Transport of acetate and sodium in sheep omasum: mutual, but asymmetric interactions

We have studied the transport of acetate across the isolated epithelium of sheep omasum; no net transport was observed (J _ms ≈ J _sm) under Ussing chamber conditions. Low mucosal pH (pH 6.4) significantly enhanced J _ms acetate and the transport rates of acetate increased linearly and significantly (r ²=0.99) with the luminal acetate concentration. The presence of another short chain fatty acid (propionate) did not affect J _ms acetate significantly. Neither addition of 1 mmol l⁻¹ DIDS to the mucosal side nor HCO₃ replacement caused changes of J _ms acetate; this does not support the assumption of acetate transport via anion exchange. Addition of 1 mmol l⁻¹ amiloride to the mucosal side significantly decreased acetate fluxes at high mucosal acetate concentration (100 mmol l⁻¹) and low pH (6.4) indicating interaction between acetate uptake in the undissociated form, intracellular release of protons and activation of Na⁺/H⁺ exchange (NHE). However, the mutual interaction between Na transport via NHE and acetate transport is asymmetric. Stimulation or inhibition of Na transport via NHE is much more pronounced than the corresponding changes of acetate fluxes. Thus, the obtained results support the conclusion that acetate is transported via simple diffusion and probably predominantly in the protonated form, thereby explaining the positive and mutual interaction between Na transport and short chain fatty acids.     

Absorption of short-chain fatty acids across ruminal epithelium of sheep

Investigations on the absorption of shortchain fatty acids across ruminal epithelium of sheep were performed both in vitro (Ussing chamber technique, using propionic acid representatively for shortchain fatty acids) and in vivo (washed, isolated reticulorumen). A pH-induced, nearly tenfold increase in the concentration of undissociated propionate led to an only twofold increase in mucosal-to-serosal flux of propionate (in vitro). Neither amiloride (1 mmol·l^-1, in vitro) nor theophylline (10 mmol·l^-1, in vivo), inhibitors of the ruminal Na⁺/H⁺ exchanger, exerted any significant influence on propionate fluxes or short-chain fatty acids absorption, respectively. Total replacement of luminal Na⁺ (by choline) did not alter short-chain fatty acids absorption (in vivo). Mucosal 4,4-diisothiocyanatostilbene-2,2-disulfonic acid (0.1 mmol·l^-1) or mucosal nitrate (40 mmol·l^-1) markedly reduced propionate net flux (in vitro). Increasing mucosal Cl^- concentration brought about a significant drop in mucosal-to-serosal flux of propionate (in vitro) and in short-chain fatty acids net absorption (in vivo), respectively. The results obtained suggest that short-chain fatty acids are absorbed both as anions and as undissociated acids across ruminal epithelium of sheep. It is concluded that short-chain fatty acids anions either compete with Cl^- for binding sites at a common anion-exchange mechanism or that they are absorbed by an short-chain fatty acids anion/HCO ₃ ^- exchanger indirectly coupled to a Cl^-/HCO ₃ ^- exchanger via intracellular bicarbonate.Abbreviations DIDS 4,4-diisothiocyanatostilbene-2,2-disulfonic acid - DMSO dimethylsulfoxide - G _t tissue conductance - HSCFA protonated - SCFA i.e. undissociated form - J _ms mucosal-to-serosal flux - J _sm serosal-to-mucosal flux - J _net net flux - I _sc short-circuit current - MOPS (3-[N-morpholino]propanesulfonic acid) - mu mucosal - Prop Propionate - SCFA ^- SCFA anions, i.e. dissociated form - SCFA short-chain fatty acids - SEM standard error of mean     

The absorption of calcium ions from the ovine reticulo-rumen

Net Ca²⁺ and Mg²⁺ absorption rates were measured in vivo from buffer solutions placed in the washed reticulo-rumen, isolated in situ in 30 conscious, trained sheep. An increase in concentration of short chain fatty acids (SCFA) in the buffer, over the range 0–50 mM, was shown to stimulate the net rates of absorption of Ca²⁺ and Mg²⁺ ions from the rumen. Similarly, the results of in vitro experiments, carried out with ovine rumen epithelium mounted in short-circuited Ussing chambers, showed that the absence of SCFA from the chamber fluid resulted in a reduction in J_net Ca²⁺ caused by reduced flux of Ca²⁺ ions in the mucosal to serosal direction (J_ms Ca²⁺). The addition of 1 mM acetazolamide, an inhibitor of carbonic anhydrase, to the ruminal buffer used in the in vivo experiments led to significant reductions in the net absorption rates of Ca²⁺and Mg²⁺ ions in the presence of SCFA (50 mmol l⁻¹) but not in the absence of SCFA. However, in the in vitro experiments, the addition of 60 μM ethoxyzolamide had no significant effect on J_net Ca²⁺. A reduction in pH of the intraruminal buffer in vivo from 6.8 to 5.4 led to significant increases in the net absorption rates of Ca²⁺and Mg²⁺ ions, an effect which was duplicated for Ca²⁺ in preliminary in vitro experiments in which the pH of the mucosal buffer was reduced from 7.4 to 5.4. This stimulatory effect was confined to J_ms Ca²⁺ and J_net Ca²⁺. Ussing chambers were also used to demonstrate that J_net Ca²⁺ was reduced by a high transmural potential difference (PD), caused by voltage clamping, independently of the mucosal K⁺ concentration. Both unidirectional Ca²⁺ fluxes consisted of a PD-dependent and a K⁺-insensitive PD-independent component. The latter may be represented by a Ca²⁺/2H⁺ antiporter. It is postulated that SCFA, and to a lesser extent H₂CO₃, can stimulate J_ms Ca²⁺ by activation of an apical Ca²⁺/2H⁺ antiporter through the provision of protons within the ruminal epithelial cell. A mild reduction in ruminal pH may also lead to a similar stimulation of this putative electroneutral exchange. Accepted: 26 July 2000     

Nacl flux in the flounder (Pseudopleuronectes americanus) intestine: Effects of ph and transport inhibitors

• 1.1. The effects of extracellular pH on Na⁺ and Cl⁻ absorption were studied in vitro in the small intestine of the winter flounder, Pseudopleuronectes americanus.
• 2.2. Reductions in bathing solution pH inhibited J_ms^na (mucosal-to-serosal flux) and J_net^na (net flux) (r = 0.90) and J_net^Cl (r = 0.92) [due to an increase in J_sm^Cl, (serosal-to-mucosal)] and decreased short circuit current (Isc).
• 3.3. Luminal bumetanide (0.1 mM) and amiloride (1 mM) inhibited Na⁺ and Cl⁻ absorption by reducing J_ms.
• 4.4. Luminal barium (5mM) and luminal copper (100 μM) decreased J_ms^Cl and increased J_sm^Cl.
• 5.5. We conclude that reductions in extracellular pH inhibit a luminal membrane NaCl absorptive process (Na⁺-K⁺-2Cl⁻) and stimulate an electrogenic Cl⁻ secretory process.

     

Luminal alkalinization in the intestine of the goby

Summary The rate of luminal alkalinization in vitro byGillichthys mirabilis posterior intestine as measured by a manual pH stat technique was 0.70±0.05 Equiv/cm² h; acidification of the mucosal medium was never observed. The rate of HCO ₃ ^– secretion (J ^HCO ₃) was reduced by ouabain, serosally-applied DIDS, removal of serosal HCO ₃ ^– and replacement of media Cl^– with gluconate. HCO ₃ ^– secretion was enhanced replacement of Cl^– with isethionate and unaffected by mucosal DIDS, furosemide or acetazolamide.J ^HCO ₃ was reduced at mucosal pH above or below 7.5. These results support active HCO ₃ ^– secretion via a Cl^–/HCO ₃ ^– exchange mechanism on the basolateral membrane and a conductive exit pathway for HCO ₃ ^– , H⁺ or OH^– on the apical membrane.Abbreviations DIDS diisothiocyanostilbene-2,2-disulfonic acid - TEP transepithelial potential - GBR Gillichthyts bicarbonate Ringer - GUR Gillichthys unbuffered, bicarbonate-free Ringer - GER Gillichthys EPPS-buffered, bicarbonate-free Ringer - EPPS N-(2-hydroxyethyl)piperazine-N-3-propanesulfonic acid     

Luminal hyperosmolarity decreases Na transport and impairs barrier function of sheep rumen epithelium

The effects of luminal hyperosmolarity on Na and Cl transport were studied in rumen epithelium of sheep. An increase of luminal osmotic pressure with mannitol (350 and 450 mosm/l) caused a significant increase of tissue conductance, G _T, which is linearly correlated with flux rates of ⁵¹Cr-EDTA and indicates an increase of passive permeability. Studies with microelectrodes revealed, that an increase of the osmotic pressure caused a significant increase of the conductance of the shunt pathway from 1.23±0.10 (control) to 1.92±0.14 mS cm⁻² (450 mosm/l) without a change of fractional resistance. Hyperosmolarity significantly increased J _sm and reduced J _net Na. The effect of hyperosmolarity on J _ms Na is explained by two independent and opposed effects: increase of passive permeability and inhibition of the Na⁺/H⁺ exchanger. Hypertonic buffer solution induced a decrease of the intracellular pH (pH_i) of isolated ruminal cells, which is consistent with an inhibition of Na⁺/H⁺ exchange, probably isoform NHE-3, because NHE-3-mRNA was detectable in rumen epithelium. These data are in contrast to previous reports and reveal a disturbed Na transport and an impaired barrier function of the rumen epithelium, which predisposes translocation of rumen endotoxins and penetration of bacteria.     

Electrogenic proton secretion in the Hindgut of the desert locust,Schistocerca gregaria

Summary The cellular mechanisms responsible for rectal acidification in the desert locust, Schistocerca gregaria, were investigated in isolated recta mounted as flat sheets in modified Ussing chambers. Previous studies conducted in the nominal absence of exogenous CO₂ and HCO ₃ ^– suggested that the acidification was due to a proton-secretory rather than bicarbonate-reabsorptive mechanism (Thomson, R.B., Speight, J.D., Phillips, J.E. 1988. J. Insect Physiol. 34:829–837). This conclusion was confirmed in the present study by demonstrating that metabolic CO₂ could not contribute sufficient HCO ₃ ^– to the lumen to account for the rates of rectal acidification observed under the nominally CO₂/ HCO ₃ ^– -free conditions used in these investigations.Rates of luminal acidification (J _H ⁺) were completely unaffected by changes in contraluminal pH, but could be progressively reduced (and eventually abolished) by imposition of either transepithelial pH gradients (lumen acid) or transepithelial electrical gradients (lumen positive). Under short-circuit current conditions, the bulk of J _H ⁺ was not dependent on Na⁺, K⁺, Cl^–,Mg²⁺, or Ca²⁺ and was due to a primary electrogenic proton translocating mechanism located on the apical membrane. A small component (10–16%) of J _H ⁺ measured under these conditions could be attributed to an apical amiloride-inhibitable Na⁺/H⁺ exchange mechanism.This work was supported by operating grants to J.E.P. and postgraduate scholarships to R.B.T. from Natural Sciences & Engineering Research Council, Canada.     

Effects of propionate on the acid microclimate of hen (Gallus domesticus) colonic mucosa

• 1.1. Short-chain fatty acid absorption in hen colon is protonated across the apical border coupled to an apical electrogenic proton pump.
• 2.2. The surface pH of the isolated colonic epithelium was 6.27 ± 0.05, when incubated in Krebs-phosphate buffer pH 7.0.
• 3.3. Propionate 7 and 40mmol/l in the incubation medium (pH 7.0) increased microclimate pH to 6.47 ± 0.04 and 6.56 ± 0.04. Inhibition of metabolic activity by potassium cyanide 1 mmol/1 increased surface pH to 6.66 ± 0.06.
• 4.4. The calculated concentration of propionic acid in the microclimate is near-linearly related to the propionate concentration. Thus, the acid microclimate is not responsible for the Michaelis-Menten like kinetics of propionate transport.

     

Active sulfate absorption in rabbit ileum: Dependence on sodium and chloride and effects of agents that alter chloride transport

Summary Unidirectional fluxes of³⁵SO₄ across and into rabbit ileal epithelium were measured under short-circuit conditions, mostly at a medium SO₄ concentration of 2.4mm. Unidirectional mucosa (m)-to-serosa (s) ands-to-m fluxes (J _ms,J _sm) were 0.456 and 0.067 moles hr^–1 cm^–2, respectively.J _ms was 2.7 times higher in distal ileum than in mid-jejunum. Ouabain abolished net SO₄ transport (J _net) by reducingJ _ms. Epinephrine, a stimulus of Cl absorption, had no effect on SO₄ fluxes. Theophylline, a stimulus of Cl secretion, reducedJ _ms without affectingJ _sm, causing a 33% reduction inJ _net. Other secretory stimuli (8-Br-cAMP, heat-stable enterotoxin, Ca-ionophore A23187) had similar effects. Replacement of all Cl with gluconate markedly reducedJ _net through both a decrease inJ _ms and an increase inJ _sm. The anion-exchange inhibitor, 4-acetoamido-4-isothiocyano-2,2-sulfonic acid stilbene (SITS), when added to the serosal side, reducedJ _ms by 94%, nearly abolishingJ _net. SITS also decreasedJ _sm by 75%. Mucosal SITS (50 m) was ineffective. 4,4-diisothiocyano-2,2-sulfonic acid stilbene (DIDS) had effects similar to SITS but was less potent. Measurements of initial rates of epithelial uptake from the luminal side (J _me) revealed the following: (1)J _me is a saturable function of medium concentration with aV _max of 0.94 moles hr^–1 cm^–2 and aK _1/2 of 1.3mm; (2) replacing all Na with choline abolishedJ _me; (3) replacing all Cl with gluconate increasedJ _me by 40%; (4) serosal SITS had no effect onJ _me; and (5) stimuli of Cl secretion had no effect onJ _me or increased it slightly. Determination of cell SO₄ with³⁵SO₄ indicated that, at steady-state, the average mucosal concentration is 1.1 mmoles per liter cell water, less than half the medium concentration. Cell SO₄ was increased to 3.0mm by adding SITS to the serosal side. Despite net transport rates greater than 1.4 Eq hr^–1 cm^–2, neither addition of SO₄ to the SO₄-free medium nor addition of SITS to SO₄-containing medium altered short-circuit current. The results suggest that (1) ileal SO₄ absorption consists of Na-coupled influx (symport) across the brush border and Cl-coupled efflux (antiport) across the basolateral membrane; (2) the overall process is electrically neutral; (3) the medium-to-cell Cl concentration difference may provide part of the driving force for net SO₄ absorption; and (4) since agents affecting Cl fluxes (both absorptive and secretory) have little effect on SO₄ fluxes, the mechanisms for their transcellular transports are under separate regulation.     

Salmonella typhimurium poultry isolate growth response to propionic acid and sodium propionate under aerobic and anaerobic conditions

Propionic acid and its sodium salt have long been used as additives in poultry feed to reduce microbial populations, including Salmonella spp. Propionic acids in poultry feed may have a potential role in inhibiting growth of Salmonella in the chicken intestine. In this study, we determined growth response of a Salmonella typhimurium poultry isolate to propionic acid and sodium propionate under aerobic and anaerobic conditions. Growth rate consistently decreased with the addition of greater concentrations of either propionic acid or sodium propionate. The extent of growth inhibition was much greater with propionic acid than the sodium form. Media pH decreased only with addition of propionic acid. Growth inhibition was more effective under anaerobic growth conditions with either propionic acid or sodium propionate. When determined at the same pH level, growth rate was significantly lowered by addition of 25 mM of either propionate or sodium propionate alone, and also by the decrease in pH levels (P<0.05). These results showed that growth inhibition of S. typhimurium by propionic acid or sodium propionate is greatly enhanced by pH decrease, and to lesser extent by anaerobiosis. We also found that sodium propionate was more inhibitory for growth of S. typhimurium than propionic acid when compared at the same pH levels.     

Cell pH and luminal acidification inNecturus proximal tubule

Summary Cellular potential and pH measurements (pH _i ) were carried out in the perfused kidney ofNecturus on proximal tubules with standard and recessed-tip glass microelectrodes under control conditions and after stimulation of tubular bicarbonate reabsorption. Luminal pH and net bicarbonate reabsorption were measured in parallel experiments with recessed-tip glass or antimony electrodes, both during stationary microperfusions as well as under conditions of isosmotic fluid transport. A mean cell pH of 7.15 was obtained in control conditions. When the luminal bicarbonate concentration was raised to 25 and 50mm, pH _i rose to 7.44 and 7.56, respectively. These changes in pH _i were fully reversible. Under all conditions intracellular H⁺ was below electrochemical equilibrium. Thus the maintenance of intracellular pH requires active H⁺ extrusion across one or both of the cell membranes. The observed rise in pH _i and the peritubular depolarization after stimulation of bicarbonate reabsorption are consistent with enhanced luminal hydrogen ion secretion and augmentation of peritubular bicarbonate exit via an anion-conductive transport pathway.     

Surface pH at the Basolateral Membrane of the Caecal Mucosa of Guinea Pig

Since the major mechanisms responsible for regulation of intracellular pH of enterocytes are located in the basolateral membrane, respective effects may be expected on pH in the compartment near the basolateral membrane. A method was established to estimate the pH at the basolateral membrane (pH _b ) of isolated caecal epithelia of guinea pig using pH-sensitive fluorescein attached to lectin (lens culinaris). In the presence of bicarbonate and a perfusion solution-pH of 7.4, pH _b was 7.70 ± 0.15. In the absence of bicarbonate or chloride as well as by inhibition of the basolateral Cl⁻-HCO⁻ ₃ exchange with H₂-DIDS, pH _b was reduced near to solution-pH. Inhibition of the basolateral Na⁺-H⁺ exchanger by adding a sodium- and bicarbonate-free, low-buffered solution increased pH _b . Decrease of pH of serosal perfusion solution to 6.4 provoked a similar decrease of pH _b to solution pH. Short-chain fatty acids (SCFA) added to the mucosal solution caused a slight decrease of pH _b . SCFA added to the serosal side alkalized pH _b . However, in the presence of bicarbonate pH _b returned quickly to the initial pH _b , and after removal of SCFA a transient acidification of pH _b was seen. These responses could not be inhibited by MIA or H₂-DIDS. We conclude that no constant pH-microclimate exists at the basolateral side. The regulation of the intracellular pH of enterocytes reflects pH _b . The slightly alkaline pH _b is due to the bicarbonate efflux. Data support the presence of an SCFA⁻-HCO⁻ ₃ exchange. Received: 17 December 1998/Revised: 24 February 1999     

Effect of pH on transepithelial electrical parameters in seawater-adapted eel intestine

The sensitivity to external pH of Cl^- absorption was studied in isolated stripped intestinal mucosa of the eel, Anguilla anguilla, mounted in Ussing chambers. Short-circuit current, transepithelial potential difference and conductance were measured in bathing solutions containing various combinations of HCO₃ ^--concentration (0–25 mmol·l^-1), partial pressure of CO₂ (0–76 mm Hg) and pH (6.9–7.9). A linear relationship was found between pH and short-circuit current in the range of pH studied both in HCO₃ ^-/CO₂ Ringer and in Hepes Ringer. The pH effect was almost completely reversible. It was not affected by the presence of mucosal Ba²⁺ (10^-3 mol·l^-1) but it was inhibited by the presence of luminal (10^-5 mol·l^-1) or serosal (10^-4 mol·l^-1) bumetanide. The results obtained suggest that the Cl^- absorption in the European eel intestine is pH sensitive. The data do not indicate whether the pH affects directly the Na⁺–K⁺–Cl^- cotransport and/or the basolateral Cl^- conductance or other mechanisms indirectly linked to Cl^- absorption.Abbreviations g _t transepithelial conductance - Hepes N-2-Hydroxyethylpiperazine-N'-2-ethanesulfonic acid - I _sc short circuit current - R _t transepithelial resistence - V _t transepithelial potential difference     

Lysine transport across the small intestine. Stimulating and inhibitory effects of neutral amino acids

Summary The ordinary aliphatic, neutral amino acids and phenylalanine have been examined for cis-inhibition of influx of alanine (J _mc ^ala ) and lysine (J _mc ^lys ) and trans-stimulation ofJ _mc ^lys across the brush border membrane of rat small intestines: and their effects on the unidirectional mucosa-to-serosa flux (J _ms ^lys ) across the short circuited intestine have been studied. The effects of alanine, -amino-n-butyric acid, leucine, and methionine on the steady-state epithelial uptake of lysine [Lys] _c have also been measured. In addition the trans-effects of alanine and leucine have been examined for sodium-dependence, and alanine was tested as trans-stimulator of influx of galactose across the brush border membrane (J _mc ^gal ).All the neutral amino acids were found to be competitive cis-inhibitors ofJ _mc ^lys , and all, except isoleucine, were trans-stimulators ofJ _mc ^lys . The magnitude of the trans-effect was unrelated to the efficiency of the amino acid as cis-inhibitor. As illustrated by alanine, the trans-effects are probably completely sodium-dependent. Alanine was also effective as trans-stimulator ofJ _mc ^gal . With respect to effects on [Lys] _c andJ _ms ^lys the neutral amino acids fall into two groups: One which reduces [Lys] _c and stimulatesJ _ms ^lys , and one which increases [Lys] _c and relatively inhibitsJ _ms ^lys . These effects are not correlated with the affinities of the neutral amino acids for the two carriers involved.It is proposed that the trans-effects onJ _mc ^lys are induced by an electrogenic, sodium-coupled efflux of the neutral amino acid across the brush border membrane, that the stimulation ofJ _ms ^lys is brought about by a selective stimulation (of unknown nature) of efflux of lysine across the basolateral membrane (J _cs ^lys ), assisted by competitive inhibition of lysine efflux across the brush border membrane (J _cm ^lys ), and that the amino acids which do not stimulateJ _cm ^lys increase [Lys] _c by competitively inhibitingJ _cs ^lys andJ _cm ^lys .The inhibitory effect of the neutral amino acids onJ _mc ^lys support the view that the carrier of basic amino acids serves as a second carrier of these amino acids.     

Time- and dose-dependent effects of protein kinase C on proximal bicarbonate transport

Summary Activation of protein kinase C has been shown to cause both stimulation and inhibition of transport processes in the brush-border membrane and renal tubule. This study was designed to examine the dose-response nature and time-dependent effect of 4 -phorbol-12-myristate-13-acetate (PMA) on the rates of bicarbonate absorption (J _HCO3) and fluid absorption (J _v) in the proximal convoluted tubule (PCT) of rat kidney. Bicarbonate flux was determined by total CO₂ changes between the collected fluid and the original perfusate as analyzed by microcalorimetry. Luminal perfusion of PMA (10^–10 10^–5 M) within 10 min caused a significant increase ofJ _HCO3 andJ _v. A peaked curve of the dose response was observed with maximal effect at 10^–8 M PMA on both bicarbonate and fluid reabsorption, which could be blocked completely by amiloride (10^–3 m) and EIPA (10^–5 M). On the other hand, with an increase of perfusion time beyond 15 min, PMA (10^–8 and 10^–6 M) could inhibitJ _HCO3 andJ _v. Amiloride (10^–3 M) or EIPA (10^–5 M) significantly inhibitsJ _HCO3 andJ _v, while there is no additive effect of PMA and amiloride or EIPA on PCT transport. An inactive phorbol-ester, 4-phorbol, that does not activate protein kinase C, had no effects onJ _HCO3 andJ _v. Capillary perfusion of PMA (10^–8 M) significantly stimulate bothJ _HCO3 andJ _v; however, PMA did not affect glucose transport from either the luminal side or basolateral side of the PCT. These results indicate that activation of endogenous protein kinase C by PMA could either stimulate or inhibit both bicarbonate and fluid reabsorption in the PCT dependent on time and dose, and these effects are through the modulation of Na⁺/H^– exchange 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.     

The effects of extremely alkaline water (pH 9·5) on rainbow trout gill function and morphology

Rainbow trout that were held under control conditions, at pH8·0, in moderately hard Hamilton tap water, had Cl^? and Na⁺ influx rates (J^CLin and J^Na, respectively) of 270 and 300 μmol kg^?1 h^?1, respectively. Exposure to pH 9·5 water led to an immediate 67% decline in J^CL_in and a 45% reduction in J^Na_in at 0–1 h. Influx rates declined further and by 4–5 h the net decreases in both J^CL_in and J^Na_in approximated 80%. By 24 h J^CL_in had recovered to rates not significantly different from those at pH 8·0; while J^Na_in only partially recovered and remained about 50% lower than control measurements through 72 h. The complete recovery of J^CL_in and partial recovery of J^Na_in may have been related to a fourfold greater branchial chloride cell (CC) fractional surface area observed in rainbow trout exposed to pH 9·5 for 72 h. Ammonia excretion (J_Amm) was about 170 μmol N kg^?1 h^?1 at pH 8·0 but was initially reduced by 90% over the first hour of high pH exposure. J_Amm rapidly recovered and by 24 h it had returned to pre-exposure levels. This recovery tended to parallel the partial recovery of J^Na_in. However, subsequent addition of amiloride (10^?4M) to the water at 75 h led to no change in J_Amm, despite a 50% reduction in J^Na_in. Thus, it does not appear that there is a linkage between Na⁺ influx and the recovery of ammonia excretion under highly alkaline conditions.     

Sulfate transport in rabbit ileum: Characterization of the serosal border anion exchange process

Summary The preceding paper [30] shows that transepithelial ileal SO₄ transport involves Na-dependent uptake across the ileal brush border, and Cl-dependent efflux across the serosal border. The present study examines more closely the serosal efflux process. Transepithelial mucosa (m)-to-serosa (s) ands-to-m fluxes (J _ms,J _sm) across rabbit ileal mucosa were determined under short-circuit conditions. SO₄ was present at 0.22mm. In standard Cl, HCO₃ Ringer's,J _ms ^SO4 was 81.3±5.3 (1se) andJ _ms ^SO4 was 2.5±0.2 nmol cm^–2 hr^–1 (n=20). Serosal addition of 4-acetamido-4-isothiocyanostilbene-22-disulfonate (SITS), 44-diisothiocyanostilbene-22-disulfonate (DIDS) or 1-anilino-8-naphthalene-sulfonate (ANS) inhibited SO₄ transport, SITS being the most potent. Several other inhibitors of anion exchange in erythrocytes and other cells had no effect on ileal SO₄ fluxes. In contrast to its effect on SO₄ transport, SITS (500 m) did not detectably alter Cl transport.Replacement of all Cl, HCO₃ and PO₄ with gluconate reducedJ _ms ^SO4 by 70% and increasedJ _ms ^SO4 by 400%. A small but significantJ _net ^SO4 remained.J _ms ^SO4 could be increased by addition to the serosal side of Cl, Br, I, NO₃ or SO₄. The stimulatory effect of all these anions was saturable and SITS-inhibitable. The maximalJ _ms ^SO4 in the presence of Cl was considerably higher than in the presence of SO₄ (73.1 and 42.2 nmol. cm^–2 hr^–1, respectively;p<0.001). TheK _1/2 value for Cl was 7.4mm, 10-fold higher than that for SO₄ (0.7mm). Omitting HCO₃ and PO₄ had no measurable effects on SO₄ fluxes.This study shows that (i) SO₄ crosses the serosal border of rabbit ileal mucosa by anion exchange; (ii) the exchange process is inhibited by SITS, DIDS and ANS, but not by several other inhibitors of anion exchange in other systems; (iii) SO₄ may exchange for Cl, Br, I, NO₃ and SO₄ itself, but probably not for HCO₃ or PO₄; (iv) kinetics of the exchange system suggest there is a greater affinity for SO₄ than for Cl, although the maximal rate of exchange is higher in the presence of Cl; and, finally (v) SITS has little or no effect on net Cl transport.     

Water absorption and bicarbonate secretion in the intestine of the sea bream are regulated by transmembrane and soluble adenylyl cyclase stimulation

In the marine fish intestine luminal, HCO₃ ^? can remove divalent ions (calcium and magnesium) by precipitation in the form of carbonate aggregates. The process of epithelial HCO₃ ^? secretion is under endocrine control, therefore, in this study we aimed to characterize the involvement of transmembrane (tmACs) and soluble (sACs) adenylyl cyclases on the regulation of bicarbonate secretion (BCS) and water absorption in the intestine of the sea bream (Sparus aurata). We observed that all sections of sea bream intestine are able to secrete bicarbonate as measured by pH?CStat in Ussing chambers. In addition, gut sac preparations reveal net water absorption in all segments of the intestine, with significantly higher absorption rates in the anterior intestine that in the rectum. BCS and water absorption are positively correlated in all regions of the sea bream intestinal tract. Furthermore, stimulation of tmACs (10???M FK?+?500???M IBMX) causes a significant decrease in BCS, bulk water absorption and short circuit current (Isc) in a region dependent manner. In turn, stimulation of sACs with elevated HCO₃ ^? results in a significant increase in BCS, and bulk water absorption in the anterior intestine, an action completely reversed by the sAC inhibitor KH7 (200???M). Overall, the results reveal a functional relationship between BCS and water absorption in marine fish intestine and modulation by tmACs and sAC. In light of the present observations, it is hypothesized that the endocrine effects on intestinal BCS and water absorption mediated by tmACs are locally and reciprocally modulated by the action of sACs in the fish enterocyte, thus fine-tuning the process of carbonate aggregate production in the intestinal lumen.     

Internal transport of CO2 from the root‐zone to plant shoot is pH dependent

We investigated the fate of carbon dioxide (CO₂) absorbed by roots or internally produced by respiration using gas exchange and stable isotopic labeling. CO₂ efflux from detached leaves supplied with bicarbonate/CO₂ solutions was followed over six cycles. CO₂ effluxes were detected when bicarbonate solution at high pH was used, corresponding to 71–85% of the expected efflux. No CO₂ efflux was detected when CO₂ solutions at low pH were used but CO₂ efflux was subsequently detected as soon as bicarbonate solutions at high pH were supplied. By sealing the leaf and petiole in a plastic bag to reduce diffusion to the atmosphere, a small CO₂ efflux signal (14–30% of the expected efflux) was detected suggesting that CO₂ in the xylem stream can readily escape to the atmosphere before reaching the leaf. When the root‐zones of intact plants were exposed to CO₂ solutions, a significant efflux from leaf surface was observed (13% of the expected efflux). However, no signal was detected when roots were exposed to a high pH bicarbonate solution. Isotopic tracer experiments confirmed that CO₂ supplied to the root‐zone was transported through the plant and was readily lost to the atmosphere. However, little ¹³C moved to the shoot when roots were exposed to bicarbonate solutions at pH 8, suggesting that bicarbonate does not pass into the xylem.