Intracellular potassium in diffuse proctocolitis

Pathogenesis of diarrhea in ulcerative colitis: new views on an old problem

BACKGROUND

Whereas water movement into the intestinal lumen occurs secondary to Cl secretion in secretory diarrheal diseases, defects in key transport processes lead to profound decreases in colonic Na, Cl, and water absorption in ulcerative colitis.

STUDIES AND RESULTS

Recent studies indicate reduced expression/activity of apical Na channels and basolateral Na, K-ATPase, leading to loss of electrogenic Na absorption in the distal colon and rectum. There is also likely to be a decrease in electroneutral NaCl cotransport, which is present throughout the colon. Preliminary work on basolateral K channel abundance and activity in colonic epithelial cells suggests that whole-cell K conductance is decreased in ulcerative colitis, leading to epithelial cell depolarization, and further limitation of Na absorption. In addition, there is a marked reduction in colonic epithelial resistance, which reflects a decrease in the integrity of intercellular tight junctions and the presence of apoptotic foci.

CONCLUSIONS

Impaired Na and Cl transport, combined with enhanced epithelial "leakiness," results in a profound decrease in the capacity of the inflamed colon to absorb salt and water. Transport abnormalities in ulcerative colitis may at least partly reflect the effects of proinflammatory cytokines, raising the possibility of novel approaches to the restoration of colonic absorptive capacity in this disease.

Cellular basis for defective electrolyte transport in inflamed human colon

Electrolyte transport pathways in distal colonic mucosa from patients with noninflammatory and inflammatory (ulcerative colitis, Crohn's colitis) disease of the large bowel were studied in vitro with electrophysiological techniques. Noninflamed tissues exhibited substantial amiloride-sensitive electrogenic sodium transport. In contrast, inflamed but structurally intact tissues exhibited only a modest degree of electrogenic sodium transport, significant increases in total tissue conductance and apical membrane conductance, and a 100% increase in the arachidonic acid content of the cell membrane fraction of mucosal homogenates. Replacement of chloride with gluconate decreased total tissue conductance to a greater extent in inflamed than in noninflamed tissues, and total tissue conductance was higher in inflamed than in noninflamed tissues in the presence of transepithelial potassium and sodium gradients, suggesting enhanced mucosal "leakiness" to anions and cations in acute colitis. Apical addition of nystatin virtually abolished amiloride-sensitive apical sodium uptake in both groups, indicating that the ionophore formed channels in the apical membrane of noninflamed and diseased mucosa. Additional studies showed that mucosal inflammation decreased maximal activity of the basolateral sodium pump by 76%. Thus, defects in the biophysical properties of colonic epithelial cell membranes are likely to be important factors in the pathogenesis of diarrhea in ulcerative and Crohn's colitis.

Risk factors in toxic megacolon

A retrospective analysis of data from a series of 22 patients with toxic megacolon complicating ulcerative colitis was performed in an attempt to detect factors associated with the fatal outcome of the attack. Of the 25 clinical findings studied, significant differences between survivors (17) and nonsurvivors (5) were observed in only seven. In nonsurvivors, duration of the disease was longer and blood pH and standard bicarbonate levels higher, whereas serum potassium, calcium, phosphorus, and beta-globulins were lower. Discriminant analysis showed that three pairs of features, namely potassium-beta-globulins, potassium-bicarbonate, and potassium-blood pH provided a good discrimination between survivors and nonsurvivors. Results of this study indicate that the severity of the electrolyte and metabolic disorder appears to be an important risk factor in toxic megacolon. Intensive fluid and electrolyte replacement should therefore be considered a crucial point for successful management of toxic megacolon.

Salt-losing diarrhoea in idiopathic proctocolitis

The faecal output of chloride, sodium and potassium was studied in 14 patients with active idiopathic proctocolitis, in an attempt to establish a relationship between faecal electrolyte output and acid-base balance data. 7 patients with ileostomy and 8 healthy volunteers were used as controls. The daily faecal excretion of chloride and sodium in proctocolitis was significantly in excess of normal, whereas that of potassium was within the normal range. From the comparison with the results in patients with ileostomy it is concluded that colitic colon has an impaired capacity to absorb chloride and sodium but retains the ability to secrete potassium. The intestinal loss of chloride in addition to that of sodium and water may be regarded as a salt-losing diarrhoea and may account for the metabolic alkalosis commonly found in proctocolitis.

Electrical potential difference and sodium and potassium fluxes across rectal mucosa in ulcerative colitis

The transmucosal electrical potential difference (pd) and the sodium and potassium flux rates (using a dialysis method) have been measured in the rectum and distal sigmoid colon of patients with ulcerative colitis and compared with measurements made in individuals having normal bowel function. In active colitis, a very low transmucosal pd was found and was associated with loss of the characteristic ability of the mucosa to absorb sodium against considerable electrochemical gradients; a marked increase in the plasma-to-lumen sodium flux rate, suggesting increased leakiness of the mucosa; and loss of the active sodium absorption mechanism. In resolving colitis, the pd was higher and all these changes of sodium transport tended to return towards normal. With full recovery, epithelial function was normal to the present tests. Potassium secretion rate showed little difference at various stages of the disease, but the nearly normal secretion of potassium in ulcerative colitis when the pd was low suggested that potassium loss to the lumen was excessive. Mucus collected from patients with ulcerative colitis had a relatively high sodium and potassium content. Measurement of pd and absorption by using a dialysis tube offers a simple means of rapid assessment of mucosal functional integrity.

The absorption and secretion of water and electrolytes across the healthy and the diseased human colonic mucosa measured in vitro

An ;in-vitro' technique is described for measuring quantitatively the absorption and secretion of water and electrolytes, the short circuiting current, and the simultaneous bidirectional flux rates of ions across the healthy and diseased colonic mucosa. The results show that the normal colon absorbs both sodium and water and secretes potassium. In both ulcerative colitis and Crohn's disease there is a reversal of sodium and water flux and potassium secretion is increased.A measure of the simultaneous bidirectional flux rates under conditions of zero potential has allowed us to use the Ussing equation to analyse some of the factors involved in sodium transport across the healthy and diseased mucosa.