Electrical potential and short circuit current of an in vitro preparation of rat colon mucosa

The effect of acute hypoxia on short-circuit current and epithelial resistivity in biopsies from human colon

BACKGROUND AND AIMS

In isolated colonic mucosa, decreases in short-circuit current (ISC) and transepithelial resistivity (RTE) occur when hypoxia is either induced at both sides or only at the serosal side of the epithelium. We assessed in human colon biopsies the sensitivity to serosal-only hypoxia and mucosal-only hypoxia and whether Na, K-ATPase blockade with ouabain interacts with hypoxia.

MATERIALS AND METHODS

Biopsy material from patients undergoing colonoscopy was mounted in an Ussing chamber for small samples (1-mm2 window). In a series of experiments we assessed viability and the electrical response to the mucolytic, dithiothreitol (1 mmol/l). In a second series, we explored the effect of hypoxia without and with ouabain. In a third series, we evaluated the response to a cycle of hypoxia and reoxygenation induced at the serosal or mucosal side while keeping the oxygenation of the opposite side.

RESULTS

1st series: Dithiothreitol significantly decreased the unstirred layer and ISC but increased RTE. 2nd series: Both hypoxia and ouabain decreased ISC, but ouabain increased RTE and this effect on RTE prevailed even during hypoxia. 3rd series: Mucosal hypoxia caused lesser decreases of ISC and RTE than serosal hypoxia; in the former, but not in the latter, recovery was complete upon reoxygenation.

CONCLUSIONS

In mucolytic concentration, dithiothreitol modifies ISC and RTE. Oxygen supply from the serosal side is more important to sustain ISC and RTE in biopsy samples. The different effect of hypoxia and Na, K-ATPase blockade on RTE suggests that their depressing effect on ISC involves different mechanisms.

Electrogenic transport, oxygen consumption, and sensitivity to acute hypoxia of human colonic epithelium

INTRODUCTION

It is recognized that epithelial ion transport depends on oxygen supply, but this dependence has not been characterized in the human colon in vitro despite its surgical and clinical implications.

PURPOSES

The aim of this study is to measure the oxygen consumption of colonic epithelium under conditions which preserve vectorial ion transport and to assess the sensitivity of the human colonic epithelium short-circuit current (I (sc)) to acute hypoxia induced in vitro.

METHODS

Isolated mucosa preparations from human sigmoid colon were placed in a modified Ussing chamber which allows simultaneous measurement of short-circuit current (I (sc)) and oxygen consumption (QO(2)). In separate experiments, the sensitivity to acute hypoxia induced in a conventional Ussing chamber was assessed.

RESULTS

Basal mean ± SEM values (n = 8) were I (sc) = 3.3 ± 0.5 μEq h(-1) cm(-2) and QO(2) = 8.09 ± 0.55 μmol h(-1) cm(-2). The contribution of the serosal side to the oxygen supply was higher than that of the mucosal side (p = 0.0023). Ouabain reduced I (sc) by 70% (P < 0.0001) and QO(2) by 26% (n = 8; P = 0.0009), suggesting that a large fraction of QO(2) is needed to support ouabain-sensitive electrogenic transport. Induction of hypoxia at both sides of the Ussing chamber caused a rapid decrease in I (sc) after 2 min. I (sc) was also significantly depressed when hypoxia was induced by 5 min in the serosal side (n = 6, P < 0.0001), but was unaffected by hypoxia induced in the mucosal side.

CONCLUSION

The present results allow a better understanding of the clinical consequences of acute hypoxia on intestinal ion transport.

Effects of chronic hypoxia on electrogenic transport and transport-related oxygen consumption in rat distal colon

The distal colon epithelium of rats submitted to chronic hypoxia shows higher short-circuit current (Isc) which, unlike non-hypoxic rat epithelium, has an amiloride-sensitive component despite low serum aldosterone levels. Isc and oxygen consumption (QO2) were simultaneously measured in mucosae from rats submitted to 0.5 atm for 10 days and from control rats in a modified Ussing chamber. Hypoxia increased Isc but not QO2. The slope of the regression line between Isc and QO2 reduction after ouabain addition was decreased in epithelia from hypoxic rats (P = 0.03). Chloride secretion blockade reduced Isc and QO2 in both groups, while sodium channel blockade did so only in the hypoxic group. Dual blockade in hypoxic rat epithelia caused correlated (P = 0.0025) additive decreases in Isc and QO2. Presented results suggest that chronic hypoxia induces an improved coupling between QO2 and electrogenic ion transport, and electrogenic sodium absorption despite low aldosterone levels.

Recent understanding of IBD pathogenesis: implications for future therapies

The inflammatory bowel diseases (IBD) are comprised of two major phenotypes, Crohn's disease (CD) and ulcerative colitis (UC). Research over the last couple of years has led to great advances in understanding the inflammatory bowel diseases and their underlying pathophysiologic mechanisms. From the current understanding, it is likely that chronic inflammation in IBD is due to aggressive cellular immune responses to a subset of luminal bacteria. Susceptibility to disease is thereby determined by genes encoding immune responses which are triggered by environmental stimuli. Based on extensive research over the last decade, there are several new and novel pathways and specific targets on which to focus new therapeutics. The following review summarizes the current view on the four basic tenets of the pathophysiological basis of IBD and its implications for therapies of IBD: genetics, immune dysregulation, barrier dysfunction and the role of the microbial flora.

Chronic hypobaric hypoxia effects on rat colon in vitro sensitivity to acute hypoxia and amiloride

Chronic hypoxia induces many physiological changes, but little is known about its effects on colonic epithelial function. Isolated distal colon mucosa from rats under normobaric conditions and rats submitted to hypobaric hypoxia for either 4 or 10 days was studied in an Ussing chamber. After 4 days of hypoxia, there was only a 15% increase in transepithelial resistivity. However, 10-day hypoxic rats showed higher short circuit current, potential difference, and resistivity. In this group, but not in normal or 4-day hypoxic animals, amiloride dose-dependently depressed short circuit current. The response to acute hypoxia in vitro was unchanged after chronic hypoxia and was not affected by amiloride. Although the amiloride-sensitive increase in short circuit current in 10-day hypoxic rats might resemble mineralocorticoid action, resistivity was increased and serum aldosterone was very low. It is suggested that chronic hypoxia may enhance electrogenic sodium transport by an aldosterone-independent mechanism.

Epithelial and subepithelial resistance of rat large intestine: segmental differences, effect of stripping, time course, and action of aldosterone

Epithelial and subepithelial electrical resistances of rat large intestine were measured by means of a 4-electrode AC impedance technique in three segments, colon ascendens, colon descendens and rectum. Epithelial resistance of colon ascendens and colon descendens was about 35 omega X cm2 and not different between these two segments. It was, however, about 3 times higher in rectum (99 omega X cm2). This finding is in accord with our previous observation of about 3-fold higher net fluxes of ions and water in colon ascendens and colon descendens than in rectum. It confirms the concept of a main functional difference between the terminal part of the large intestine (rectum) and the more proximal segments (colon). The acutely (within hours) varied level of aldosterone by keeping the rats for 7 h in anaesthesia caused in the rectum a more than 10-fold increase in short circuit current (Isc) and transepithelial voltage but no significant decrease in resistance. Similarly, the decline in Isc, as regularly observed in the early phase of in vitro measurements on partially stripped large intestine, was paralleled by voltage changes but not by changes in resistance. We conclude that the wide range of resistance values published so far was caused to a great extent by including various portions of colon or rectum. By comparing intact (not stripped) and partially stripped preparations (muscularis propria removed) of the rectum it was shown that partial stripping did not alter the epithelial resistance but reduced the subepithelial resistance in this segment from 26 to 8 omega X cm2, or by 68%.(ABSTRACT TRUNCATED AT 250 WORDS)

Segmental differences in electrical properties and Na-transport of rabbit caecum, proximal and distal colon in vitro

Electrical potential difference, short circuit current, tissue conductance, and unidirectional Na-fluxes were measured in four segments of the rabbit large intestine in vitro. Compared to the relatively tight, low conductance distal colon, caecum and proximal parts of the colon are leaky epithelia with high conductances. Net Na-absorption was highest in caecum, and then decreased gradually towards the distal colon, whereas potential and short circuit current where high in the caecum and proximal colon, low in the middle part of the colon, and high again in the distal colon. Unidirectional Na-fluxes of all four segments were different. The discrepancy between the short circuit current and net Na-absorption in the two segments of the proximal colon indicates electrogenic transport of other ions. 0.1 mM ouabain virtually abolished short circuit current and Na-absorption in all segments, whereas 0.1 mM amiloride was not effective in the caecum and the proximal colonic parts. The present study focuses on the comparative aspects of Na-transport. It demonstrates the marked segmental heterogeneity of the basic electrical properties and suggests four different segmental organizations of large intestinal electrolyte transport.

Active secretion of bicarbonate by rabbit colon

A serosa-mucosa movement of bicarbonate against concentration gradient was detected at this intestinal level. This movement is partially dependent on oxidative phosphorylation since it is abolished by DNP administration, but as it is not affected by acetazolamide administration it can be supposed as dependent on carbonic anhydrase activity.

Segmental variations in the surface architecture of the normal rat colonic mucosa

A microdissection technique used previously in the small intestine has been adapted for the rat colon in order to determine morphometric data and to calculate surface parameters in various proximal and distal segments (caecum, ascending and descending colon, rectum) of normal colonic mucosa. From the caecum to the rectum there was a gradual significant decrease in the diameter of the middle portions of the crypts, in the number of crypts per unit area and the inner intestinal circumference, as well as in the total mucosal surface per mm intestinal length and the surface of all crypts per unit area. However, there was an increase in the surface of the lining epithelium between the crypts from the caecum to the rectum. These findings are of importance when interpreting alterations in colonic function in terms of segmental changes in the mucosal architecture.

Ion fluxes and electrical characteristics of the short-circuited rat colon in vitro

The unidirectional fluxes of sodium and chloride across stripped rat colon mucosa were measured and compared with the electrical characteristics of the tissue under voltage-clamped conditions. The relationship between the serosal-mucosal fluxes of the two ions and an imposed potential revealed that the serosal-mucosal flux of sodium was entirely paracellular, whereas there was also a transcellular component of the corresponding chloride flux. In the absence of sodium, the short-circuit current and net chloride flux were abolished; in the absence of chloride, the net sodium flux was reduced but not abolished, and the short-circuit current was unchanged. From an analysis of the effects of the inhibitors, amiloride, theophylline, acetazolamide, furosemide and piretanide, a plausible model was developed to explain the characteristics of these transports. It was proposed that both membranes possess Cl-/HCO-3 antiports, though their sensitivities to inhibitors were different. There is also a Na+/Cl- symport and an electrogenic sodium entry mechanism in the brushborder membrane.

Kinins stimulate net chloride secretion by the rat colon

1 Short circuit current (SCC), transepithelial conductance and ion fluxes were measured across the isolated descending colon of the rat in response to bradykinin or kallidin. 2 Kinins added to the serosal bath caused immediate increases in SCC but were ineffective when added to the mucosal bath. Increases in SCC were accompanied by significant increases in transepithelial conductance. Threshold kinin concentration was 0.5nM and maximal increases were seen at 50-100 nM. 3 A rat glandular kallikrein (7nM) or mellitin (2 microM) also increased SCC if added to the serosal bath. 4 Responses to kinins were unaffected by mucosal amiloride (100 micron) but attenuated or blocked by serosal frusemide (100 microM), indomethacin (1 microM) or mepacrine (50 microM). 5 Replacement of chloride ion in the serosal bath by gluconate and sulphate ions abolished responses to kinins which reappeared after chloride re-addition. 6 Measurement of 36Cl, 22Na and 86Rb fluxes across the tissue showed that the kinin-induced increase in SCC resulted principally from increased net chloride secretion. Effects upon 22Na or 86Rb flux were minimal and made no contribution to the current responses observed in this tissue. 7 The results prove that kinins stimulate net chloride secretion in the rat colon, most probably via a prostaglandin-dependent pathway.

The short-circuited everted sac of rat colon mucosa

A short-circuited preparation of everted rat colon sacs is described. The serosal current electrode is a AgAgCl wire. A cylindrical agar bridge or AgAgCl electrode may be employed on the mucosal side. Effects of Ag+ ions liberated from the electrodes on ion transport could not be demonstrated. Fluid and sodium are absorbed ad bicarbonate secreted. Potassium and chloride movements are not significantly different form zero. The preparation remains stable for at least 2 h. Sodium absorption is diminished by 50% and bicarbonate secretion abolished in the absence of glucose. In principle, similar ion transport properties were found as in Ussing-chamber preparations. The advantage of the everted sac is the capability of measuring net transport of fluid and electrolytes simultaneously and directly because of the large surface/inner volume ratio of the sac.

Gold colitis, therapy and confirmation of mucosal recovery by measurement of rectal potential difference

A 38-year-old man with rheumatoid arthritis developed colitis after a course of 320 mg sodium aurothiomalate. Recovery comparable with that reported with corticosteroids or chelating agents was achieved by fluid replacement alone. Mucosal recovery was confirmed by measurement of rectal potential differences.

Maternofetal electrical potential difference in conscious sheep: effect of fetal death or acidosis

Maternofetal electrical potential difference (pd) was measured in conscious pregnant ewes during the last 40 days of gestation. Recordings were made via catheters filled with saline solution and chronically implanted in at least two blood vessels on either side of the placenta. The pd ranged between -14 and 93 mV, mother positive. The mean potential declined from 54 +/- 8 mV (n = 5) immediately after operation to 28 +/- 7 mV after 48 hours and did not alter significantly thereafter. The pd increased with fetal acidosis induced by infusion of acid. The mean slope was 54 +/- 11 mV per pH unit (n = eight infusions). The pd rose when the fetus died, but was abolished by killing the ewe. The pd 1 day postoperatively fell with increasing gestational age (pd = 164 - 1.00 x days; p < 0.05, n = 41 sheep).

Oxalate transport across the isolated rat colon. A re-examination

A net absorption of oxalate and chloride was observed when isolated, short-circuited segments of rat colon were bathed by a calcium-containing buffer. Removal of calcium promoted a two-fold decrease in transmural resistance, while the net chloride flux was reduced and the net oxalate transport abolished. It was concluded that net oxalate absorption was not observed in previous studies (employing calcium-free buffers) because calcium is reqiured to maintain the integrity of the conductive pathways across colonic epithelia.

Calcium and inorganic phosphate transport in rat colon: dissociated response to 1,25-dihydroxyvitamin D3

In the small intestine, 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] stimulates both calcium (Ca) and inorganic phosphate (Pi) absorption. This is mediated through an increase in mucosal-to-serosal flux (Jms) whereas the serosal-to-mucosal flux (Jsm) remains unchanged. We now report that in rat proximal colon, 1,25(OH)(2)D(3) produces active Ca absorption without affecting Pi transport, and that this induced active Ca absorption is associated with alterations in kinetics of both Jms and Jsm so that both processes demonstrate saturable components. Vitamin D-deficient rats were given daily injections of solvent (-D) or 270 ng 1,25(OH)(2)D(3) (+D) for 3 d. (45)Ca and [(32)P]phosphate fluxes were measured employing the Ussing technique using a modified Krebs-Ringer-HCO(3) buffer ([Ca] 1.25, [Pi] 1.18, [glucose] 11 mM). In -D rats there was no net flux (Jnet) of either Ca or Pi. In +D rats net active Ca absorption was observed (-D = 3.3 nmol/cm(2) per h +/-3.4 (SEM); +D = 27.3 +/-3.8, n = 11, P < 0.001) whereas Pi transport was unchanged, i.e., still no Jnet. Pi Jms was not different from Pi Jsm measured at the following buffer [Pi]: 0.0118, 0.118, 1.18, and 2.36 mM. Ca saturation kinetics were estimated using buffer [Ca] from 0.0125 to 5.0 mM. Saturable processes were demonstrated for both Jms and Jsm. Jnet for Ca across colon from +D rats exhibited saturation at [Ca] > 3 mM, with an estimated V(max) of 44.0 nmol/cm(2) per h and a K(m) of 0.9 mM. This colonic model may provide a useful system for studying 1,25(OH)(2)D(3)-induced molecular events related to Ca but not Pi transport. The apparent action of 1,25(OH)(2)D(3) on Ca secretory process may furnish new insights into the mechanism of action of vitamin D.

A comparison of some extra-renal effects of spironolactone and canrenone

1 Measurement of changes in trans-mural rectal potential difference (t.m.r.p.d.) and plasma aldosterone levels have been used in a comparison of the extra-renal activities of spironolactone and its major metabolite canrenone. 2 A characteristic pressure artifact was observed during measurement of t.m.r.p.d. When pressure artifacts were eliminated, there was a log-linear relationship between increasing doses of intravenous aldosterone and maximum increase in t.m.r.p.d. 3 Pre-treatment for 5 days with spironolactone or canrenone produced a similar attenuation of the increase in t.m.r.p.d. produced by infused aldosterone, suggesting that canrenone is the active metabolite of spironolactone in the rectum. This is in contrast to the significantly greater renal activity of spironolactone that has been demonstrated after a similar treatment period. Neither antagonist treatment produced significant changes in pre-infusion plasma aldosterone concentrations. 4 The need for assay of the extra-renal activities of aldosterone antagonists in the assessment of their therapeutic potential is discussed.

Localization of the effect of acetylcholine in regulating intestinal ion transport

1. The location of the site involved in the secretory response of rat jejunum and colon to ACh was investigated by selectively damaging either the villi of the jejunum and the surface epithelium of the colon or the crypts. 2. The secretory response induced by ACh was measured both in terms of changes in electrical activity and chloride fluxes. 3. Exposure of the mucosa to 2 M-Na2SO4 for 30 min selectively damaged the jejunal villi and colonic surface epithelium but did not reduce the increased potential difference and current generated by ACh. 4. When resistance changes were taken into account the colonic response was markedly increased after Na2SO4 treatment although the jejunal response was unchanged. Under control conditions ACh reduced net Na absorption and stimulated Cl secretion by the colon. After exposure to Na2SO4 only the Cl secretory component of the ACh response remained, thus accounting for the enhanced effect. 5. Cycloheximide, administered I.V. at a dose of 12 mg/kg, damaged the crypts after 2 hr without affecting the villi of the jejunum or the surface epithelium of the colon. After cycloheximide treatment the increased potential difference, current and net Cl secretion induced by ACh were significantly reduced. 6. The crypts therefore appear to be the site primarily involved in the secretory response of rat jejunum and colon to ACh, although in the colon an inhibitory effect on the Na transport process located in the surface epithelium was observed.

Structure, function, and long-term maintenance of the isolated turtle colon

We describe the 5-day maintenance of sacs of turtle colonic mucosa in enriched bathing solutions. The mean maximum transepithelial potential difference (PD) developed by the sacs in Ringer solution enriched with tissue-culture medium and gassed with 95% air-5% CO2 was 126 mV at 24 hours. Lower values were observed in other solutions. The PD of 24-hour sacs was partially or totally inhibited by ouabain, replacement of Na by choline in mucosal bathing fluids, or removal of Ca from serosal bathing fluids. The sacs transported Na in excess of H2O forming a dilute mucosal solution. The response of four different sac preparations (normally oriented or everted, and stripped normally oriented or everted) to long incubation were compared. Stripped normally oriented tissue developed the highest PD and maintained the lowest water content. The morphology of fresh and long-incubated tissue was examined. This investigation demonstrates that the turtle colon can be maintained in vitro for long periods, and it provides information on the morphology and physiology of this tissue.

Effect of aldosterone on ion transport by rabbit colon in vitro

Segments of descending colon obtained from rabbits, that had been maintained on drinking water containing 25 mM NaCl and an artificial diet which contains 1% Na and is nominally K-free, respond to aldosterone in vitro (after a 30 to 60-min lag period) with a marked increase in the short-circuit current (Isc), an equivalent increase in the rate of active Na absorption (JNa net) and a decline in tissue resistance (Rt). Aldosterone also brings about a marked increase in the unidirection influx of Na into the cells across the mucosal membrane ("zero-time" rate of uptake) which does not differ significantly from the inrease m Isc. Treatment of control tissues with amphotericin B brings about sustained increases in Isc and JNa net to levels observed in aldosterone-treated tissues. However, addition of amphotericin B to the mucosal solution of aldosterone-treated tissues does not result in a sustained increase in Isc or JNa net and these values do not differ markedly from those observed in control tissues treated with amphotericin B. These findings, together with other evidence that Na entry in the presence of amphotericin B is sufficiently rapid to saturate the active Na extrusion mechanism at the baso-lateral membrane, are consistent with the notion that the aldosterone-induced protein increases the permeability of the mucosal membrane to Na but does not increase the "saturation level" of the active Na "pump" within the time-frame of these studies (3 hr). Finally, aldosterone has no effect on the bidirectional or net transepithelial movements of K under short-circuit conditions, suggesting that the enhanced secretion of K observed in vivo is the result of increased diffusion of K from plasma to lumen via paracellular pathways in response to an increased transepithelial electrical potential difference (lumen negative).

The role of acetylcholine in the regulation of ion transport by rat colon mucosa

1. Acetylcholine increases the potential difference across rat proximal colon both in vivo and in vitro.2. There is a sigmoid relationship between the change in potential difference and the logarithm of the dose of acetylcholine. The dose-response curve is shifted to the left by neostigmine and to the right by atropine, suggesting that the action of acetylcholine is mediated by a muscarinic type of receptor.3. The dose-response curve for acetylcholine in vivo is not altered by the ganglion-blocking agents hexamethonium and pentolinium, suggesting a direct effect of this transmitter on the colon.4. Acetylcholine causes an increase in potential difference, a small decrease in resistance and hence a rise in the current generated by both normal and stripped everted sacs of rat colon.5. In the absence of sodium, the calculated current change produced by acetylcholine is reduced, and the removal of chloride has a similar inhibitory effect. The absence of bicarbonate does not significantly affect the response.6. Acetylcholine virtually abolished net sodium movement and induced net chloride secretion and these changes accounted for the increased short-circuit current.7. Acetylcholine had no effect on oxygen consumption by rings of colon.8. Tracts staining for acetylcholinesterase were observed running from the submucous plexus towards the mucosal epithelium.9. This study shows that acetylcholine can influence ion movement by rat colonic mucosa and suggests that the autonomic nervous system might be involved in the regulation of transport mechanisms in this tissue.

[Intestinal uptake and transfer of zinc after supplement of 2,4-dinitrophenol and ouabain]

The effect of the inhibitors 2,4-dinitrophenol (DNP) and g-strophantin (ouabain) on the intestinal uptake and cellular transfer of zinc (Zn) and the effect of the organic ligands histidine and EDTA were studied in vitro with everted intestinal sacs of normally maintained rats. Zn was added to the incubation solution as ZnCl2 or as ZnCl2 mixed with histidine in a molar ratio of 200 : 1 and mixed with EDTA in a molar ratio of 1 : 1 or 10 : 1. The DNP in a concentrationof 0.05 mM proved sufficient to greatly inhibit the uptake and transfer of Zn from the solution containing ZnCl2 mixed with an excess of histidine. DNP also reduced Zn absorption when ZnCl2 alone as added. On the other hand, the presence of DNP in the solution containing ZnCl2 with EDTA was found to increase the transfer of Zn and its ligand. In the presence of EDTA, Zn absorption may, therefore, be controlled by the uptake and transfer of the ligand. Ouabain at the contration of 5 mM and 10 mM greatly reduced Zn uptake by the intestinal wall and the celluar Zn transfer from the solution containing ZnCl2 and histidine. There was only a small effect of ouabain on Zn absorption whether ZnCl2 is added alone or with EDTA. The addition of an excess of histidine as ligand to ZnCl2 in the control groups markedly improved Zn absorption. In contrast to this, the addition of EDTA to ZnCl2 reduces the intestinal Zn uptake and increases the transfer of Zn compared to ZnCl2 alone.

Ion transport by rabbit colon. I. Active and passive components

Descending rabbit colon, stripped of muscularis externa, absorbs Na and Cl under short-circuit conditions and exhibits a residual ion flux, consistent with HCO3 secretion, whose magnitude is approximately equal to the rate of active Cl absorption. Net K transport was not observed under short-circuit conditions. The results of ion replacement studies and of treatment with ouabain or amiloride suggest that the short-circuit current ISC is determined solely by the rate of active Na transport and that the net movements of Cl and HCO3 are mediated by a Na-independent, electrically-neutral, anion exchange process. Cyclic AMP stimulates an electrogenic Cl secretion, abolishes HCO3 secretion but does not affect the rate of Na absorption under short-circuit conditions. Studies of the effect of transepithelial potential difference on the serosa-to-mucosa fluxes Jism of Na, K and Cl suggest that JNasm,JIsm and one-third of JCl-sm may be attributed to ionic diffusion. The permeabilities of the passive conductance pathway(s) are such that Pk:PNa:PCl= 1.0:0.07:0.11. Electrolyte transport by in vitro rabbit colon closely resembles that reported from in vivo studies of mammalian colon and thus may serve as a useful model for the further study of colonic ion transport mechanisms.

Lithium transport by the colon of normal and sodium-depleted rats

1. The transport of Li by colonic epithelium has been examined in normal and Na-depleted rats. 2. Substitution of Li for Na with lumen of the conon causes the transepithelial electrical potential difference (p.d.) and short-circuit current to fall to low levels and the electrical resistance of fall moderately. Recovery occurs by fairly slowly after removal of Li. 3. Li absorption increases linearly with increasing concentration in the lumen and is significantly faster in Na-depleted rats. Increasing the luminal Na concentration reduces Li absorption from solutions of low Li concentration. 4. Comparison of absorption rates with secretion rates in rats given Li systemically, together with measurements of Li distribution across the epithelium in relationship to the transepithelial p.d. indicate that Li transport is predominatly or entirely passive. Interference with Li absorption by Na suggests, however a mucosal membrane carrier which, since Li absorption rises after Na depletion, may be increased in the Na-depleted state.

Intestinal absorption of water and electrolytes

1. Recent advances in knowledge of intestinal physiology have provided some insight into disturbed mechanisms and their clinical effects; for example, diarrhoea can now be defined biochemically as excessive fluid and electrolyte loss due to their malabsorption or excessive secretion. 2. Because of differences in structure and in absorptive and secretory mechanisms, the various parts of the gut perform different functions. In the jejunum, transport activity is extensive and the rapid equilibration of its content provides the optimal absorptive mixture. Functionally, the ileum and colon are similar; compared with the jejunum, they have greater absorptive capacity for electrolytes and generate significantly higher transmural electrical potentials. In the colon, some transport mechanisms are potentiated by adrenocortical steroids. 3. Water and electrolyte absorption and secretion are the end-products of bidirectional fluxes across the intestinal wall that are several times greater than net movement in either direction. Secretion is the surplus of negative flux (into the lumen) and absorption the surplus of positive flux (out of it). 4. Many electrolyte transport mechanisms require the absorption of other electrolytes or non-electrolytes, and some are concerned with electrolyte exchange. Water transport is always passive, in the direction of solute flow, but its solvent drag can move solutes across the intestinal membrane.

Adrenergic influences on the electrical potential across the colonic mucosa of the rabbit

1. An investigation was undertaken of the role of adrenergic influences on the transient change in colonic potential difference (p.d.) induced by handling in the rabbit. 2. P.d. was increased from 9-5 +/- 0-5 to 27-05 +/- 1-94 mV during a 2 hr period of handling and wrapping. Atropine and alpha-adrenergic block during this 2 hr period did not alter the magnitude of the response and the response in adrenalectomized animals was not significantly different from that of normal animals. 3. 'Total' adrenergic block and selective beta-adrenergic block abolished the change in p.d. with handling and infusion of an alpha-receptor agonist during the 2 hr period significantly reduced the response. 4. The changes in colonic p.d. with handling were greatly increased by the infusion of a beta-receptor agonist during the 2 hr test period. 5. The results indicate that the change in colonic p.d. with handling is a direct effect of the autonomic nervous system mediated via beta-receptors and that the changes were not related to the effects of circulating catecholamines or to changes in aldosterone production.

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.

Effect of luminal ions on the transepithelial electrical potential difference of human rectum

Skin electrodes are the most convenient reference electrodes for clinical measurements of electrical potential differences (pd) across the epithelium of the alimentary tract but the presence of an electrical charge on normal skin introduces an error. In the present study, by comparison with results obtained using subcutaneous and intravenous electrodes, it was shown that an intradermal injection of saline abolished the skin potential differences. This simple method, therefore, allows skin electrodes to be used to measure the true transepithelial potential differences of gut mucosa. The method was applied to investigate the effect on the rectal potential difference of altering the composition of the luminal solutions. Changes in the cations (sodium, potassium, magnesium) showed that sodium was the most important cationic determinant of the potential difference, especially when sodium absorption was stimulated by giving mineralocorticoids. Changes in the anions (chloride, iodide, bromide, nitrate, bicarbonate, sulphate, phosphate, citrate, and acetate) indicated that the molecular size of the anion rather than its chemical nature was the significant factor and suggested that the ions had to cross a barrier relatively impermeable to anions of radius greater than 3.5 to 4 A degrees . Changes in osmolality and glucose concentration were without effect.

Studies on the electrical potential profile across rabbit ileum. Effects of sugars and amino acids on transmural and transmucosal electrical potential differences

When isolated strips of mucosal rabbit ileum are bathed by physiological electrolyte solution the electrical potential difference (PD) across the brush border (psi(mc)) averages 36 mv, cell interior negative. Rapid replacement of Na in the mucosal solution with less permeant cations, Tris or choline, results in an immediate hyperpolarization of psi(mc). Conversely, replacement of choline in the mucosal solution with Na results in an abrupt depolarization of psi(mc). These findings indicate that Na contributes to the conductance across the brush border. The presence of actively transported sugars or amino acids in the mucosal solution brings about a marked depolarization of psi(mc) and a smaller increase in the transmural PD (Deltapsi(ms)). It appears that the Na influx that is coupled to the influxes of amino acids and sugars is electrogenic and responsible for the depolarization of psi(mc). Under control conditions Deltapsi(ms) can be attributed to the depolarization of psi(mc) together with the presence of a low resistance transepithelial shunt, possibly the lateral intercellular spaces. However, quantitatively similar effects of amino acids on psi(mc) are also seen in tissues poisoned with metabolic inhibitors or ouabain. Under these conditions Deltapsi(mc) is much smaller than under control conditions. Thus, the depolarization of psi(mc) might not account for the entire Deltapsi(ms), observed in nonpoisoned tissue. An additional electromotive force which is directly coupled to metabolic processes might contribute to the normal Deltapsi(ms).

Sodium transport and short-circuit current in rat colon in vivo and the effect of aldosterone

1. A method for measurement of short-circuit current and for applying a voltage clamp to segments of rat colon in vivo is described.2. The mucosa behaved as an ohmic resistor of average resistance 154 Omega/cm(2) although brief transient effects were frequently observed. Tissue resistance was independent of considerable changes in ionic strength and composition of the luminal solution.3. The short-circuit current averaged 120 muA/cm(2) in normal rats. Aldosterone intravenously raised the p.d., short-circuit current rising proportionately and tissue resistance being unchanged. The effects of various modifications of the intraluminal solution in respect to composition, hydrostatic pressure and pH were examined. An increase in the osmolality of the luminal solution sufficient to abolish water absorption did not affect p.d. or short-circuit current.4. The short-circuit current measured with 150 mM-NaCl in the lumen was almost completely accounted for by active Na absorption both in normal and aldosterone-treated rats. The changes in Na efflux rate produced by voltage clamping suggested that only part of Na efflux was due to simple diffusion. With lower [NaCl] in the lumen, the short-circuit current exceeded that atributable to active Na absorption, the discrepancy increasing with reduction of [NaCl].5. The luminal [Na] at which Na efflux and influx rates were equal was reduced by aldosterone, an effect which is probably responsible for the low stool [Na] of aldosterone treated animals. The significance of this finding in terms of the mode of action of aldosterone is discussed.

Electrical potentials of the sigmoid colon and rectum in irritable bowel syndrome and ulcerative colitis

Using a simple and rapid method, electrical potential differences across rectal and colonic mucosa have been measured at routine sigmoidoscopy in patients with irritable bowel syndrome and ulcerative colitis. In patients with irritable bowel syndrome, all of whom had diarrhoea, the mucosa was charged negatively on the luminal side and potential differences were not significantly different from those of normal subjects. In acute exacerbations of ulcerative colitis, the potential difference was reversed, the luminal side being positive. This characteristic change was seen even in mild attacks. The potential difference was usually restored to normal within a few weeks of commencing treatment. In some cases, however, it was persistently abnormal for months and failed to show the normal response to stimulation by the mineralocorticoid, fludrocortisone. The way in which measurements of potential difference can be useful in diagnosis, prognosis, and as a guide to treatment is discussed.

Measurement of electrical potentials of the human rectum and pelvic colon in normal and aldosterone-treated patients

A method is described which allows rapid measurement of the electrical potential difference across colonic mucosal epithelium to be carried out during routine sigmoidoscopy. The potential differences measured had a mean value of 25 mV (range 4 to 51 mV) in 27 subjects with normal bowel. Six hours after two intravenous injections of 0.5 mg aldosterone the potential difference had risen to 60 mV (range 37 to 101 mV). The time course of responses studied after a single injection of aldosterone showed that the potential allowance rose within four hours and had fallen again after 18 hours. Urinary sodium concentrations and sodium/potassium ratios fell after aldosterone injections, the time course of the changes being similar to that of the potential differences of the colon. Sodium concentration of stool fluid also fell. The concentration of chloride in the stool fluid was consistent with a passive distribution of chloride according to the electrochemical gradient, but that of potassium was considerably greater than expected from a passive distribution, suggesting that potassium is actively secreted into the lumen of the colon.

Kinetics of potassium in colonic mucosa of normal and sodium-depleted rats

1. The transport of K across the mucosal epithelium of the colon of normal and Na-depleted rats has been investigated in vivo by methods involving I.V. administration of solutions containing (42)K and by labelling the mucosa with (42)K from the lumen and observing the ;soaking-out' rates into radionucleide-free solutions.2. During the infusions, the specific activity of the mucosal K was always less than the simultaneous specific activity of the secreted K. On stopping the infusion the specific activity of the mucosal K fell although the plasma specific activity remained at all times greater than that of the mucosa. The blood flow rate to the mucosa estimated from these results was 0.95 ml./min.g wet wt.3. Experiments with (42)K in the lumen showed that the steady-state specific activity of the mucosal K was considerably less than expected if the K had been in a single compartment. The (42)K efflux curves were also consistent in showing two components. The curves were analysed on the basis of a model having two compartments in the mucosa with half-times of 6 and 33 min.4. Na depletion produced two significant changes, (a) K secretion rate doubled, (b) the steady-state specific activity of the mucosal K after luminal exposure to (42)K was increased. The half-times of the compartments were unaffected.5. The results were discussed in terms of a working hypothesis which represented the mucosal K in two compartments. Most of the K exchanges across the mucosa took place through the smaller compartment which probably contained 5% or less of the mucosal K, but which had a rapid turnover rate. The change of K permeability of the luminal face produced by Na depletion and probably due to increased aldosterone action, may well be an important factor in producing the rise of K secretion rate.

Factors influencing the electrical potential across the mucosa of rat colon

1. An investigation has been carried out into various factors which influence the transmucosal potential difference (p.d.) of rat colon in vivo when the p.d. is either high (> 30 mV) or low (< 20 mV).2. The p.d. was uninfluenced by short duration anaesthesia with ether or pentobarbitone. When anaesthesia was prolonged for several hours, p.d. rose steadily. The gradient of p.d. along the descending colon which developed and its elimination by adrenalectomy suggested that the rise was due to increased secretion of adrenal steroids.3. P.d. was increased by Na depletion after a delay of about 18 hr and fell again following Na repletion with a similar time delay. A characteristic gradient of p.d. along the descending colon was seen.4. Both haemorrhage and anoxia caused a rapid fall of p.d. P.d. was restored rapidly to its previous level when anoxia was corrected.5. Vasopressin (I.V.) in low dose was without effect; in high dose it caused a transient fall of p.d. associated with intense vasoconstriction of gut blood vessels.6. The following factors studied were without effect on p.d.: presence of glucose within the lumen; considerable osmotic gradients across the mucosa; variation of luminal pH over the range 5.2-9.8; intravenous administration of acetazolamide, chlorothiazide, frusemide, triamterene, ethacrynic acid or ouabain. Ouabain in the luminal solution also had no effect in all but two rats in which a small fall of p.d. was seen.7. 2,4-dinitrophenol, 10(-2)M, in the lumen caused a small fall of p.d. only if the p.d. was high.8. Experiments were done to determine the effect on p.d. of altering the ionic composition of the luminal solution. When the p.d. was low (< 20 mV) alteration of [Na], [K] or [Cl] produced small absolute changes of the p.d., all of comparable magnitude. The changes could be interpreted as due to diffusion potentials resulting from the ionic gradients across the mucosa. When the p.d. was high (> 30 mV), it showed a striking dependence on the luminal [Na] only, consistent with the presence of a large p.d. due to active Na transport.