Electrolyte transport across the rabbit caecum in vitro

Electrolyte transport across rabbit caecal epithelium was investigated in vitro using conventional shortcircuiting and radioisotope techniques. In standard saline the caecum exhibited a relatively high short-circuit current (I _sc=4.4 μEq · cm⁻² · h⁻¹) and conductance (6.43 mS · cm⁻²). Both sodium and chloride were absorbed (J _net ^Na =6.40 andJ _net ^Cl =3.40 μEq · cm⁻² · h⁻¹) and potassium was secreted (J _net ^K =−0.5 μEq · cm⁻² · h⁻¹). Removal of Na⁺ abolishedI _sc andJ _net ^Cl whereas removal of Cl⁻ reducedJ _net ^Na to 2.92 μEq · cm⁻² · h⁻¹ but did not alterI _sc. In HCO ₃ ⁻ free salines containing 10⁻⁴ M acetazolamideJ _net ^Cl was abolished andJ _net ^Na andI _sc were reduced to 2.3 and 2.5 μEq · cm⁻² · h⁻¹ respectively. A positive residual ion flux (∼ 1 μEq · cm⁻² · h⁻¹) was detected in standard and Cl⁻-free salines but not in Na⁺-free or HCO ₃ ⁻ buffers. Mucosal amiloride (10⁻³ M) decreased net Na⁺ and Cl⁻ absorption but did not decreaseI _sc. Mucosal DIDS (10⁻⁴ M) decreasedJ _net ^Cl while mucosal bumetanide (10⁻⁴ M) did not affect any of the measured parameters. Finally, addition of theophylline (8 mM) stimulated Cl⁻ secretion and increasedI _sc. It is concluded that net sodium absorption by caecal epithelia occurs by both electrogenic and electroneutral mechanisms whereas net chloride absorption occurs only by an electroneutral process. Coupling of the absorptive fluxes of Na⁺ and Cl⁻ may result from Na⁺/H⁺ and Cl⁻/HCO ₃ ⁻ antiport systems in this tissue. Finally, it is proposed that up to half of theI _sc is due to a Na⁺-dependent secretion of bicarbonate ion.     

Mechanisms of oxalate absorption and secretion across the rabbit distal colon

To further evaluate the mechanisms of oxalate (Ox^2–) transport in the intestine the following studies were performed using isolated, short-circuited segments of the rabbit distal colon (DC). In control buffer, the DC absorbed Ox^2– (net Ox^2– flux, J _Net ^Ox =5.4±0.7 pmol · cm^–2 · h^–1). Replacement of Na⁺ with N-methyl-d-glucamine (NMDG⁺) abolished Ox^2– absorption by decreasing mucosal to serosal Ox^2– flux (J _ms ^Ox ), without affecting Cl^– transport, while gluconate substitution for Cl^– did not affect J _Net ^Ox or net Na⁺ flux (J _Net ^Na ). Addition of Na⁺ to the serosal side of tissues bathed by NMDG⁺ buffer increased J _ms ^Ox 40% without altering mucosal to serosal Cl^– flux (J _ms ^Cl ). Serosal amiloride or dimethyl amiloride (10^–3 M) abolished J _Net ^Ox by decreasing J _ms ^Ox , it increased serosal to muscosal Cl^– flux (J _sm ^Cl ) and it gradually inhibited short-circuit current (I _sc). Mucosal amiloride (10^–4 M) abolished I _sc but had no effect on Ox^2– or Cl^– fluxes. Serosal 4,4-diisothiocyanatostilbene-2,2-disulfonic acid (DIDS, 10^–6 M) reduced J _ms ^Ox by 20% and J _Net ^Ox by 43% without affecting J _ms ^Cl or J _Net ^Cl . Dibutyryl cyclic adenosine monophosphate (dB-cAMP, 5×10^–4 M, both sides) stimulated Ox^2– secretion (J _Net ^Ox = –12.6±3.3 pmol · cm^–2 · h^–1). The dB-cAMP-induced secretion of Ox^2– and Cl^– were fully abolished by serosal furosemide (10^–4 M) and partially inhibited (35%) by 5×10^–4 M mucosal NPPB [5-nitro-2-(3-phenylpropylamino)-benzoic acid], a putative Cl^– channel blocker. It is proposed that: (1) basal absorption of Ox^2–, but not Cl^–, is dependent upon a previously undescribed basolateral Na⁺-H⁺ exchanger that may be coupled to a DIDS-sensitive, basolateral anion exchange system that mediates Ox^2– flux; (2) the DC secretes Ox^2– in response to dB-cAMP by a mechanism that is indistinguishable from the pathway for Cl^– secretion.     

Sodium and chloride transport in the isolated human cornea

²²Na and³⁶Cl fluxes in isolated human cornea preparations were measured under short-circuited conditions. At pH 7.3–pH 7.6 sodium net flux was directed from aqueous humour to tear side, chloride net flux was statistically not different from zero. Alkalinization of the bathing solution to pH 8.6 stimulated unidirectional sodium and chloride fluxes from aqueous humour to tear side resulting in net sodium and chloride fluxes towards the tear side which both were statistically different from zero. Cyclic AMP (10^–3 M) was found to stimulate sodium and chloride uridirectional fluxes from aqueous humour to tear side, thus leading to significant net sodium and chloride fluxes in the same direction. Epinephrine (10^–4 M) increased the unidirectional chloride flux from aqueous humour to tear side more pronounced than in the opposite direction, producing a significant net chloride flux towards the tear side. The results are consistent with the hypothesis that the electrolyte pumps may under certain conditions contribute to the dehydration of the stroma.Abbreviations PD transcorneal potential difference - SCC short circuit current - R dc electrical resistance - TA unidirectional tear to aqueous humour side flux - AT unidirectional aqueous humour to tear side flux - net net flux Supported by Deutsche Forschungsgemeinschaft (Wi 328)     

In situ characterization of oxalate transport across the basolateral membrane of the proximal tubule

 Oxalate transport across the contraluminal membrane of the proximal tubule was studied in vivo using the ”capillary stopped flow microperfusion method” (Pflügers Arch 400:250–256, 1984). Cellular uptake of oxalate was characteristic of a carrier-mediated transport process (J _max = 1.6 ± 0.6 pmol/s per cm proximal tubular length, K _m = 2.03 ± 0.77 mmol/l). Sulphate inhibited oxalate transport in a dose-dependent manner (K _i-value = 1.53 ± 0.38 mmol/l). Sulphate transport across the basolateral membrane was also characteristic of a carrier- mediated transport process (J_max = 1.83 ± 0.56 pmol/s per cm proximal tubular length, K _m = 1.37 ± 0.57 mmol/l). Oxalate inhibited the sulphate transport in a dose-dependent manner (K _i = 2.06 ± 0.82 mmol/l). No significant differences were found between the K _i values and the K _m values of the two substances, indicating that oxalate and sulphate are transported by the same carrier across the basolateral membrane of the proximal tubule. Oxalate transport was not dependent on the extracellular sodium or potassium concentration. Bicarbonate competitively inhibited the oxalate transport. Chloride significantly inhibited the oxalate transport, but not dose dependently. It is, therefore, suggested that oxalate is transported into the cell of the proximal tubule in exchange for sulphate or bicarbonate. The dose-independent inhibition by chloride is suggested to be mediated by the coupling of the sulphate (bicarbonate)/oxalate exchanger with the chloride/bicarbonate exchanger at the basolateral membrane of the proximal tubule. This, furthermore, suggests that the transport of oxalate or sulphate across the basolateral membrane might be indirectly coupled with the reabsorption of chloride at this membrane side. Received: 5 August 1997 / Accepted: 8 January 1998     

Ion transport and electrophysiology of the early proximal colon of rabbit

The ion transport properties of the mammalian descending colon have been the subject of numerous investigations during the last decade. In contrast, relatively few studies have investigated proximal segments of this organ. In the present study, we assessed transepithelial transport of Na⁺, K⁺ and Cl^– in the isolated initial segment (P1) of rabbit colon in vitro using radioisotopic tracer fluxes and electrophysiological techniques. Like the rabbit descending colon, the proximal colon actively absorbs sodium and chloride, howeveer, its transport systems are markedly different. In vivo, this segment absorbs potassium, however in vitro active potassium secretion was observed. Unlike the descending colon, Na⁺ absorption is relatively insensitive to amiloride and only a slight inhibition was obtained even at 1 mM concentrations of this drug. Na⁺ and Cl^– absorption appeared to be coupled (directly or indicrectly) since the absorption of each ion was inhibited by the removal of the other. Serosal ouabain also inhibited Na⁺ and Cl^– absorption and net K⁺ secretion. Unlike the descending colon, the proximal P1 segment did not have a net absorptive K⁺ transport system that was detectable in the presence of ouabain. Electrically, the early proximal colon has a low transepithelial resistance compared to descending colon (R _T=133±7 cm²) but a larger short-circuit current (l _sc=178±12 A/cm²). The transepithelial potential averaged –21±1 mV, in excellent agreement with values measured in vivo. The apical and basolateral membrane potentials averaged –21±1 mV and –42±1 mV and intracellular potassium activity was 70±2 mM. The findings indicate active K⁺ uptake across the basolateral membrane and passive exit across the apical membrane. The basolateral membrane conductance may be a potassium conductance that is blockable by barium. It is likely that K⁺ transport normally occurs by both cellular and paracellular routes in this epithelium. Because of the numerous differences between this segment and the descending colon, we conclude that the P1 segment of proximal colon has a distinct function in colonic electrolyte transport     

Sodium dependence of the amino acid transport in the proximal convolution of the rat kidney

Summary With the technique of stop flow microperfusion with simultaneous capillary microperfusion the zero net flux transtubular concentration differences (c) of labelled amino acids which are equivalent to their active transport rates were measured. Alll-amino acids tested (phenylalanine, histidine, aminobicycloheptane-carboxylic acid, aminoisobutyric acid; lysine, ornithine, arginine; aspartic acid; proline and glycine) showed a considerable c, i.e. active transport rate. When, however, the ambient sodium was replaced by choline the c values dropped to zero. An analysis of the Na⁺ dependence of the ornithine transport revealed that the sodium-dependence is of the mixed type, i.e. thatK _m decreased andV _max increased with increasing Na⁺ concentration to the same extent.In contrast to other biological systems no mutual interaction between the Na⁺-dependentd-glucose andl-histidine transport could be observed.Incidental to these studies it was observed that the active transport rate ofd-histidine was in the range of 40% of that of thel-isomer while ford-phenylalanine it was only in the range of 10% of the active transport of thel-isomer. Furthermore it was found that thel-aspartic acid transport was already saturated at a luminall-aspartic acid concentration of 0.05 mmol/l while that ofl-phenylalanine was not saturated even at a luminal concentration of 9 mmol/l.     

The effect of harmaline on intestinal sodium transport and on sodium-dependentd-glucose transport in brush-border membrane vesicles from rabbit jejunum

Harmaline inhibition of sodium uptake and of sodium-dependentd-glucose transport was investigated using brush-border membrane vesicles from frozen rabbit jejunum. Under sodium-gradient conditions, initiald-glucose uptake (20 s) was inhibited by harmaline at concentrations above 0.5 mM, but at lower harmaline concentrationsd-glucose uptake was stimulated by 10–15%. When a similar potassium gradient was used, harmaline had no effect. At concentrations upt to 2 mM, harmaline did not alter the equilibrium uptake ofd-glucose ord-mannitol. After pre-equlibration with sodium (25 mM),d-glucose uptake was inhibited at harmaline concentrations ranging from 0.1 to 2 mM. Sodium (10 mM) uptake was also inhibited by harmaline. Increasing the sodium concentration reduced the inhibitory effect of harmaline on tracer sodium uptake as well as on sodium-dependentd-glucose uptake. Similar to phlorizin, harmaline (1 mM) was able to prevent glucose-induced sodium influx across the brush-border membrane.Sodium uptake into brush-border membrane vesicles seems to be inhibited at lower harmaline concentrations than sodium-dependentd-glucose uptake. At high (2 mM) inhibitor concentrations, however, sodium-dependent glucose uptake is more strongly inhibited than sodium uptake. These results suggest that harmaline inhibits both sodium and sodium-dependent transport across intestinal brush-border membranes by interacting with specific sodium-binding sites.     

Amiloride and the mode of action of aldosterone on sodium transport across toad bladder and skin

Summary The characteristics of the inhibitory influence exerted by amiloride on active sodium transport by toad bladder and skin were taken advantage of to shed additional light on the mechanism whereby hormones-chiefly aldosterone-stimulate sodium transport across such epithelia.Toad skin treated with amiloride was as responsive to vasopressin and to aldosterone as the matched control preparation, while the response of toad skin to insulin was blunted under similar circumstances.When aldosterone-stimulated toad skin was exposed to amiloride, the resulting depression of sodium-transporting activity was less pronounced than was the case with matched control; on the other hand, insulin-treated membranes were as sensitive to amiloride as matched controls. Similar observations were made with triamterene.This is taken to indicate that the density of the permeases apparently required for sodium to cross the apical cell border of these specialized epithelia, is increased under the influence of aldosterone.The affinity of amiloride for toad bladder tissue was not influenced by aldosterone, as indicated by tissue labelling experiments.On leave of absence from the University of Chicago, Department of Medicine. Recipient of a U.S.P.H.S. Career Development Award K2-GM21.842.     

Phenothiazines increase active sodium transport across the isolated toad skin

Fluphenazine (FPZ) and trifluoperazine (TFP) are phenothiazine derivatives commonly used as antipsychotic transquilizers. Their mechanism of action is incompletely understood. Epidermal addition of each drug promoted biphasic short-circuit current (SCC) changes across isolated pelvic skin ofBufo arenarum toads. By means of radiotracers fluxes, SCC was found to be given by the algebraic sum of net sodium and chloride transport. A readily stimulant effect was detected a low concentrations (from 1·10^–6 mol/l up to 1·10^–4 mol/l for FPZ, from 1·10^–5 mol/l up to 3.2·10^–4 mol/l for TFP) above which inhibition prevailed. Dermal FPZ also stimulated SCC. A higher concentration and time threshold were required. Epidermal 1·10^–5 mol/l FPZ stimulation was partially reversible, with a diminished membrane resistance and enhancement of sodium influx, without alteration of sodium efflux or net chloride transport. It could be prevented by amiloride pretreatment, or diminished by dermal sodium removal. Variation of epidermal bulk pH from 5.8 to 8.7 demonstrated that ionized and nonionized molecules contribute to FPZ's effect. Our results suggest that SCC stimulation elicited by FPZ and TFP may be a consequence of direct or indirect modifications on apical sodium conductance.     

Endothelin-1 stimulates chloride and potassium secretion in rabbit descending colon

The vasoactive peptide endothelin-1 (ET-1) which is present in high concentrations in the colon, causes concentration-dependent electrogenic Cl^– secretion in rabbit descending colon. This effect is half-maximal at 0.11 mol/l. Like other secretagogues, ET-1 also stimulates K⁺ secretion. The secretory effect of ET-1 is associated with increased release of prostaglandin E₂ from the serosal surface of the mucosa. ET-1-induced Cl^– secretion is completely inhibited by the loop diuretic bumetanide and by indomethacin and quinacrine, inhibitors of prostaglandin synthesis. Neuronal mechanisms do not seem to be involved, as tetrodotoxin did not affect the secretory response to ET-1 significantly. On the other hand, neither the catalytic activity nor the transport function of the Na⁺/K⁺-ATPase of rabbit colon epithelium is affected by endothelin-1 (ET-1) in concentrations up to 10 mol/l. It is concluded that ET-1 causes Cl^– and K⁺ secretion by stimulating phospholipase A₂ and release of prostaglandins, whereas Na⁺ transport is not altered.Parts of this study were presented at the 10th meeting of the European Intestinal Transport Group [26]     

The role of bicarbonate and other buffers on isotonic fluid absorption in the proximal convolution of the rat kidney

Summary The fluid reabsorption from the proximal convolution of the rat kidney was measured with the Gertz shrinking droplet technique. Simultaneously, the peritubular capillaries were perfused with artificial solutions. In some experimental series, fluid from the shrinking droplet was withdrawn and analysed for Cl^–, Na⁺, and osmolality so that the transtubular transport of Na⁺, Cl^–, and HCO ₃ ^– could be calculated. Capillary perfusate in some experiments was also withdrawn and its pH was measured. The following results were obtained: 1. With increasing concentration of HCO ₃ ^– in the capillary perfusate, the transtubular water, sodium, chloride, and bicarbonate reabsorption increased. 2. The sulfonamide buffers sulfamerazine and glycodiazine (Redul^®), which easily penetrate the tubular wall, could, in equimolar concentrations, substitute totally for the bicarbonate buffer in promoting isotonic fluid absorption. 3. Butyrate, propionate, and acetate were also effective; pyruvate, lactate, and paraaminohippurate, however, were not. 4. The effect of HCO ₃ ^– and glycodiazine on isotonic absorption was shown to depend exclusively on the concentration of the buffer anion and not on the concentration of undissociated acid or pH. From these data it is suggested that for proximal isotonic absorption of water, sodium, and chloride, the reabsorption of buffer anions via H⁺ secretion and nonionic diffusion may be essential. The H⁺ secretion or the buffer anion absorption across the luminal cell wall may secondarily influence the active Na⁺ transporting mechanism located at the basal cell site either by a luminal H⁺–Na⁺ exchange mechanism or by a lyotropic effect which would increase the Na⁺ permeability of the luminal cell site. Thereby more Na⁺ would be delivered to the Na⁺ pumping site and the rate of Na⁺ pumping would be augmented.     

Chloride and sodium transport across bovine tracheal epithelium: Effects of secretagogues and indomethacin

The regulation of ion transport in bovine tracheal epithelium was studied in vitro. In the absence of exogenous midifiers of ion transport, average values for transepithelial electrical potential difference (_t), short-circuit-current (I _sc) and tissue resistance (R _t) were 35.4 mV (lumen negative), 5.4 Eq·h^–1·cm^–2 and 187 ·cm² respectively; net Cl secretion (3.2 Eq·h^–1·cm^–2) and net Na absorption (1.3 Eq·h^–1·cm²) accounted for 82% of theI _sc. Amiloride reduced (₁) andI _sc, and increasedR _t. The values of (_t),R _t andI _sc obtained following addition of theophylline, epinephrine or prostaglandin E₁ (PGE₁) were not different from control values. Theophylline aldo did not alter Na and Cl fluxes but it increased tissue cAMP content 3-fold. Indomethacin did not affect (_t) but it increasedR _t and net Na absorption, and decreasedI _sc and net Cl secretion; it did not significantly reduce tissue cAMP. When added to indomethacin-treated tissues, epinephrine restoredI _sc,R _t and Na and Cl fluxes to control levels and increased tissue cAMP 3-fold. Similary, when PGE₁ was added to indomethacin-treated tissues,I _sc andR _t were restored to control levels.We conclude that: (1) bovine tracheal epithelium, like its canine counterpart, absorbs Na and secretes Cl; the two tissues differ, however, in two ways: the spontaneous rate of Na absorption is higher in bovine trachea and the spontaneous rate of Cl secretion cannot be further increased in bovine trachea by secretagogues; (2) Cl secretion and Na absorption in bovine trachea are normally regulated by endogenous prostaglandins; (3) although cAMP may mediate changes in ion transport, a strict correlation between tissue cAMP content and Na and Cl transport rates is not evident; and (4) Na absorptive and Cl secretory rates are reciprocally related suggesting that both processes are present in the same cells.     

Asymmetry of the chloride transport system in human erythrocyte ghosts

The concentration dependence of the unidirectional chloride flux and the inhibition of the unidirectional chloride flux by sulfate were studied in human red cell ghosts. The concentration dependence of the unidirectional chloride flux and its inhibition by sulfate were asymmetric. The unidirectional chloride flux can be saturated from the inner and from the outer membrane surface. For the inner membrane surface, lower chloride half-saturation constants were obtained than for the outer membrane surface. The inhibition of the unidirectional chloride flux by sulfate is competitive. In contrast to the chloride half-saturation constants, the inhibition constants of sulfate for the inner membrane surface were higher than the inhibition constants of sulfate for the outer membrane surface. Either there are fixed anion binding sites at the inner and at the outer membrane surface which control the access of anions to a pore, or there is a mobile carrier which is in contact with both membrane surfaces. The asymmetry of the concentration response and of the inhibition of the unidirectional chloride flux suggest that the anion binding sites at the inner and at the outer membrane surface differ with respect to their affinities for chloride and for sulfate. Alternatively, the asymmetry of the chloride transport system could indicate an asymmetric distribution of a mobile anion carrier across the erythrocyte membrane.A part of the results was presented at the 48th meeting of the German Physiological Society at Regensburg, March 15–18, 1977 [26] and at the XXVIIth International Congress of Physiological Sciences at Paris, July 18–23, 1977 [27]     

Hormonal regulation of electrolyte and water transport in the Colon

Summary The colon participates in water and electrolyte homoiostasis by the absorption of sodium (Na) and water as well as by potassium (K) secretion. The primary step of colonic transport is the active Na transport via a transcellular route. Steroidal hormones considerably increase Na absorption by utilizing two mechanisms: (1) passive Na entry into the cells is enhanced by an increased membrane permeability; (2) active transport capacity is increased by a stimulation of ATPase synthesis. Mineralocorticoid versus glucocorticoid actions of steroids have not yet been clearly differentiated; parallel influences are possible. Active chloride (Cl) secretion is found in the colon under certain pathological conditions and is induced by a number of factors, e.g., hormones produced by pancreas tumors. Cellular events involve a rise of intracellular cAMP and calcium (Ca) concentrations, and altered Cl permeabilities. Functional changes of colonic epithelial cells caused by hormones assume a significant role in the etiology of diarrhea, as well as in compensatory processes by which an intestinal loss of electrolytes and water is prevented.     

Regulation of ion transport via apical purinergic receptors in intact rabbit airway epithelium

We investigated purinergic receptors involved in ion transport regulation in the intact rabbit nasal airway epithelium. Stimulation of apical membrane P2Y receptors with ATP or UTP (200 M) induced transient increases in short-circuit current (I_sc) of 13 and 6% followed by sustained inhibitions to 8 and 17% below control level, respectively. Serosal application of nucleotides had no effect. The ATP-induced response appeared to involve additional activation of apical adenosine (P1) and P2X receptors. The inhibitory effect of ATP and UTP on I_sc was eliminated by pretreatment with amiloride (100 M), while the stimulatory effect was potentiated, indicating that ATP and UTP inhibit Na⁺ and stimulate Cl^– current. Ionomycin (1 M) induced responses similar to UTP and ATP and desensitized the epithelium to the nucleotides, indicating involvement of intracellular Ca²⁺ (Ca²⁺_i). Furthermore, ATP, UTP and ionomycin induced 21, 24, and 21% decreases, respectively, in transepithelial conductance. Measurements of unidirectional isotope fluxes showed a 39% decrease in the dominant net Na⁺ absorption in response to ATP, while the smaller net Cl^– secretion increased only insignificantly and unidirectional Cl^– fluxes decreased significantly. The results suggest that nucleotides released to the airway surface liquid exert an autocrine regulation of epithelial NaCl absorption mainly by inhibiting the amiloride-sensitive epithelial Na⁺ channel (ENaC) and paracellular anion conductance via a P2Y receptor-dependent increase in Ca²⁺_i, while stimulation of Cl^– secretion is of minor importance.     

Relevance of sodium transport pool measurements in toad bladder tissue for the elucidation of the mechanism whereby hormones stimulate active sodium transport

Summary The size of toad bladder sodium transport pool, defined as that amount of sodium of apical origin and recovered in tissue at equilibrium, was compared with sodium transport rate, derived from short-circuit current read immediately before tissue analysis.Provided certain precautions be taken, the relationship between both variables can be described by a curve starting at the intersect ofX (pool, in Eq) andY (transport, in Eq/hr) axes, with a tendency forX to increase faster thanY. Assuming sodium transport pool forms one compartment, its calculated half-life averages 2–3 min.sodium transport pool measurements are thought to shed light on mechanism of sodium transport by toad bladder because pool size was large with respect to transport rate when tissue was exposed to several inhibitors of sodium transport. Conversely, upon stimulation of activity of (substrate — depleted) preparations by glucose, a relative reduction of pool size was observed.Aldosterone, vasopressin (and adenosine 3,5-phosphate) increased sodium pool size and transport rate, proportionately; insulin stimulated sodium transport more than it increased pool size. Thus, insulin presumably exerts its effect at the sodium pump while such a site of action need not be postulated for aldosterone and vasopressin: these 2 hormones would instead induce, permeability changes faciliting sodium movement at the apical border of toad bladder epithelial cells.with the technical assistance ofJ. Scarlata)     

Gastric fundic inhibition of sugar transport across the intestinal mucosa of guinea-pig

A low molecular weight peptide designated gastric fundic factor (GFF), extracted from porcine fundic mucosa and administered to the serosal surface of mucosal sheets from guinea-pig intestine, decreased the transport of luminal glucose across the sheets by up to 70%. The results show that gastric fundic inhibition of glucose absorption observed in different animal models in vivo can be reproduced in vitro, and suggest that the intestinal mucosa itself is the target for peptide hormone(s) released by the gastric fundic mucosa. Simultaneous transport of -aminoisobutyric acid, a nonmetabolisable amino acid, through the jejunal mucosa was unaffected as was paracellular permeation by inulin. However, amino acid transport was also reduced when GFF was administered to sheets of ileal mucosa. The intestinal mucosal sheet in vitro is a sensitive and convenient model with which to follow the purification of GFF to homogeneity.     

Concentration dependence of the chloride selfexchange and homoexchange fluxes in human red cell ghosts

The concentration dependence of the unidirectional chloride flux in human red cell ghosts was studied under selfexchange and under homoexchange conditions. Under selfexchange conditions the intracellular concentration of chloride [Cl]_in is equal to the extracellular concentration [Cl]_ex and [Cl]_in and [Cl]_ex are raised concomitantly. Under homoexchange conditions [Cl]_in or [Cl]_ex were varied separately at a fixed trans-concentration of chloride. The chloride fluxes were calculated from the rate of the tracer efflux and the intracellular chloride. All experiments were executed in isotonic (330 mosM) KCl/K-citrate/sorbitol solutions containing 0–100 mM KCl, 40 mM K-citrate and different concentrations of sorbitol for isoosmotic substitution.The chloride selfexchange and the chloride homoexchange fluxes exhibit a pure saturation kinetics. The halfsaturation constant for the chloride selfexchange was 20 mM, the maximal flux was approx. 3.5·10^–4 mol/(min g cells). The apparent chloride halfsaturation constants from the homoexchange experiments were in the range of 0.9–4.5 mM for the outer and of 5.5–14.5 mM (0° C, pH 7.3) for the inner membrane surface, both halfsaturation constants increase with increasing trans-concentrations. At infinite trans-concentrations of chloride, the halfsaturation constant for the outer and the inner membrane surface amounts to 5 mM and 15 mM, respectively. The slope of the double reciprocal plots of flux versus cis-chloride concentration decreases with increasing trans-concentration of chloride.The kinetics of the chloride transport provides evidence for a carrier mediated transport mechanism with a single reciprocating transport site. The translocation of the loaded carrier appears to be much faster than the translocation of the unloaded carrier. At low concentrations of chloride, however, the slippage of the empty carrier seems to contribute to the back-diffusion of carrier to the cis-side of the membrane.     

Electrochemical forces for chloride transport in the proximal tubules of the rat kidney

The electrochemical forces for chloride transport in the proximal tubule of the rat kidney were studied using micropuncture techniques. Electrical transmembrane potentials were recorded in randomly punctured tubules with Ling-Gerhard electrodes. Chloride activities in the luminal, cellular and interstitial compartments were measured with ion selective micro-electrodes. Electrical potential measurements between cell to interstitium and lumen to interstitium were -72.1 ± 2.6 mVand ± 0.5±1.4mV(mean ± S.D.)respectively. The calculated chloride concentrations for lumen, cell and interstitium were 133.0±10.3 mM, 8.5±1.0 mM and 99.1 ± 3.2 mM (mean ± S.D.) respectively. The net electrochemical forces, qualitatively, offer a passive chloride ion pathway through the tubular wall and a chloride equilibrium over the luminal membrane seems to exist.