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.     

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.     

Bioelectric properties and ion transport across excised canine fetal and neonatal airways

Knowledge of liquid secretion by fetal lung stems from studies of sheep. We extended these studies to dogs and examined the persistence of the fetal pattern of airway epithelial permeability and ion transport in the neonatal animal. Plasma and lung liquid from fetal dogs were analyzed for Na+, K+, Cl-, and HCO3-. Only the Cl- concentration of fetal lung liquid (129 meq/l) was significantly different from that of fetal plasma (111 meq/l). Segments of trachea from fetal and neonatal (less than 1, 7-10, and 21-46 days after birth) dogs were excised and mounted in flux chambers. The transepithelial potential difference (PD) of all tissues was oriented lumen negative (9.8-14.8 mV). Under short-circuit conditions, unidirectional Na+ flows were symmetrical. Cl- was secreted, and the secretion was equivalent to short-circuit current (Isc). Cl- secretion persisted under open-circuit conditions. Lobar bronchi from 21- to 46-day neonates absorbed Na+ (1.9 mueq.cm-2.h-1), but unidirectional flows of Cl- were symmetrical. Amiloride (10(-4) M) reduced Isc of neonatal bronchi by 47% but did not affect fetal bronchi. Isoproterenol increased Isc of both fetal (33%) and neonatal (40%) bronchi. These responses suggest that fetal bronchi do not absorb Na+ but can be stimulated to secrete Cl-. We conclude that Cl- secretion by epithelium of large airways may contribute to fetal lung liquid production, but it is unlikely that the tracheal epithelium is involved in fluid absorption at birth. Whereas fetal bronchi appear to secrete Cl-, neonatal bronchi absorb Na+.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

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.     

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-.     

Bioelectric properties and ion transport of airways excised from adult and fetal sheep

Segments of fetal and maternal trachea, maternal bronchi from near-term sheep, and trachea and bronchi from nonpregnant adult sheep were excised and mounted as sheets in Ussing chambers. The conductance (G) for each group of tissues was similar (approximately 4 mS/cm-2); the short circuit current (Isc) ranged from 45-90 microA/cm-2. Under short-circuit or open-circuit conditions trachea and bronchi from pregnant and nonpregnant adult animals absorbed Na+, whereas fetal trachea secreted Cl-. Short-circuited maternal bronchi secreted K+, whereas maternal and fetal trachea did not. Isoproterenol induced an increase in Isc, G, and Cl- secretion of fetal trachea. Maternal trachea and bronchi were not affected. Amiloride reduced Na+ absorption and Isc of maternal trachea and bronchi, but had little effect on fetal trachea. The permeability of fetal trachea to 14C-mannitol was 17 X 10(-7) cm/s and was not affected by isoproterenol. The permeation of dextran (10 K) and horseradish peroxidase across fetal trachea and of all three probes across maternal airways did not reach steady state, but the relative rates were compatible with an equivalent pore radius greater than 4 nm. We conclude that ion transport in fetal large airways contributes to the Cl- and liquid secretion by the entire fetal pulmonary epithelium, whereas resting ion transport of large airways from adult sheep, like that of mature airways of many species, is dominated by Na+ absorption. All of these airway epithelia are characterized by large paracellular aqueous paths.     

Ion transport across cat and ferret tracheal epithelia

Sheets of trachea from ferret and cat were mounted in Ussing chambers and continuously short circuited. Under resting conditions, in both the cat and ferret there was little or no Cl secretion, and Na absorption accounted for most of the short-circuit current (Isc). Ouabain (10(-4) M, serosal bath) reduced Isc to zero in 30-60 min. This decline was matched by a decrease in net Na absorption. Amiloride (10(-4) M, luminal bath) caused a significant decrease in Isc and conductance (G) in both species. Bumetanide (10(-4) M, serosal bath) had negligible effects on Isc and G. In both species, isoproterenol increased Isc by stimulating Cl secretion. Methacholine induced equal amounts of Na and Cl secretion, with little change in Isc. In the cat, prostaglandins E2 and F2 alpha and bradykinin increased Isc, responses which were abolished in Cl-free medium. In open-circuited cat tissues, Na flux from the serosal to mucosal side was measured simultaneously with the secretion of nondialyzable 35S. Prostaglandins E1, E2, and F2 alpha, histamine, bradykinin, methacholine and isoproterenol all increased both Na and 35S-mucin secretion.     

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.     

Cl- transport by gastric mucosa: cellular Cl- activity and membrane permeability

The mechanism of Cl- secretion in the isolated, resting (i.e. cimetidine-treated) gastric mucosa of Necturus has been investigated with radioisotopic and electrophysiological techniques. Measurement of transepithelial 36Cl- fluxes (mucosal to serosal (M leads to S), Jms Cl-; S leads to M, Jsm Cl-) during control conditions show that at open circuit, when the transepithelial potential difference psi ms = 20 mV (S ground), Jms Cl- = Jsm Cl-, i.e. Jnet Cl- = 0, but during short-circuit current conditions Jnet Cl- = I sc = 2 mu equiv cm-2 h. Experiments with low [Cl-] solutions indicate that Cl- exchange diffusion does not contribute significantly to either Jms Cl- or Jsm Cl-. Double-barrelled, Cl- -selective microelectrodes showed that in open circuit, the cellular (C) chemical potential for Cl-, psi c Cl- = 31 mV (apparent [Cl-] = 29 mM), the electrical potential across the M membrane, psi m = -34 mV (mucosa ground) while that across the S membrane, psi s = -52 mV (serosa ground). During short-circuit current conditions, psi m = psi s = -49 mV and [Cl-]c = 30 mM. The permeability of the M membrane to Cl- (Pm Cl-) was calculated both from the tracer experiments and the electrode measurements by using the constant-field equation. Short-term (45 s) uptake of 36Cl- at [Cl]m = 96 mM during short circuit conditions gave Pm Cl- = 2.6 x 10(-5) cm s-1. Measurement of [Cl-]c by means of the electrodes when [Cl-]m was changed from 96 to 2 mM or from 2 to 96 mM gave Pm Cl- = 2.9-5.7 x 10(-5) cm s-1. Our results indicate that during open circuit conditions Cl- is accumulated across the S membrane into gastric cells in an energy-requiring step, but since Jnet Cl- = 0, Cl- must leak back into the S solution at a rate equal to the entry rate. When the tissue is short-circuited, Cl- secretion occurs (Jnet Cl- = Isc) owing to the same energy-requiring accumulation of Cl- by the cells and a passive (apparently electrodiffusive) movement across the mucosal membrane.     

In vitro effects of tetrodotoxin and hexamethonium on electrolyte transport in rabbit ileum treated with cholera toxin.

To determine if there was a role for the submucosal nerves in cholera toxin (CT)-induced secretion, we studied the effects of serosal addition of two neurotoxins, the nerve conduction blocking agent, tetrodotoxin (TTX), and the nicotinic ganglionic blocking agent, hexamethonium (HXM), on electrolyte secretion in control isolated rabbit ileum and in that stimulated by CT. 1). In the absence of CT, the short circuit current (Isc) decreased after TTX (10(-7) M) (P less than 0.01) and was unaltered by HXM (10(-5) M). In the presence of CT, Isc increased but was not modified by 10(-7) M TTX or 10(-5) M HXM. 2) In control tissues the mean isotopic Na+ and Cl- fluxes were not significantly altered by TTX addition. Cl- absorption alone was significantly reduced by HXM (delta JCl- = 1.95 +/- 0.81 microEq.hr-1.cm-2; P less than 0.02). After stimulation with CT, TTX significantly inhibited Na+ and Cl- secretion (delta JNa+ = 2.15 +/- 0.61 and delta JCl- = 2.15 +/- 0.76 microEq.hr-1.cm-2; P less than 0.01). Similarly, HXM significantly inhibited CT-stimulated Na+ and Cl- secretion (delta JNa+ = 1.73 +/- 0.70 and delta JCl- = 1.46 +/- 0.62 microEq.hr-1.cm-2; P less than 0.02). 3) In TTX and HXM treated tissues there was no difference in the increase in Isc caused by cAMP (2 x 10(-3) M), calcium ionophore A 23187 (4 x 10(-6) M) and glucose (10(-3) M) compared to the untreated tissues in the presence or absence of CT.(ABSTRACT TRUNCATED AT 250 WORDS)     

Octreotide inhibits increases in short-circuit current induced in rat colon by VIP, substance P, serotonin and aminophylline.

We investigated the effect of octreotide (OCT), a stable somatostatin analog, (OCT) on changes in short-circuit current (Isc) induced by vasoactive intestinal peptide (VIP), aminophylline, serotonin (5-HT) and substance P. OCT significantly decreased basal Isc at a concentration of 10(-9) M; the maximum decrease in Isc was observed at 10(-6) M. OCT (10(-7) M) significantly inhibited the intestinal secretory response to all the secretagogues studied. The maximum Isc response was reduced when tissues were stimulated with VIP (184.9 +/- 18.0 vs. 119.7 +/- 14.1, P less than 0.05), 5-HT (135.1 +/- 14.4 vs. 79.5 +/- 13.4, P less than 0.05) and substance P (156.0 +/- 19.2 vs. 30.7 +/- 5.4, P less than 0.01). In the case of aminophylline, the concentration-response curve was shifted to the right but the maximum response was not reduced. Because VIP and aminophylline increase cAMP while 5-HT and substance P stimulate intestinal secretion principally by a calcium linked mechanism, we conclude that OCT inhibits Isc in rat colon by more than one mechanism.     

Effect of chloride on pH microclimate and electrogenic Na+ absorption across the rumen epithelium of goat and sheep

Active Na+ absorption across rumen epithelium comprises Na+/H+ exchange and a nonselective cation conductance (NSCC). Luminal chloride is able to stimulate Na+ absorption, which has been attributed to an interaction between Cl-/HCO3- and Na+/H+ exchangers. However, isolated rumen epithelial cells also express a Cl- conductance. We investigated whether Cl- has an additional effect on electrogenic Na+ absorption via NSCC. NSCC was estimated from short-circuit current (Isc) across epithelia of goat and sheep rumen in Ussing chambers. Epithelial surface pH (pHs) was measured with 5-N-hexadecanoyl-aminofluorescence. Membrane potentials were measured with microelelectrodes. Luminal, but not serosal, Cl- stimulated the Ca2+ and Mg2+ sensitive Isc. This effect was independent of the replacing anion (gluconate or acetate) and of the presence of bicarbonate. The mean pHs of rumen epithelium amounted to 7.47 +/- 0.03 in a low-Cl- solution. It was increased by 0.21 pH units when luminal Cl- was increased from 10 to 68 mM. Increasing mucosal pH from 7.5 to 8.0 also increased the Ca2+ and Mg2+ sensitive Isc and transepithelial conductance and reduced the fractional resistance of the apical membrane. Luminal Cl- depolarized the apical membrane of rumen epithelium. 5-Nitro-2-(3-phenylpropylamino)-benzoate reduced the divalent cation sensitive Isc, but only in low-Cl- solutions. The results show that luminal Cl- can increase the microclimate pH via apical Cl-/HCO3- or Cl-/OH- exchangers. Electrogenic Na+ absorption via NSCC increases with pH, explaining part of the Cl- effects on Na+ absorption. The data further show that the Cl- conductance of rumen epithelium must be located at the basolateral membrane.     

Electrical properties and fluxes of Na+ and Cl- across lizard intestine

Electrical parameters and unidirectional Na+ and Cl- fluxes were determined in vitro across the duodenum, ileum and colon of lizard (Gallotia galloti). Electrical potential difference (PD) and short circuit current (Isc) were low in the three segments studied, whilst tissue conductance (Gt) was high. A net active transport of Na+ and Cl- was observed in the three segments. Net Na+ absorption was higher across duodenum and ileum than across the colon, while net Cl- absorption was similar in duodenum, ileum and colon. Ouabain virtually abolished Isc, PD and net Na+ and Cl- fluxes in all the segments. Amiloride abolished net Cl- flux in duodenum, ileum and colon, whereas net Na+ flux was abolished in colon but decreased in duodenum and ileum. PD and Isc were not affected by the presence of the diuretic.     

Stimulation by aldosterone of a conductive chloride pathway in toad skin

As a rule, chloride movement (JC1-) across amphibian skin is considered to be passive; this is implied in fact for preparations incubated in Ringer's fluid, since short-circuit current (Isc) is the quantitative expression of net, active sodium transport (JNa+). The nature of the Cl- pathway(s) was investigated by incubating amphibian skin (mostly Bufo marinus) with Cl- present on the epithelial side only, and after blocking JNa+ by combined treatment with ouabain and amiloride. In such conditions, JCl- was found to be equal to (reversed) Isc; furthermore, when JCl- was "translated" in terms of conductance, gCl-, the latter accounted almost quantitatively for transepithelial conductance, g1. When residual intratissue (i.e. intracellular) electronegativity was eliminated by replacing Na+ with K+, JCl- was larger but Isc and JCl- were still found to reflect each other, and gCl- again accounted for most, if not all, of g1. JCl- in the opposite direction, as a result of Cl- being present only on the dermal side, was negligible, and g1 was very low. Thus, in the absence of sodium transport, when experimental conditions are such that a net inward JCl- obtains, the anion apparently flows only through (a) conductive pathway(s). Aldosterone is probably involved in the regulation of this pathway, as JCl- was much lower when toads were maintained in dilute saline than in water or on moist peat; so was the fraction of the apical surface corresponding to mitochondria-rich cells.     

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.     

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.     

Ion transport in an immortalized rat submandibular cell line SMG-C6

The immortalized rat submandibular epithelial cell line, SMG-C6, cultured on porous tissue culture supports, forms polarized, tight-junction epithelia facilitating bioelectric characterization in Ussing chambers. The SMG-C6 epithelia generated transepithelial resistances of 956+/-84Omega.cm2 and potential differences (PD) of -16.9 +/- 1.5mV (apical surface negative) with a basal short-circuit current (Isc) of 23.9 +/- 1.7 microA/cm2 (n = 69). P2 nucleotide receptor agonists, ATP or UTP, applied apically or basolaterally induced a transient increase in Isc, followed by a sustained decreased below baseline value. The peak DeltaIsc increase was partly sensitive to Cl- and K+ channel inhibitors, DPC, glibenclamide, and tetraethylammonium (TEA) and was completely abolished following Ca2+ chelation with BAPTA or bilateral substitution of gluconate for Cl-. The major component of basal Isc was sensitive to apical Na+ replacement or amiloride (half-maximal inhibitory concentration 392 nM). Following pretreatment with amiloride, ATP induced a significantly greater Isc; however, the poststimulatory decline was abolished, suggesting an ATP-induced inhibition of amiloride-sensitive Na+ transport. Consistent with the ion transport properties found in Ussing chambers, SMG-C6 cells express the rat epithelial Na+ channel alpha-subunit (alpha-rENaC). Thus, cultured SMG-C6 cells produce tight polarized epithelia on permeable support with stimulated Cl- secretory conductance and an inward Isc accounted for by amiloride-sensitive Na+ absorption.     

Effect of short-chain fatty acids on cyclic 3',5'-guanosine monophosphate-mediated colonic secretion

Short chain fatty acids (SCFA) prevent and reverse cyclic 3',5'-adenosine monophosphate (cAMP) but not Ca(2+)-mediated Cl- secretion. Mucosal [HCO3-]i has an opposite effect on these secretagogues. We examined whether SCFA and [HCO3-]i affect cyclic 3',5'-guanosine monophosphate (cGMP)-induced secretion. Stripped segments of male Sprague-Dawley rat (Rattus norvegicus) proximal and distal colon, and cultured T84 cells were studied in Using chambers, and pHi and [HCO3-]i were determined. Mucosal [cGMP] was measured in proximal colon. In T84 cells, the increase in Cl- secretion (measured as Isc) induced by mucosal 0.25 microM Escherichia coli heat-stable enterotoxin (STa) was prevented/reversed by bilateral 50 mM Na+ butyrate (71%/73%), acetate (58%/76%), propionate (68%/73%) and (poorly metabolized) isobutyrate (80%/79%). In proximal colon in HCO3- Ringer, basal Cl- secretion was not affected by [HCO3-]i or 25 mM butyrate. Mucosal 0.25 microM STa decreased net Na+ and Cl- absorption. Bilateral but not mucosal 25 mM SCFA reversed STa-induced effects on Na+ absorption and Cl- secretion. Bilateral and mucosal 25 mM SCFA but not [HCO3-]i prevented STa-induced Cl- secretion and increases in mucosal [cGMP]. STa did not produce Cl- secretion in distal colon. It was concluded that SCFA but not [HCO3-]i can prevent and reverse cGMP-induced colonic Cl- secretion.     

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.