Measurement of rectal electrical potential difference as an instant screening-test for hyperaldosteronism

Aldosterone sensitivity: an opportunity to explore the pathogenesis of hypertension

Aldosterone sensitivity is defined as an outcome variable for a given circulating level of aldosterone. In basic and translational studies, aldosterone sensitivity has been measured in differential tissue responses, e.g., lower urine sodium and higher urine potassium, as an index of the renal response; in clinical studies, aldosterone sensitivity has been measured in differential blood pressure responses. The concept of aldosterone sensitivity disrupts the conventional wisdom of the renin-angiotensin-aldosterone system and has the potential to uncover novel mechanisms of hypertension. Here, we review basic and translational science studies that uncovered differential renal responses to aldosterone and connect this earlier work to more recent observational studies and randomized trials that have demonstrated differential blood pressure responses for a given level of aldosterone in healthy and hypertensive persons. Black race and older age are associated with higher aldosterone sensitivity and blood pressure. We also discuss gaps in the field and how future basic and clinical studies might inform mechanisms of differential sensitivity.

Salt- and angiotensin II-dependent variations in amiloride-sensitive rectal potential difference in mice

1. In the rectum and distal nephron, sodium reabsorption is mediated by the amiloride-sensitive epithelial sodium channel (ENaC). The ENaC-mediated sodium transport is electrogenic and creates an amiloride-sensitive transepithelial potential difference (PD). 2. We have evaluated the salt- and angiotensin (Ang)II-dependent variations in amiloride-sensitive rectal PD in mice and assessed their relationship with renal sodium handling. 3. Rectal PD was measured in vivo in mice maintained on a medium-, low- or high-sodium diet. On a medium-salt diet, the mean (+/- SEM) amiloride-sensitive PD was larger in the afternoon than in the morning (-26.1 +/- 0.9 and -11.2 +/- 0.7 mV, respectively; P = 0.001), indicating a circadian cyclicity. Rectal PD increased on a low-sodium diet and decreased on a high-sodium diet. 4. Amiloride-sensitive rectal PD correlated significantly with the urinary Na+/K+ ratio (P < 0.001) and with sodium reabsorption in the distal nephron as measured by the lithium clearance technique (P < 0.001). 5. In mice treated with an AngII AT1 receptor antagonist, amiloride-sensitive rectal PD was increased in the afternoon compared with controls (-32.8 +/- 2.0 vs -24.4 +/- 0.9, respectively; P < 0.001). 6. At high doses, AngII decreased the amiloride-sensitive rectal PD and this effect was blunted by an AT1 receptor antagonist. 7. These results show the presence of a salt-dependent daily cyclicity of sodium transport in the mouse rectum that follows circadian changes in sodium handling in the distal nephron. Angiotensin II appears to modulate this diurnal pattern of rectal amiloride-sensitive sodium transport.

Segmental heterogeneity of basal and aldosterone-induced electrogenic Na transport in human colon

Recent in vitro studies in human colon have demonstrated marked segmental differences in electrogenic Na transport. In the present study, the Na channel blocker amiloride was used further to characterise basal and aldosterone-induced electrogenic Na transport in isolated human distal and proximal colon. Bathed in NaCl Ringer solution, distal and proximal colon exhibited similar basal electrical properties, but the amiloride-sensitive short-circuit current (Isc) was 200% greater in the distal than in the proximal segment. Bathed in choline-Cl Ringer solution, total Isc decreased by 97% in distal colon and by 88% in proximal colon, indicating that Na dependent transport process(es) account almost entirely for the Isc in both segments. Substituting Na2SO4 for NaCl Ringer solution (i) increased amiloride-sensitive Isc by 56% (p less than 0.01) in distal colon but had no effect on amiloride-sensitive Isc in proximal colon, and (ii) decreased amiloride-insensitive Isc in distal and proximal colon by 52% (p less than 0.05) and 81% (p less than 0.001) respectively. After the addition of nystatin to the apical membrane, the relationship between total Isc and mucosal Na concentration indicated that the activity of the basolateral membrane Na pump was similar in both colonic segments. In a further series of experiments, exposure of distal colon to 1 mumol/l aldosterone for 5 h increased total Isc by 52% (p less than 0.05), which reflected stimulation of its amiloride-sensitive component; in contrast, aldosterone had no effect on proximal colon.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Sodium potassium ATPase activity in human rectal mucosa with and without renal insufficiency

Studies in rats have shown that fecal potassium excretion and colonic mucosa Na-K-ATPase activity are elevated during dietary potassium loading and in chronic renal insufficiency. We studied Na-K-ATPase activity in human rectal mucosa in normal subjects as well as in patients with chronic renal insufficiency (creatinine clearance 2 to 72 mL/min). In normals, Na-K-ATPase activity was 4.34 +/- 0.83 mumol P/mg protein. After 2 weeks on a potassium intake of 300 mmol/d the mean activity did not differ significantly from the control value (2.49 +/- 1.30). In none of the patients with renal failure was Na-K-ATPase activity beyond the range found in the normal subjects, irrespective of serum potassium; the mean activity was 3.50 +/- 0.85. Like others, however, we found a two-fold increase in Na-K-ATPase activity in potassium loaded rats. Possible explanations for these differences are discussed.

Disorders of mineralocorticoid activity

The investigation of syndromes of mineralocorticoid excess and deficiency has been reviewed. For screening for primary aldosteronism, repeated measures of plasma potassium on high sodium intake is the most practicable method. Further investigation should include saline infusion and, in cases of doubt, fludrocortisone/sodium loading. Differential diagnosis of adenoma from hyperplasia is best achieved by the aldosterone response to posture, CT scanning and, if necessary, adrenal venous catheterization. Suitable techniques for the investigation of secondary aldosteronism of unknown cause and for primary and secondary hypoaldosteronism are described, as well as the adjustment of dosage for replacement therapy where mineralocorticoid secretion is deficient.

Disorders of the adrenal cortex: their effects on electrolyte metabolism

The adrenal cortex is an important factor in the control of electrolyte and water balance and in blood pressure homeostasis. Not surprisingly, therefore, hyper- and hyposecretion of one or more of its products has extensive repercussions. Hypersecretion of aldosterone, as seen in primary hyperaldosteronism and related diseases, and of other mineralocorticoids such as corticosterone and/or 11-deoxycorticosterone, as seen in 17 alpha-hydroxylase deficiency or 11 beta-hydroxylase deficiency syndromes, respectively, are associated with hypertension, sodium retention, potassium wastage and a metabolic alkalosis. On the other hand, impaired secretion, as in Addison's disease or in congenital deficiencies of other steroid-synthesizing enzymes, leads to hypotension, sodium loss with hypovolaemia, and hyperkalaemia. In each case, these disturbances of electrolyte metabolism may cause neurological and muscle dysfunction. The relationship between glucocorticoid hypertension and electrolyte metabolism is less clear and the importance of the adrenal cortex in the aetiology of essential hypertension is still being assessed.

Hormonal regulation of electrolyte and water transport in the colon

The colon participates in water and electrolyte homeostasis by the absorption of sodium (Na) and water as well as by potassium (K) secretion. The primary step of colonic transport is the active Na transport via a transcellular route. Steroidal hormones considerably increase Na absorption by utilizing two mechanisms: (1) passive Na entry into the cells in enhanced by an increased membrane permeability; (2) active transport capacity is increased by a stimulation of ATPase synthesis. Mineralocorticoid versus glucocorticoid actions of steroids have not yet been clearly differentiated; parallel influences are possible. Active chloride (Cl) secretion is found in the colon under certain pathological conditions and is induced by a number of factors, e.g., hormones produced by pancreas tumors. Cellular events involve a rise of intracellular cAMP and calcium (Ca) concentrations, and altered Cl permeabilities. Functional changes of colonic epithelial cells caused by hormones assume a significant role in the etiology of diarrhea, as well as in compensatory processes by which an intestinal loss of electrolytes and water is prevented.

Nutritional modulation of mineralocorticoid and prostaglandin production: potential role in prevention and treatment of gastric pathology

Various lines of evidence indicate that aldosterone and prostaglandins may play physiological roles in protecting the gastric mucosa. This would suggest that low-sodium, high-potassium diets, and supplementation with essential fatty acids that are efficient prostaglandin precursors (as in evening primrose oil), may have value in the prevention and treatment of gastric ulcer and gastritis. A low-sodium, high-potassium diet may also reduce the risk of gastric cancer.

The effect of cathartic agents on transmucosal electrical potential difference in the human rectum

Active ion transport in the colon is generating a transmucosal electrical potential difference (PD) of about 40 mV. Cathartic agents inhibit electrolyte and water net-absorption or cause net-secretion which should be reflected in a change of PD. In 83 normal subjects the effect of an isotonic eletrolyte solution (control) and different cathartic agents on rectal PD was tested: Laxatives (bisacodyl, rhein), bile acids (cholic and deoxycholic acid), fatty acids (oleic and ricinoleic acid) and cardiac glycosides (meproscillarin, digitoxin, digoxin). Bisacodyl, deoxycholic acid in high concentration, meproscillarin and digitoxin significantly decreased PD, while the other substances did not. Cathartics act on different transport mechanisms which together with different absorption characteristics of the proximal and distal colon may explain the difference in effecting the PD. Rectal PD measurement provides an easy and convenient tool to document effects of cathartic agents on electrolyte transport, otherwise difficult to obtain, and is applicable for clinical use.

Subtraction potential difference: In-vivo assay for mineralocorticoid activity

Parallel fluctuations of potential difference (P.D.) across oral and rectal mucosa are probably related to the activity of autonomic nerves rather than adrenal steroids. Unlike rectal mucosa, oral mucosa does not respond to exogenous or endogenous aldosterone. Therefore subtraction of oral P.D. from rectal P.D. gives a closer indication of mineralocorticoid activity than does rectal P.D. alone. In normal subjects plasma-aldosterone correlated well with subtraction P.D. (r = 0.74; P is less than 0.001). A subtraction P.D. higher than 26 mV in subjects on a normal sodium intake indicated primary or secondary hyperaldosteronism; if the level was lower than 11 mV four hours after intramuscular injection of 0.25 mg tetracosactrin this suggested mineralocorticoid deficiency. Measurement of oral and rectal P.D. permits rapid and inexpensive diagnosis of aldosterone excess and deficiency. The method may also be used in study of the mineralocorticoid effect of other adrenal steroids: as assessed with this bioassay, the plasma 18-OH-deoxycorticosterone, which is raised in some patients with essential hypertension, lacked any in vivo mineralocorticoid activity.

Rectal potential difference and histology in Crohn's disease

The rectal potential difference (PD) was measured in 27 patients with Crohn's disease, and in 16 subjects without gastrointestinal disease to establish a normal range. Sigmoidoscopic assessment and rectal biopsy were performed in all patients with Crohn's disease, and the mean resting rectal PD was significantly reduced in patients with sigmoidoscopically active disease and in those with abnormalities of the superficial epithelium on rectal biopsy. Patients with diarrhoea had a significantly lower mean resting PD than those with normal bowel habit, suggesting that an abnormality of rectal sodium transport may be contributing to the diarrhoea in these patients. The response of rectal PD to mineralocorticoid stimulation with oral fludrocortisone was measured in 13 patients. The PD failed to rise only with patients with sigmoidoscopically active disease, and the test proved to be a less sensitive indication of minor mucosal abnormalities than sigmoidoscopy of biopsy.

Adrenal aldosterone-producing adenoma: use of colonic potential in diagnosis and subtraction scanning technique for localisation

Primary hyperaldosteronism is a potentially curable cause of hypertension, and much interest has been shown in methods of diagnosing the associated hypokalaemic hypertension and localising the adrenal adenoma. In two patients the diagnosis of primary aldosteronism was confirmed by colonic potential measurement and the adenoma localised by a new subtraction technique for early adrenal imaging applied to the use of 131I-19-iodocholesterol. Both patients underwent adrenalectomy and in each case an adenoma was removed. Blood pressure and electrolyte levels returned to normal after operation. In one patient bilateral adrenal phlebography had failed to show the tumour, and sampling of aldosterone concentrations in the adrenal veins had been unsatisfactory.

Spironolactone in essential hypertension: evidence against its effect through mineralocorticoid antagonism

The effect of a six-week course of spironolactone 300 mg/day was examined in 25 unselected patients with essential hypertension. In the blood spironolactone produced a significant rise in urea and potassium concentrations and a fall in sodium and bicarbonate concentrations. In six patients blood pressure was normal at the end of the course, while in five patients there was almost no change. Studies of the effects of spironolactone on various indices usually affected by mineralocorticoids-namely, blood electrolytes, total body potassium, and rectal electrical properties-showed no differences between responding and non-responding patients. Mineralocorticoid excess therefore seems to be rarely responsible for essential hypertension and the influence of spironolactone cannot at present be fully explained.

Electrical potential difference, sodium absorption and potassium secretion by the human rectum during carbenoxolone therapy

The transmucosal electrical potential difference (pd) and the sodium and potassium net fluxes were measured in the rectum of subjects taking carbenoxolone. There was a rise in transmucosal pd persisting throughout treatment in all subjects which was accompanied by an increase in sodium absorption and potassium secretion. Comparison of the pd changes produced by carbenoxolone with those due to the mineralocorticoid 9-alpha-fluorocortisol showed that carbenoxolone had about 1/1000th the potency on a weight basis and the two drugs appeared to be additive in their effects. Topical instillation of carbenoxolone into the rectum produced an elevation of pd which persisted for three days. Amiloride and bendrofluazide did not interfere with these actions of carbenoxolone but spironolactone abolished them. One patient who developed fluid retention and hypokalaemia had a rectal pd similar to that of the other patients who had no side effects.

Rectal potential difference in the diagnosis of aldosterone excess

Rectal potential difference (pd) is directly related to the plasma aldosterone concentration, and rises when aldosterone is stimulated by sodium deprivation. However, when the measurement of rectal pd was tested at a screening test for hyperaldosteronism in 19 hypertensive subjects, four of the eight with primary hyperaldosteronism had a normal pd and four of the eight without aldosterone excess had an abnormally raised potential difference. The technique cannot therefore be recommended as a routine screening test for hyperaldosteronism. No relationship was found between rectal pd and hypertension associated with excess of deoxycorticosterone. Rectal pd rises in response to the mineralocorticoid-like agent carbenoxolone.

Electrical polarization of rectal mucosa and excretion of tetrahydroaldosterone in patients with cystic fibrosis of pancreas and in normal subjects

The electrical potential difference (PD) across the rectal wall was measured in 26 patients with cystic fibrosis of pancreas (CFP) and in 18 healthy subjects. The PDs obtained in normal children were identical to those previously obtained in normal adults. A significantly greater dispersion of the values was observed in CFP. When the patients were divided into groups according to metachromasia in fibroblast cultures, the mean PD was increased only in the ametachromatic group. True enough, this observation suggests a difference between various forms of CFP, distinguished by metachromasia, and thus is a further indication of the heterogeneity of the disease. The greater abnormalities in metachromasia negative patients may, however, be due solely to the fact that these patients are more severely affected by the disease. The urinary excretion of tetrahydroaldosterone in patients was within the ranges obtained in controls, which excludes the possibility of secondary hyperaldosteronism as the source of increased PD. No evidence was provided in favour of a basic defect in the intestinal transport of Na+ or Cl minus, but K+ concentrations in faecal fluids of patients were significantly lower than in controls. The equilibrium concentration of K+ could be accounted for by simple passive diffusion, suggesting that the epithelium behaved inertly with respect to this ion in CFP.

Periodic hypokalaemic paralysis, adrenal adenoma, and normal colonic transport of sodium and potassium

A 47-year-old woman was cured of hypokalaemia and recurrent paralysis by the excision of an adrenal adenoma. Hypertension was initially ameliorated but was not cured. Suppression of plasma renin activity was abolished when the adenoma was excised. Repeated measurement of plasma corticosteroids before operation showed a slight increase in aldosterone and normal plasma concentrations of deoxycorticosterone, corticosterone, and cortisol. No evidence of excess mineralocorticoid was obtained from measurement of the electrolyte composition of colonic fluid or of rectal potential difference, although both these variables responded normally to salt depletion and exogenous aldosterone. The diagnostic importance of the paradoxically normal colonic measurements is emphasized and the possibility is considered that the adenoma may have secreted an unidentified hormone.

Rectal hyperpolarization following intestinal bypass for obesity

The electrical potential difference across the rectal mucosa in six shunt-operated women was compared with that obtained in 26 normal females. The potential difference was considerably increased in all the patients, whereas the plasma concentrations and the transmural fluxes of sodium, chloride, and potassium were within normal ranges. Based on measurements of plasma renin and plasma aldosterone concentrations in three of the patients, the possibility of secondary hyperaldosteronism could be ruled out.