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

Regional characterisation of TRPV1 and TRPA1 signalling in the mouse colon mucosa

Capsaicin and allyl isothiocyanate (AITC) activate transient receptor potential (TRP) vanilloid-1 (TRPV1) and TRP ankyrin-1 (TRPA1), respectively. TRPV1 and TRPA1 expression have been identified in the gastrointestinal (GI) tract. GI mucosal functions remain largely undefined for TRPV1 and TRPA1 with side-dependence and regional differences in signalling unclear. Here we investigated TRPV1- and TRPA1-induced vectorial ion transport as changes in short-circuit current (ΔIsc), in defined segments of mouse colon mucosa (ascending, transverse and descending) under voltage-clamp conditions in Ussing chambers. Drugs were applied basolaterally (bl) or apically (ap). Capsaicin responses were biphasic, with primary secretory and secondary anti-secretory phases, observed with bl application only, which predominated in descending colon. AITC responses were monophasic and secretory, with ΔIsc dependent on colonic region (ascending vs. descending) and sidedness (bl vs. ap). Aprepitant (neurokinin-1 (NK1) antagonist, bl) and tetrodotoxin (Na+ channel blocker, bl) significantly inhibited capsaicin primary responses in descending colon, while GW627368 (EP4 receptor antagonist, bl) and piroxicam (cyclooxygenase inhibitor, bl) inhibited AITC responses in ascending and descending colonic mucosae. Antagonism of the calcitonin gene-related peptide (CGRP) receptor had no effect on mucosal TRPV1 signalling, while tetrodotoxin and antagonists of the 5-hydroxytryptamine-3 and 4 receptors, CGRP receptor, and EP1/2/3 receptors had no effect on mucosal TRPA1 signalling. Our data demonstrates the regional-specificity and side-dependence of colonic TRPV1 and TRPA1 signalling, with involvement of submucosal neurons and mediation by epithelial NK1 receptor activation for TRPV1, and endogenous prostaglandins and EP4 receptor activation for TRPA1 mucosal responses.

Comparative Study of Three Regimens of Bowel Preparation Before Transabdominal Ultrasonography of the Colon

The objective of the study was to compare the efficacy of three bowel preparation regimens for transabdominal colon ultrasonography. A total of 192 consecutive patients were given one of three regimens (senna, magnesium sulfate or polyethylene glycol electrolyte powder) before ultrasonographic examinations. The cleaning grade (I = emptying; II = filled or filled + empty; III = I or II with some retention; and IV = retention [grades I and II were termed "qualified"]) and cleaning range (A = all seven colon sections were qualified; B = four to six sections were qualified; C = three or less sections were qualified) were evaluated retrospectively. Senna was found more effective than polyethylene glycol in terms of cleaning grade (p < 0.001), qualified rate (p < 0.001) and cleaning range (p = 0.003). Senna was better than magnesium sulfate in cleaning grade (p < 0.001). Our results suggest that senna seems to be the preferred regimen for bowel preparation before transabdominal colonic ultrasonography.

Mechanisms of lysophosphatidic acid (LPA) mediated stimulation of intestinal apical Cl-/OH- exchange

Lysophosphatidic acid (LPA), a potent bioactive phospholipid, is a natural component of food products like soy and egg yolk. LPA modulates a number of epithelial functions and has been shown to inhibit cholera toxin-induced diarrhea. Antidiarrheal effects of LPA are known to be mediated by inhibiting chloride secretion. However, the effects of LPA on chloride absorption in the mammalian intestine are not known. The present studies examined the effects of LPA on apical Cl(-)/OH(-) exchangers known to be involved in chloride absorption in intestinal epithelial cells. Caco-2 cells were treated with LPA, and Cl(-)/OH(-) exchange activity was measured as DIDS-sensitive (36)Cl(-) uptake. Cell surface biotinylation studies were performed to evaluate the effect of LPA on cell surface levels of apical Cl(-)/OH(-) exchangers, downregulated in adenoma (DRA) (SLC26A3), and putative anion transporter-1 (SLC26A6). Treatment of Caco-2 cells with LPA (100 muM) significantly stimulated Cl(-)/OH(-) exchange activity. Specific agonist for LPA2 receptor mimicked the effects of LPA. LPA-mediated stimulation of Cl(-)/OH(-) exchange activity was dependent on activation of phosphatidylinositol 3-kinase/Akt signaling pathway. Consistent with the functional activity, LPA treatment resulted in increased levels of DRA on the apical membrane. Our results demonstrate that LPA stimulates apical Cl(-)/OH(-) exchange activity and surface levels of DRA in intestinal epithelial cells. This increase in Cl(-)/OH(-) exchange may contribute to the antidiarrheal effects of LPA.

Segmental difference of water and electrolyte transport in rat colon in vivo

The segmental difference of water and electrolyte transport in the rat colon was studied in vivo. The proximal and distal colon segments were perfused separately but simultaneously at a constant rate with physiological solution, and net movements of water, sodium and chloride were determined. The effects of osmolality and sodium concentration of perfusate were assessed. The effect of a sodium channel blocker on the net transport of water and electrolytes was also studied in each colon segment. The net absorption of water, sodium and chloride correlated with the sodium concentration and osmolality of the perfusion solution in both colon segments and were dominant in the distal colon segment in each condition, compared with that in the proximal colon segment. The concentrations of three electrolytes in the collected fluid were almost the same as those of the perfusion solutions in both segments and these results indicated that water was transported isotonically through the colon lumen. Benzamil, a specific sodium ion channel blocker, inhibited net water and sodium absorption by 58.8% and 63.1% in the proximal colon segment and by 52.0% and 43.6% in the distal colon segment, respectively. These results suggest the existence of an electrogenic sodium transport mechanism and a paracellular pathway in normal (i.e., not treated with corticosteroids or sodium-depleted food) rats which has not been detected in in vitro studies with both apical and basolateral membrane vesicles.

Chloride transport in human proximal colonic apical membrane vesicles

The mechanism(s) of Cl- transport across the human colonic apical membranes are not well understood. Apical membrane vesicles (AMV) purified from organ donor proximal colonic mucosa and a rapid millipore filtration technique were utilized to study 36Cl- uptake into these vesicles. Outwardly directed OH- and HCO3- gradient stimulated 36Cl- uptake into these vesicles demonstrating a transient accumulation over equilibrium uptake. Voltage clamping the membrane potential of the vesicles or making them inside positive with K+/valinomycin failed to influence chloride uptake, indicating that the conductive Cl- uptake pathway is minimal in proximal colonic AMV. Anion exchange inhibitors, DIDS and SITS (1 mM) inhibited OH- and HCO3- stimulated 36Cl- uptake by approximately 60%. Furosemide also demonstrated a small but significant inhibition of chloride uptake. Amiloride, bumetanide and acetazolamide (1 mM) failed to inhibit 36Cl uptake. HCO3- and pH gradient stimulated 36Cl- uptake exhibited saturation kinetics with an apparent Km for chloride of 4.0 +/- 0.7 mM and Vmax of 17.8 +/- 3.9 nmol/mg per min. Bromide, chloride, nitrate and acetate (50 mM each) inhibited 5 mM 36Cl uptake. Inwardly directed gradients of Na+, K+, or Na+ and K+ did not stimulate 36Cl- uptake into these vesicles, indicating that uptake of Na+ and Cl- in human proximal colonic AMV does not involve Na-Cl or Na-K-2Cl cotransport. The above findings indicate that chloride transport in human proximal colonic AMV involves an electroneutral Cl-HCO3- (OH-) exchange process. In view of the previous demonstration of Na+-H+ antiporter in these vesicles, dual ion exchange mechanism of Na+-H+ and Cl-HCO3- in apical membrane domain of human colonocytes is postulated to be the primary mechanism for NaCl absorption in the human proximal colon.

Stimulation of butyrate absorption in the human rectum in vivo

BACKGROUND

Models of short-chain fatty acid absorption have focused on the stimulation of sodium absorption, an effect mainly located in the proximal colon of man. With the present efforts to utilize butyrate enemas as a treatment of ulcerative colitis, it seemed important to assess the transport in the rectum.

METHODS

Non-equilibrium dialysis of the rectum was applied by placing dialysis bags containing various electrolyte solutions in the rectum of volunteers for 30 min. In this period changes in ion concentrations were linear with time. Net absorption and secretion rates were calculated from the change in fluid composition.

RESULTS

Sodium absorption was highest (24 +/- 8 mumol/cm2 h) in the presence of chloride and lowest (16 +/- 2 mumol/cm2 h) in the presence of bicarbonate and butyrate. Butyrate (70 mmol/l) was absorbed at a high rate of 7.1 +/- 2.2 mumol/cm2 h, independent on the presence of chloride, and was accompanied by increased bicarbonate secretion. Butyrate absorption increased to 9.6 +/- 1.8 mumol/cm2 h in sodium-free high-potassium media containing bicarbonate.

CONCLUSION

The results show that it is possible to increase butyrate uptake by manipulation of the electrolyte composition in the rectal lumen. Maximal uptake occurred with an electrolyte composition that was similar to the natural rectal content. The information gathered could be useful in designing enemas for trial in ulcerative colitis, provided the findings can be confirmed in these patients.

Pathophysiology of potassium absorption and secretion by the human intestine

When normal people ingest 90 mEq/day of K+ in their diet, they absorb about 90% of intake (81 mEq) and excrete an equivalent amount of K+ in the urine. Normal fecal K+ excretion averages about 9 mEq/day. The vast majority of intestinal K+ absorption occurs in the small intestine; the contribution of the normal colon to net K+ absorption and secretion is trivial. K+ is absorbed or secreted mainly by passive mechanisms; the rectum and perhaps the sigmoid colon have the capacity to actively secrete K+, but the quantitative and physiological significance of this active secretion is uncertain. Hyperaldosteronism increases fecal K+ excretion by about 3 mEq/day in people with otherwise normal intestinal tracts. Cation exchange resin by mouth can increase fecal K+ excretion to 40 mEq/day. The absorptive mechanisms of K+ are not disturbed by diarrhea per se, but fecal K+ losses are increased in diarrheal diseases by unabsorbed anions (which obligate K+), by electrochemical gradients secondary to active chloride secretion, and probably by secondary hyperaldosteronism. In diarrhea, total body K+ can be reduced by two mechanisms: loss of muscle mass because of malnutrition and reduced net absorption of K+; only the latter causes hypokalemia. Balance studies in patients with diarrhea are exceedingly rare, but available data emphasize an important role for dietary K+ intake, renal K+ excretion, and fecal K+ losses in determining whether or not a patient develops hypokalemia. The paradoxical negative K+ balance induced by ureterosigmoid anastomosis is described. The concept that fecal K+ excretion is markedly elevated in patients with uremia as an intestinal adaptation to prevent hyperkalemia is analyzed; we conclude that the data do not convincingly show the existence of a major intestinal adaptive response to chronic renal failure.

Mechanisms of Na+ transport in human distal colonic apical membrane vesicles

Apical membrane vesicles purified from mucosal scrapings obtained from distal segments of organ donor colons and a 22Na-uptake technique were used to characterize the mechanism(s) of Na+ transport into these vesicles. An outwardly directed H+ gradient (pH 5.5in/7.5out) markedly increased uptake of 22Na into these vesicles. Osmolarity studies demonstrated that 22Na was taken up into the intravesicular space with minimal binding observed to the surface of the vesicles. Voltage clamping in the presence of K+/valinomycin reduced the H+ gradient-dependent 22Na uptake into these vesicles by approximately 45% and generation of an inside negative membrane potential significantly increased 22Na uptake. Under non voltage clamped conditions, H+ gradient-dependent 22Na uptake into these vesicles was significantly inhibited by specific inhibitors of Na(+)-H+ exchange (DMA, HMA and EIPA) as well as by inhibitor of epithelial Na+ channels (phenamil). Under voltage clamped conditions, H+ gradient-dependent 22Na uptake, however, was unaffected by phenamil (20 microM), but was almost completely inhibited by DMA, HMA and EIPA (20 microM each). The mechanism of amiloride inhibition of electroneutral Na(+)-H+ exchange was noncompetitive with a Ki for amiloride of 340 microM. Electroneutral 22Na uptake exhibited saturation kinetics with an apparent Km for Na+ of 8.7 +/- 1.7 mM and a Vmax of 2.02 +/- 0.45 nmol/mg per 5 s. The Na(+)-H+ exchange demonstrated cation specificity similar to the Na(+)-H+ exchangers described in other epithelia. These studies demonstrate for the first time that Na+ transport across the apical membranes of human distal colon involves both conductive Na+ uptake and an electroneutral Na(+)-H+ exchange process.

Na+ transport in human proximal colonic apical membrane vesicles

BACKGROUND/AIMS

The mechanisms of Na+ movement across colonocyte plasma membranes in the human colon are not well understood. Current studies were undertaken to investigate Na+ transport pathways in apical membranes of proximal organ donor colons.

METHODS

Purified apical membrane vesicles and rapid filtration 22Na-uptake techniques were used.

RESULTS

An outwardly directed H(+)-gradient (pH 5.5 in/7.5 out) increased 22Na uptake into these vesicles. H+ gradient-driven 22Na uptake was significantly reduced by voltage clamping with K+/valinomycin, but was significantly stimulated by creation of an inside-negative potential. Potential sensitive 22Na uptake was inhibited by Na+ channel inhibitors phenamil and benzamil. Electroneutral 22Na uptake was insensitive to phenamil and benzamil, but was inhibited by amiloride, 5-(N,N-dimethyl)amiloride, 5-(N,N-hexamethylene)amiloride, and 5-(N-ethyl-N-isopropyl)amiloride. Electroneutral 22Na uptake showed saturation kinetics with an apparent Michaelis constant for Na+ of 11.8 +/- 2.4 mmol/L and a maximal velocity of 2.5 +/- 0.6 nmol.mg protein-1 x 5 s-1. The mechanism of amiloride inhibition was noncompetitive with an inhibitor constant for amiloride of 325 mumol/L. Acetazolamide, furosemide, bumetanide, 4-acetamido-4'-isothiocyano-2,2'-disulfonic acid stibene, and 4,4'-di-isothiocyanatostilbene-2,2'-disulfonic acid (1 mmol/L each) failed to inhibit 22Na uptake. Li+ and NH4+ (but not Cs+, K+, or choline+) inhibited H(+)-gradient driven 22Na uptake.

CONCLUSIONS

Na+ transport in human proximal colonic apical membrane vesicles involves both conductive Na+ transport and an electroneutral Na(+)-H+ exchange.

Apical plasma membrane vesicles formed from organ donor colon demonstrate Na+ and H+ conductances and Na+/H+ exchange

Apical plasma membrane vesicles were prepared from human organ donor colon mucosal scrapings. These vesicles were enriched 10-fold in cysteine-sensitive alkaline phosphatase activity compared to starting homogenates, and showed minimal contamination of microsomal, mitochondrial or basolateral membranes. Transport studies using [22Na] uptake into proximal colonic vesicles demonstrated Na+ and H+ conductances, Na+/H+ exchange and amiloride inhibition of Na+ uptake. The isolation of these apical vesicles will permit detailed study of human colonic transport processes.