Electrical PD,short-circuit current and fluxes of Na and Cl across avian intestine

In vitro measurements were made of transmural potential difference (PD), short-circuit current (Isc), resistance and unidirectional fluxes of 22Na and 36Cl across the duodenum, jejunum, ileum and colon of normal sodium-replete domestic fowl (Gallus domesticus). The PD ranged from about 1 mV across the duodenum to 8 mV across the colon while the Isc was, respectively, 2.8 and 64 microA X cm-2. The jejunum and ileum exhibited values between these extremes. Unidirectional fluxes (under short-circuit conditions) of Na and Cl were lowest across the duodenum where there was no evidence of active transport of these ions. Unidirectional fluxes of Na and Cl were less across the jejunum than across the ileum or colon. A net active transport of Na (but not Cl) was observed in the ileum (= 106% of the Isc) and colon (= 50% of Isc). The possible physiological significance of these observations in the domestic fowl are discussed and are compared to that of a mammal, the rabbit.     

Effects of vasopressin on electrolyte transport in lizard intestine

Summary Vasopressin applied serosally had no effect on electrical parameters and unidirectional Na and Cl fluxes across anin vitro short circuited preparation of lizard ileum. Short circuit current (Isc) and transmural potential difference (PD) across colon were decreased by vasopressin and increased by cyclic AMP. Vasopressin increased the mucosal-to-serosal flux of sodium and chloride across short circuited colon. Cyclic AMP had no effect on the rate of Na absorption but reversed Cl absorption to secretion. Vasopressin enhanced the net absorption of water across the colon but had no effect on absorption across ileum. Cyclic AMP activity in homogenates of colon was not altered by vasopressin but was increased by theophylline. It is concluded that the colonic response of the lizard colon to vasopressin is mediated by a noncyclic AMP mechanism.     

Bioelectric properties and ion flow across excised human bronchi

Bioelectric properties and ion transport of excised human segmental/subsegmental bronchi were measured in specimens from 40 patients. Transepithelial electric potential difference (PD), short-circuit current (Isc), and conductance (G), averaged 5.8 mV (lumen negative), 51 microA X cm-2, and 9 mS X cm-2, respectively. Na+ was absorbed from lumen to interstitium under open- and short-circuit conditions. Cl- flows were symmetrical under short-circuit conditions. Isc was abolished by 10(-4) M ouabain. Amiloride inhibited Isc (the concentration necessary to achieve 50% of the maximal effect = 7 X 10(-7) M) and abolished net Na+ transport. PD and Isc were not reduced to zero by amiloride because a net Cl- secretion was induced that reflected a reduction in Cl- flow in the absorptive direction (Jm----sCl-). Acetylcholine (10(-4) M) induced an electrically silent, matched flow of Na+ (1.7 mueq X cm-1 X h-1) and Cl- (1.9 mueq X cm-12 X h-1) toward the lumen. This response was blocked by atropine. Phenylephrine (10(-5) M) did not affect bioelectric properties or unidirectional ion flows, whereas isoproterenol (10(-5) M) induced a small increase in Isc (10%) without changing net ion flows significantly. We conclude that 1) Na+ absorption is the major active ion transport across excised human bronchi, 2) Na+ absorption is both amiloride and ouabain sensitive, 3) Cl- secretion can be induced by inhibition of the entry of luminal Na+ into the epithelia, and 4) cholinergic more than adrenergic agents modulate basal ion flow, probably by affecting gland output.     

Paths of ion transport across canine fetal tracheal epithelium

Fluid secretion by the fetal sheep lung is thought to be driven by secretion of Cl- by the pulmonary epithelium. We previously demonstrated Cl- secretion by tracheal epithelium excised from fetal dogs and sheep. In this study we characterized the ion transport pathways across fetal canine tracheal epithelium. The transport of Na+ and Cl- across trachea excised from fetal dogs was evaluated from transepithelial electrical properties and isotope fluxes. Under basal conditions the tissues were characterized by a lumen-negative potential difference (PD) of 11 mV and conductance of 5.2 mS/cm2. The short-circuit current (Isc) was 43 microA/cm2 (1.6 mueq.cm-2.h-1). Basal Na+ flows were symmetrical, but net Na+ absorption (1.1 mueq.cm-2.h-1) could be induced by exposure of the luminal surface to amphotericin B (10(-6) M). Bilateral replacement of Na+ reduced Isc by 85%. Replacement of submucosal Na+ or exposure to submucosal furosemide (10(-4) M) reduced net Cl- secretion by 60-70%. Luminal exposure to indomethacin (10(-6) M) induced a 50% decrease in Isc, whereas isoproterenol (10(-6) M) increased Isc by 120%. The properties of the Cl- secretory pathway across fetal dog trachea are consistent with the model proposed for Cl- secretion across adult dog trachea and other Cl- -secreting tissues (e.g., bullfrog cornea and shark rectal gland). The absence of basal Na+ absorption by fetal dog trachea probably reflects limited apical membrane Na+ permeability.     

The electrical potential difference, short circuit current and Na+ and Cl- transport across different segments of sheep colon

1. The electrical potential difference (pd) and short circuit current (Isc) across the sheep colon descendens was significantly higher than across the sheep colon ascendens. 2. The ion equivalent of the Isc and the net Na+ transport from the mucosal (m) to the serosal (s) side of the short-circuited sheep colon descendens were identical, while the net Na+ transport across the colon ascendens exceeded the ion equivalent of the Isc. 3. There was a net m-s Cl- transport across both short-circuited colon segments, indicating that Cl-, like Na+, is absorbed by active transport. 4. The results suggest that active Na+ transport across the sheep colon descendens occurs entirely by an electrogenic mechanism, whereas active Na+ transport across the sheep colon descendens occurs entirely by an electrogenic mechanism, whereas active Na+ transport across sheep colon ascendens probably occurs by both an electrogenic and an electrically silent mechanism.     

Uncoupled basal sodium absorption and chloride secretion in prairie dog (Cynomys ludovicianus) gallbladder.

1. Prairie dog gallbladders mounted in a Ussing-type chamber and bathed with symmetrical Ringer's solutions exhibited a transepithelial resistance (Rt) of 51 +/- 5 omega cm2, a lumen negative potential difference (Vms) of 11.5 +/- 0.7 mV and a short-circuit current (Isc) of 6.9 +/- 0.3 microEq/hr/cm2. 2. Radioisotopic ion flux experiments revealed that the basal Isc of 6.9 +/- 0.3 microEq/hr/cm2 was mostly accounted for by net Na+ absorption of 3.2 +/- 0.5 microEq/hr/cm2 and net Cl- secretion of 2.9 +/- 0.3 microEq/hr/cm2. 3. In HCO3- free Ringer's, net Na+ flux was virtually abolished, net Cl- flux decreased by 50% and Isc was reduced by 77%. 4. 10(-3) M mucosal amiloride and DIDS reduced Isc by 28 and 24%, respectively. 5. Mucosal NaCl diffusion potentials indicated that the paracellular pathway was cation selective. 6. Thin section electron micrographs showed a single cell population in this epithelium suggesting that net Na+ absorption and Cl- secretion may emerge from the same cells. 7. We conclude that prairie dog gallbladder epithelium is an electrogenic tissue and, in contrast to gallbladders of most other species, simultaneously but independently absorbs Na+ and secretes Cl-.     

Bile salt alteration of ion transport across jejunal mucosa.

The effect of conjugated dihydroxy and trihydroxy bile salts on electrolyte transport across isolated rabbit jejunal mucosa was studied. Both taurochenodeoxycholic acid and taurocholic acid increased the short-circuit current (Isc) in bicarbonate-Ringer solution but not in a bicarbonate-free, chloride-free solution. Taurochenodeoxycholic acid was significantly more effective than taurocholic acid in increasing Isc. The presence of theophylline prevented the taurochenodeoxycholic acid- and taurocholic acid-induced increase in Isc. Transmural ion fluxes across jejunal mucosa demonstrated that 2 mM taurochenodeoxycholic acid decreased net Na+ absorption, increased net Cl- secretion and increased the residual flux (which probably represents HCO3- secretion). These studies support the hypothesis that cyclic AMP may be a mediator of intestinal electrolyte secretion.     

Mechanisms of transport of Na+ and Cl- in the lizard colon

1. Ionic fluxes of sodium and chloride across lizard colon mucosa were measured and compared with the electrical characteristics of the tissue under voltage-clamped conditions. 2. In a Ringer-bicarbonate solution there was both a net sodium flux (JNanet) and a net chloride flux (JClnet) from mucosa to serosa. The net flux residual (JR) was near zero, indicating that net sodium and chloride transport is the result of an electrically neutral transport mechanism. 3. In the presence of sodium, the net chloride flux was abolished and the short-circuit current (Isc) and the electrical potential difference (PD) were unchanged. In the absence of chloride the net sodium flux was abolished and the short-circuit current and electrical potential difference were not modified. 4. From an analysis of the effects of the inhibitors, furosemide, amiloride and disulfonic stilbene (DIDS), a plausible model was developed to explain the characteristics of sodium and chloride absorption.     

Effect of theophylline on ion transport in the lizard colon

1. The effect of theophylline on ion transport was examined using an in vitro short-circuited preparation of lizard colon. 2. Theophylline increased short circuit current (Isc) and transmural potential difference (PD). This increase caused by theophylline was accompanied by a small increase in transmural conductance (Gt). 3. Theophylline did not inhibit the absorption of Na+ but reversed Cl- absorption to secretion. This latter effect was due to an increase of the serosal-to-mucosal flux of Cl-. 4. Ion substitution experiments revealed that the effect of theophylline was Na+- and HCO3(-)-dependent and that these ions were required in the bathing solution. 5. These results with lizard colon are compared with those reported for mammalian colon and the mechanism of theophylline-induced Cl- secretion in these epithelia is discussed.     

The identity of the current carriers in canine lingual epithelium in vitro

Ion transport across the lingual epithelium has been implicated as an early event in gustatory transduction. The fluxes of isotopically labelled Na+ and Cl- were measured across isolated canine dorsal lingual epithelium under short-circuit conditions. The epithelium actively absorbs Na+ and to a lesser extent actively secretes Cl-. Under symmetrical conditions with Krebs-Henseleit buffer on both sides, (1) Na+ absorption accounts for 46% of the short-circuit current (Isc); (2) there are two transcellular Na+ pathways, one amiloride-sensitive and one amiloride-insensitive; (3) ouabain, added to the serosal solution, inhibits both Isc and active Na+ absorption. When hyperosmotic (0.25 M) NaCl is placed in the mucosal bath, both Isc and Na+ absorption increase; net Na+ absorption is at least as much as Isc. Ion substitution studies indicate that the tissue may transport a variety of larger ions, though not as effectively as Na+ and Cl-. Thus we have shown that the lingual epithelium, like other epithelia of the gastrointestinal tract, actively transports ions. However, it is unusual both in its response to hyperosmotic solutions and in the variety of ions that support a transepithelial short-circuit current. Since sodium ion transport under hyperosmotic conditions has been shown to correlate well with the gustatory neural response, the variety of ions transported may likewise indicate a wider role for transport in taste transduction.     

Intestinal glucose and galactose transport in the cultured gilthead bream (Sparus aurata)

1. Electrical parameters and transepithelial glucose and galactose transport were determined in vitro across anterior and posterior intestine of the culture fish Sparus aurata. 2. Electrical potential difference (PD) and short-circuit current (Isc) were serosa-positive in anterior intestine, while they were serosa-negative or near zero in posterior intestine. 3. Tissue conductance (Gt) was higher in posterior than in anterior intestine. In both parts it was decreased when the Na ion was omitted in mucosal and serosal reservoirs. 4. Addition of glucose or galactose to the mucosal side of intestine caused an increase in PD and Isc in posterior intestine but did not significantly change PD and Isc in anterior intestine. 5. Isotopic flux of glucose and galactose measurements in short-circuit conditions showed a net active glucose and galactose absorption in posterior intestine, while in anterior intestine active transport of glucose or galactose was not observed. 6. The net transport of glucose and galactose in posterior intestine was decreased to zero in the absence of Na in mucosal and serosal reservoirs or in the presence of ouabain (1 mM) in serosal solution.     

Effect of E. coli heat-stable enterotoxin on colonic transport in guanylyl cyclase C receptor-deficient mice

We studied the functional importance of the colonic guanylyl cyclase C (GCC) receptor in GCC receptor-deficient mice. Mice were anesthetized with pentobarbital sodium, and colon segments were studied in Ussing chambers in HCO3- Ringer under short-circuit conditions. Receptor-deficient mouse proximal colon exhibited similar net Na+ absorption, lower net Cl- absorption, and a negative residual ion flux (J(R)), indicating net HCO3- absorption compared with that in normal mice. In normal mouse proximal colon, mucosal addition of 50 nM Escherichia coli heat-stable enterotoxin (STa) increased the serosal-to-mucosal flux of Cl- (J(s-->m)(Cl)) and decreased net Cl- flux (J(net)(Cl)) accompanied by increases in short-circuit current (I(sc)), potential difference (PD), and tissue conductance (G). Serosal STa had no effect. In distal colon neither mucosal nor serosal STa affected ion transport. In receptor-deficient mice, neither mucosal nor serosal 500 nM STa affected electrolyte transport in proximal or distal colon. In these mice, 1 mM 8-bromo-cGMP produced changes in proximal colon J(s-->m)(Cl) and J(net)(Cl), I(sc), PD, G, and J(R) similar to mucosal STa addition in normal mice. We conclude that the GCC receptor is necessary in the mouse proximal colon for a secretory response to mucosal STa.     

Effect of furosemide on unidirectional fluxes of sodium and chloride across the skin of the frog, Rana pipiens.

1. The diuretic furosemide, when added to the outside solution at a concentration of 5-10-4 M, increases the electrical potential difference (PD) across the isolated frog skin, but the short-circuit current (Isc) is unchanged. Lower concentrations had no significant effect on these electrical parameters. 2. When SO42- or NO3- are substituted for Cl- in the Ringer's solution furosemide has no effect on the PD or Isc. 3. Simultaneous unidirectional fluxes of Na+ and Cl- show that furosemide (5-10-4 M outside) reduces both the influx and outflux of Cl-, while the Na+ fluxes are not altered. 4. Furosemide (5-10-4 M) on the corium side of the frog skin had no significant effect on either PD, Isc or undirectional fluxes of Cl-. 5. It is suggested that furosemide reduces passive Cl- transfer, possibly by interacting with the Cl-/Cl- exchange diffusion mechanism which has been observed in this tissue. These observations further suggest that perhaps the diuretic action of furosemide may be mediated by such an effect on passive Cl- permeability which is linked to the active Cl- transport mechanism in the renal tubule.     

Ion transport in the intestine of Gobius niger in both isotonic and hypotonic conditions

Ion transport in the intestine of Gobius niger, a euryhaline teleost, was studied in both isotonic and hypotonic conditions. Isolated tissues, mounted in Ussing chambers and bilaterally perfused with isotonic Ringer solution, developed a serosa negative transepithelial voltage and a short circuit current indicating a net negative current in absorptive direction. Bilateral removal of Cl- and Na+ from the bathing solutions as well as the luminal removal of K+in the presence of Ba2+(10(-3) M) almost abolished both Vt and Isc. Similar results were obtained by adding bumetanide (10(-5)M) to the luminal bath while other inhibitors of Cl- transport mechanisms were ineffective. These observations suggest that salt absorption begins with a coupled entry of Na+, Cl-, and K+ across the apical membrane; a Ba2+inhibitable K+ conductance, demonstrated also by micropuncture experiments, recycles the ion into the lumen. Salt entry into the cell is driven by the operation of the basolateral Na+/K(+)-ATPase since serosal ouabain (10(-4)M) completely abolished both Vt and Isc; this pump also completes the Na(+) absorption. The inhibitory effect of both serosal bumetanide (10(-4)M) and SITS (5 x 10(-4)M) suggests that Cl- would leave the cell via the KCl cotransport, the Cl/HCO3- antiport and/or conductive pathways. Bilateral exposure of tissues to hypotonic media produced a reduction of both the transepithelial voltage and the short circuit current probably due to the activation of homeostatic ionic fluxes involved in cell volume regulation. The results of experiments with both isolated enterocytes and intestine exposed to hypotonic solution suggested that the recovery of cell volume, after the initial cell swelling, involves a parallel opening of K+ and Cl- channels to facilitate net solute and water effluxes from the cell. J. Exp. Zool. 301A:49-62, 2004.     

Avian colonic ion transport: effects of corticosterone and dexamethasone

1. Corticosterone, a natural corticosteroid hormone in birds, when injected into domestic fowl (Gallus domesticus) (2000 micrograms.kg-1, 4-5 h before experiment) increases both the basal Isc (short-circuit current) and amiloride-sensitive Isc as well as the PD across the colon in vitro. Dexamethasone, a synthetic analogue (650 micrograms.kg-1, 4-5 h before experiment) also increases the basal and amiloride-sensitive Isc as well as PD in these preparations. 2. In marked contrast, longer term injection or infusion of dexamethasone (650 micrograms.kg-1) for 3 or more days caused a decline in basal Isc and PD (the PD often reversed with the serosal side becoming electronegative) and a drop in resistance. However in these preparations, the amiloride-sensitive Isc was significantly elevated which could be accounted for by an increase in net Na flux. 3. No significant change occurs in net flux of Cl or K although unidirectional fluxes in both directions were increased for both ions in birds given dexamethasone for 3 days. 4. A disparity between the basal Isc and the amiloride-sensitive Isc appeared in these preparations from dexamethasone injected birds reflecting the transport of other ions, possibly HCO3- or H+. The possible role of corticosterone in mineral metabolism of birds is discussed.     

Effects of metabolic inhibition on ion transport by dog bronchial epithelium

Mammalian bronchial epithelium absorbs Na+ under basal conditions, but Cl- secretion can be induced. We studied the effects of several modes of metabolic inhibition on the bioelectric properties and solute permeability of dog bronchial epithelium mounted in Ussing chambers. Net Na+ absorption and short-circuit current were inhibited by approximately 75% by hypoxia or by 10(-3) M NaCN. The reduced net Na+ absorption was characterized by a decrease in absorptive flux and an increase in backflux. The latter change was proportional to an increase in permeability to [14C]mannitol, implying that solute flow through a paracellular shunt was increased. In contrast, the reduction of conductance expected from exposure to amiloride (0.94 +/- 0.15 ms/cm2 or 12%) was abolished by NaCN pretreatment. Metabolic inhibition also decreased epithelial conductance and unidirectional Cl- fluxes by approximately 25%. NaCN rapidly and reversibly inhibited the hyperpolarization of potential difference (PD) induced by low luminal bath [Cl-]. This effect was mimicked by the Cl- channel blocker, 5-nitro-2-(3-phenylpropylamino) benzoic acid. Because the transepithelial Cl- diffusion PD reflects, in part, the depolarization of the Cl- -conductive apical cell membrane, metabolic inhibition appears to affect this path. We conclude that metabolic inhibition not only decreased net ion transport by dog bronchial epithelium but also inhibited cellular Na+- and Cl- -conductive pathways and increased paracellular permeability.     

5-(N,N-Dimethyl)-amiloride to discriminate the Unidirectional electrolyte transports in rat small intestine and proximal colon in vivo

The effect of dimethyl-amiloride (DMA), a selective Na+/H+ exchange blocker, was studied on electrolyte net fluxes and unidirectional fluxes of Na and Cl at four levels of rat intestine in vivo in basal conditions. DMA was applied intraluminally at concentrations of 10(-4) and 10(-3) M in the model of ligated loops prepared from duodenum, proximal jejunum, distal ileum and ascending colon in fasted Sprague Dawley rats. Two iso-osmotic test solutions were used: (1) hypo-ionic: Na+ 80 mM and (2) iso-ionic: Na+ 148 mM, pH 8.2. 22Na was placed in the loop and 36Cl was given by intravenous route at the beginning of the experiment. Na+/H+ was calculated by two different means, one was based on pH variation following amiloride inhibition of Na influx, the other on the calculation of the passive Na transport. The quantitative evaluation shows that Na/H exchange largely contributes to the electroneutral absorption and luminal pH regulation. The exchanger activity decreases from duodenum, jejunum, ileum and colon where it is completed by K/H exchange to assure low colon luminal pH.     

Net uptake of chloride across the posterior gills of the Chinese crab (Eriocheir sinensis)

Two methods are commonly used for the determination of transbranchial net fluxes of Na+ and Cl-: direct analysis of changes in ion concentrations in the external medium using flame spectrophotometry or titration (net flux method), and measurement of unidirectional ion fluxes by means of radioactive tracers (tracer method). When we applied both methods in the same preparation, the isolated perfused posterior gill of freshwater-acclimated Eriocheir sinensis, to determine net fluxes of Cl-, the results differed substantially. In artificial fresh water (AFW) containing NaCl, the net flux method yielded a net uptake, but the tracer method showed a net efflux of Cl-. The net uptake of Cl- was abolished in Na(+)-free AFW indicating that Cl- uptake is coupled with the uptake of Na+. Applying the tracer method, net efflux of Cl- remained almost unchanged in Na(+)-free AFW. This suggests the opposite mechanism, i.e. uncoupled uptake of Na+ and Cl-. The discrepancy in the results obviously depends on the method employed. Since the data obtained with the net flux method explain the osmoregulatory performance of crabs living in fresh water, we consider this method as appropriate for determining net transbranchial ion fluxes.     

The colon of Leucophaea maderae: fine structure and physiological features

The colon of L. maderae consists of a single columnar epithelium covered with a cuticle and of a musculo-connective sheath. The apical plasma membranes form a system of leaflets with numerous mitochondria inserted in association with microfilaments. Lateral plasma membranes are linked together by junctional complexes consisting of a zonula adherens and a long convoluted septate junction of the pleated type. In the basal region of the cell, numerous membrane infolds and scattered scalariform junctions with associated mitochondria are present. These cell specializations are typical of arthropod transporting organs, being distinctive features of ion and fluid transporting epithelia. The isolated colon exhibited a transepithelial electrical potential difference (PD) of about 100 mV, lumen side positive with respect to the haemolymph side. The PD was almost abolished by metabolic inhibitors, it was reduced by acetazolamide and SITS, and it was unaffected by ouabain. These effects suggest that HCO3- and Cl- are involved in the genesis of the PD, whereas Na+ is not directly responsible of the PD. Measurements of Na+ and Cl- fluxes across the colon wall confirm that Na+ moves following the PD across the tissue, while Cl- movement occurs against an electrochemical potential difference. The electrical profile of the epithelial cells is of the well type and it suggests that the primary or secondary active step for Cl- transport across the epithelium should be located at the mucosal border of the cell.     

Colonic absorption of inorganic ions and volatile fatty acids in the rabbit

Net fluxes of water, Na+, K+, Cl-, HCO3- and volatile fatty acids (VFA) were investigated in three different segments of rabbit colon. Two opposite phenomena occurred: secretion of water and inorganic ions in the oral part of the colon and absorption in the remaining colon; VFA were always absorbed. The movement of cations was closely correlated with those of VFA and Cl-. Results are consistent with the presence of exchange: Na+/H+, K+/H+, in the colon brush border membrane. In fact net absorption of cations and VFA seems linked to the availability of protons. In the absence of net cation transport an additional source of protons may be provided by hydration of luminal CO2. So VFA could enter mucosa by passive diffusion as the undissociated acids.