Pathophysiology of potassium absorption and secretion by the human intestine

The Unappreciated Role of Extrarenal and Gut Sensors in Modulating Renal Potassium Handling: Implications for Diagnosis of Dyskalemias and Interpreting Clinical Trials

In addition to the classic and well-established "feedback control" of potassium balance, increasing investigative attention has focused on a novel and not widely recognized complementary regulatory paradigm for maintaining potassium homeostasis-the "feed-forward control" of potassium balance. This regulatory mechanism, initially defined in rumen, has recently been validated in normal human subjects. Studies are being conducted to determine the location for this putative potassium sensor and to evaluate potential signals, which might increase renal potassium excretion. Awareness of this more updated integrative control mechanism for potassium homeostasis is ever more relevant today, when the medical community is increasingly focused on the challenges of managing the hyperkalemia provoked by renin-angiotensin-aldosterone system inhibitors (RAASis). Recent studies have demonstrated a wide gap between RAASi prescribing guidelines and real-world experience and have highlighted that this gap is thought to be attributable in great part to hyperkalemia. Consequently we require a greater knowledge of the complexities of the regulatory mechanisms subserving potassium homeostasis. Sodium polystyrene sulfonate has long been the mainstay for treating hyperkalemia, but its administration is fraught with challenges related to patient discomfort and colonic necrosis. The current and imminent availability of newer potassium binders with better tolerability and more predictive dose-response potassium removal should enhance the management of hyperkalemia. Consequently it is essential to better understand the intricacies of mammalian colonic K+ handling. We discuss colonic transport of K+ and review evidence for potassium (BK) channels being responsible for increased stool K+ in patients with diseases such as ulcerative colitis.

Potassium homeostasis and dyskalemias: the respective roles of renal, extrarenal, and gut sensors in potassium handling

Integrated mechanisms controlling the maintenance of potassium homeostasis are well established and are defined by the classic "feedback control" of potassium balance. Recently, increasing investigative attention has focused on novel physiological paradigms that increase the complexity and precision of homeostasis. This review briefly considers the classic and well-established feedback control of potassium and then considers subsequent investigations that inform on an intriguing and not widely recognized complementary paradigm: the "feed-forward control of potassium balance." Feed-forward control refers to a pathway in a homeostatic system that responds to a signal in the environment in a predetermined manner, without responding to how the system subsequently reacts (i.e., without responding to feedback). Studies in several animal species, and recently in humans, have confirmed the presence of a feed-forward control mechanism that is capable of mediating potassium excretion independent of changes in serum potassium concentration and aldosterone. Knowledge imparted by this update of potassium homeostasis hopefully will facilitate the clinical management of hyperkalemia in patients with chronic and recurrent hyperkalemia. Awareness of this updated integrative control mechanism for potassium homeostasis is more relevant today when the medical community is increasingly focused on leveraging and expanding established renin-angiotensin-aldosterone system inhibitor treatment regimens and on successfully coping with the challenges of managing hyperkalemia provoked by renin-angiotensin-aldosterone system inhibitors. These new insights are relevant to the future design of clinical trials delineating renal potassium handling.

Point-of-Admission Serum Electrolyte Profile of Children less than Five Years Old with Dehydration due to Acute Diarrhoea

BACKGROUND/OBJECTIVE

Fluid, electrolytes and acid base disturbances are responsible for most deaths due to acute diarrhoea. The aim of this study is to describe the point-of-admission serum electrolyte profile of children with dehydration due to acute diarrhoea.

METHODS

In this cross-sectional study, the serum electrolyte levels of 185 children with dehydration due to acute diarrhoea were assessed at the point of admission at the Diarrhoea Treatment and Training Unit of the University of Benin Teaching Hospital. The age of the study population ranged from 29 days to 59 months.

RESULTS

Out of a total of 185 subjects, 30 (16.2%), 114 (61.6%), and 41 (22.2%) had severe, moderate and mild dehydration, respectively. In addition, hyponatraemic dehydration was the most common type of dehydration, accounting for 60.5% of cases. Metabolic acidosis and hypokalaemia occurred in 59.5% and 44.3% of cases, respectively. Only the serum bicarbonate level was significantly affected by degree of dehydration (p = 0.001). Age of more than 12 months and presence of vomiting were significantly associated with hyponatraemia (p = 0.005 & p = 0.02), while age of less than or equal 12 months and absence of vomiting were associated with metabolic acidosis (p = 0.04 & p = 0.03).

CONCLUSION

The degree of dehydration appears to be a good predictor of the occurrence of metabolic acidosis while age is a risk factor for hyponatraemia and metabolic acidosis.

Serum potassium decline during hospitalization for acute decompensated heart failure is a predictor of 6-month mortality, independent of N-terminal pro-B-type natriuretic peptide levels: An individual patient data analysis

BACKGROUND

Limited data exist for the role of serum potassium changes during hospitalization for acute decompensated heart failure (ADHF). The present study investigated the long-term prognostic value of potassium changes during hospitalization in patients admitted for ADHF.

METHODS

Our study is a pooled individual patient data analysis assembled from 3 prospective cohorts comprising 754 patients hospitalized for ADHF. The endpoint was all-cause mortality within 180 days after discharge. Serum potassium levels and N-terminal pro-B-type natriuretic peptide (NT-proBNP) levels were measured at admission and at discharge.

RESULTS

A percentage decrease >15% in serum potassium levels occurred in 96 (13%) patients, and an absolute decrease of >0.7 mmol/L in serum potassium levels occurred in 85 (12%) patients; and both were predictors of poor outcome independent of admission or discharge serum potassium. After the addition of other strong predictors of mortality-a 30% change in NT-proBNP during hospitalization, discharge levels of NT-proBNP, renal markers, and other relevant clinical variables-the multivariate hazard ratio of serum potassium percentage reduction of >15% remained an independent predictor of 180-day mortality (hazard ratio 2.06, 95% CI 1.14-3.73).

CONCLUSIONS

A percentage serum potassium decline of >15% is an independent predictor of 180-day all-cause mortality on top of baseline potassium levels, NT-proBNP levels, renal variables, and other relevant clinical variables. This suggest that patients hospitalized for ADHF with a decline of >15% in serum potassium levels are at risk and thus monitoring and regulating of serum potassium level during hospitalization are needed in these patients.

Comparative dissolution test of modified release pharmaceutical products for potassium replacement

Introduction: Oral potassium replacement is still inevitable. To reduce the irritation of the gastric and intestinal mucosa, pellet and matrix based formulations ensuring extended release of potassium chloride are used. The dissolution tests may help to understand the in vivo steps of the release of potassium chloride and the absorption of potassium. Aim: Using dissolution tests extended to 12 hours the authors evaluated potassium chloride release characteristics of pellet and matrix tablet based formulations used for potassium replacement. Method: The tests were performed in line with the CPMP/EWP/QWP/1401/98 guideline at nine time points (0, 1, 2, 3, 4, 5, 7, 9 and 12 hours) in three dissolution media (0.1 M hydrochloric acid, pH 1.2; acetate buffer, pH 4.5; phosphate buffer, pH 6.8). Results: Similar results were found in all three dissolution media. Conclusions: It is conceivable, that the release of potassium chloride begins already in the stomach (pH = 1.2) and at an average speed of gastrointestinal transit – in about 6–7 hours – 80% of the potassium chloride content of both formulations is dissolved by the time of the entrance to the large bowel. It seems likely, that in vivo in the proximal section of the gastrointestinal tract more potassium chloride is dissolved out of the matrix based formulation, than from the pellet based one. Both formulations meet the clinical requirements of the effective potassium chloride release. Orv. Hetil., 2015, 156(12), 479–482.

Chronic renal disease progression: treatment strategies and potassium intake

Disordered potassium homeostasis is a common complication of chronic kidney disease and traditional management focuses on restricting potassium intake to avoid hyperkalemia. Permissive potassium intake carries the risk of hyperkalemia and hyperphosphatemia, and possibly may contribute to the development of uremic neuropathy. Excessive potassium restriction and removal by dialysis carries the risk of worsened chronic hypertension, intradialytic hypotension, renal fibrosis and cyst formation, and ventricular arrhythmias. Cohort studies have associated both hypokalemia and hyperkalemia with increased mortality in CKD. A single study of potassium intake in hemodialysis patients found increased intake associated with increased mortality despite adjustment for serum potassium concentration. We recommend avoiding mandatory potassium restriction in early chronic kidney disease. We endorse routine potassium restriction in advanced chronic kidney disease requiring hemodialysis and close monitoring of serum potassium concentration in any patients receiving renin-angiotensin-aldosterone system blockers.

Manual ventilation therapy and aggressive potassium supplementation in the management of respiratory failure secondary to severe hypokalaemia in a cat with exocrine pancreatic insufficiency

A domestic shorthair cat was referred for progressive muscle weakness and dyspnoea. The cat had a 2-month history of severe weight loss, small intestinal diarrhoea, polyphagia and polyuria/polydipsia. Biochemical analysis and venous blood gas evaluation revealed severe hypokalaemia [1.7 mmol/l; reference interval (RI): 3.5–5.1 mmol/l] and hypoventilation (partial pressure of carbon dioxide = 68 mmHg; RI: 34–38 mmHg). Aggressive potassium supplementation was initiated. The cat was manually ventilated until serum potassium increased to 3 mmol/l. A diagnosis of exocrine pancreatic insufficiency (EPI) was made based on clinical signs and serum feline trypsin-like immunoreactivity (0.1 μg/l; RI: 12–82 μg/l). Medical management of the EPI resulted in clinical recovery.

Severe hyperkalemia following colon diversion surgery in a patient undergoing chronic hemodialysis: a case report

Introduction

Potassium (K⁺) homeostasis in healthy subjects is maintained mainly by urinary excretion of K⁺. In patients with end-stage renal disease, the intestinal tract might assume an accessory K⁺ excretory role in the face of declining renal excretory function. Here, we report the case of a patient with end-stage renal disease who developed severe hyperkalemia following colon diversion surgery.

Case presentation

A 56-year-old Caucasian woman undergoing hemodialysis experienced ischemic colitis, leading to ileocecal resection and a temporary ileostomy. She made a good recovery and her dietary intake improved. However, her pre-dialysis serum K⁺ level three weeks later was 7.2mmol/L, which was much higher than her previous level (range 4.9 to 6.1mmol/L). Despite dietary restriction of K⁺ and use of oral cation-exchange resin and low K⁺ dialysate, her serum K⁺ level remained high (6.1 to 8.3mmol/L). Six months later, her bowel continuity was restored and her serum K⁺ decreased to the previous level. Her fecal K⁺ concentration before and after stoma reversal showed a marked difference: 23mmol/L before and 60mmol/L after.

Conclusions

We assume that the severe hyperkalemia seen in our patient was caused by reduced colonic K⁺ secretion due to the colon diversion. Our patient’s case demonstrates the importance of colonic K⁺ secretion for the maintenance of K⁺ homeostasis in patients with end-stage renal disease.

The practical value of comprehensive stool analysis in detecting the cause of idiopathic chronic diarrhea

The practical diagnostic value of fecal analysis in the evaluation of patients with chronic nonbloody diarrhea is controversial. It is possible that variations in its value depend on how it is done and how the results are interpreted rather than on its intrinsic value. In the authors' city, stool analysis has been made easily accessible, with a commitment to quality assurance and interpretation. To evaluate its practical value, the results of stool analysis obtained on stool specimens submitted by gastroenterologists were retrospectively reviewed. The results indicate that stool analysis has substantial practical diagnostic value in patients with chronic diarrhea.

PEARL-HF: prevention of hyperkalemia in patients with heart failure using a novel polymeric potassium binder, RLY5016

The treatment of heart failure has seen considerable advances in the past decades. In particular, a therapeutic focus on the renin-angiotensin-aldosterone system has provided significant improvements in outcomes. Multiple inhibition points in the renin-angiotensin-aldosterone system, including direct renin inhibitors, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers and mineralocorticoid receptor antagonists, have the common feature of either blocking aldosterone production (direct renin inhibitor, angiotensin-converting enzyme inhibitor, angiotensin receptor blocker) or the mineralocorticoid receptor. As a consequence of this inhibition, sodium and water reabsorption is blocked, and potassium (K(+)) excretion is reduced. Hyperkalemia may result from the use of multiple renin-angiotensin-aldosterone inhibitors or blockers, particularly in patients with heart failure and concomitant chronic kidney disease. Interventions to reliably control serum K(+) during renin-angiotensin-aldosterone inhibition have not been available to date, and would be of particular value with the use of mineralocorticoid receptor antagonists that have been shown to reduce mortality in patients with heart failure and a reduced left ventricular ejection fraction. In this review, we examine the PEARL-HF study, which has tested the combined use of RLY5016, a novel nonabsorbed K(+) binding polymer, with spironolactone in heart failure patients receiving standard care but with previous documented hyperkalemia or chronic kidney disease. RLY5016 significantly lowered serum K(+) levels from baseline relative to placebo, lowered the incidence of hyperkalemia and allowed a higher proportion of heart failure patients to receive spironolactone at a dose of 50 mg/day.

Cardiac arrest due to lymphocytic colitis: a case report

INTRODUCTION

We present a case of cardiac arrest due to hypokalemia caused by lymphocytic colitis.

CASE PRESENTATION

A 69-year-old Caucasian man presented four months prior to a cardiac arrest with watery diarrhea and was diagnosed with lymphocytic colitis. Our patient experienced a witnessed cardiac arrest at his general practitioner's surgery. Two physicians and the emergency medical services resuscitated our patient for one hour and four minutes before arriving at our university hospital. Our patient was defibrillated 16 times due to the recurrence of ventricular tachyarrhythmias. An arterial blood sample revealed a potassium level of 2.0 mmol/L (reference range: 3.5 to 4.6 mmol/L) and pH 6.86 (reference range: pH 7.37 to 7.45). As the potassium level was corrected, the propensity for ventricular tachyarrhythmias ceased. Our patient recovered from his cardiac arrest without any neurological deficit. Further tests and examinations revealed no other reason for the cardiac arrest.

CONCLUSION

Diarrhea can cause life-threatening situations due to the excretion of potassium, ultimately causing cardiac arrest due to hypokalemia. Physicians treating patients with severe diarrhea should consider monitoring their electrolyte levels.

Colonic potassium handling

Homeostatic control of plasma K+ is a necessary physiological function. The daily dietary K+ intake of approximately 100 mmol is excreted predominantly by the distal tubules of the kidney. About 10% of the ingested K+ is excreted via the intestine. K+ handling in both organs is specifically regulated by hormones and adapts readily to changes in dietary K+ intake, aldosterone and multiple local paracrine agonists. In chronic renal insufficiency, colonic K+ secretion is greatly enhanced and becomes an important accessory K+ excretory pathway. During severe diarrheal diseases of different causes, intestinal K+ losses caused by activated ion secretion may become life threatening. This topical review provides an update of the molecular mechanisms and the regulation of mammalian colonic K+ absorption and secretion. It is motivated by recent results, which have identified the K+ secretory ion channel in the apical membrane of distal colonic enterocytes. The directed focus therefore covers the role of the apical Ca2+ and cAMP-activated BK channel (KCa1.1) as the apparently only secretory K+ channel in the distal colon.

Influence of traumatic brain injury on potassium and phosphorus homeostasis in critically ill multiple trauma patients

OBJECTIVE

The intent of this study was to ascertain whether multiple trauma patients with traumatic brain injury (TBI) had lower serum concentrations of potassium and phosphorus and required more aggressive supplementation than multiple trauma patients without TBI.

METHODS

Ventilator-dependent adult patients without renal impairment who were admitted to the trauma intensive care unit or neurosurgical intensive care unit and who received enteral nutrition were evaluated for the first 14 d after hospital admission. Patients were grouped according to the presence or absence of TBI. Target serum concentrations for potassium and phosphorus were 4 mEq/L and 4 mg/dL, respectively. Electrolyte repletion therapy was given according to the nutritional support service guidelines.

RESULTS

Fifty trauma patients (25 with and without TBI) were studied. Daily serum potassium concentrations were consistently lower for those with TBI (P < or = 0.001), whereas the mean net potassium intake was greater (1.3 +/- 0.5 versus 0.7 +/- 0.3 mEq x kg(-1) x d(-1), respectively, P < or = 0.001). Serial serum phosphorus concentrations were similar between groups (P = NS) except for a significantly lower serum phosphorus concentration for trauma patients with TBI on day 3 after hospital admission (2.5 +/- 0.5 versus 2.9 +/- 0.7 mg/dL, respectively, P < or = 0.05). However, the mean net phosphorus intake was significantly greater for trauma patients with TBI (0.65 +/- 0.25 versus 0.45 +/- 0.17 mmol x kg(-1) x d(-1), P < or = 0.001).

CONCLUSION

Potassium and phosphorus requirements are greater for multiple trauma patients with TBI compared with those without TBI.

Nutritional deficiencies in intestinal failure

Intestinal failure (IF) is the ultimate malabsorption state, with multiple causes, requiring long-term therapy with enteral or intravenous fluids and nutrient supplements. The primary goal during management of children with potentially reversible IF is to promote intestinal autonomy while supporting normal growth, nutrient status, and preventing complications from parenteral nutrition therapy. This article presents how an improved understanding of digestive pathophysiology is essential for diagnosis, successful management, and prevention of nutrient deficiencies in children with IF.

Regulation of colonic apical potassium (BK) channels by cAMP and somatostatin

High-conductance apical K+ (BK) channels are present in surface colonocytes of mammalian (including human) colon. Their location makes them well fitted to contribute to the excessive intestinal K(+) losses often associated with infective diarrhea. Since many channel proteins are regulated by phosphorylation, we evaluated the roles of protein kinase A (PKA) and phosphatases in the modulation of apical BK channel activity in surface colonocytes from rat distal colon using patch-clamp techniques, having first increased channel abundance by chronic dietary K+ enrichment. We found that PKA activation using 50 micromol/l forskolin and 5 mmol/l 3-isobutyl-1-methylxanthine stimulated BK channels in cell-attached patches and the catalytic subunit of PKA (200 U/ml) had a similar effect in excised inside-out patches. The antidiarrheal peptide somatostatin (SOM; 2 micromol/l) had a G protein-dependent inhibitory effect on BK channels in cell-attached patches, which was unaffected by pretreatment with 10 micromol/l okadaic acid (an inhibitor of protein phosphatase type 1 and type 2A) but completely prevented by pretreatment with 100 micromol/l Na+ orthovanadate and 10 micromol/l BpV (inhibitors of phosphoprotein tyrosine phosphatase). SOM also inhibited apical BK channels in surface colonocytes in human distal colon. We conclude that cAMP-dependent PKA activates apical BK channels and may enhance colonic K+ losses in some cases of secretory diarrhea. SOM inhibits apical BK channels through a phosphoprotein tyrosine phosphatase-dependent mechanism, which could form the basis of new antidiarrheal strategies.

The effects of angiotensin converting enzyme inhibitors on potassium homeostasis in dialysis patients with and without residual renal function

Hyperkalemia is exacerbated by angiotensin converting enzyme inhibitors (ACE-I). Distal potassium (K(+)) secretion is negligible in anuric patients. ACE-I therapy may reduce renal, peritoneal, and colonic K(+) losses. We examined the effect of ACE-I therapy on serum, urinary, and dialysate K(+) in a cross-section of peritoneal and hemodialysis patients. Serum, 24-h urine K(+), and peritoneal dialysate excretion K(+) levels were measured and the results were compared in the various dialysis and treatment groups. Eighty-one hemodialysis (HD) and 32 peritoneal dialysis (PD) patients were included. Serum K(+) in HD patients with no residual renal function (RRF) was higher in those receiving ACE-I therapy (P = 0.02). Serum K(+) levels in HD patients receiving ACE-I treatments with RRF was similar to that in oligoanuric HD patients not receiving an ACE-I. Urinary K(+) excretion was significantly reduced in those on ACE-I therapy versus those not on an ACE-I (P < 0.05). Mean serum K(+) was lower in PD versus HD patients (P < 0.05). PD patients with no RRF on ACE-I therapy had higher serum K(+) concentrations (P = 0.002) and dialysate K(+) excretion was lower (P = 0.05), in comparison with PD patients not on an ACE-I. PD patients with RRF on ACE-I therapy had higher serum K(+) concentrations compared with those not on ACE-I therapy (P = 0.03). Both urinary and dialysate K(+) excretion were reduced (P = 0.001 and P = 0.002, respectively). ACE-I therapy increases serum K(+) concentration in dialysis patients. PD patients have relatively lower serum K(+) levels compared with HD patients. In PD patients, ACE-I therapy reduces dialysate K(+). These changes may result from reduced peritoneal movement of K(+).

Secretory diarrhoea with high faecal potassium concentrations: a new mechanism of diarrhoea associated with colonic pseudo-obstruction? Report of five patients

OBJECTIVE

To report the mechanism of diarrhoea in patients with subacute colonic pseudo-obstruction, profuse secretory diarrhoea and hypokalemia.

PATIENTS

Five consecutive patients who developed colonic pseudo-obstruction, profuse watery diarrhoea and severe hypokalemia. Investigations excluded mechanical intestinal obstruction. Usual cause of diarrhoea were ruled out. Abdominal distension and diarrhoea improved simultaneously in all cases after colonoscopic decompression or intravenous neostigmine.

RESULTS

Faecal ionograms showed a low osmotic gap and high faecal potassium concentration explaining the hypokalemia: 100 to 180 mEq/kg (usually inferior than 50 mEq/l in case of secretory diarrhoea) and low faecal sodium concentrations. Potassium salts were the only factor identified as the driving osmotic force for the diarrhoea.

CONCLUSION

Secretory diarrhoea is classically due to chloride active secretion with passive sodium secretion or to inhibition of sodium absorption. In five cases of Ogilvie's syndrome we evidenced an original mechanism of secretory diarrhoea due to active potassium secretion responsible of a profound hypokalemia. This novel type of diarrhoea may be a hallmark of colonic pseudo-obstruction due to colonic distension.

Renal and gastrointestinal potassium excretion in humans: new insight based on new data and review and analysis of published studies

OBJECTIVES

Little is known about the relationship between the renal and gastrointestinal excretion of potassium in humans. This information is important in light of strong associations of potassium intake with hypertension and occlusive stroke.

METHODS

We determined the relationship between fecal and urinary excretion of potassium under both fixed and variable potassium intakes using our unpublished archival data and published data of others. Twenty-five subjects were evaluated.

RESULTS

On a fixed, low oral potassium intake (61.2 +/- 4.7 mmol/day; mean +/- SD), there was an inverse relationship between fecal and urinary potassium excretion (r = -0.66, p = 0.040). In studies in which potassium intake varied between 61-135 mmol/day, fecal and urinary potassium excretions were positively correlated (r = 0.58, p = 0.024). Considerable within-and-between-subject variation was observed in the relationship between fecal and urinary potassium excretion.

CONCLUSIONS

Inter-individual variation in fecal potassium excretion may arise from both variation in dietary potassium intake and intrinsic individual differences in the renal versus gastrointestinal handling of potassium.

[Disturbances in volume and electrolytes with intestinal and kidney diseases]

The intestines and kidney are the most important excretion organs. Both organ systems are key players in keeping the homeostatic balance regarding hydration and electrolytes. Disturbances of function can lead to enormous and sometimes life-threatening complications. Intestinal diseases lead often to diarrhoea, which can be associated with fluid loss of up to 20 l per day. The accompanying electrolyte disturbances can be hypo- or hypernatremia in combination with hypokalemia. The therapy is substitution guided by knowledge of the pathophysiology. Kidney diseases lead to excessive volume and electrolyte balances, depending on the underlying molecular or pathological defect, but deficiencies can also be found. In case of kidney impairment with 30-50% total loss of function, calcium and phosphate metabolism is impaired.

Stimulated active potassium secretion in a patient with colonic pseudo-obstruction: a new mechanism of secretory diarrhea

BACKGROUND & AIMS

Secretory diarrhea is caused by inhibition of intestinal active sodium absorption and stimulation of active chloride secretion. The resulting increase in fecal sodium salts causes an isotonic increase in fecal water output. Abnormalities in potassium transport are not known to be a cause of secretory diarrhea. The aim of our report is to describe a patient with secretory diarrhea that was mediated by excess intestinal secretion of potassium.

METHODS

A 78-year-old woman developed colonic pseudo-obstruction, complicated by severe diarrhea and hypokalemia. Her stools were collected quantitatively on 11 occasions and analyzed for electrolyte concentrations. Rectosigmoid potential difference was measured.

RESULTS

The diarrheal fluid had a very high potassium concentration (130-170 mEq/L) and a very low sodium concentration (4-15 mEq/L). Stool potassium losses were as high as 256 mEq/day (normal, 9 mEq/day), and fecal sodium losses were never higher than 13 mEq/day. Potential difference between colonic lumen and a peripheral reference electrode was -14 mV (lumen side negative).

CONCLUSIONS

Fecal potassium salts were the exclusive driving force for severe secretory diarrhea in a patient with colonic pseudo-obstruction. The high fecal output of potassium was due to stimulation of active colonic potassium secretion, possibly because of changes in autonomic nervous system activity and distention of the colon in association with colonic pseudo-obstruction. The extremely low fecal excretion of sodium indicates that active sodium absorption was not inhibited. This case study reveals an ion transport mechanism of secretory diarrhea that has not been previously appreciated.

Protective effects of high dietary potassium: nutritional and metabolic aspects

Potassium (K+) requirements have been largely overlooked because severe deficiencies are uncommon due to the ubiquity of this element in foods. However, a transition toward modern ("Westernized") diets has led to a substantial decline of K+ intake compared with traditional food habits, and a large fraction of the population might now have suboptimal K+ intake. A high K+ intake was demonstrated to have protective effects against several pathologic states affecting the cardiovascular system, kidneys, and bones. Additionally, fruits and vegetables contain K/organic anion salts (malate, citrate), which exert alkalinizing effects, through KHCO(3)(-) generation, which serves to neutralize fixed acidity in urine. Low-grade metabolic acidosis, when not properly controlled, may exacerbate various catabolic processes (bone Ca++ mobilization, proteolysis), especially in the elderly. Fruits and vegetables are therefore receiving great attention in a strategy to increase the nutritional value of meals while reducing energy density and intake. The need to ensure a 2.5- to 3.5-g daily K+ supply from fruits and vegetables represents a strong rationale for the "5-10 servings per day" recommendations.

Hyperkalemia with concomitant watery diarrhea: an unusual association

Four patients presented to the emergency room with life-threatening hyperkalemia and concomitant watery diarrhea. Hypovolemia, acidosis, and renal insufficiency were present in all 4 cases. In 2 patients, hyperkalemia followed initiation of angiotensin-converting enzyme (ACE) inhibitor therapy, whereas 1 patient experienced hyperkalemia after a dose increase of an ACE inhibitor, and the fourth patient was on continuous ACE-inhibitor therapy at the time of the hyperkalemia episode. Two of the 3 patients with functioning kidneys required hemodialysis to correct the hyperkalemia, whereas the other patient was on long-term hemodialysis therapy. In the 2 patients in whom transtubular potassium (K+) gradients were available, their values ranged far below normal, indicating tubular failure to secrete K+. This abnormality was attributed to decreased distal delivery of sodium and water and to renin/angiotensin II/aldosterone blockade. It has been proposed that aldosterone blockade impairs the capacity of the colonic epithelial cells to secrete K+. In all 4 patients the watery diarrhea ceased in parallel with the correction of serum K+ to normal values. It is suggested that hyperkalemia, most likely by stimulating intestinal motility, induced the watery diarrhea in all 4 patients. The watery diarrhea, however, failed to compensate for the renal tubular failure to secrete K+.

In vitro potassium transport in the mouse small intestine

Ingested K+ is believed to be absorbed mainly in the small intestine by passive diffusion through the paracellular pathway. To further clarify K+ absorption in the small intestine, we determined the unidirectional flux values of Rb+ in vitro by atomic absorption spectroscopy in the mouse ileum mounted in Ussing chambers under short-circuit conditions. The mucosal-to-serosal Rb+ flux (J(ms)) was larger than the serosal-to-mucosal Rb+ flux (J(sm)), resulting in positive net Rb+ absorption (J(net)). The effect of changing the transmucosal potential (V(t)) showed that J(ms) was composed of both a V(t)-dependent diffusion component and a V(t)-independent non-diffusion component, while J(sm) was composed mainly of a V(t)-dependent component. A forskolin treatment eliminated J(net) mainly due to the increase in J(sm). When animals were fed a low-Na diet, J(net) was mainly eliminated as a result of the increase in J(sm). These findings suggest that K+ is absorbed not only by passive diffusion through the paracellular pathway, but also by an active transport mechanism operating through the cellular pathway. In addition, cAMP and aldosterone may be involved in regulating intestinal K+ transport.

Chronic microscopic enterocolitis with severe hypokalemia responding to subtotal colectomy

The authors present the first case report of a 50-year-old woman with a 33-year history of severe, chronic watery diarrhea and hypokalemia secondary to chronic active microscopic enterocolitis with patterns similar to lymphocytic colitis but with acute cryptitis and terminal ileum involvement microscopically. The progressive nature of her illness resulted in multiple hospital admissions secondary to hypokalemia with subsequent chronic renal failure. High continuous doses of oral potassium supplements failed to correct the hypokalemic episodes. After subtotal colon resection, the patient made a marked clinical improvement with normal serum potassium levels without receiving potassium supplementation.

Role of skeletal muscle in plasma ion and acid-base regulation after NaHCO3 and KHCO3 loading in humans

This paper examines the time course of changes in plasma electrolyte and acid-base composition in response to NaHCO3 and KHCO3 ingestion. It was hypothesized that skeletal muscle is involved in the correction of the ensuing plasma disturbance by exchanging ions, gasses, and fluids between cells and extracellular fluids. Five male subjects, with catheters in a brachial artery and antecubital vein, ingested 3.57 mmol/kg body mass NaHCO3 or KHCO3. While seated, blood samples were taken 30 min before ingestion of the solution, at 10-min intervals during the 60-min ingestion period, and periodically for 210 min after ingestion was complete. Blood was analyzed for gases, hematocrit, plasma ions, and total protein. With NaHCO3, arterial plasma Na+ concentration ([Na+]) increased from 143 +/- 1 to 147 +/- 1 (SE) meq/l, H+ concentration ([H+]) decreased by 6 +/- 1 neq/l, and PCO2 increased by 5 +/- 1 mmHg. There was no detectable net Na+ uptake by tissues. An increased plasma strong ion difference ([SID]) accounted fully for the decrease in plasma [H+]. With KHCO3, K+ concentration increased from 4.25 +/- 0.10 to 7.17 +/- 0.13 meq/l, plasma volume decreased by 15.5 +/- 2.3%, [H+] decreased by 4 +/- 1 neq/l, and there was no change in PCO2. The decrease in [H+] in the KHCO3 trial primarily arose in response to the increased [SID]. Net K+ uptake by tissues accounted for 37 +/- 5% of the ingested K+. In conclusion, ingestion of NaHCO3 and KHCO3 produced markedly different fluid and ionic disturbances and associated regulatory responses by skeletal muscle. Accordingly, the physicochemical origins of the acid-base disturbances also differed between treatments. The tissues did not play a role in regulating plasma [Na+] after ingestion of NaHCO3. In contrast, the net influx of K+ to tissues played an important role in removing K+ from the extracellular compartment after ingestion of KHCO3.

Ouabain-sensitive H,K-ATPase: tissue-specific expression of the mammalian genes encoding the catalytic alpha subunit

Human ATP1AL1 and corresponding genes of other mammals encode the catalytic alpha subunit of a non-gastric ouabain-sensitive H,K-ATPases, the ion pump presumably involved in maintenance of potassium homeostasis. The tissue specificity of the expression of these genes in different species has not been analyzed in detail. Here we report comparative RT-PCR screening of mouse, rat, rabbit, human, and dog tissues. Significant expression levels were observed in the skin, kidney and distal colon of all species (with the exception of the human colon). Analysis of rat urogenital organs also revealed strong expression in coagulating and preputial glands. Relatively lower expression levels were detected in many other tissues including brain, placenta and lung. In rabbit brain the expression was found to be specific to choroid plexus and cortex. Prominent similarity of tissue-specific expression patterns indicates that animal and human non-gastric H,K-ATPases are indeed products of homologous genes. This is also consistent with the high sequence similarity of non-gastric H,K-ATPases (including partial sequences of hitherto unknown cDNAs for mouse and dog proteins).

Beta-adrenergic stimulation of cellular K+ uptake in rat distal colon

We recently demonstrated that the ratio between colonic K+ absorptive and K+ secretive pathways was higher in infant than in adult rats. To test the hypothesis that hormones selectively affect these pathways during ontogeny we examined the effect of adrenergic agonists on cellular K+ uptake in distal colon from infant (10-day-old) and adult (50-day-old) rats. Here we describe that adrenaline (10(-5) M) increased total and ouabain-insensitive 86Rb uptake in both age groups, but it did not affect ouabain-sensitive 86Rb uptake. This stimulation was more pronounced in adult than in infant rats. The effect of adrenaline was mediated via beta-adrenergic receptors. Incubation in vitro with beta-agonist, isoproterenol, stimulated SCH-28080-sensitive, i.e. H+, K(+)-ATPase-dependent, 86Rb uptake in adult but not in infant rats. The threshold dose of beta-agonist was at 10(-7) M, and the maximal activation was observed at 10(-5) M. In vivo inhibition of beta-adrenergic system with propranolol caused a significant decrease in H+, K(+)-ATPase-dependent 86Rb uptake in infant but not in adult colon. In conclusion, this study suggests that the higher colonic K+ absorption in infant rats may be as a result of a selective beta-adrenergic up-regulation leading to stimulation of the apical H+, K(+)-ATPase.

The rat distal colon P-ATPase alpha subunit encodes a ouabain-sensitive H+, K+-ATPase

The functional properties and the pharmacological profile of the recently cloned cDNA colonic P-ATPase alpha subunit (Crowson, M.S., and Shull, G.E. (1992) J. Biol. Chem. 267, 13740-13748) were investigated by using the Xenopus oocyte expression system. Xenopus oocytes were injected with alpha subunit cRNAs from Bufo marinus bladder or rat distal colon and/or with beta subunit cRNA from B. marinus bladder. Two days after injection, K+ uptake was measured by using 86 Rb+ as a K+ surrogate, and pH measurements were performed by means of ion-selective microelectrodes. Co-injection of alpha and beta subunit cRNAs led to a large increase in 86Rb+ uptake, an intracellular alkalinization, and an extracellular medium acidification, as compared to alpha or beta injection alone. These results indicate that the colonic P-ATPase alpha subunit, like the bladder alpha subunit, acts as a functional H+,K+-ATPase, and that co-expression of alpha and beta subunits is required for the function. External K+ activation of the 86Rb+ uptake had a K1/2 of approximately 440 microM for the bladder isoform (consistent with the previously reported value (Jaisser, F., Horisberger, J.D., Geering, K., and Rossier, B.C. (1993) J. Cell. Biol. 123, 1421-1431) and a K1/2 of approximately 730 microM for the colonic isoform. Sch28080 was ineffective to reduce 86Rb+ uptake whereas ouabain inhibited the activity expressed from rat colon alpha subunit with a Ki of 970 microM when measured at the Vmax of the enzyme. We conclude that, when expressed in Xenopus oocytes, the rat colon P-ATPase alpha subunit encodes a ouabain-sensitive H+,K+-ATPase.

Effect of three laxatives and a cation exchange resin on fecal sodium and potassium excretion

BACKGROUND/AIMS

The treatment of hyperkalemia in patients with renal insufficiency often includes the ingestion of sorbitol and a cation exchange resin. Sorbitol alone may be used to remove sodium and water from overloaded patients. The efficacy of these regimens has never been compared with other laxative or laxative-resin combinations. The aim of the study was to compare the relative effect of three laxatives with different mechanisms of action, alone and in combination with resin, on fecal sodium and potassium excretion.

METHODS

Sodium, potassium, and water excretion in 12-hour stool collections were analyzed after various laxative-resin combinations in normal subjects.

RESULTS

Correctol (yellow phenolphthalein) (Schering Plough Health Care Products, Memphis, TN) was more effective than sorbitol or sodium sulfate in causing fecal sodium and potassium loss. Resin recovery in stool was much greater with phenolphthalein than with other laxatives, and more potassium was excreted in stool with phenolphthalein-resin than with phenolphthalein alone or other laxative-resin combinations. Sorbitol caused more undesirable gastrointestinal symptoms than did sodium sulfate or phenolphthalein.

CONCLUSIONS

In normal people, phenolphthalein (1) is preferable to other laxatives in causing fecal sodium and potassium excretion, (2) hastens resin transit through the intestine compared with other laxatives, and (3) produces greater fecal potassium excretion when combined with resin than phenolphthalein alone or other laxative-resin combinations.