Factors influencing the electrical potential across the mucosa of rat colon

Distribution and changes of glycoconjugates in rat colonic mucosa during development. A histochemical study using lectins

A study was made of the modifications of glycoconjugates in rat colonic mucosa during development. Sections of the caecum, and proximal and distal portions of the colon from Sprague Dawley rats at different stages of development (embryos, fetuses, suckling, weaning and adult rats) were examined. The sections were incubated with a battery of eight fluoresceinated lectins: DBA, SBA, WGA, LFA, PNA, GS-I, UEA-I and Con A. Some sections were treated with neuraminidase, and others were submitted to sequential saponification-neuraminidase treatment prior to incubation with the lectin (WGA, PNA or LFA). The intensity of the fluorescence was evaluated and graded from absent (-) to very positive (4+). Gradual and progressive changes were seen in colonic glycoconjugates during development. These changes revealed a unique developmental pattern for each lectin, which was independent for each cellular compartment (goblet cells, luminal surface and supranuclear region). Local and regional differences, observed between the different colonic sections, were already present from early stages of development. Moreover, our study showed that for several glycoconjugates, the differentiation process in colonic mucosa began in the distal region and continued through to the proximal region, the former being the first to reach the adult pattern. In the caecum, some lectins maintained a fetal pattern throughout all the periods of development up to the adult stage.

Ion transport in human colon in vitro

Ion transport in the human colon was studied in vitro under short-circuit conditions. The proximal, transverse, and distal colon all actively absorbed Na and Cl at similar rates. Tissue conductance was lower in proximal colon, but there were no other regional differences in basal electrophysiologic parameters. There was a gradient of amiloride-sensitive electrogenic Na transport. Whereas amiloride had only a minimal effect in proximal colon, it inhibited 70% of short-circuit current and 50% of net Na absorption in distal colon. Ion substitution experiments demonstrated an electroneutral, coupled Na-Cl cotransport system in proximal and distal colon. Neither amphotericin nor impermeant anions had a consistent stimulatory effect on short-circuit current in human colon. Theophylline (10(-3) M), increased short-circuit current by 4 microEq X cm-2 X h-1, stimulated net Cl secretion, but did not block net Na absorption. Epinephrine, via an alpha 2-adrenergic mechanism, significantly decreased short-circuit current but did not alter Na or Cl transport. These results suggest that all segments of human colon actively absorb Na and Cl, Na absorption occurs by both electrogenic Na absorption and electroneutral Na-Cl cotransport, there is an aboral gradient of increasing electrogenic Na transport, theophylline stimulates secretion in a pattern most consistent with electrogenic Cl secretion, and epinephrine does not increase Na-Cl cotransport in human distal colon. These studies demonstrate that human colon in vitro has distinct transport properties that must be considered both in clinical situations and in comparison to animal models.

Regulation of cation transport by low doses of glucocorticoids in in vivo adrenalectomized rat colon

A dose response curve for glucocorticoid-induced proximal and distal colonic cation transport in vivo was established in adrenalectomized rats. All doses (0.5-50 nmol/100 g body wt) stimulated sodium absorption. Distal sodium absorption did not saturate at dexamethasone levels that saturate the glucocorticoid receptor but also bind to greater than 35% of aldosterone receptors. Saturation of the pure glucocorticoid response occurred in both segments with RU26988, a synthetic glucocorticoid that does not occupy aldosterone receptors. Maximum velocities for pure glucocorticoid-induced sodium absorption were 15 and 16 mu eq/min per g dry tissue, and Michaelis constants (Km) were 4.2 and 4.6 X 10(-9) mol/liter for proximal and distal colon. Kms are similar to the dissociation constant for the colonic glucocorticoid receptor and too low for significant aldosterone receptor occupancy. Dexamethasone increased sodium absorption significantly within 30 min of injection, suggesting the response is not dependent on new protein synthesis. Similar time and dose responses in proximal and distal colon suggest glucocorticoids stimulate the same pathway in both segments.

Calcium transport across the colon ascendens and the influence of 1,25-dihydroxyvitamin D3 and dexamethasone

Concentration dependence of unidirectional calcium fluxes across the rat colon ascendens were measured in a modified Ussing chamber. Both the mucosa (m) to serosa (s) and the s to m calcium flux exhibited saturation kinetics. The maximum transport rates and the affinity to the transporter of calcium was higher in the m to s direction than that from s to m, resulting in a remarkable net calcium absorption. The results obtained from measurements of unidirectional calcium fluxes in dependence on clamped transepithelial potentials showed that: (i) calcium transport in both direction had a voltage-independent component; (ii) the voltage-independent, i.e. non diffusive fraction of the m to s calcium flux was 3.2 times greater than that in the opposite direction; (iii) the voltage dependent, i.e. diffusional fraction of the m to s calcium flux, was about two times greater than the voltage-dependent fraction of the calcium flux in the s to m direction; and (iv) in the m to s direction 62%, and the s to m direction 73%, of the total unidirectional flux was voltage-dependent. Dexamethasone, known to enhance sodium and water absorption in the colon, had no significant influence on net calcium absorption but increased the unidirectional calcium fluxes in both directions. The increase in unidirectional calcium fluxes parallel to that of the extracellular marker mannitol suggests that dexamethasone has no influence on the transcellular calcium transport but increases the calcium flux along the paracellular way. Amiloride had no influence on the dexamethasone-induced changes of the epithelial electrical parameters as distinguished from the colon descendens.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of carbonic anhydrase activity in the developing rat colon

Carbonic anhydrase activity was localized histochemically by light and electron microscopy in the proximal and distal colon of developing rats. Fixed tissue was taken for normal morphology and carbonic anhydrase localization from fetal (20-22 days gestation), suckling (1-19 days postnatal), weanling (20-25 days postnatal), and adult rats. The proximal colon had distinct villi at birth which were diminished between days 5 and 11 postnatally. The distal colon lacked villi at birth but had rudimentary crypts (ridges and furrows) which were replaced during the suckling period by a flat mucosa interspersed with true crypts. Carbonic anhydrase first appeared in both proximal and distal colonic epithelial cells on the day of birth (22 days gestation). Goblet cells were nonreactive at each developmental period. In neonatal rats, epithelial cells in the upper half of the villi of the proximal colon and on the surface and upper crypts of the distal colon were positive for carbonic anhydrase throughout the cytoplasm. Cells at the villar base (proximal colon) or in the deep crypt (distal colon) had reaction product in the intercellular spaces but not the cytoplasm. By 11 days postnatal, cytoplasmic reaction product was present in proximal colonic cells in the upper three-fourths of the crypt and was concentrated in a heavy band in the apical cytoplasm. In the distal colon, cytoplasmic positive cells did not extend as deeply into the crypts and the apical banding pattern was weak. Intercellular spaces in the deeper crypt epithelium were positive in both proximal colon and distal colon, suggesting a membrane-bound carbonic anhydrase. It was concluded that carbonic anhydrase appeared suddenly at birth and was continuously present in mid- to upper-crypt (or upper villus in early neonatal proximal colon) non-goblet cells into adulthood. This suggests a functional role for carbonic anhydrase in chloride-bicarbonate exchange across the neonatal and adult colonic mucosa.

Effects of vasopressin on electrolyte transport across isolated colon from normal and dexamethasone-treated rats

Vasopressin enhanced the absorption of Na+ and Cl- across the short-circuited colon descendens from normal rats. This effect of vasopressin results from an increase in the mucosal to serosal movement of Na+ and Cl- and a decrease in the serosal to mucosal movement of Cl- and was accompanied with a decrease in the short-circuit current (ISC). Neither the base-line absorption of Na+ and Cl-, the vasopressin-induced increase in Na+ and Cl- absorption nor the decrease in ISC were inhibited by amiloride in the colon from normal rats. Colon descendens from rats treated for 3 days with dexamethasone had remarkably higher transmural potential difference (p.d.), tissue conductance (Gt) and ISC. The absorption of Na+ across the short-circuited colon descendens from dexamethasone-treated rats was increased 3-fold when compared to colon from normal rats. The absorption of Cl- in normal rats was reversed to Cl- secretion in treated rats. Amiloride rapidly and reversibly decreased the p.d., Gt and ISC in colon from dexamethasone-treated rats. The transport of Na+ was nearly completely inhibited by amiloride in treated rats. In contrast to its enhancing effects on Na+ absorption in colon from normal rats vasopressin did not enhance Na+ absorption in colon from dexamethasone-treated rats. This enhancement of Cl- absorption by vasopressin was retained in colon from treated rats. This enhancement of Cl- transport was due solely to a decrease in the serosal to mucosal movement of Cl- and was accompanied with a decrease in ISC and Gt. The results support the hypothesis that vasopressin causes inhibition of the electrogenic secretion of Cl- in colon from dexamethasone-treated rats. Furthermore, the results suggest that the increase in the mucosal to serosal movement of Na+ and Cl- and the decrease in the serosal to mucosal movement of Cl- in colon from normal rats are caused by independent effects of vasopressin.

Evidence for size and charge permselectivity of rat ascending colon. Effects of ricinoleate and bile salts on oxalic acid and neutral sugar transport

We have measured unidirectional transmural fluxes of oxalate and neutral sugars across rat ascending colon in vitro, under short-circuit conditions, to characterize permeability barriers selective for size and charge. Ionic oxalate appears to be transported preferentially to sodium oxalate. Mucosal addition of taurocholate (1 mM), deoxycholate (1 mM), or ricinoleate (1 mM) increased bidirectional oxalate fluxes, and the ricinoleate effects were independent of medium calcium. Bidirectional fluxes of uncharged sugar molecules fell sharply at molecular weights above 76 (molecular radius above 3 A), and oxalate transport was retarded relative to that of uncharged molecules of similar size, suggesting that there is both size and charge permselectivity. Ricinoleate increased fluxes of all neutral molecules tested but changed neither the exclusion limits nor the cation selectivity of the epithelium. Bile salts and ricinoleate increase oxalate transport, probably by making more channels available, but do not alter size and charge selectivity.

Effect of aldosterone and dexamethasone pretreatment on sodium transport in rat distal colon in vitro

Aldosterone and dexamethasone stimulate sodium absorption in the rat colon in vivo. In vitro, increased amiloride inhibitable short-circuit current (ISC) has been demonstrated following aldosterone or dexamethasone treatment. Since ISC bears no relationship to sodium flux (JNa) in the untreated rat colon, we measured JNa in partially stripped voltage clamped segments of rat distal colon. Our results demonstrate directly that continuous infusion of aldosterone or dexamethasone for 4-7 days stimulated amiloride inhibitable JNa by stimulating JNaM -S. The amiloride inhibitable portion of JNaM -S was highly correlated with and approximately equal to the amiloride inhibitable ISC. Amiloride had no effect in controls. We conclude that JNaM -S in the rat distal colon is only sensitive to mucosal amiloride after treatment with aldosterone or dexamethasone. The amiloride sensitive ISC in these treated tissues was a good measure of the amiloride sensitive JNa. Small differences between aldosterone and dexamethasone treatment were noted in the effect on transepithelial resistance, potential difference, and the ISC after amiloride.

Time course of aldosterone and corticosterone plasma levels in rats during general anaesthesia and abdominal surgery

The transepithelial voltage (psi ms) of rat rectum in vivo increases for several hours in experiments under general anaesthesia. So far this was attributed by indirect evidence to increasing aldosterone plasma levels during the course of the experiment. We performed direct measurements of aldosterone and corticosterone plasma concentrations during intestinal perfusion experiments on barbiturate anaesthetized rats. Experiments were terminated for blood sampling at 10, 75, 300, 400, 800, or 1,800 min, respectively. (i) After 75 min of anaesthesia, surgical preparation was finished and plasma levels of aldosterone and of corticosterone were found increased by the factors 5 and 3, respectively, as compared to conscious controls. (ii) During the following 12 h, aldosterone further increased to levels 10 times as high as those of controls. In contrast, during the same period corticosterone slowly decreased but still remained elevated as compared to controls. (iii) The increase of both hormones was attenuated when abdominal surgery was omitted. (iv) The use of pentobarbital (Nembutal) instead of thiobarbital (Inactin) did not influence the adrenal response. (v) In adrenalectomized rats a continuous substitution with 65 ng X h-1 X kg-1 BWT aldosterone resulted in plasma levels as high as in conscious intact animals. (vi) Rectal psi ms started to move to higher lumen-negative values with a time delay of 1-1 1/2 h as compared with the increase of hormone levels. psi ms then stayed elevated until to the end of the experiments. We conclude that in vivo experiments of several hours duration in thio- or pentobarbital anaesthetized rats take place under conditions of aldosterone and corticosterone plasma levels which are high as compared to those of conscious unstressed animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Segmental heterogeneity of epithelial transport in rat large intestine

Functionally isolated segments of rat colon and rectum were perfused in situ in a closed loop system. Rectum was defined as the lower 25--35% of the length of large intestine (cecum excluded). Perfusion conditions were optimized at 0.5 ml.min-1 and 3 cm H2O luminal pressure. Variation of perfusion rate between 0.2 and 2 ml.min-1 did not influence net volume transport (JNV). Luminal distension following elevation of hydrostatic pressure to 18 cm H2O reversibly increased Jnv. Under control conditions Jnv and Na+-transport rates (JnNa) of colon were 2--3 times higher than those of rectum. In colon transepithelial electrical potential difference (psims) was time independent --12 mV (lumen negative) whereas rectal psims increased with time from --6 mV, reaching a plateau of --67 mV within 6 h. Amiloride 10(-4) mol.l-1 had no effect on psims, Jnv, and JnNa in colon but did slightly depress K+-secretion in colon descendens. In contrast, psims in rectum was dose-dependently depressed, being reversed to +7 mV at 10(-4) mol.l-1. Jnv and JnNa were decreased by half. Acetazolamide in addition to amiloride lowered the positive post-amiloride rectal psims by half. Adrenalectomy had no effect on colonic psims, but abolished psims of the rectum. A single dose of 40 microgram.kg-1 b.w. aldosterone during the experiment restored the typical time course of rectal psims, but did not affect psims in colon. It is concluded that aldosterone induces an amiloride-sensitive Na+-pathway only in rectum, but not in colon, and that colon and rectum differ basically in their transport properties, quantitatively as well as qualitatively, as do the kidney distal convoluted tubule and the cortical collecting duct.

The effect of bradykinin on the electrical activity of rat jejunum

Bradykinin increased the potential difference across both the jejunum and colon of the rat. This effect was significantly reduced by indomethacin, suggesting that it was mediated by prostaglandins. The possibility that bradykinin may contribute to the diarrhoea of the carcinoid syndrome by inducing a net secretory state in the intestine is discussed.

Potassium adaptation after reduction of nephron population

Two weeks after 75 percent nephrectomy in rats fed a normal diet glomerular filtration rate was found to be reduced by 2/3 and there was no hyperkalemia. Normal K balance was maintained by a threefold increase of fractional urinary potassium excretion. When infused with 0.5 M KCl solution, both normal and 75 percent nephrectomized rats increased their fractional excretion, while normal rats kept on a very high K-diet did not further increase their fractional potassium excretion. Adaptation of fractional excretion to infused KCl was blunted in 75 percent nephrectomized rats given a low K diet.Addition of 0.1 M KCl to the drinking water resulted in a three- to fourfold increase of potassium intake in normal rats: within 7 days, the Na-K-ATPase in the outer medulla of the kidney rose by 30 percent but no change occurred in the cortex. Further increases in dietary K load induced an increase of Na-K-ATPase activity, both in outer medulla and cortex, but not in other tissues. After 75 percent nephrectomy, specific Na-K-ATPase activity increased by 20-25 percent in the outer medulla and in the cortex.Dietary K loading, in normal rats, also resulted in a large increase of net potassium secretion into the perfused colon and of specific Na-K-ATPase activity of the colonic mucosa. These effects of potassium loading were not abolished by adrenalectomy and were accompanied by an increase of transmural PD. It was concluded that chronic potassium loading may enhance secretion of potassium into lower nephron tubular fluid and into colonic contents by primarily stimulating the synthesis of Na-K-ATPase and the resulting increase of the number of pumping sites. 75 percent nephrectomy may induce similar changes in the remaining nephrons.

Correlation of variations in intraluminal pressure and potential differences in the perfused colen

To investigate the nature of variations in the large intestine potential differences, a continuous perfusion of isotonic saline was carried out in the colon of 14 rats. Intraluminal pressure and potential differences between the lumen and the peritoneal cavity were continuously and simultaneously recorded, while impedance of the system and respiration were also constantly monitored. To obtain a quantitative evaluation of the data, Fast Fouier Transform was performed on the signals and their derivatives which were auto- and cross-correlated. While there was no obvious relation between pressure and potential in the unperfused colon, there was clear visual qualirative evidence that, during steady state conditions of perfusion, an increase in intraluminal pressure was accompanied by a decrease in potential differences, while impedance of the recording system remained unchanged. Computer analysis disclosed four narrow ranges of stable frequencies for both pressure and potential. They were centred around 0-3, 1-75, 10-7, and 75 cycles per minute, the latter being synchronous with respiration. It is concluded that the variations of potential differences recorded during perfusion, a well-know phenomenon, are not electrical artefacts: the fast rhythm is probably induced by respiration, which increases intracolonic pressure and that, in turn, reduces the absolute value of potential differences, which remain negative mucosa versus serosa. The slower rhythms are synchronous for pressure and potential. Mechanisms responsible for the decrease in potential related to the increase in pressure remain unknown.

Hormones and the stimulated sodium transport in cecum hypertrophy

Sodium transport per unit tissue is stimulated in dietetically produced cecum hypertrophy of the rat. Presumably this reflects an adaptive process. The possibility was tested whether transport adaptation was mediated by hormones, particularly by the pituitary-adrenal system, Cecum hypertrophy was induced by dissolving polyethylene glycol in the drinking water, and cecal sodium and water net absorption was measured in vivo. In both the adapted and normal mucosa, sodium and water absorption per unit macrosurface or dry weight was increased by aldosterone and decreased by adrenalectomy, hypophysectomy and volume expansion while the decrease following adrenalectomy was reversed by cortisol and the absence of antidiuretic hormone in hereditary diabetes insipidus rats had little effect on absorption. However, none of the test conditions abolished the relatively larger absorption of the adapted compared to the normal mucosa. It is concluded that the hormonal effects were additive but not causally related to transport adaptation.

Salt and water permeability of the epithelium of the coprodeum and large intestine in the normal and dehydrated fowl (Gallus domesticus). In vivo perfusion studies

1. The transmural net flow of salt and water in the coprodeum and large intestine of normal and dehydrated hens was investigated by means of an intraluminal in vivo perfusion technique.2. The lumen was perfused with a raffinose-electrolyte solution having a low sodium concentration (Na(+) = 1 m-equiv/l.). The osmolality of the solution was adjusted in the range 66-585 m-osmolal by adding raffinose. Polyethylene glycol 4000 (PEG) served as a water marker. The experiments permitted estimation of the passive transport parameters: the reflexion coefficient (sigma) of the penetrating solutes (predominantly NaCl), the Na(+) mobility (omega(s)), and the osmotic water permeability coefficient (P(osm)).3. When the luminal fluid had the same osmolality as plasma the net water flow (J(v)) was zero, indicating a sigma of unity. The net flow of Na(+) was zero, (at J(v) = 0) and a transmural electric potential difference close to zero was present both in normal and in dehydrated birds. This indicates an omega(s) of zero. When lumen osmolality was higher than that of plasma, no ;solvent drag' effect on Na(+) was demonstrated in the serosa to mucosa (s-m) direction.4. The P(osm) appeared to be independent of the luminal osmolality in the range of +/-200 m-osmolal from plasma osmolality. In normal birds the P(osms-m) was 3.2 mul./kg.hr.m-osmolal, the P(osmm-s) 5.8 mul./kg.hr.m-osmolal. In dehydrated birds these values were 3.6 and 10.0 respectively. Thus there seems to be rectification of water flow, and it varies with the state of hydration.5. A net K(+) flow of 15-50 mu-equiv/kg.hr in the m-s direction and a net Cl(-) flow of 10-50 mu-equiv/kg.hr in the s-m direction were observed. No relationship was observed between the flow of these solutes and the net water flow.

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.

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.

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

1. Using a preparation of rat colon mucosa mounted in vitro in small chambers, some factors which influence the electrical properties of the mucosa have been investigated.2. The mucosa behaved mainly as an ohmic resistance although a very brief transient occurred on first passing current. At 32 degrees C, the fresh preparation had a mean resistance of 108Omega/cm(2) and a mean short circuit current (s.c.c.) of 143 muA/cm(2). Tissues taken from Na-depleted and adrenalectomized rats differed little from normal tissues in electrical resistance but those from Na-depleted rats had higher potential difference (p.d.) and s.c.c.3. Increase of temperature led to a rise of conductance of similar order to that found for ions in aqueous solution. S.c.c. also rose with increase of temperature but the effect was relatively greater consistent with its being dependent on metabolic processes.4. Anoxia or the addition of cyanide, iodoacetate or 2,4-dinitrophenol to the bath fluid caused considerable fall in the p.d. and s.c.c.5. Ouabain decreased the p.d. and s.c.c. when added to the serosal side but had no effect when on the luminal side.6. Aldosterone and acetazolamide had no effect.7. Varying serosal side [K] produced only minor changes in p.d.8. Reducing [Na] of the luminal solution caused a considerable fall of p.d. but similar reduction of [Na] on the serosal side had little effect.9. The frequently employed model which represents the transepithelial p.d. as the sum of diffusion potentials originating at the luminal and serosal sides of the cell layer is not consistent with the present results. The colonic transmucosal p.d. probably originates in the electrogenic transport of Na by a mechanism located on the serosal side of the epithelium.