Effect of amiloride on sodium transport in the proximal, distal, and entire human colon in vivo.

In vitro studies under short-circuited conditions suggest that amiloride, a diuretic agent which is thought to block apical membrane sodium entry, has significant effects on sodium absorption by the human colon. To evaluate this in vivo, we studied the effect of amiloride applied in concentrations of 10(-5) and 10(-4) M on sodium transport and potential difference (PD) in human colon during steady-state perfusion. Net sodium absorption was reduced 25% by amiloride and chloride absorption by 15%; potassium and bicarbonate secretion rates were enhanced. In other studies the colon was divided into a proximal and distal test segment by endoscopic introduction of a collection channel to the descending colon-sigmoid junction. Comparison of tritiated water absorption by the two segments indicated that the distal segment comprised approximately 20% of the total colon surface area. However, the distal test segment only accounted for 5-7% of total sodium, chloride, or water absorption; in contrast, 17-20% of total potassium or bicarbonate secretion occurred there. In the proximal test segment, amiloride reduced net sodium absorption by almost one third from 21.0 to 14.4 mmol/hr (P less than 0.02) but had no significant effect on PD. In the distal test segment, amiloride produced a 25% reduction in mean sodium absorption from 1.2 to 0.9 mmol/hr, but this reduction was not statistically significant; however, potential difference was significantly reduced by 33% (P less than 0.02). These results suggest that most sodium absorption in normal human colon in vivo is mediated by transport processes which are insensitive to these doses of amiloride.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies of the prevalence and significance of radiolabeled bile acid malabsorption in a group of patients with idiopathic chronic diarrhea.

We studied radiolabeled fecal bile acid excretion in 11 normal subjects and 17 patients with idiopathic chronic diarrhea for three major purposes: to establish normal values for this test in the presence of increased stool volumes (induced in normal subjects by ingestion of poorly absorbable solutions); to test for bile acid malabsorption in the patients and to correlate this with an independent test of ileal function, the Schilling test; and to compare the results of the bile acid excretion test with the subsequent effect of a bile acid binding agent (cholestyramine) on stool weight. In normal subjects fecal excretion of the radiolabel was increased with increasing stool volumes. As a group, patients with idiopathic chronic diarrhea excreted radiolabeled bile acid more rapidly than normal subjects with induced diarrhea (t1/2 56 +/- 8 vs. 236 +/- 60 h, respectively, p less than 0.005). There was a statistically significant positive correlation between t1/2 of radiolabeled bile acid and Schilling test results in these patients. Although 14 of 17 patients absorbed labeled taurocholic acid less well than any of the normal subjects with comparable volumes of induced diarrhea, cholestyramine had no statistically significant effect on stool weight in the patient group, and in none of the patients was stool weight reduced to within the normal range. In summary, most patients with idiopathic chronic diarrhea have bile acid malabsorption (as measured by fecal excretion of labeled bile acid), but they do not respond to cholestyramine therapy with a significant reduction in stool weight. Although the significance of these findings was not clearly established, the most likely interpretation is that bile acid malabsorption is a manifestation of an underlying intestinal motility or absorptive defect rather than the primary cause of diarrhea.

The absorption of dietary phosphorus and calcium in hemodialysis patients.

Absorption of dietary phosphorus plays a critical role in the development of metabolic bone diseases in patients with chronic renal failure. However, phosphorus absorption is difficult to quantitate in dialysis patients because the dialysis treatments complicate metabolic balance studies. Utilizing a recently developed technique which permits measurement of net absorption of dietary constituents after a single meal, we measured phosphorus absorption in dialysis patients. The following observations were made: A.) Following a meal containing approximately 300 mg phosphorus, mean phosphorus absorption in five hemodialysis patients (with severe vitamin D deficiency) was only slightly less than in matched controls (186 +/- 35 vs. 242 +/- 30). B.) After dialysis patients were treated with 1,25(OH)2-D3, phosphorus absorption increased from 186 +/- 35 to 272 +/- 16 mg (P less than 0.025). C.) The effect of three aluminum containing antacids on phosphorus absorption was studied; each slightly reduced the absorption of phosphorus compared to placebo (P less than 0.01), but there was no significant difference between them. D.) Aluminum hydroxide and calcium carbonate each reduced dietary phosphorus absorption to approximately the same extent. Calcium carbonate ingestion was associated with sharply increased calcium absorption. The absorption of dietary phosphorus is influenced only modestly by 1,25(OH)2-D3 and is inhibited to an equal but only modest degree by various aluminum antacids and by calcium carbonate.

Urinary excretion of polyethylene glycol 3350 and sulfate after gut lavage with a polyethylene glycol electrolyte lavage solution.

Ingestion of an electrolyte lavage solution containing polyethylene glycol 3350 and sulfate is an effective method of cleansing the colon for diagnostic studies. Polyethylene glycol and sulfate are considered poorly absorbed from the gastrointestinal tract. Because of the quantities administered, concern exists about potential toxicity of absorption of even a small percentage, particularly for polyethylene glycol. We measured the urinary excretion of both polyethylene glycol and sulfate in normal subjects and inflammatory bowel patients. Absorption of polyethylene glycol can be assessed by measuring recovery in urine, as 85%-96% of an intravenous load is excreted in urine. Similarly, appreciable sulfate absorption would exceed renal tubular reabsorption and result in increased urinary excretion. Mean percent polyethylene glycol load recovered in urine was minimal and similar for normal (0.06%) and inflammatory bowel (0.09%) subjects. Urinary sulfate excretion after lavage was also similar for both groups and was not different from baseline. These results do not suggest the likelihood of toxicity due to polyethylene glycol 3350 or sulfate absorption during gut lavage with this solution.

Effect of magnesium on active and passive sodium transport in the human ileum.

The effect of 12.5 mM magnesium chloride on sodium transport in the human ileum in vivo was investigated using segmental perfusion. Choline chloride was used as a control. During perfusion of a balanced electrolyte solution containing isotopes of sodium and chloride, magnesium reduced unidirectional flux of sodium in both directions across the ileum; magnesium had no statistically significant effect on net sodium absorption, on chloride fluxes, or on potential difference. When sodium-free test solutions were infused, magnesium (and calcium) reduced net sodium secretion compared with choline and potassium. These results suggest that magnesium (and calcium) reduce passive sodium movement across ileal mucosa.

Studies of the antidiarrheal action of clonidine. Effects on motility and intestinal absorption.

Clonidine, an alpha 2-adrenergic agonist, has been reported to stimulate the rate of electrolyte absorption in vitro, to alter intestinal motility in vivo, and to have antidiarrheal effects in animals. Experiments were performed in 8 healthy volunteers in order to evaluate the antidiarrheal effect of clonidine in humans. When diarrhea was induced by intragastric infusion of 2700 ml of balanced electrolyte solution over 90 min, oral administration of 0.3 mg of clonidine reduced the volume of rectal effluent by 48% (from 1233 +/- 62 to 640 +/- 77 ml, p less than 0.001), a clear-cut antidiarrheal effect. Clonidine increased total gut volume significantly (from 987 +/- 91 to 1830 +/- 142 ml, p less than 0.001), suggesting that clonidine exerted its antidiarrheal effect by altering gut motility, i.e., increasing the capacity of the gut and slowing the transit of fluid through the intestine. In other experiments, the net absorption rate of the whole gut during steady state total gut perfusion was measured. The rate of absorption of fluid was transiently stimulated by clonidine by 15% (from 696 +/- 77 to 799 +/- 55 ml/h, p less than 0.02), indicating an additional effect on mucosal cell function. These studies indicate that in this experimental diarrhea model, clonidine has antidiarrheal properties that are due largely to effects on motility of the gut but that clonidine also modestly stimulates the net rate of absorption by intestinal mucosa.

Gas production after reaction of sodium bicarbonate and hydrochloric acid.

Ingestion of sodium bicarbonate has been implicated as one of the proximate causes of spontaneous gastric rupture. However, the volume and rate of gas released from the reaction of ingested sodium bicarbonate and gastric acid has not been previously studied in detail. We, therefore, developed an in vitro method for measuring gas release after addition of sodium bicarbonate to a solution containing hydrochloric acid. From the results of our studies, we conclude that even though hydrochloric acid and sodium bicarbonate react instantaneously, the resulting gas production is slow, mainly because CO2 produced from the dehydration of carbonic acid dissolves in water and is only slowly released into the gas phase. The major exogenous factors that determine the rate of gas release are the volume of the solution, the quantity of reactants, the air volume over the reaction mixture, the partial pressure of CO2 of the acid solution before the addition of bicarbonate, and the stirring rate. The presence of food, alcohol, and carbonic anhydrase had relatively little if any effect. Based on our results, we believe that ingestion of the recommended dose of sodium bicarbonate (one-half teaspoon) would result in only small amounts of sudden gas release, probably not enough to be an important factor in causing spontaneous gastric rupture. On the other hand, we measured the amount of sodium bicarbonate that people actually select to take for indigestion, and all exceeded the recommended dose. Some people selected doses of bicarbonate that would result in several hundred milliliters of gas release within 3 min; it seems likely that such injudicious ingestion of sodium bicarbonate, if taken when the stomach was distended with air, food, and liquid, could be an important factor in spontaneous gastric rupture.

Mechanism of the antidiarrheal effect of loperamide.

To determine whether the antidiarrheal effect of loperamide is due to an effect on intestinal motor function or to an acceleration of the rate of absorption by the intestine (as has been suggested recently), we studied absorption during experimental diarrhea produced by the rapid intragastric infusion of electrolyte solution. In studies in which a 2700-ml bolus of electrolyte solution was infused into the stomach over 90 min, loperamide delayed the appearance of rectal effluent in each of 5 subjects and decreased the volume of rectal effluent from 1090 +/- 118 to 770 +/- 73 ml (p = 0.05). When intragastric infusion was continued for 5 h, producing steady-state total gut perfusion, the volume of effluent produced per unit time and the concentration of a nonabsorbable polyethylene glycol marker in rectal effluent was not different with or without loperamide, indicating that loperamide did not alter the rate of absorption by intestinal mucosal cells. Loperamide also had no effect during steady-state perfusion when absorption rates were reduced by intravenous infusion of vasoactive intestinal polypeptide. Loperamide did substantially increase the intraluminal volume of the total gut, from 985 +/- 131 to 1764 +/- 195 ml (p less than 0.02). These results suggest that loperamide exerts its antidiarrheal effect by a change in the motor function of the intestine, which results in increased capacitance of the gut and a delay in the passage of fluid through the intestine. This change in motor function, rather than a change in the rate of absorption by intestinal mucosal cells, is responsible for the antidiarrheal effect of loperamide in our experimental diarrhea model.

An evaluation of the importance of gastric acid secretion in the absorption of dietary calcium.

Since calcium solubility is a prerequisite to calcium absorption, and since solubility of calcium is highly pH-dependent, it has been generally assumed that gastric acid secretion and gastric acidity play an important role in the intestinal absorption of calcium from ingested food or calcium salts such as CaCO3. To evaluate this hypothesis, we developed a method wherein net gastrointestinal absorption of calcium can be measured after ingestion of a single meal. A large dose of cimetidine, which markedly reduced gastric acid secretion, had no effect on calcium absorption in normal subjects, and an achlorhydric patient with pernicious anemia absorbed calcium normally. This was true regardless of the major source of dietary calcium (i.e., milk, insoluble calcium carbonate, or soluble calcium citrate). Moreover, calcium absorption after CaCO3 ingestion was the same when intragastric contents were maintained at pH 7.4 (by in vivo titration) as when intragastric pH was 3.0. On the basis of these results, we conclude that gastric acid secretion and gastric acidity do not normally play a role in the absorption of dietary calcium. Other possible mechanisms by which the gastrointestinal tract might solubilize ingested calcium complexes and salts are discussed.

Purging and calorie absorption in bulimic patients and normal women.

Self-induced purging with laxatives is common among bulimic persons, who assume that purging reduces intestinal absorption of ingested calories. However, the efficacy of purging in reducing calorie absorption has never been studied, probably because the standard calorie balance procedure is expensive and time consuming. With a recently devised method, calorie absorption during a single day was measured to determine to what extent phenolphthalein or saline purge reduced calorie absorption. In two bulimic patients who regularly used laxatives for weight control and five normal young women, even extreme purging producing 4 to 6 L of diarrhea caused calorie absorption to decrease by only about 12% of calorie intake. The theoretical basis on which laxatives are taken for weight control is unsound.

Evaluation of chloride/bicarbonate. Exchange in the human colon in vivo.

During perfusion of a plasma-like solution, colonic absorption rate of chloride was much higher than the secretion rate of bicarbonate (34 vs. 3.5 meq/h, respectively). This might suggest that anion exchange (Cl/HCO3) accounts for only a small fraction of total chloride absorption. However, if the colon absorbs as well as secretes bicarbonate, this reasoning would underestimate the magnitude of the anion exchange. To see if the colon absorbs bicarbonate, we perfused a chloride-free solution (which would eliminate bicarbonate secretion via (Cl/HCO3 exchange) and found that the colon absorbed bicarbonate at a rate of 5.1 meq/h. Calculation of electrochemical gradients and measurement of luminal fluid PCO2 indicated that this bicarbonate absorption was mediated passively in response to electrical gradients, rather than via reversed Cl/HCO3 exchange or acid secretion. The combined results of the plasma-like and chloride-free perfusion experiments suggest Cl/HCO3 exchange at a rate of 8.6 meq/h (the sum of bicarbonate movements, 3.5 and 5.1 meq/h, observed in the two experiments). To obtain a second estimate under different experimental conditions, a choline chloride-choline bicarbonate (sodium-free) solution was perfused; with this solution, chloride and bicarbonate absorption dependent on active sodium transport should be eliminated or markedly reduced, and the magnitude of Cl/HCO3 exchange should be revealed. This experiment suggested a Cl/HCO3 exchange rate of 9.3 meq/h, similar to the first estimate. As chloride was absorbed at a rate of 34 meq/h during perfusion of the plasma-like solution, the Cl/HCO3 exchange provides for approximately one-fourth of total chloride absorption.

Lithium absorption in tight and leaky segments of intestine.

There is significant absorption of Li+ by human jejunum and ileum, but negligible absorption by human colon. Thus, a proximal-to-distal gradient of decreasing Li+ absorption and increasing junctional tightness exists in intestine as well as in renal tubule. For six leaky epithelia the relative permeabilities of K+, Na+, and Li+ by the junctional route are in the sequence PK greater than PNa greater than PLi and all fall within a factor of 2.5. In contrast, for tight epithelia PLi approximately PNa much greater than PK in the amiloride-sensitive channel of the apical membrane, but PK much greater than PLi approximately PNa in the basolateral membrane. The ability of several tight epithelia to sustain nonzero transepithelial Li+ absorption despite this basolateral barrier may be due to Na+/Li+ countertransport at the basolateral membrane, resulting in secondary active transport of Li+ across the epithelium.

Starch blockers--their effect on calorie absorption from a high-starch meal.

It has been known for more than 25 years that certain plant foods, such as kidney beans and wheat, contain a substance that inhibits the activity of salivary and pancreatic amylase. More recently, this antiamylase has been purified and marketed for use in weight control under the generic name "starch blockers." Although this approach to weight control is highly popular, it has never been shown whether starch-blocker tablets actually reduce the absorption of calories from starch. Using a one-day calorie-balance technique and a high-starch (100 g) meal (spaghetti, tomato sauce, and bread), we measured the excretion of fecal calories after normal subjects had taken either placebo or starch-blocker tablets. If the starch-blocker tablets had prevented the digestion of starch, fecal calorie excretion should have increased by 400 kcal. However, fecal calorie excretion was the same on the two test days (mean +/- S.E.M., 80 +/- 4 as compared with 78 +/- 2). We conclude that starch-blocker tablets do not inhibit the digestion and absorption of starch calories in human beings.

Studies of the mechanism of the antidiarrheal effect of codeine.

To determine whether the antidiarrheal action of opiate drugs in humans is due to enhanced intestinal absorption rates, as suggested by recent experiments in animals, or is due to altered intestinal motility, as traditionally thought, we studied the effect of therapeutic doses of codeine on experimental diarrhea and on the rate of intestinal absorption of water and electrolytes in normal human subjects. Our results show that codeine (30-60 mg i.m.) markedly reduced stool volume during experimental diarrhea induced by rapid intragastric infusion of a balanced electrolyte solution. There was, however, no evidence that codeine stimulated the rate of intestinal absorption in the gut as a whole or in any segment of the gastrointestinal tract, either in the basal state or when absorption rates were reduced by intravenous infusion of vasoactive intestinal polypeptide. We also measured segmental transit times to determine whether and where codeine delayed the passage of fluid through the intestine. Codeine caused a marked slowing of fluid movement through the jejunum, but had no effect on the movement of fluid through the ileum or colon. In other studies, we found that the opiate antagonist naloxone did not significantly affect water or electrolyte absorption rates in the jejunum or ileum. We conclude (a) that therapeutic doses of codeine increase net intestinal absorption (and thereby reduce stool volume) by increasing the contact time of luminal fluid with mucosal cells, not by increasing the rate of absorption by the mucosal cells; and (b) that endogenous opiates do not regulate intestinal absorption in humans.

Permeability characteristics of human jejunum, ileum, proximal colon and distal colon: results of potential difference measurements and unidirectional fluxes.

In order to assess the passive permeability characteristics of the human intestine in vivo, we measured potential difference in the jejunum, ileum, proximal colon, and distal colon during perfusion of various test solutions that were designed to establish chemical gradients for sodium or chloride, or both or neither. In addition, unidirectional fluxes of sodium and chloride were measured in 30-cm segments of the jejunum and ileum and entire colon during perfusion of balanced electrolyte solution. These studies indicate that there are marked differences in the pathways for passive ion movement in the areas of the intestine studied. In the jejunum, this pathway appears to be highly permeable to both sodium and chloride with modest cation selectivity. In the ileum this pathway is much more cation selective, predominantly because of a relative impermeability to chloride. In the colon, on the other hand, these passive pathways appear to be more anion than cation selective. The implication of these results for normal transport physiology are discussed.

Effect of vasoactive intestinal polypeptide on active and passive transport in the human jejunum.

The effect of intravenous vasoactive intestinal polypeptide (VIP) on normal transport mechanisms in the human jejunum in vivo was examined with the triple-lumen, steady-state perfusion technique. By using special test solutions that revealed different aspects of jejunal transport, we were able to evaluate the effect of VIP on specific transport processes, such as active bicarbonate absorption, active chloride secretion, and passive absorption or secretion of sodium chloride. At an infusion rate of 200 pmol/kg per h, VIP inhibited active bicarbonate absorption by approximately 42%, stimulated active chloride secretion to a slight extent, and slightly reduced passive sodium chloride absorption. A larger dose of VIP, 400 pmol/kg per h, had essentially the same effect on active bicarbonate absorption and active chloride secretion, but it markedly depressed passive sodium chloride absorption and also inhibited passive secretion induced by mannitol. VIP reduced the lumen-to-plasma unidirectional sodium and chloride flux rates, while the plasma-to-lumen flux rates were decreased to a lesser extent or remained unchanged. The potential difference became more lumen-negative with VIP, but the sodium diffusion and glucose-stimulated potential were not affected. We conclude that the major effect of VIP in the human jejunum is to decrease the normal absorption of water and electrolytes--not only active bicarbonate-mediated absorption, but also the passive absorption in response to osmotic forces generated by active or facilitated absorptive processes. Although an increase in chloride secretion does occur, this does not appear to be of major importance.

Inhibition of water and electrolyte absorption by polyethylene glycol (PEG).

Polyethylene glycol (PEG) is commonly used as a nonabsorbable volume marker in intestinal perfusion and flow studies. It has been assumed that PEG does not affect water and electrolyte movement, but this has not been extensively investigated. Using triple-lumen tube perfusion technique, we examined the effect of various PEG concentrations (0, 2, 5, 10, and 20 g/liter) on water and electrolyte absorption by the jejunum and ileum in normal subjects. 14C-labeled PEG served as the nonabsorbable marker in the 0 PEG concentration solution. There was a progressive reduction in water, sodium, and chloride absorption as the concentrations of PEG was increased from 0 to 20 g/liter. Though further studies are necessary to establish the mechanism responsible for this PEG effect, the observed changes in luminal fluid osmolality and electrolyte concentrations suggest that the reduction in absorption most likely results from an osmotic effect rather than an inhibition of active absorption or stimulation of secretion.

Development of a lavage solution associated with minimal water and electrolyte absorption or secretion.

Ingestion of large volumes of a balanced electrolyte solution has previously been shown to be an effective method of cleaning the colon for diagnostic studies. However, in this paper we have shown that total gut perfusion with such a solution results in absorption of 2400 ml water and 375 meq of sodium over 3 hr, which is the approximate time required to clean the colon by this technique. This might be hazardous to patients who are unable to readily excrete a salt and water load. We, therefore, designed a solution containing mainly sodium sulfate that was associated with only trivial amounts of water and sodium absorption or secretion during total gut perfusion. This new solution might be useful in colon cleansing before colonoscopy, barium enema, and surgery. In addition, such a solution may have some therapeutic indications, including bowel cleaning in patients with hepatic encephalopathy or as a rapid washout technique for ingested toxins.

Chronic diarrhea of unknown origin.

We studied 27 patients with severe chronic diarrhea for which extensive investigations carried out at other institutions had failed to reveal a diagnosis. They were studied by standard diagnostic methods as well as by careful fecal analysis and intestinal perfusion. If they were incontinent of feces, anal sphincter function tests were performed. Although many were suspected of having pancreatic cholera syndrome, this diagnosis could not be established in a single patient. The most common diagnosis that could be established was surreptitious ingestion of drugs (laxatives in 7 patients and diuretics in 2). Other specific diagnoses included ulcerative colitis in 2 patients, allergy to beef in 1, and bacterial overgrowth of the small intestine in 1. Thus, we were able to establish a specific diagnosis in 13 patients. Of the remaining 14 patients, 8 had findings suggestive of irritable bowel syndrome, and 2 others had anal sphincter dysfunction as the major cause of their disability. The other 4 undiagnosed patients had severe secretory (3 patients) or osmotic (1 patient) diarrhea. Follow-up interviews at 6 mo-6 yr failed to reveal evidence of a cause for diarrhea that had been overlooked during our studies. The diagnostic approach to patients with unexplained diarrhea is discussed. The importance of a search for surreptitious drug ingestion and accurate measurement of bowel movement frequency and stool weight is emphasized.