 1- and beta 1-mRNA induction occurs independently of 11 beta OHSD, whereas the DHCS-mediated alpha 1- and beta 1-mRNA induction occurs through 11 beta OHSD-dependent mechanisms, possibly via conversion of inactive DHCS into active CS.

Low Na+ diet inhibits Na+ and water transport response to vasopressin in rat cortical collecting duct

BACKGROUND

We previously demonstrated that vasopressin (AVP) produces a sustained increase in Na+ reabsorption by the isolated perfused cortical collecting duct (CCD) from rats on a normal diet, and that this effect is synergistic with that of pharmacological doses of deoxycorticosterone (DOC) or physiological levels of aldosterone. The present experiments examined the effect of AVP under the more physiological circumstances when plasma aldosterone was elevated by prior volume depletion.

METHODS

Rats were volume depleted by a single dose of furosemide followed by a low-salt diet (0.3% NaCl) for four to nine days. Some of these rats were also implanted with a pellet containing 2.5 mg DOC. Rats in a third group were not injected with furosemide but were implanted with the DOC pellet and maintained on a standard (approximately 1% NaCl) diet. CCD were perfused and the lumen-to-bath Na+ flux (JNA), transepithelial voltage (VT), and osmotic water permeability (Pf) were measured in the presence and absence of 200 pm AVP.

RESULTS

Although Na+ depletion by a single injection of furosemide and the low salt diet elevated plasma aldosterone and Vt, JNA remained low and there was a decreased response to AVP in comparison with DOC-treated rats on a standard diet. In CCD from rats on the low salt-diet with DOC, JNa was less than observed in CCD from DOC-treated rats on a standard diet. AVP-dependent Pf in CCD from rats on the low salt-diet, with or without DOC treatment, was also markedly lower.

CONCLUSIONS

We interpret the results to demonstrate that maximal rates of Na+ reabsorption in the CCD depend not only on the synergistic stimulatory effects of aldosterone and AVP, but also require normal to high rates of salt delivery in vivo for the effects of the hormones on Na+ transport to be maximized in vitro.

Aldosterone stimulates intestinal Na+ absorption in rats by increasing NHE3 expression of the proximal colon

Na+ retention by the colon in response to salt deprivation is mediated in part by the resulting secondary hyperaldosteronism. We show that experimental hyperaldosteronism, to levels seen with salt deprivation, causes an increase in the selective expression and activity of NHE3, an apically located isoform of the Na+/H+ exchange family that functions in transepithelial Na+ absorption. The effect of aldosterone on NHE3 expression is tissue specific, occurring in intestine and not in kidney. Within the intestine, these effects are regional, being observed only in proximal colon, and different in distribution from that observed with glucocorticoids, where the predominant effect occurs in ileum. Although glucocorticoids are well known to exert many effects via regulation of transcript levels, the present study demonstrates that aldosterone stimulates intestinal Na+ absorption by increasing cellular NHE3 expression, a response that is tissue and region specific.

Role of aldosterone in the remnant kidney model in the rat

The renin-angiotensin-aldosterone system (RAAS) participates in the injury sustained by the remnant kidney. Our studies assessed the importance of aldosterone in that model and the response of aldosterone to drugs interfering with the RAAS. Initially, four groups of rats were studied: SHAM-operated rats, untreated remnant rats (REM), REM rats treated with losartan and enalapril (REM AIIA), and REM AIIA rats infused with exogenous aldosterone (REM AIIA + ALDO). The last group was maintained with aldosterone levels comparable to those in untreated REM rats by constant infusion of exogenous aldosterone. REM rats had larger adrenal glands and a > 10-fold elevation in plasma aldosterone compared to SHAM. REM AIIA rats demonstrated significant suppression of the hyperaldosteronism as well as marked attenuation of proteinuria, hypertension, and glomerulosclerosis compared to REM. REM AIIA + ALDO rats manifested greater proteinuria, hypertension, and glomerulosclerosis than REM AIIA rats. Indeed, by 4 wk of observation all of these features of the experimental disease were similar in magnitude in REM AIIA + ALDO and untreated REM. In separate REM rats spironolactone administration did not reduce glomerular sclerosis but did transiently reduce proteinuria, lowered arterial pressure, and lessened cardiac hypertrophy. In summary, aldosterone contributes to hypertension and renal injury in the remnant kidney model.

Different effects of three aldosterone treatments on plasma aldosterone and salt intake

Adrenalectomized male rats received a nominal dose of 47.6 micrograms/day aldosterone for 14 days by daily injections, osmotic minipumps, or controlled-release pellets. Plasma aldosterone concentrations were barely detectable (< 20 pg/ml) 24 h after rats received aldosterone by injection, remained constant at 200 pg/ml in rats with osmotic minipumps, and dropped from > 500 to 75 pg/ml during the first week after implantation of controlled-release pellets. For the most part, the effects of the different treatments on NaCl intake were related to their effects on plasma aldosterone levels according to a U-shaped function. However, NaCl intake was dissociated from plasma aldosterone levels when treatment first began or was discontinued. NaCl intake may be a function of the amount of aldosterone delivered but not necessarily plasma aldosterone concentration.

Effect of furosemide-induced hypokalemic metabolic alkalosis on renal transport enzymes

Hypokalemic metabolic alkalosis is one of the most common complications of chronic furosemide administration. In this study we examined acid-base composition and ATPase enzyme activities in medullary thick ascending limb of Henle's loop (MTAL) and collecting tubule (CCT and MCT) after seven days of chronic furosemide therapy. All of the studies were conducted in adrenal intact (AI) rats or in adrenalectomized (ADX) glucocorticoid replete rats replaced with a physiological dose of aldosterone (Aldo). Furosemide (F) was administered to each rat by mini-osmotic pump. In the AI+F group, plasma Aldo was high and obvious metabolic alkalosis occurred (HCO3- = 37 +/- 2 mEq/liter vs. 22 +/- 2 mEq/liter in controls, P < 0.005); activities of H-K-ATPase, H-ATPase, and Na-K-ATPase were increased approximately twofold in both CCT and MCT. In the ADX+F group (HCO3- = 28 +/- 2 mEq/liter, P < 0.05 from control), H-ATPase activity was normal in CCT and it was slightly increased in MCT. CCT and MCT H-K-ATPase activities were markedly increased (approximately twofold). Na-K-ATPase activity was the same as control in CCT but it was increased in MCT. In ADX+F+Vanadate (V) group which also had normal Aldo levels, acid-base changes were modest (20 +/- 2 mEq/liter, NS from control); in CCT and MCT H-K-ATPase and Na-K-ATPase activities were markedly reduced, but H-ATPase activity in MCT was increased. In all three experimental groups Na-K-ATPase activity in MTAL was reduced fivefold. Hypokalemia developed in both intact and ADX animals receiving furosemide.(ABSTRACT TRUNCATED AT 250 WORDS)

Mucosal ouabain and Na+ inhibit active Rb+(K+) absorption in normal and sodium-depleted rat distal colon

To determine the effect of mucosal sodium and mucosal ouabain on active Rb+(K+) absorption, unidirectional and net 86Rb+ fluxes were measured under voltage-clamp conditions in the distal colon of normal and sodium-depleted rats. The role of mucosal sodium (independent of serosal sodium) was evaluated in a model of Rb+(K+) absorption in which serosal ouabain markedly enhanced active Rb+(K+) absorption. In normal rats, mucosal sodium was a competitive inhibitor of Rb+(K+) absorption, and Rb+(K+) absorption consisted of a mucosal sodium-sensitive component and a mucosal sodium-insensitive component. Further, mucosal ouabain almost completely inhibited the mucosal sodium-insensitive component but did not affect the mucosal sodium-sensitive component. In sodium-depleted rats, both mucosal sodium-sensitive and mucosal sodium-insensitive fractions of Rb+(K+) absorption were also identified. Aldosterone markedly stimulated the mucosal sodium-sensitive component (1.68 +/- 0.15 vs. 0.60 +/- 0.10 muEq.h-1.cm-2) but not the sodium-insensitive component (0.88 +/- 0.09 vs. 0.64 +/- 0.06 muEq.h-1.cm-2) component of Rb+(K+) absorption; however, in contrast to normal animals, mucosal sodium in sodium-depleted animals was a noncompetitive inhibitor of Rb+(K+) absorption. The mucosal sodium-insensitive component of Rb+(K+) absorption in sodium-depleted animals was substantially inhibited by mucosal ouabain, but the mucosal sodium-sensitive component, unlike that in normal animals, was partially inhibited by mucosal ouabain. These studies indicate that the characteristics of the Rb+(K+) absorptive process in sodium-depleted animals differ significantly from those present in normal animals, suggesting that aldosterone induces an Rb+(K+) absorptive mechanism not present in normal animals.

Aldosterone and PCO2 enhance K-dependent chloride absorption in rat distal colon

We have previously demonstrated that, in the absence of Na+ in vitro, the rate of colonic K+ absorption is increased by increasing PCO2. Chronic secondary hyperaldosteronism induced by dietary Na-depletion further stimulated K+ absorption under these conditions. Because the observed increments in CO2-dependent K+ absorption were not accompanied by corresponding changes in short-circuit current, macroscopic electroneutrality must have been maintained either by anion absorption or by cation secretion. Colonic Cl- absorption is known to respond to increased PCO2 both in vivo and in vitro, but its response under Na-free conditions and the relationship to K+ absorption have not been examined. To determine the relationship of Cl- absorption to K+, we measured unidirectional fluxes of 36Cl and the response to PCO2 in voltage-clamped segments of rat distal colon. Our findings indicate that the rate of Cl- absorption is increased by increasing CO2, both in the presence and absence of Na+. Under Na-free conditions, Cl- absorption is inhibited by acetazolamide and by the absence of K+;K+ absorption (86Rb or 42K flux) is inhibited in a reciprocal fashion by the absence of Cl-. The rates of K+ and Cl- absorption are similar in controls and after secondary hyperaldosteronism due to a Na-deficient diet. These findings suggest that K- and Cl- absorption are closely coupled under Na-free conditions, most likely due to the operation of parallel, aldosterone-responsive H(+)-K+ and Cl(-)-HCO3- exchange pathways.

Secondary hyperaldosteronism stimulates acidification in rat distal colon

Recent studies from this laboratory have determined that colonic K+ absorption is altered by the PCO2 and by secondary hyperaldosteronism. Partial inhibition by vanadate and mucosal ouabain suggested the operation of an H+/K+ exchange pump. To determine the mechanism of acidification in rat distal colon, we measured in vitro acidification using the pH-stat technique by voltage-clamped segments of colonic epithelium in controls and in the presence of secondary hyperaldosteronism, induced by a sodium-deficient diet. Chronic stimulation with aldosterone resulted in increased mucosal acidification in vitro for at least 2 h. This effect could not be accounted for by lactate production and was not altered by elimination of the aldosterone-induced increase in voltage and short-circuit current with 10 microM amiloride. Studies with inhibitors and ion substitution revealed that mucosal acidification resulted from both Na-dependent and Na-independent mechanisms. Na-dependent acidification was inhibited by ATPase inhibitors and was mediated in part by a luminal Na+/H+ exchanger in the presence of secondary hyperaldosteronism. Na-independent acidification was mediated by a pathway dependent on luminal K+ that was inhibited by vanadate and mucosal ouabain, consistent with the operation of an H+/K+ exchange pump.

Regulation of active sodium and potassium transport in the distal colon of the rat. Role of the aldosterone and glucocorticoid receptors

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.

Characterization of aldosterone-induced potassium secretion in rat distal colon

The role of apical and basolateral membranes in aldosterone-induced active potassium (K) secretion in rat distal colon was investigated by measuring mucosal-to-serosal (Jms) and serosal-to-mucosal (Jsm) 42K fluxes (mueq.h-1.cm-2) across isolated stripped mucosa under short-circuit conditions in normal and secondary-hyperaldosterone animals. In normal colons mucosal tetraethylammonium (TEA; 30 mM) or barium (Ba; 5 mM), but not cesium (Cs; 15 mM), reduced Jsm without affecting Jms. In aldosterone animals (a) net K secretion (-0.54 +/- 0.11) was converted to net K absorption (0.63 +/- 0.15) by mucosal TEA, which produced a marked reduction in Jsm (0.82 +/- 0.07) and an increase in Jms (0.35 +/- 0.07). In contrast mucosal Ba resulted in a relatively smaller reduction in JK(sm) without altering JK(ms), whereas mucosal Cs was ineffective; (b) serosal bumetanide or the removal of serosal Na or Cl markedly inhibited JK(sm and abolished net K secretion; and (c) serosal ouabain (1 mM) produced qualitatively similar effects to those of serosal bumetanide. These results demonstrate that (a) normal rat distal colon contains apical TEA- and Ba-sensitive K channels; (b) aldosterone induces TEA-sensitive and Ba-sensitive apical K channels; (c) aldosterone-induced K secretion requires both the Na,K-pump and Na-K-2Cl cotransport for K uptake across the basolateral membrane; and (d) alteration of any of these processes results in inhibition of aldosterone-induced active K secretion simultaneously with stimulation of K absorption.

Effects of corticosteroid hormones on the electrophysiology of rat distal colon: implications for Na+ and K+ transport

1. Conventional microelectrodes, the Na+ channel blocker amiloride (0.1 mM), and the K+ channel blocker tetraethylammonium chloride (TEA, 30 mM) were used to examine the effects of corticosteroid hormones administered in vivo on the Na+ and K+ transport properties of isolated rat distal colon. The cell membrane changes induced by aldosterone (a specific mineralocorticoid), RU 28362 (a synthetic glucocorticoid with negligible affinity for mineralocorticoid receptors), and dexamethasone (an activator of both mineralocorticoid and glucocorticoid receptors) were compared. 2. In control animals, there was no amiloride-sensitive apical Na+ conductance, and only a relatively small TEA-sensitive apical K+ conductance. 3. Hyperaldosteronism secondary to dietary Na+ depletion for 10-14 days, dexamethasone (600 micrograms 100 g-1 day-1 for 3 days), and RU 28362 (600 micrograms 100 g-1 day-1 for 3 days) induced amiloride-sensitive electrogenic Na+ transport, with the potency of aldosterone greater than dexamethasone greater than RU 28362. 4. With each corticosteroid, increased electrogenic Na+ transport reflected enhanced apical Na+ conductance, and in the case of aldosterone and dexamethasone, 3.3-fold and 2-fold increases respectively in the maximum activity of the basolateral Na+-K+ pump. In contrast, RU 28362 suppressed the maximum activity of the basolateral Na+-K+ pump by 45%. 5. All three corticosteroids enhanced the K+ conductance of the apical membrane, with the potency of aldosterone greater than dexamethasone greater than RU 28362. 6. Co-administration of spironolactone (5 mg 100 g-1 day-1) inhibited the effects of aldosterone on Na+ and K+ transport, but in dexamethasone-treated animals spironolactone resulted in a pattern of response similar to that found in RU 28362-treated animals. 7. The results support the view that mineralocorticoid receptors mediate changes in colonic Na+ and K+ transport which differ quantitatively and qualitatively from those mediated by glucocorticoid receptors. Dexamethasone and similar 'glucocorticoids' activate both types of receptor, with an overall epithelial response which mimics that induced by aldosterone.

Effect of selective aldosterone deficiency on acidification in nephron segments of the rat inner medulla

Mineralocorticoid plays a role in urinary acidification and acid-base balance, but the response of the inner medulla to aldosterone has not been elucidated. A model of selective aldosterone deficiency (SAD) with hyperkalemia and hyperchloremic metabolic acidosis was employed to assess segmental acidification by measuring in situ pH, titratable acidity (TA) and total ammonia (Am). Hydrogen ion secretion was also examined as a function of the increment in in situ PCO2 in the collecting duct during bicarbonate loading. SAD rats were compared to ADX controls that received adrenalectomy and chronic replacement of gluco- and mineralocorticoid and to rats with chronic metabolic acidosis induced by oral NH4Cl (CMA). Both fractional and absolute delivery of Am to the loop of Henle was lower in SAD vs. CMA rats (1.34 to 3.63 mM, P less than 0.01). Delivery of Am to the base and tip collecting duct (BCD and TCD) was also markedly lower in SAD (1.50 vs. 0.52 and 1.77 vs. 0.47 mM, respectively, P less than 0.01). Net addition of Am and net acid between BCD and TCD, observed in CMA rats, was not observed in SAD despite equivalent degrees of systemic metabolic acidosis. Similarly, the concentration gradient favoring transfer of NH3 between loop of Henle and CD was reduced in SAD. During bicarbonate loading the increment in PCO2 at BCD, TCD and in final urine was significantly lower in SAD rats than in adrenal intact bicarbonate-loaded rats. Therefore, the acidification defect in this model of SAD appears to be a result of a decrease in ammonia production and delivery to the loop of Henle, impaired transfer from loop to collecting duct and reduction in the rate of H+ secretion by the collecting duct.

Electrophysiology of rat distal colon after partial nephrectomy. Implications for K transport

Previous in vivo studies in rat and man indicate that chronic renal insufficiency leads to an increase in the capacity of the large intestine for K secretion. The present studies were performed in isolated rat distal colon with conventional and K-sensitive microelectrodes to determine the cellular basis for enhanced colonic K secretion after 70% nephrectomy. The data revealed that in animals fed a regular diet, nephrectomy had no effect on the Na or K conductance of the apical membrane, or the kinetics of the basolateral membrane Na-K pump, but intracellular K activity decreased from 70 +/- 4 mmol/l to 58 +/- 4 mmol/l (P less than 0.005). In control (non-nephrectomised) animals, feeding a diet modestly (4-fold) enriched with K resulted in small but significant increases in the Na and K conductance of the apical membrane, no change in the kinetics of the basolateral membrane Na-K pump, and a rise in intracellular K activity from 70 +/- 4 mmol/l to 94 +/- 7 mmol/l (P less than 0.005). In contrast, in animals fed the K enriched diet, nephrectomy resulted in (i) large, amiloride-sensitive increases in transepithelial voltage and total tissue conductance (consistent with an appreciable degree of secondary hyperaldosteronism), (ii) marked increases in the Na and K conductance of the apical membrane, (iii) significant hyperpolarization of the basolateral membrane, (iv) a 100% increase (P less than 0.02) in the maximum activity of the basolateral membrane Na-K pump, and (v) a rise in intracellular K activity from 94 +/- 7 mmol/l to 129 +/- 7 mmol/l (P less than 0.0025). These data suggest that the combination of modest dietary K enrichment and 70% nephrectomy stimulated an active K secretory process which reflected an increase in the K excretory load applied to the colonic mucosa, and the effects of aldosterone. In this model of renal insufficiency, enhanced K secretion by the transcellular and paracellular (potential-dependent) pathways results in a marked rise in the K excretory capacity of the colon.

Aldosterone reverses potassium-induced food aversions in adrenalectomized rats

Young adult male rats were individually housed and given a standard ration (66 ml) of a liquid diet (Nutrament) each day. The animals were divided into 7 groups: five groups were bilaterally adrenalectomized (ADX) and given one of 5 doses of aldosterone and/or dexamethasone by continuous, osmotic minipump infusions. The remaining two groups served as intact and sham operated controls. Each of the seven groups were subdivided into 3 dietary groups: a basal potassium dietary group, a moderately potassium-supplemented dietary group, and a highly potassium-supplemented dietary group. All rats with intact adrenals as well as those ADX rats given basal or 10 X basal aldosterone treatment consumed all of their allotted 66 ml of diet each day, independent of the level of potassium supplementation. ADX rats given little or no aldosterone treatment that were given access to the moderately or highly supplemented diets became anorexic, eating little or none of the diet. These data are discussed with reference to the factors controlling the intake of ADX rats.

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)

Effect of adrenalectomy on transport in the rat medullary thick ascending limb

Previous studies in adrenalectomized (Adx) rats suggest that aldosterone may regulate ion transport in the ascending portion of Helen's loop. In order to examine directly the effect of adrenalectomy on transport, medullary thick ascending limb (Mtal) segments were isolated from Adx, Adx replaced with aldosterone (Adx + Ald, 0.5 micrograms X 100 g X body wt X d), and control Sprague-Dawley rats. Both net sodium and net chloride fluxes were significantly less in the Mtal segments from Adx rats compared with those in the control or Adx + Ald group. Physiologic levels of exogenous aldosterone increased net sodium chloride flux toward control values in the Adx + Ald group. Net potassium flux was not different among the three groups. We conclude that adrenalectomy impairs reabsorptive NaCl but not K transport in the Mtal, and that aldosterone restores this process. This reabsorptive defect may contribute to the urinary concentrating and diluting abnormality associated with adrenal insufficiency.

Regulation of cation transport by low doses of glucocorticoids in in vivo adrenalectomized rat colon

A dose response curve for glucocorticoid-induced proximal and distal colonic cation transport in vivo was established in adrenalectomized rats. All doses (0.5-50 nmol/100 g body wt) stimulated sodium absorption. Distal sodium absorption did not saturate at dexamethasone levels that saturate the glucocorticoid receptor but also bind to greater than 35% of aldosterone receptors. Saturation of the pure glucocorticoid response occurred in both segments with RU26988, a synthetic glucocorticoid that does not occupy aldosterone receptors. Maximum velocities for pure glucocorticoid-induced sodium absorption were 15 and 16 mu eq/min per g dry tissue, and Michaelis constants (Km) were 4.2 and 4.6 X 10(-9) mol/liter for proximal and distal colon. Kms are similar to the dissociation constant for the colonic glucocorticoid receptor and too low for significant aldosterone receptor occupancy. Dexamethasone increased sodium absorption significantly within 30 min of injection, suggesting the response is not dependent on new protein synthesis. Similar time and dose responses in proximal and distal colon suggest glucocorticoids stimulate the same pathway in both segments.

Contribution of the urea appearance rate to diuretic-induced azotemia in the rat

Studies were performed to evaluate the contribution of the urea appearance rate to the elevated plasma urea concentration found during diuretic-induced sodium depletion. Negative sodium balance of -1162 + 29 microEq/100 g body wt was induced over a four day period by the administration of furosemide, 20 to 30 mg/kg/d i.p., to rats ingesting a sodium free diet. When compared with sodium replete controls, sodium depletion significantly increased the plasma urea concentration (65.0 +/- 3.1 vs. 26.4 +/- 1.1 mg/dl) through both an increase in the urea appearance rate (160 +/- 5.2 vs. 125 +/- 3.5 mg/day/100 g body wt), and a decrease in the urea clearance rate (1.99 +/- 0.14 vs. 3.16 +/- 0.12 ml/min/kg). The urea appearance rate increased on the first day of diuretic administration, remained elevated three days after stopping diuretics, rapidly returned to control levels after sodium repletion, and was significantly correlated with the magnitude of sodium deficit. Similar results were obtained when diuretic-induced sodium depletion was produced in adrenalectomized animals. After four days of sodium depletion the plasma concentration was increased for some amino acids but not for the plasma total amino acid, nitrogen concentration. The results indicate that sodium depletion increases the urea appearance rate through a mechanism that is independent of adrenal function. Thirty to sixty percent of the elevation in plasma urea concentration that occurs in the rat during diuretic-induced sodium depletion can be accounted for by an enhanced urea appearance rate.

Amiloride-sensitive sodium transport of the rat distal colon during early postnatal development

To evaluate developmental changes in colonic sodium transport, the sensitivity of the transepithelial potential and short-circuit current to amiloride was investigated. The amiloride-sensitive short-circuit current (IscNa), which represents the electrogenic sodium transport through Na+ channels, rose significantly from day 5, reached a peak on day 10, and entirely disappeared after weaning. The maximum rate of electrogenic, amiloride-sensitive sodium transport was 12.0 microEq/cm2 X h. The IscNa was suppressed by adrenalectomy and/or premature weaning but not by a mineralocorticoid antagonist, spironolactone. On the contrary, treatments which increase aldosterone levels in vivo (low-sodium diet, furosemide-induced natriuresis, high dietary intake of potassium) stimulated the IscNa. The effect of adrenalectomy increased during postnatal development. The sensitivity of IscNa to aldosterone was highest at the end of the weaning period. High-sodium diet, which causes a decrease in circulating aldosterone, was associated with a partial inhibition of IscNa (P less than 0.016). These data suggest that the distal colon of neonatal rats can transport sodium via an electrogenic, amiloride-sensitive mechanism and that adrenocortical hormones exert the main regulatory control of this pathway.

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.

Role of aldosterone in the mechanism of renal potassium adaptation

Chronic potassium loading results in an adaptive change in renal tubular epithelium which increases the capacity for potassium excretion. The present study was performed to evaluate the role of aldosterone in renal potassium adaptation, since hyperaldosteronism stimulates potassium secretion, and potassium loading increases the production of aldosterone. Experiments were performed in animals with intact adrenal glands, and in adrenalectomized animals (Adx) replaced with basal physiologic amounts of corticosterone, which were not replaced with aldosterone, or were chronically infused with aldosterone to achieve either basal plasma levels or elevated levels. Chronic potassium loading in adrenal intact animals was associated with a statistically significant higher rate of urinary potassium excretion (3.57 +/- 0.30 microEq/min/100 g BW) compared to the control rate (2.54 +/- 0.25 microEq/min/100 g BW, p less than 0.05), during acute infusion of KCl. In potassium loaded Adx animals, with selective replacement of adrenal hormones, the maximum rate of potassium excretion was blunted in the absence of aldosterone, compared to potassium loaded animals with intact adrenal glands. In contrast, when Adx animals were infused chronically with aldosterone, to achieve basal or elevated plasma levels, the maximum rate of potassium excretion was not blunted, although at basal aldosterone levels increased potassium excretion was due, at least in part, to hyperkalemia. These results indicate that the induction of renal potassium adaptation after a week or more of chronic potassium loading is dependent on the action of hyperaldosteronism on renal tubular epithelium.

Ion transport in proximal colon of the rat. Sodium depletion stimulates neutral sodium chloride absorption

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.

Modulation of renal sodium-potassium-adenosine triphosphatase by aldosterone. Effect of high physiologic levels on enzyme activity in isolated rat and rabbit tubules

The purpose of this study was to determine the nephron site, time course, and mechanism of mineralocorticoid action on renal tubular Na-K-ATPase in rats and rabbits, without dietary manipulation and by using the natural mineralocorticoid aldosterone. Sustained, high physiologic levels of circulating aldosterone mimicking those produced endogenously during potassium loading or sodium deprivation were provided by constant delivery of the hormone in doses of 5 or 50 micrograms/100 g body wt per 24 h, respectively, from osmotic minipumps implanted subcutaneously. In adrenal-intact rats receiving the 5-microgram dose, aldosterone levels were similar to those seen in animals fed a high K diet and produced a time-dependent increase in Na-K-ATPase activity in the cortical-collecting tubule (CCT) to a level 103% higher than in controls after 7 d (2,007 +/- 178 vs. 989 +/- 72 pmol/mm per h, P less than 0.001); the enzyme activity in the proximal convoluted tubule, medullary thick ascending limb, and the inner stripe of the medullary-collecting tubule did not change significantly. The increment in CCT Na-K-ATPase was larger (142%) in animals receiving for the same period of time the 50-micrograms dose, which produced circulating aldosterone levels similar to those of sodium-deprived rats. A significant stimulation of Na-K-ATPase activity was seen in the CCT of adrenalectomized rats after 24 h of treatment with either dose of the hormone, and at 12 h only in animals receiving the 50 micrograms/100 g per 24 h regimen. To determine whether the enhanced Na-K-ATPase activity produced by aldosterone is due to synthesis of new enzyme units or to alteration in its kinetics, we examined the ouabain-binding capacity and the affinity for Na and K of the enzyme from CCT of rabbits treated with 5 micrograms/100 g body wt per 24 h aldosterone for 3 d. These experiments revealed a parallel increment on Na-K-ATPase activity and specific [3H]ouabain binding in aldosterone-treated rabbits, while the affinity of the enzyme for either sodium or potassium was unaltered. The results of this study indicate that high physiologic levels of aldosterone simulating those measured during K loading or Na deprivation lead to a segment-specific increase in Na-K-ATPase activity in the CCT. This effect was time-and dose-dependent and was due to an increase in the number of active enzyme units. The segmental specificity and time course of the increase in enzyme activity suggest that modulation of Na-K-ATPase by aldosterone plays a role in the chronic adaptation of the CCT to altered availability of sodium and potassium, and therefore in the homeostasis of these cations by the kidney.

Effects of adrenalectomy and chronic adrenal corticosteroid replacement on potassium transport in rat kidney

Clearance experiments were carried out in pair-fed rats to examine the long-term effects of adrenalectomy and selective adrenal corticosteroid replacement in physiological amounts on renal potassium transport. To this end, clearance studies were conducted in rats that were sham operated, or adrenalectomized (ADX). ADX animals were given either vehicle, aldosterone (0.5 microgram/100 g body wt per day), dexamethasone (1.2 micrograms/100 g body wt per day), or aldosterone and dexamethasone, by osmotic minipump for 7-9 d whereupon clearance experiments were conducted. After chronic hormone treatment, during basal conditions when only Ringers solution was infused, all groups excreted similar amounts of potassium. However, in all ADX animals without mineralocorticoid replacement, the maintenance of urinary potassium excretion at control levels was associated with hyperkalemia, increased urine flow, and natriuresis; all are factors known to stimulate urinary potassium excretion. During acute potassium infusion, the increase in urinary potassium excretion was less in ADX rats than in controls. This functional deficiency in potassium excretion was partially corrected by dexamethasone and was uniformly associated with a significant increase in urine flow. Aldosterone replacement or aldosterone and dexamethasone given together chronically, sharply increased potassium excretion but did not restore excretion to control levels. Only acute aldosterone infusion (0.2 microgram/100 g body wt bolus plus 0.2 microgram/100 g body wt per hour), superimposed upon chronic aldosterone and dexamethasone treatment, fully restored potassium excretion to control levels. This aldosterone induced enhancement of potassium excretion, both chronic and acute, was not associated with hyperkalemia, and increased urine flow or natriuresis. Thus, physiological levels of both classes of adrenal corticosteroids stimulate renal potassium excretion albeit by different mechanisms. Mineralocorticoids stimulate tubular potassium excretion directly, whereas glucocorticoids augment excretion indirectly by increasing fluid and sodium delivery along the distal nephron.

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.

Role of aldosterone and dietary potassium in potassium adaptation in the distal colon of the rat

The present experiments were designed to differentiate the effects of aldosterone and chronic potassium loading in the mechanism of colonic potassium adaptation, using a recently described animal model that permits systematic manipulation of aldosterone and dietary potassium. Unidirectional 42K fluxes were performed under short-circuit conditions across isolated distal colonic mucosa from rats with intact adrenal glands and in adrenalectomized, hormone-replaced animals. Administration of a potassium-enriched diet reversed net potassium absorption (+0.43 +/- 0.10 muEq/h.cm2) in intact animals to net potassium secretion (-0.76 +/- 0.08 muEq/h.cm2). Infusion of aldosterone in adrenalectomized animals, in an amount equivalent to that produced by potassium loading in intact rats, reduced JnetK to zero (-0.03 +/- 0.06 muEq/h.cm2). Similarly, zero net potassium transport (-0.14 +/- 0.08 muEq/h.cm2) was observed when experimental rats were fed a potassium-enriched diet and maintained with basal aldosterone levels. Therefore, both elevated aldosterone levels and a potassium-enriched diet had an effect on net potassium transport, but neither produced the full effects of chronic potassium loading. An increase in net potassium secretion (-0.57 +/- 0.07 myEq/h.cm2) comparable to that of intact potassium-loaded animals did, however, occur when potassium loading and elevated aldosterone levels were combined in experimental animals. These studies demonstrate that (a) chronic potassium loading alters active potassium transport by an aldosterone-independent mechanism and (b) the mechanism of chronic potassium adaptation represents the additive effects of increased dietary potassium and aldosterone.

Role of aldosterone in the mechanism of potassium adaptation in the initial collecting tubule

Studies were performed on the initial collecting tubule of the rat to determine whether potassium adaptation in this nephron segment is aldosterone-dependent. Previous studies demonstrated that chronic potassium loading, in animals with intact adrenal glands, caused an increase in transepithelial potential difference in late distal convolution, an increase in surface density of the basolateral cell membrane, SVBLM, of principal cells in the initial collecting duct, and a rise in plasma aldosterone levels. The present study shows that epithelial changes that characterize dietary potassium loading are not dependent on hyperaldosteronism, since potential difference (-47 +/- 1 vs. 40 +/- 3 mV, lumen negative) and SVBLM (2.91 +/- 0.11 vs. 2.53 +/- 0.09 micron2/micron3) increased significantly (P less than 0.05) in the late distal convolution of adrenalectomized, hormone-replaced animals in which plasma aldosterone levels were maintained at basal values of approximately 5 ng/dl. In addition, these experiments suggest that the initial collecting tubule is sensitive to the action of aldosterone, at physiological plasma levels, since chronic hyperaldosteronism, in the absence of potassium loading, increased SVBLM in initial collecting tubule cells. In contrast to other mineralocorticoid-sensitive tissues, however, neither the acute or chronic administration of aldosterone caused an increase in potential difference in late distal convolution. These results suggest that the mechanism by which aldosterone stimulates electrolyte movement is not identical in all target tissues.

Differential acute effects of aldosterone, dexamethasone, and hyperkalemia on distal tubular potassium secretion in the rat kidney

To determine the specific effects on renal potassium transport of acute elevations in plasma aldosterone, dexamethasone, and potassium concentrations, we studied adrenalectomized rats prepared such that each factor could be varied independently. Clearance data alone could not be used to deduce the underlying tubular transport effects, however, since infusion of each of these agents was associated with a marked change in urinary flow rate, which may itself have influenced potassium excretion. We therefore used a technique of continuous microperfusion, in vivo, of single superficial distal tubules to evaluate potassium secretion at constant luminal flow rate during each experimental maneuver. Acute aldosterone infusion was associated with a 90% stimulation of potassium secretion by microperfused tubules. However, total kidney sodium excretion and urinary flow rate were markedly reduced, and these factors opposed the direct tubular action of aldosterone, resulting in no net change in the amount of potassium excreted into the final urine. Conversely, dexamethasone had no direct effect on potassium secretion by single microperfused tubules, but it caused a sharp increase in urinary flow and sodium excretion, and secondarily enhanced urinary potassium excretion by 50%. Hyperkalemia per se stimulated renal potassium excretion both via a direct tubular effect and by increasing urinary flow rate. We conclude that urinary potassium excretion after infusion of each of these agents represents the net result of direct tubular effects and secondary flow-mediated changes.

Regulation of renal Na-K-ATPase in the rat. Role of the natural mineralo- and glucocorticoid hormones

Both mineralo- and glucocorticoids stimulate renal Na-K-ATPase, but their relative role in the regulation of the enzyme remains controversial. In this study we measured Na-K-ATPase activity in the cortical collecting tubule (CCT) of adrenalectomized rats replaced with either the native mineralocorticoid (aldosterone) or glucocorticoid (corticosterone) in doses calculated to yield previously determined physiologic concentrations of these hormones (5 ng X dl-1 and 5 micrograms X dl-1, respectively). This was achieved by continuous delivery of aldosterone (1 microgram X 100 g-1 X d-1) from an osmotic minipump or of corticosterone (2 pellets of 20 mg each), implanted subcutaneously either at adrenalectomy or 7 d later, when Na-K-ATPase activity reached its nadir. Adrenalectomized rats not receiving hormone replacement and adrenal-intact animals served as controls. The CCT was chosen because it contains the highest concentration of binding sites for both hormones. Na-K-ATPase activity declined 52% in the CCT of untreated adrenalectomized rats after 7 d, and remained unchanged thereafter. Physiologic replacement doses of aldosterone prevented this decline and restored the activity of the enzyme after it had been allowed to decrease maximally following adrenal ablation, whereas similar replacement of corticosterone was without effect. These observations suggest that under physiologic conditions Na-K-ATPase in the CCT, a probable target nephron segment of both hormones, is under mineralocorticoid rather than glucocorticoid control.