Active sodium transport by the colon of Bufo marinus: stimulation by aldosterone and antidiuretic hormone

Salt- and angiotensin II-dependent variations in amiloride-sensitive rectal potential difference in mice

1. In the rectum and distal nephron, sodium reabsorption is mediated by the amiloride-sensitive epithelial sodium channel (ENaC). The ENaC-mediated sodium transport is electrogenic and creates an amiloride-sensitive transepithelial potential difference (PD). 2. We have evaluated the salt- and angiotensin (Ang)II-dependent variations in amiloride-sensitive rectal PD in mice and assessed their relationship with renal sodium handling. 3. Rectal PD was measured in vivo in mice maintained on a medium-, low- or high-sodium diet. On a medium-salt diet, the mean (+/- SEM) amiloride-sensitive PD was larger in the afternoon than in the morning (-26.1 +/- 0.9 and -11.2 +/- 0.7 mV, respectively; P = 0.001), indicating a circadian cyclicity. Rectal PD increased on a low-sodium diet and decreased on a high-sodium diet. 4. Amiloride-sensitive rectal PD correlated significantly with the urinary Na+/K+ ratio (P < 0.001) and with sodium reabsorption in the distal nephron as measured by the lithium clearance technique (P < 0.001). 5. In mice treated with an AngII AT1 receptor antagonist, amiloride-sensitive rectal PD was increased in the afternoon compared with controls (-32.8 +/- 2.0 vs -24.4 +/- 0.9, respectively; P < 0.001). 6. At high doses, AngII decreased the amiloride-sensitive rectal PD and this effect was blunted by an AT1 receptor antagonist. 7. These results show the presence of a salt-dependent daily cyclicity of sodium transport in the mouse rectum that follows circadian changes in sodium handling in the distal nephron. Angiotensin II appears to modulate this diurnal pattern of rectal amiloride-sensitive sodium transport.

Changes in the electrophysiological parameters of the posterior intestine of Anguilla anguilla (Pisces) induced by oxytocin, urotensin II and aldosterone

In view of the importance of the intestine in the osmoregulation of freshwater fishes, we determined the effects of oxytocin, urotensin II (UII), and aldosterone added to the serosal side of the isolated posterior intestine of the freshwater-adapted teleost Anguilla anguilla on electrophysiological parameters. Oxytocin decreased the short-circuit current (SCC) and transepithelial potential difference (TPD) at concentrations of 1 and 10 mU/ml (to 50% and 42% of control values, respectively), but did not alter these parameters at a concentration of 0.1 mU/ml. UII reduced SCC and TPD at concentrations of 10 nM, 50 nM and 100 nM (to 85% of control values), but increased these parameters at the concentration of 500 nM (to 115% of control values). Aldosterone did not alter SCC or TPD at the concentrations tested (10 nM and 100 nM). Oxytocin may open Na+ channels in the apical membrane, allowing the flow of Na+ to the serosa, reducing SCC and TPD. Should this hypothesis be correct, oxytocin would be important for freshwater adaptation, since it would increase Na+ absorption. The reduction of SCC and TPD in the posterior intestine of A. anguilla induced by UII is evidenced that this neurohormone is also important for freshwater adaptation in teleosts. Aldosterone did not show this effect probably due to the lack of receptors in this organ.

Different modes of electrogenic Na+ absorption in the coprodeum of the chicken embryo: role of extracellular Ca2+

Transepithelial electrogenic Na+ transport (INa) was investigated in the coprodeum of 20-days-old chicken embryos in Ussing chambers. Short circuit current (Isc) and transepithelial resistance (Rt) were 14.7 +/- 4.8 microA.cm-2 (n = 12) and 0.53 +/- 0.09 k omega.cm-2 (n = 12), respectively. INa was calculated from changes in Isc by substitution of mucosal Na+ by (N-methyl-D-glucamine) (NMDG). Isc inversed during Na+ removal, and INa was found to be 27.8 +/- 4.7 microA.cm-2 (n = 12). Amiloride (100 mumol.l-1) inhibited only about 60% of INa. Analysis of Isc fluctuations revealed a Lorentzian component in the power density spectrum with a corner frequency of about 57 Hz. This component was not correlated to INa, and its origin is still unclear. Removal of mucosal Ca2+ increased INa about 2.5-fold due to an increase of the amiloride-insensitive component of INa in additionally investigated adult tissues. The results clearly show that this is due to a non-selective cation channel with an "apparent" order of selectivity Cs+ greater than Na+ = K+ greater than Rb+ greater than Li+. The Ca2+ concentration required to block 50% of the Isc was about 18 mumol.l-1. The IscCa could also be suppressed by other divalent cations such as Mg2+ and Ba2+. Additionally, an INa-linked Lorentzian component occurred which dominated the control spectrum with a significantly higher corner frequency (about 88 Hz). The results indicate that Na+ absorption in the coprodeum of the chicken embryo is more complex than in adult hens.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca-sensitive sodium absorption in the colon of Xenopus laevis

Transepithelial electrogenic Na transport (INa) was investigated in the colon of the frog Xenopus laevis with electrophysiological methods in vitro. The short circuit current (Isc) of the voltage-clamped tissue was 24.2 +/- 1.8 microA.cm-2 (n = 10). About 60% of this current was generated by electrogenic Na transport. Removal of Ca2+ from the mucosal Ringer solution stimulated INa by about 120%. INa was not blockable by amiloride (0.1 mmol.l-1), a specific Na-channel blocker in epithelia, but a fully and reversible inhibition was achieved by mucosal application of 1 mmol.l-1 lanthanum (La3+). No Na-self-inhibition was found, because INa increased linearly with the mucosal Na concentration. A stimulation of INa by antidiuretic hormones was not possible. The analysis of fluctuations in the short circuit current (noise analysis) indicated that Na ions pass the apical cell membrane via a Ca-sensitive ion channel. The results clearly demonstrate that in the colon of Xenopus laevis Na ions are absorbed through Ca-sensitive apical ion channels. They differ considerably in their properties and regulation from the amiloride-sensitive Na channel which is "typically" found in the colon of vertebrates.

Noise analysis of cAMP-stimulated Na current in frog colon

The effects of oxytocin and cAMP on the electrogenic Na+-transport in the short-circuited epithelium of the frog colon (Rana esculenta, Rana temporaria) were investigated. Oxytocin (100 mU.ml-1) elevated the short-circuit current (Isc) transiently by 70% whereas cAMP (1 mmol.l-1) elicited a comparable sustained response. The mechanism of the natriferic action of cAMP was studied by analysing current fluctuations through apical Na+-channels induced by amiloride or CDPC (6-chloro-3,5-diaminopyrazine-2-carboxamid). The noise data were used to calculate Na+-channel density (M) and single apical Na+-current (iNa). iNa-Values obtained with amiloride and CDPC were 1.0 +/- 0.1 pA (n = 5) and 1.1 +/- 0.2 pA (n = 6) respectively and unaffected by cAMP. On the other hand, cAMP caused a significant increase in M from 0.23 +/- 0.08 micron-2 (n = 5) to 0.49 +/- 0.17 micron-2 (n = 5) in the amiloride experiments. In our studies with CDPC we obtained smaller values for M in control (0.12 +/- 0.04 micron-2; n = 6) as well as during cAMP treatment (0.19 +/- 0.06 micron-2; n = 6). However, the cAMP-induced increase in M was also significant. We conclude that cAMP stimulates Na+-transport across the frog colon by activating "silent" apical Na+-channels. Thus, the mechanism of regulation of colonic Na-transport in frogs differs considerably from that in other vertebrates as mammals and birds.

Electrophysiological analysis of sodium-transport in the colon of the frog (Rana esculenta). Modulation of apical membrane properties by antidiuretic hormone

Sodium transport and apical bioelectrical membrane properties were investigated in frog colonic epithelium in the absence and presence of the antidiuretic hormone arginine-vasotocin (AVT). Apical Na-permeability and intracellular Na-activity were evaluated by analysis of current-voltage relationships in the serosally K-depolarized tissue. Tissue- and apical membrane capacitance were measured by voltages step analysis. The frog colon was found to be a tight epithelium with a transepithelial resistance of 2.63 +/- 0.25 k omega.muF (n = 17). 85-90% of short circuit current (11.2 +/- 1.1 microA.microF.l-1; n = 17) was related to electrogenic Na-transport from mucosa to serosa. Graded doses of amiloride (less than 50 mumol.l-1) induced Michaelis-Menten-type inhibition kinetics. Serosal addition of 10(-6) mol.l-1 AVT induced a significant increase in sodium current (25%), apical sodium permeability (19%) and tissue capacitance (4.3%) whereas intracellular Na-activity remained unchanged. There was a good correlation between increased Na-current and apical Na-permeability. No correlation was found between Na-current and membrane capacitance. Our results demonstrate that in contrast to other species the amphibian colon shows a natriferic reaction to AVT. We suggest that the regulation of Na-transport in frog colon is similar to that in the toad urinary bladder. It is caused by an activation of preexisting apical Na-channels and not by fusion of subapical cytoplasmic vesicles with the apical membrane.

The role of aldosterone in water and electrolyte transport across the colonic epithelium of the lizard, Gallotia galloti

1. The effects of D-aldosterone on the electrical properties and ionic transport have been analysed (in vitro) in the colonic epithelium of the lizard, Gallotia galloti. 2. The injection of 30 g of D-aldosterone induced a significant rise of plasma aldosterone concentration, sustained for 4 hr after administration. 3. Intraperitoneal injections of D-aldosterone caused a slight reduction of transmural potential difference (PD) and short-circuit current (Isc). 4. Net absorption of Na+ across the colon from aldosterone-treated lizards did not increase when compared to colon from normal lizards. Cl- absorption was increased in treated lizards. 5. Addition of mucosal barium to treated tissues brought about a sustained increase of PD and Isc. 6. The absorption of fluid in normal lizards was reversed to fluid secretion in treated lizards. 7. The effects of D-aldosterone on lizard colon are compared with those reported for the colon of mammals and birds.

Characterization of steroid receptors in the gut and kidney of the frog (Rana catesbeiana) and in the gut of the turtle (Chrysemys picta)

Paraglucocorticoid- and paramineralocorticoid-binding cytosolic receptors (pGR, pMR) were demonstrated in the intestine and kidney of the frog, Rana catesbeiana and in the intestine of the turtle, Chrysemys picta, in the presence of sodium molybdate. These receptors were of high affinity and low capacity with the following binding parameters: pGR:Kd:frog intestine (FI), triamcinolone acetonide (TA): 3.3 nM, corticosterone (B): 3.4 nM; frog kidney (FK), TA:4.3 nM, B: 9.3 nM; turtle intestine (TI), TA: 4.8 nM; Nmax: FI, TA: 357, B: 371; FK, TA: 301, B: 157; TI, TA: 350 fmol/mg protein. pMR:Kd: FI, aldosterone: 0.9 and 90 nM (biphasic curves); FK, aldosterone: 0.6 and 36 nM (biphasic curves); Nmax: FI, 13 and 147 fmol/mg protein; FK, 78 and 109 fmol/mg protein. The receptor had the following ligand affinities: pGR: FI and FK: triamcinolone acetonide greater than DOC greater than 11 beta-hydroxyprogesterone greater than progesterone greater than corticosterone greater than cortisol greater than aldosterone greater than 11-dehydrocorticosterone greater than 17 alpha-hydroxyprogesterone greater than cortisone; TI: triamcinolone acetonide greater than corticosterone greater than progesterone greater than DOC greater than cortisol greater than aldosterone; pMR: FI and FK: corticosterone greater than 11 beta-hydroxyprogesterone greater than aldosterone greater than triamcinoline acetonide = cortisol greater than DOC greater than 11-dehydrocorticosterone greater than progesterone greater than 17 alpha-hydroxyprogesterone greater than cortisone. Androgens, estrogens or 18-hydroxycorticosterone did not compete for binding in either tissue. The heat activated frog receptors did not bind to naked DNA, though the turtle receptor did. It was possible to show that cytosol receptor-ligand complexes from all tissues were bound by nuclear acceptor sites. On linear sucrose gradients, the FI TA-receptor complex sediments with a single peak (7.5S), the FK TA-receptor complex gave two peaks (8.0 and 4.4S) and the TI TA-receptor complex showed a single peak (9.0S). The hydrodynamic parameters of the pGR's were determined by gel exclusion on Sephacel S-300. The following results were obtained: Mr: FI, 265, 80, 40 kDa (multiple proteins); FK, 280, 60, 20 kDa (multiple proteins); TI, 366 kDa; Rs: FI, 6.9, 3.9 nm; FK, 6.9, 2.9 nm; TI, 7.6 nm; f/f0: FI, 1.6; FK, 1.6; TI, 1.6. It is suggested on the basis of the binding and hydrodynamic parameters that non-mammalian epithelia corticosterone receptors have undergone biochemical evolution from one class of vertebrates to another.

Effect of chronic hyperaldosteronism on the electrophysiology of rat distal colon

Microelectrodes have been used to study the effects of aldosterone on the barriers and forces controlling sodium and potassium transport in rat distal colon. Compared to control tissues, hyperaldosteronism induced by dietary sodium depletion resulted in a 7-fold increase in transepithelial voltage (VT) and a 52% decrease in total resistance (RT). Increased VT reflected both a rise in the basolateral membrane voltage (Vbl) and a fall in the apical membrane voltage (VA). RT was resolved into its separate membrane components using nystatin (585 U X ml-1), and the decrease in RT produced by aldosterone was found to be due entirely to a 66% decrease in the apical membrane resistance (RA). Amiloride had no effect on the control tissues, but restored VT, Vbl and VA in tissues from sodium deprived animals to control values. Amiloride also increased RT in the experimental tissue, but the post-amiloride values remained significantly lower than those in controls. These results indicate, therefore, that hyperaldosteronism results in an increase in VT by hyperpolarizing the basolateral membrane, as well as depolarizing apical membrane in rat distal colon. The fall in RT, however, is due only to a fall in RA since Rbl and junctional resistance (Rj) were unaffected. The data are consistent with the concept that aldosterone acts to stimulate sodium absorption by increasing the rate of cell entry of sodium, through the induction of amiloride-sensitive sodium channels in the apical membrane, and enhances the rate of potassium secretion by increasing the electrical driving force towards the mucosal solution.

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).

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.

Sodium transport by the colon of Bufo marinus: Na uptake across the mucosal border

Na transport by the isolated toad colon has been studied by measuring transmural Na fluxes and by direct measurement of the Na influx across the mucosal border. Net Na transport accounts for 88% of the short circuit current in the presence and in the absence of exogenous aldosterone. Na influx across the mucosal border appears to consist of two components. One component is highly correlated with short circuit current, is a saturable function of mucosal Na concentration, and is inhibited by lithium ions in the mucosal medium. The second component is a linear function of mucosal Na concentration, is unaffected by lithium, and is apparently not related to net Na transport by the tissue.

Disappearance of insulin response after enzymatic treatment of sodium-transporting amphibian epithelia

Amphibian epithelia specialized in trans-cellular sodium transport lose their capacity to react to insulin by a stimulation of this process upon treatment with collagenase; baseline activity and responsiveness to other hormones (vasopressin, aldosterone) bringing about such a stimulation are preserved. This renders it likely that proteases contaminating most collagenase preparations exert a detrimental effect on the receptors held responsible for interaction between insulin and its target cells in the tissues examined.

Pathway of sodium moving from blood to intestinal lumen under the influence of oxyphenisatin and deoxycholate

The transfer of (51)CrEDTA and inulin--substances which are distributed only in the extracellular space--across the rat colonic mucosa in vivo is increased by oxyphenisatin O (3.5 times 10(-5)M) and deoxycholate D(3 times 10(-3)M). O and D do not change the size of the intra- and extracellular fluid compartments of the mucosa as measured with (51)CrEDTA from the blood side. The sodium and potassium content of the mucosal tissue is not altered. Therefore the calculated intracellular concentrations of sodium and potassium remain constant. The time course of the (22)Na uptake into the mucosal epithelium is not influenced by O and D up to 5 min after i.v. injection. The specific activity of sodium, however, in the luminal fluid increases under the influence of O(twofold) and D(fivefold). The uptake of (22)Na into the mucosal tissue after administration of (22)Nainto the intestinal lumen is not changed in presence of O and D. We conclude that the net transport of sodium and water from blood to lumen under the influence of O and D occurs mainly via the intercellular way.

Transport of electrolytes across the helicoidal colon of the new-born pig

1. The Na, K, Cl and water content of faeces removed from different parts of the pig helicoidal colon were determined for 1-day-old and adult animals. Faecal Na, Cl and water content fell in both cases during passage of contents through the colon. K content increased in the distal colon of the adult pig. This did not occur in the 1-day-old animal. 2. The colon of the 1-day-old pig removed a larger proportion of water from its contents than did that of the adult. The absorption of both water and Cl was found to extend into the distal colon of the 1-day-old animal; little or no net absorption took place in this region in the adult. 3. Colons taken from new-born pigs maintained stable short-circuit currents of about 60-80 muA cm-2 with open-circuit voltages of about 10 mV. Similar values were found for proximal and mid regions of colons taken from 1-day-old, suckled animals. In the distal colon, however, both short-circuit current and open-circuit potential doubled after suckling. 4. Measurements of Na flux in vitro showed no regional difference at birth. The amount of Na absorbed, about 4.5 muequiv cm-2 h-1, was twice that predicted from the short-circuit current, supposing that to be due solely to the electrogenic transport of Na. 5. Colons taken from suckled pigs transported Na at double the rate found in the new-born animal. This applied to both the proximal region, where the short-circuit current remained constant, and the distal region, where the short-circuit was double that of the new-born. 6. Fluxes of Cl and K were also measured across the proximal colon of the one-day-old, suckled pig. There was a net absorption of Cl and secretion of K (1.3 and -0.05 muequiv cm-2 h-1 respectively). These fluxes, taken together with that for Na, could not wholly account for the short-circuit current measured across these preparations. 7. The pig colon seems well able to cope with both electrolyte and water absorption during the first 24 hr of post-natal life. Part of the absorbed sodium appears to follow a non-electrogenic, possibly pinocytotic, route, but the full ionic contribution to the measured short circuit current has still to be determined.

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.

Electrical polarization of rectal mucosa and excretion of tetrahydroaldosterone in patients with cystic fibrosis of pancreas and in normal subjects

The electrical potential difference (PD) across the rectal wall was measured in 26 patients with cystic fibrosis of pancreas (CFP) and in 18 healthy subjects. The PDs obtained in normal children were identical to those previously obtained in normal adults. A significantly greater dispersion of the values was observed in CFP. When the patients were divided into groups according to metachromasia in fibroblast cultures, the mean PD was increased only in the ametachromatic group. True enough, this observation suggests a difference between various forms of CFP, distinguished by metachromasia, and thus is a further indication of the heterogeneity of the disease. The greater abnormalities in metachromasia negative patients may, however, be due solely to the fact that these patients are more severely affected by the disease. The urinary excretion of tetrahydroaldosterone in patients was within the ranges obtained in controls, which excludes the possibility of secondary hyperaldosteronism as the source of increased PD. No evidence was provided in favour of a basic defect in the intestinal transport of Na+ or Cl minus, but K+ concentrations in faecal fluids of patients were significantly lower than in controls. The equilibrium concentration of K+ could be accounted for by simple passive diffusion, suggesting that the epithelium behaved inertly with respect to this ion in CFP.

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.

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.

Use of a miniature GM counter and a whole body counter in the study of potassium transport by the colon of normal, sodium-depleted and adrenalectomized rats in vivo

1. A method is described employing a whole-body counter and a miniature GM counter placed within the lumen of the gut which can be used to study the absorption rates and kinetics of labelled substances in the mucosal epithelium in vivo. The method was applied to the study of K transport by the mucosa of the descending colon of normal, Na-depleted and adrenalectomized rats.2. The K influx rate was linearly related to luminal [K] over the range 5-80 mM. NaCN in the lumen produced a fall of electrical potential difference (p.d.) and of K secretion rate but did not reduce K influx rate. The findings were consistent with passive K influx.3. The mucosal (42)K content reached a steady value after 60-90 min of exposure to (42)K in the lumen. The value was similar in normal and adrenalectomized rats but was greater in Na-depleted rats.4. After replacement of luminal solution by a non-radioactive solution the fall of mucosal (42)K content could be described, in the majority, by a curve having two exponential components with half-times of 8-10 and 65-106 min. Approximately half of the (42)K outflow passed to the lumen. The rate constants were similar in all groups of rats.5. The results analysed on the basis of representing the mucosal K as in two compartments, indicated that the observed absorption rate could not be accounted for by transport through these compartments alone and that most of the (42)K crossed the mucosa by a fast pathway not detectable by the miniature GM counter. Most of the mucosal K was relatively inaccessible from the lumen.6. Na depletion raised the transmucosal p.d., increased the K secretion rate and appeared to increase K permeability of the mucosa but the kinetics were unaltered. Accumulation of (42)K in the mucosa was greater, suggesting that the permeability of the luminal face had increased. Adrenalectomy produced only one change, namely a reduction of K secretion rate.

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.

Effect of a saline environment on sodium transport by the toad colon

1. Colons isolated from saline-adapted or aldosterone-injected toads maintained transmural potential differences with the serosal side positive to the mucosa. The short-circuit currents of colons taken from aldosterone-injected toads could be expressed quantitatively by the net flux of sodium measured isotopically. This did not apply to saline-adapted colons where the net sodium flux was from serosa to mucosa.2. The short-circuit currents of colons taken from aldosterone-injected animals increased as the sodium concentration was raised from 10 to 50 mM, then decreased as the sodium concentration was further increased to 115 mM. Adaptation to saline changed this relationship, the short-circuit current becoming directly dependent on the sodium concentration.3. Faecal sodium was higher than serum sodium in saline-adapted toads. There was little or no change in the level of serum sodium or potassium. The urine of saline-adapted toads also contained high concentrations of sodium.4. The total and ouabain-sensitive ATPase activities of mucosal scrapings taken from saline-adapted colons were about half those found in the aldosterone-injected animal. Ten per cent of the total ATPase activity could be inhibited by ouabain.5. Microsomal fractions of mucosal scrapings taken from saline-adapted toads contained 4 times less Na(+) + K(+)-activated ATPase than did corresponding fractions from aldosterone-injected animals. High concentrations of sodium inhibited the microsomal ATPase activity irrespective of the previous conditions of adaptation.6. Regulation of sodium movements across the toad colon appears to be a complex process with the mucosal cells changing their properties so that they either absorb or secrete sodium ions depending on the physiological state of the animal.

Amiloride: a potent inhibitor of sodium transport across the toad bladder

1 Amiloride inhibits Na transport and short-circuit current (SCC) across the toad bladder. It is 1000 times more active at the mucosal than serosal surface. The lowest effective concentration was 10(-7)M.2. The inhibition was non-competitive with the sodium on the mucosal side of the bladder.3. Vasopressin, cyclic adenosine monophosphate (AMP) and aldosterone increased Na transport and SCC across the bladder and these effects were inhibited by amiloride.4. The antagonism of amiloride for vasopressin was non-competitive.5. Amphotericin B also increases Na transport across the bladder but its action was not changed by amiloride.6. Amiloride was without effects on SCC and diffusion potentials in bladders metabolically inhibited with CN(-) and iodoacetic acid (IAA).7. Neither plasma albumin, Ca(2+) nor adenosine triphosphate (ATP) altered the effects of amiloride.8. The only structural analogue of amiloride found to reduce SCC similarly was guanidine which was 1000 times less active. Pyrazine and a substituted pyrazine analogue were without effect. Neither guanidine nor the substituted pyrazine compound were competitive with amiloride.9. Amiloride had no effect on the osmotic permeability of the toad bladder either in the presence or absence of vasopressin.10. Na transport across the toad colon was also reduced by 10(-5)M amiloride at the mucosal surface.11. The possible mechanism of action of amiloride is discussed.

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.