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.     

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 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.     

Electrogenic Na(+)-dependent L-alanine transport in the lizard duodenum. Involvement of systems A and ASC

L-Alanine transport across the isolated duodenal mucosa of the lizard Gallotia galloti has been studied in Ussing chambers under short-circuit conditions. Net L-alanine fluxes, transepithelial potential difference (PD), and short-circuit current (Isc) showed concentration-dependent relationships. Na(+)-dependent L-alanine transport was substantially inhibited by the analog alpha-methyl aminoisobutyric acid (MeAIB). Likewise, MeAIB fluxes were completely inhibited by L-alanine, indicating the presence of system A for neutral amino acid transport. System A transport activity was electrogenic and exhibited hyperbolic relationships for net MeAIB fluxes, PD, and Isc, which displayed similar apparent K(m) values. Na(+)-dependent L-alanine transport, but not MeAIB transport, was partially inhibited by L-serine and L-cysteine, indicating the participation of system ASC. This transport activity represents the major pathway for L-alanine absorption and seemed to operate in an electroneutral mode with a negligible contribution to the L-alanine-induced electrogenicity. It is concluded from the present study that the active Na(+)-dependent L-alanine transport across the isolated duodenal mucosa of Gallotia galloti results from the independent activity of systems A and ASC for neutral amino acid transport.     

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.     

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.     

Effects of NPPB (5-nitro-2-(3-phenylpropylamino)benzoic acid) on chloride transport in intestinal tissues and the T84 cell line.

NPPB (5-nitro-2-(3-phenylpropylamino)benzoic acid) has been reported to block Cl- channels in isolated rabbit nephrons with high potency (IC50 = 80 nM). The effects of this compound on Cl(-)-mediated transport processes in intestinal tissues have been studied using agonist-stimulated short-circuit current (T84) in Ussing chamber experiments and 36Cl- fluxes in monolayers of a colonic cell line (T84). NPPB inhibited PGE1-stimulated Isc in rabbit distal colon and ileum at concentrations in the range 20 to 100 microM. However, NPPB at the same concentrations also inhibited glucose-stimulated Isc in rabbit ileum, suggesting that its effects were not restricted to those on Cl- transport. Consistent with this, exposure of rabbit distal colon to 100 microM NPPB was found to reduce endogenous ATP levels by 69%, implying that, at these concentrations, NPPB could impair active transport processes by an effect on cellular energy metabolism. Clear evidence for a direct effect of NPPB on epithelial chloride channels was found in studies on Cl- fluxes in T84 cell monolayers. NPPB inhibited VIP-stimulated Cl- uptake into T84 cells with an IC50 of 414 microM. NPPB (1 mM) also inhibited Cl- efflux from pre-loaded cells confirming its effect as a weak Cl- channel blocker in this system.     

Melittin-induced changes in Na and Cl movements across the skin and cornea (in vitro) of the toad Bufo marinus

Melittin, from bee venom, increases short-circuit current (Isc) across the skin and cornea of toads. In skin this reflects a rise in the influx of Na and is inhibited by meclofenamic acid (inhibits prostaglandin synthetase). In corneas with melittin on the inside the rise in Isc is inhibited by bumetanide (inhibits Cl transport) and meclofenamic acid. Melittin on the tear side of the cornea causes a biphasic change in Isc, and a rise in all undirectional fluxes of Na and Cl. This effect was not changed by bumetanide or meclofenamic acid. Melittin apparently has two types of effects, one mediated by prostaglandins while the other is more direct.     

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.     

Cationophore properties of the new polyether antibiotic Salinomycin investigated in distal rabbit colon in vivo and in vitro

The distal rabbit colon was used as a model to investigate the influence of the cationophore Salinomycin in vivo with a single-pass perfusion, and in vitro with a modified Ussing chamber technique. For in vivo experiments with labelled 14C-PEG as a volume marker in the perfusate, a dose of 10E-4 mol/l Salinomycin was used. Net water (53 microliters/h/cm), net chloride (3 mumol/h/cm) and net sodium (3.6 mumol/h/cm) absorption was not significantly influenced, but net potassium secretion (-3 mumol/h/cm) was decreased to zero and transepithelial potential (PD) reduced from -45 mV to -33 mV. 10E-4 mol/l Salinomycin, applied in vitro on the muscosal side, decreased PD in 80 min and 10E-3 mol/l in 30 min from 18 mV to zero. Both concentrations decreased the short-circuit current (Isc = 77 microA/cm2) in 60 min, respectively 30 min to 40 microA/cm2. After 60 min mucosal 10E-4 mol/l Salinomycin the Isc increased, resulting from a transepithelial conductance (Gt) increase from 3 to 40 mS/cm2. A dose-related effect was present on PD, Isc and Gt at concentrations between 10E-7 and 10E-6 mol/l. The unidirectional 22Na fluxes were increased to 20 times the control values and net Na transport disappeared. We conclude that Salinomycin when applied in usual doses (10E-4 mol/l) as a coccidiostatic feed additive, profoundly affects colonic electrolyte transport.     

Ion transport across the chick ileum: a good model for transport studies.

1. The young chick (5-8 days) has been found to be an excellent preparation for the study of transepithelial intestinal ion transport. Due to the thinness of the intestinal tissue, it is not necessary to remove the serosal layers (serosal membranes, circular, and longitudinal muscles), thus circumventing the problems inherent in "stripping" the tissue. 2. The intact chick ileum had a significantly greater short-circuit current (Isc) and lower resistance than did intact adult ileum and transport parameters remained stable over the 6 hr experimental period. 3. Compared to the adult tissue, unidirectional fluxes of Na and Cl were greater in the chick ileum. Net flux of Na (absorption) was about 3 times greater in the chick ileum and the flux was equivalent to the Isc, thus this preparation appears to be characterized by electrogenic Na absorption. 4. Several ileal preparations from day old chicks were studied over an 18 hr period and these preparations were found to remain viable for this period of time with the Isc at the end of 18 hr being nearly identical to that at 2 hr. 5. Besides the advantage of not having to strip the intestinal tissue, and the long-term viability of the tissue, the chick is very inexpensive and easy to obtain and maintain.     

Na, Cl, and Water Transport by Rat Colon

Segments of the colon of anesthetized rats have been perfused in vivo with isotonic NaCl solutions and isotonic mixtures of NaCl and mannitol. Unidirectional and net fluxes of Na and Cl and the net fluxes of water and mannitol have been measured. Net water transport was found to depend directly on the rate of net Na transport. There was no water absorption from these isotonic solutions in the absence of net solute transport, indicating that water transport in the colon is entirely a passive process. At all NaCl concentrations studied, the lumen was found to be electrically negative to the surface of the colon by 5 to 15 mv. Na fluxes both into and out of the lumen were linear functions of NaCl concentration in the lumen. Net Na absorption from lumen to plasma has been observed to take place against an electrochemical potential gradient indicating that Na is actively transported. This active Na transport has been interpreted in terms of a carrier model system. Cl transport has been found to be due almost entirely to passive diffusion.     

Sodium and potassium concentrations in the intestinal fluids of the fowl, Gallus domesticus

1. Na and K concentrations in the luminal fluids on the jejunum, ileum and colon were measured in domestic fowl on diets containing different amounts of Na and K. 2. Physiological adjustments of the Na and K content of these fluids were observed to occur in all three intestinal segments. 3. Regulation of gastrointestinal losses of Na and K appears to be initiated in the anterior regions of the intestines of the domestic fowl and is maintained or amplified as the ingesta moves posteriorly.     

Electrogenic ion transport in the intestine of the Australian common brushtail possum, <Emphasis Type="Italic">Trichosurus vulpecula</Emphasis>: indications of novel transport patterns in a marsupial

In this study, electrogenic ion transport in the intestine of the Australian common brushtail possum, Trichosurus vulpecula was investigated. In the ileum, a Na(+)-dependent, phloridzin- and amiloride-insensitive short-circuit current ( Isc) was present. Mucosal glucose stimulated a further phloridzin-sensitive, dose-dependent increase in Isc. A Na(+)-dependent, ouabain-sensitive Isc was also present in the caecum and colon. In the proximal and distal colon, amiloride (100 micro mol l(-1), mucosal) inhibited this Isc by 81+/-4% and 65+/-3%, respectively and the Ki for amiloride (approximately 1 micro mol l(-1)) was consistent with the inhibition of a classical epithelial Na(+) channel. In the caecum, 50% of the Isc was inhibited by amiloride (100 micro mol l(-1), mucosal). The amiloride-insensitive Isc in the colon was not due to electrogenic Cl(-) secretion, as serosal bumetanide (100 micro mol l(-1)) had no effect on the Isc. Furthermore, the secretagogues forskolin (10 micro mol l(-1)), carbachol (100 micro mol l(-1)) and dibutyryl-cAMP or dibutyryl-cGMP (100 micro mol l(-1)) did not stimulate electrogenic Cl(-) secretion by the colon. These results indicate that the transport properties of the hindgut of the possum differ significantly from those of eutherian mammals and may be associated with different functions of the hindgut of possums when compared to eutherian mammals.     

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.     

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.     

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.     

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.     

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.     

Absorption of electrolytes and water by the jejunum and colon in milk-fed lambs

Net absorption of electrolytes (Na, Cl, K, Ca) and water from ligated loops was studied at various intestinal sites in milk-fed lambs. The unidirectional fluxes of Na across the intestinal mucosa were also investigated using 22Na. Net Na and water absorption in the mid-jejunum were about two-fold higher than in the proximal and distal jejunum and the colon descendens. With the exception of the proximal jejunum, Na and Cl absorption did not differ significantly. The unidirectional Na fluxes in both directions were much higher in the proximal and mid-jejunum than in the distal jejunum and colon descendens. K was also absorbed most efficiently from the mid-jejunum. In the colon descendens mean net K absorption was about zero. Ca absorption in the upper and mid-jejunum exceeded that of the distal jejunum and colon descendens, where the values were close to zero. The results show that in the whole jejunum of young milk-fed lambs net absorptive fluxes of Na, Cl, K, Ca and water occur, whereas the colon descendens appears to play a role only in Na, Cl and water absorption.