Net Cl Research Articles

The cyanobacterium Synechococcus R‐2 (Anacystis nidulans, S. leopoliensis) PCC 7942 has a sodium‐dependent chloride transporter

ABSTRACT Synechococcus R‐2 (PCC 7942) actively accumulated Cl− in the light and dark, under control conditions (BG‐11 media: pHo, 7·5; [Na+]o, 18 mol m−3; [Cl−]o, 0·508 molm−3). In BG‐11 medium [Cl−], was 17·2±0·848 mol m−3 (light), electrochemical potential of Cl− (ΔμCl−i,o) =+211±2mV; [Cl−]i= 1·24±0·11 mol m−3(dark), ΔμCl−i,o=+133±4mV. Cl− fluxes, but not permeabilities, were much higher in the light: ϕCl−i,o= 4·01±5·4 nmol m−2 s−1, PCl−i,o= 47±5pm s−1 (light); ϕCl−i,o= 0·395±0·071 nmol m−2 s−1, PCl−i,o= 69±14 pm s−1 (dark). Chloride fluxes are inhibited by acid pHo (pHo 5; ϕCl−i,o= 0·14±0·04 nmol m−2 s−1); optimal at pHo 7·5 and not strongly inhibited by alkaline pHo (pHo 10; ϕCl−1i,o= 1·7±0·14 nmol m−2 s−1). A Cl−in/2H+in coporter could not account for the accumulation of Cl− alkaline pHo. Permeability of Cl− is very low, below 100pm s−1 under all conditions used, and appears to be maximal at pHo 7·5 (50–70 pm s−1) and minimal in acid pHo (20pm s−1). DCCD (dicyclohexyl‐carbodiimide) inhibited ϕCl−i,o in the light about 75% and [Cl−]i fell to 2·2±0·26 (4) mol m−3. Valinomycin had no effect but monensin severely inhibited Cl− uptake ([Cl−]i= 1·02±0·32 mol m−3; ϕCl−i,o= 0·20±0·1 nmol m−2 s−1). Vanadate (200 mmol m−3) accelerated the Cl− flux (ϕCl−i,o= 5·28±0·64 nmol m−2 s−1) but slightly decreased accumulation of Cl− ([Cl−], = 13·9±1·3 mol m−3) in BG‐11 medium but had no significant effect in Na+‐free media. DCMU (dichlorophenyldimethylurea) did not reduce [Cl−], or ϕCl−i,o to that found in the dark ([Cl−]i= 8·41±0·76 mol m−3; ϕCl−i,o= 2·06±0·36 nmol m−2 s−1). Synechococcus also actively accumulated Cl− in Na+‐free media, [Cl−]i was lower but ΔΨi,o hyperpolarized in Na+‐free media and so the ΔμCl−i,o was little changed ([Cl−]i= 7·98±0·698 mol m−3; ΔμCl−i,o=+203±3 mV). Net Cl− uptake was stimulated by Na+; Li+ acted as a partial analogue for Na+. Synechococcus has a Na+ activated Cl− transporter which is probably a primary 2Cl−/ATP pump. The Cl− pump is voltage sensitive. ΔμCl−i,o is directly proportional to ΔΨi,o(P»0·01%): ΔμCl−i,o= ‐1·487 (±0·102) ×ΔΨi,o, r= ‐0·983, n= 31. The ΔμCl−i,o increased (more positive) as the Δμi,o became more negative. The ΔμCl−i,o has no known function, but might provide a driving force for the uptake of micronutrients.

Phorbol esters stimulate and inhibit Cl- secretion by different mechanisms in a colonic cell line.

Phorbol 12-myristate 13-acetate (PMA) was found to increase both the short-circuit current (Isc) and the efflux of 125I- or 36Cl- in the colonic epithelial cell line HT-29.cl19A. Neither the PMA-provoked rise in Isc nor the stimulation of 125I- efflux was affected by the cyclooxygenase inhibitor indomethacin. The PMA-induced increase in Cl- efflux was not accompanied by a rise in adenosine 3',5'-cyclic monophosphate (cAMP) levels. A prolonged incubation with PMA (3 h), however, inhibited the PMA- and the cAMP-stimulated Isc by greater than 90%, whereas the cAMP-provoked 125I- and 36Cl- efflux was not inhibited. The long-term PMA treatment was found to inhibit the basal and cAMP-provoked 86Rb+ efflux by 65 +/- 9 and 86 +/- 7%, respectively. A 3-h incubation with PMA also strongly inhibited the Ca2+ ionophore A23187-induced increase in 86Rb+ efflux, whereas the A23187-stimulated 125I- efflux was only marginally inhibited. These data suggest that phorbol esters, presumably by activation of protein kinase C, can provoke Cl- secretion in HT-29.cl19A colonocytes independently of a prostaglandin- or cAMP-mediated pathway. Prolonged exposure to PMA, however, causes an inhibition of net electrogenic Cl- secretion by downregulation of the activity of K+ transporters.

Purinergic receptor activation of Cl- secretion in T84 cells.

The regulation by ATP of Cl- secretion in T84 cells grown on filters was investigated by measuring short-circuit current (Isc = net Cl- secretion). ATP (greater than or equal to 10 microM) added to the basolateral side markedly stimulated Isc both in the presence and absence of forskolin-activated Isc. Fluorescence microscopy of cells loaded with the Ca2+ indicator fura-2 showed that ATP stimulated a transient increase in intracellular free Ca2+ concentration [Ca2+]i. The augmentation of forskolin-stimulated Isc by ATP was at least partly caused by mobilization of Ca2+ from an internal store because prior depletion of the store using ionomycin prevented the response. The activity sequence for stimulation of Isc in the presence of forskolin was adenosine 5'-O-(3-thiotriphosphate) = 5'-adenylylimidodiphosphate (AMP-PNP) greater than ATP greater than ADP greater than AMP, suggesting the presence of a P2 purinergic receptor. Neither beta, gamma-methyleneadenosine 5'-triphosphate nor alpha, beta-methyleneadenosine 5'-triphosphate increased the Isc. Stimulation of Isc by ATP in the absence of forskolin was at least partly due to the breakdown of ATP to AMP and adenosine, which act at P1 receptors to stimulate Isc, since 1) inhibition of the ecto-phosphohydrolase 5'-nucleotidase by alpha, beta-methylene-ADP partially inhibited stimulation of Isc by ATP, 2) the adenosine receptor antagonists caffeine and 8-phenyltheophylline markedly inhibited the ATP-stimulated Isc, and 3) AMP-PNP, a weakly hydrolyzable analogue of ATP, caused a much smaller increase in Isc compared with ATP. Adenosine had no effect on [Ca2+]i.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of increased ureteropelvic pressure on fluid and NaCl absorption across the jejunum.

The aim of the present study was to elucidate the effects of an increased ureteropelvic pressure (UPP) on the net jejunal fluid, Na+, and Cl- absorption in anesthetized dogs. UPP was changed under hydrostatic pressure with warmed Ringer's solution. At a UPP of 0 mmHg, the net jejunal fluid, Na+, and Cl- absorption were 10.1 +/- 0.5 ml, 2.4 +/- 0.1, and 2.0 +/- 0.1 mEq, and were significantly reduced to 5.9 +/- 0.4 ml, 2.0 +/- 0.1, and 1.6 +/- 0.1 mEq, respectively, by an increase in UPP to 60 mmHg. After lowering the UPP to 0 mmHg, the net absorption recovered to the control values. The same experiments were performed after ipsilateral renal denervation. Ipsilateral renal denervation completely abolished this response. This result suggests that the afferent pathway of this response is the renal nerves. We also assessed the validity of the method using a jejunal loop by examining the effects of repetition of the absorption experiment on the net absorption. The net absorption was not altered by 6 times repetition of the absorption experiment. To determine the collection ratio, phenol red was used in the first and sixth absorption experiments. The collection ratios were 92.2 +/- 1.1 and 90.3 +/- 0.9%, respectively. There was no significant difference in collection ratio between the first and sixth values. This is the first report in which an increased UPP was found to inhibit the net jejunal fluid, Na+, and Cl- absorption.

Mechanisms of Na and Cl absorption across the distal colon epithelium of the pig.

Porcine distal colon epithelium was mounted in Ussing chambers and bathed in plasma-like Ringer solution. Tissue conductances ranged from 10 to 15 mS and the short-circuit current (Isc) ranged from -15 to 220 microA.cm-2. Variations in basal Isc resulted from differences in the amount of amiloride (10 microM mucosal addition)-sensitive Na+ absorption. Ion substitution and transepithelial flux experiments showed that 10 microM amiloride produced a decrease in the mucosal-to-serosal (M-S) and net Na flux, and that this effect on Isc was independent of Cl- and HCO3- replacement. When the concentration of mucosal amiloride was increased from 10 to 100 microM, little change in Isc was observed. However, increasing the concentration to 1 mM produced a further inhibition, which often reversed the polarity of the Isc. The decrease in Isc due to 1 mM amiloride was dependent on both Cl- and HCO3-, and was attributed to reductions in the M-S and net Na+ fluxes as well as the M-S unidirectional Cl- flux. Ion replacement experiments demonstrated that Cl- substitution reduced the M-S and net Na fluxes, while replacement of HCO3- with HEPES abolished net Cl- absorption by reducing the M-S unidirectional Cl- flux. From these data it can be concluded that: (1) Na+ absorption is mediated by two distinct amiloride-sensitive transport pathways, and (2) Cl- absorption is completely HCO3- dependent (presumably mediated by Cl-/HCO3- exchange) and occurs independently of Na+ absorption.

Altered Na+ and Cl? flux during diet-induced mixed gallstone formation in the prairie dog

Recent studies suggest that altered gallbladder absorptive function may be an important and previously unrecognized factor in the pathogenesis of experimentally induced gallstones. The present study was designed to define the specific changes in gallbladder epithelial ion transport that occur during mixed gallstone formation. Fifteen prairie dogs were fed either control or corn-alfalfa chow for six months. No control animals developed gallstones or crystals. Three of eight corn-alfalfa-fed animals had large black stones, and the remaining five had crystals ("pregallstone" group). Corn-alfalfa-fed animals had significant increases in gallbladder bile cholesterol, phospholipids, and calcium as compared to controls. Gallbladders were removed and mounted in a Ussing chamber for electrophysiologic and ion flux studies. Gallbladders from animals fed corn-alfalfa demonstrated significant decreases in short-circuit current and potential difference as compared to controls (P less than 0.05). 22Na and 36Cl were used to determine unidirectional ion fluxes. While net ion fluxes were similar in pregallstone animals and controls, stone-forming animals exhibited a significant decrease in net Na+ flux and a significant reversal in the direction of net Cl- flux (from secretion to absorption) as compared to controls (P less than 0.05). These data indicate that mixed gallstone formation is associated with alterations in gallbladder ion transport. The role of these changes in the pathogenesis of mixed gallstones remains to be determined.

Ca2+ and cAMP activate different chloride efflux pathways in HT-29.cl19A colonic epithelial cell line.

The mechanism of adenosine 3',5'-cyclic monophosphate (cAMP)- and Ca(2+)-induced Cl- secretion was studied in monolayers of the colon carcinoma cell line HT-29.cl19A by combined short-circuit current (Isc) and 125I- or 36Cl- efflux measurements. Forskolin, a specific adenylate cyclase activator, was found to induce a large increase in Isc and a two- to threefold increase in 36Cl- efflux solely across the apical border. The fractional efflux of 36Cl-compared with 125I- (basal ratio 1.71 +/- 0.28) did not change significantly in the presence of forskolin (1.91 +/- 0.45). In contrast, the Ca2+ ionophore A23187 did not appreciably affect the Isc but enhanced 36Cl- and 125I- efflux at the apical and basolateral side of the monolayer. Furthermore, the fractional efflux ratio of 36Cl- to 125I- changed dramatically to a value of 0.36 +/- 0.14. Both forskolin- and A23187-induced 36Cl- or 125I- efflux were only weakly inhibited by the putative Cl- channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoicacid. Carbachol, a Ca(2+)-linked agonist, mimicked the effects of A23187 on the 36Cl- and 125I- efflux but additionally provoked a significant increase in Isc. These data show that Ca2+ and cAMP activate different Cl-efflux pathways in HT-29.cl19A cells. Most likely these pathways represent a cAMP-activated conductance in the apical membrane and a separate Ca(2+)-activated Cl- conductance expressed in both apical and basolateral membranes. Apparently cholinergic agonists induce net electrogenic Cl- secretion through an intracellular signaling pathway (e.g., protein kinase C activation) different from the one activated by Ca2+/Ca2+ ionophore alone.

Mucosal histamine inhibits Na absorption and stimulates Cl secretion across equine tracheal epithelium

When the equine tracheal epithelium is mounted in Ussing chambers and bathed in plasma-like Ringer solution, the tissue generates a lumen-negative transepithelial potential (PD) of 22 mV and a short-circuit current (Isc) of 70-200 microA/cm2. Mucosal addition of 10 microM histamine produces a transient increase in the Isc followed by a return to baseline or below. Mucosal addition of 2 microM diphenhydramine inhibits the Isc response to mucosal histamine, whereas 100 microM mucosal cimetidine produces no effect. The average initial increases in Isc over time for mucosal vs. serosal histamine addition are significantly different (17.32 +/- 2.8 and 3.76 +/- 0.69 microA/min, respectively). Pretreatment with mucosal amiloride significantly prolongs the effect of mucosal histamine on Isc over a 20-min period from 4.73 +/- 0.33 to 15.48 +/- 3.16 microA. When Cl is replaced by gluconate, mucosal histamine addition results in a gradual decrease in Isc and significantly reduces the effect of mucosal amiloride on Isc from 80.8% to 54.9%. Mucosal histamine inhibits the net transepithelial Na flux by 42% and stimulates the secretion of Cl by 106%. Subsequent addition of serosal bumetanide decreases net Cl secretion by 70% These results suggest that histamine stimulates bumetanide-sensitive Cl secretion and inhibits amiloride-sensitive Na absorption; these effects are mediated by H1 receptors at the apical membrane surface.

Low-dose glucocorticoids maintain Na-H exchange in distal colon of adrenalectomized rats.

With in vivo perfusion we demonstrated that physiological doses of glucocorticoids restore Na and Cl absorption in adrenalectomized rat colon. The absorption is spironolactone and amiloride resistant and is inhibited by the Na-H inhibitor, 5-(N-ethyl-N-isopropyl)amiloride (EIPA), suggesting that glucocorticoids modulate Na-H antiport. The present in vitro study examines pathways mediated by glucocorticoids in adrenalectomized rat distal colon and rectum. In vivo administration of 2.5 micrograms/100 g body wt dexamethasone did not alter serosal-to-mucosal flux or tissue electrical parameters but restored mucosal-to-serosal flux and net Na and Cl absorption within 2-3 h of administration to levels found in intact rat colon. Transport was not inhibited by 10(-5) M amiloride but was eliminated by 10(-5) M EIPA. After 26 h of dexamethasone, an amiloride-resistant short-circuit current was stimulated, accompanied by increased residual ion flux in rectum, but not distal colon, suggesting that a delayed or secondary effect of glucocorticoids is stimulation of electrogenic anion secretion. Thus adrenalectomy reduces net ion flux in distal colon by its effect on electroneutral mucosal-to-serosal NaCl flux. Small doses of glucocorticoids completely ameliorate this effect via stimulation of the Na-H antiport. Glucocorticoids maintain basal electroneutral NaCl absorption in distal rat colon.

Endothelin stimulates chloride secretion across canine tracheal epithelium.

The endothelins (ET) are a group of isopeptides produced by a number of cells, including canine tracheal epithelial cells. Because these compounds are endogenous peptides that may activate eicosanoid metabolism, we investigated the effects of ET on Cl secretion in canine tracheal epithelium. Endothelin 1 (ET-1) was found to produce a dose-dependent change in short-circuit current (Isc) that increased slowly and reached a maximal value within 10-15 min. When isopeptides of ET were compared, 300 nM ET-1 and ET-2 produced comparable maximal increases in Isc, whereas ET-3 produced smaller changes in Isc (half-maximal concentrations of 2.2, 7.2, and 10.4 nM, respectively). Ionic substitution of Cl with nontransported anions, iodide and gluconate, reduced ET-1-induced changes in Isc. Furthermore, the response was inhibited by the NaCl cotransport inhibitor, furosemide. In paired tissues, ET-1 significantly increased mucosal net 36Cl flux without significant effect on 22Na flux. The increase in Isc induced by ET was diminished by pretreatment with indomethacin. The second messengers mediating the increase in Isc were investigated in cultured canine tracheal epithelial cells. ET-1 stimulated the release of [3H]arachidonate from membrane phospholipids, increased intracellular Ca2+ (occasionally producing oscillations), and increased adenosine 3',5'-cyclic monophosphate accumulation. The latter was diminished by indomethacin. Thus ET is a potent agonist of Cl secretion (with the isopeptides having the following potency: ET-1 greater than or equal to ET-2 greater than ET-3) and acts, in part, through a cyclooxygenase-dependent mechanism.

Ontogeny of ion transport across fetal pulmonary epithelial cells in monolayer culture

The transition of the fetal lung from a fluid-secreting to a fluid-absorbing organ is dependent on ion transport across the pulmonary epithelium. This study examined the ion transport characteristics of distal pulmonary epithelial cells isolated from rat fetuses in late gestation and maintained in differentiation-arrested monolayer cultures. The response to inhibitors of active ion transport suggested the presence of apical to basolateral Na+ transport in monolayers derived from each gestational age. However, amiloride inhibition of short-circuit current (Isc) varied with gestational age, decreasing Isc by 30% in monolayers derived from day 18 fetuses and by 55% in monolayers from day 21 fetuses. A portion (10%) of the residual Isc remaining after amiloride addition to monolayers from day 18 fetuses could be inhibited by bumetanide, suggesting the induction of net Cl- transport. Ion-substitution experiments confirmed the presence of Na+ and inducible Cl- transport mechanisms in monolayers from day 18 fetuses and only Na+ transport mechanisms in monolayers from day 21 fetuses. beta-Adrenergic stimulation increased Isc but maintained the age-dependent characteristics of Na(+)- and Cl(-)-dependent ion transport. In summary, monolayer cultures of fetal pulmonary epithelial cells exhibit age-dependent differences in ion transport properties that are consistent with a transition or maturation of the distal pulmonary epithelium from an epithelium capable of Na+ absorption and Cl- secretion preterm to one capable of only Na+ absorption at term.

Antisecretory effect of prostaglandin D2 in rat colon in vitro: action sites

The effect of prostaglandin D2 (PGD2) on colonic ion transport was studied in the Ussing chamber. PGD2 (10(-6) M) decreased baseline short-circuit current (Isc) in two preparations of rat colon descendens, a mucosa-submucosa preparation with and a mucosa preparation without the submucosal plexus. In both preparations, PGD2 inhibited the neuronally mediated secretory responses to electric field stimulation, the sea anemone toxin ATX II, and different cholinergic agents. Unidirectional flux measurements revealed that PGD2 diminished the secretagogue-induced increase in the serosal-to-mucosal flux of Cl- and thereby inhibited net Cl- secretion. PGD2, however, had no effect on the adenosine 3',5'-cyclic monophosphate-mediated response to forskolin or vasoactive intestinal peptide or on guanosine 3',5'-cyclic monophosphate-mediated secretion induced by the heat-stable enterotoxin of Escherichia coli. The PGD2 also blocked the increase in Isc evoked by two neuronally acting inflammatory mediators, i.e., bradykinin and PGI2 in the mucosa-submucosa preparation, but had no effect on the response to PGE2. Consequently, PGD2 exerts an indirect antisecretory effect caused by an inhibition of enteric secretomotor neurons of both the submucosal and the mucosal plexus.

Inability of Ehrlich ascites tumor cells to volume regulate following a hyperosmotic challenge

Ehrlich cells shrink when the osmolality of the suspending medium is increased and behave, at least initially, as osmometers. Subsequent behavior depends on the nature of the hyperosmotic solute but in no case did the cells exhibit regulatory volume increase. With hyperosmotic NaCl an osmometric response was found and the resultant volume maintained relatively constant. Continuous shrinkage was observed, however, with sucrose-induced hyperosmolality. In both cases increasing osmolality from 300 to 500 mOSM initiated significant changes in cellular electrolyte content, as well as intracellular pH. This was brought about by activation of the Na+/H+ exchanger, the Na/K pump, the Na+ + K+ + 2Cl cotransporter and by loss of K+ via a Ba-sensitive pathway. The cotransporter in response to elevated [Cl-]i (approximately 100 mM) and/or the increase in the outwardly directed gradient of chemical potential for Na+, K+ and Cl-, mediated net loss of ions which accounted for cell shrinkage in the sucrose-containing medium. In hyperosmotic NaCl, however, the net Cl- flux was almost zero suggesting minimal net cotransport activity. We conclude that volume stability following cell shrinkage depends on the transmembrane gradient of chemical potential for [Na+ + K+ + Cl-], as well as the ratio of intra- to extracellular [Cl-]. Both factors appear to influence the activity of the cotransport pathway.

Reconstruction of immune‐mediated ion secretion in gut mucosa of the athymic rat

A defect in an immune-mediated, physiological reaction in the small intestine of congenitally immunosuppressed rats has been identified and corrected by recoupling the immunological trigger in the mucosa with epithelial effector cells. Antigenic challenge of jejunum from euthymic rats sensitized by infection with Trichinella spiralis evoked an anaphylactic response in vitro that was characterized by an elevation in transmural short-circuit current (Isc) and caused by net Cl- secretion. In contrast, jejunum from previously infected athymic counterparts failed to respond electrophysiologically to antigenic stimulation. Passive immunization of athymic rats with anti-Trichinella serum restored jejunal responsiveness to antigen. Jejunal unresponsiveness to antigenic challenge in the athymic rat compared with the euthymic rat was not due to a difference in the duration of exposure to the sensitizing antigens, mast cell numbers, inflammation-related histological changes nor to reduced responsiveness of epithelium to Cl- secretagogues. Because the transduction of the antigenic signal into epithelial ion transport changes in euthymic rats requires IgE, mucosal mast cells, mast cell-derived mediators and enteric nerves, the results support the conclusion that an antibody deficiency is the only functional defect preventing the antigen-induced change in mucosal ion transport in the athymic host.

Avian cecum: role of glucose and volatile fatty acids in transepithelial ion transport

In the fowl cecum in vitro, the influence of glucose and the three most prevalent naturally occurring volatile fatty acids (acetate, propionate, butyrate) on short-circuit current (Isc), electrical resistance, and transport of Na and Cl was determined. When glucose, acetate, or butyrate was present, ion transport was characterized by electrogenic Na absorption, greater than 65% of which was amiloride inhibitable, and Cl secretion, which also was electrogenic. Isc could be completely accounted for by net fluxes of Na and Cl. When glucose, acetate, or butyrate (10 mM both sides) was included in the incubation medium, cecal tissue maintained its Isc and a constant rate of net Na absorption and Cl secretion for a 5-h period. When no substrate was present or propionate was included in the medium, a marked fall in Isc and net Na and Cl fluxes was seen. Glucose caused an increase in Isc when added only to the serosal side. As 3-O-methylglucose (not metabolized) was not effective in stimulating Isc of the cecum (serosal or mucosal addition), it appeared that glucose increased Isc by acting as an energy substrate for active Na transport. Acetate and butyrate appeared to be equally effective in stimulating Na transport and Isc when placed on either side of the membrane. When the preparation was supplied with glucose (serosal side) and acetate was added to the mucosal side, no further stimulation of Isc occurred. Thus it appeared that acetate and butyrate were acting as substrates for active Na transport rather than stimulating Na transport by some other mechanism such as a cotransport with Na.(ABSTRACT TRUNCATED AT 250 WORDS)

Brain natriuretic peptide stimulates K and Cl secretion across porcine distal colon epithelium

Porcine distal colon epithelium was mounted in Ussing chambers and bathed with porcine Ringer solution. The serosal addition of brain natriuretic peptide (BNP; 50 nM) or atriopeptin III (AP-III; 500 nM) produced significant increases (50-75 microA/cm2) in short-circuit current (Isc). These increases in Isc were not inhibited by pretreatment with tetrodotoxin (TTX) or 5,8,11,14-eicosatetraynoic acid (ETYA). Analysis of concentration-response relationships revealed that BNP was 5.8-fold more potent than AP-III in stimulating the Isc. BNP and AP-III significantly increased the serosal-to-mucosal (S----M) Cl flux and reduced net Cl absorption by 38 and 41%, respectively. The BNP-stimulated S----M Cl flux was abolished when HCO3 was removed. In contrast, the vasoactive intestinal peptide (VIP)-stimulated S----M Cl flux was not affected by HCO3 replacement. In addition to their effects on Cl transport, BNP and AP-III increased net Rb secretion by 79 and 58%, respectively. BNP-stimulated Rb secretion was reduced by 76% after HCO3 replacement. These results indicate that natriuretic peptides stimulate K- and HCO3-dependent Cl secretion which is not present under basal conditions or after VIP stimulation. The difference in potency between BNP and AP-III suggests that ANP-B receptors may mediate their effects on ion transport in the porcine colon.

Contrasting effects of PGE2 and PGD2: ion transport in the canine proximal colon

Our earlier study [Am. J. Physiol. 256 (Gastrointest. Liver Physiol. 19): G673-G679, 1989] showed that the isomers prostaglandin E2 and D2 (PGE2 and PGD2) had contrasting effects on the canine proximal colonic epithelium in vitro. Whereas PGE2 produced sharp increases in short-circuit current (Isc), PGD2 rapidly reversed these changes. We report here that PGD2 reversed increases in Isc produced by other agonists (forskolin, carbachol) and was thus not a specific antagonist to PGE2. The transient increase in Isc and conductance produced by PGE2 were accompanied by alterations in Cl- but not Na+ fluxes. These were partitioned into early and late phases. In the early phase, net Cl- flux (JCl-net) decreased due to a reduction in Cl- mucosal to serosal flux (JCl-m----s) and increases in JCl-s----m. In the later phase, these changes appeared to revert to prestimulation values. Because forskolin produced more sustained increases in Isc, we used it as an agonist to define the effects of PGD2 on ion fluxes. Forskolin produced a marked reduction in JCl-net due to a decrease in JCl-m----s and an increase in JCl-s----m. PGD2 not only reversed the increases in Isc and conductance produced but also reversed the changes in Cl- flux.

Effects of environmental conditions on mitochondrial-rich cell density and chloride transport in toad skin

Chloride flux across amphibian skin is usually passive, yet largely conductive; previous reports have suggested that aldosterone influences this pathway. The conductive Cl- pathway and its regulation were examined further, across the abdominal skin of toads (Bufo marinus) adapted to various environments. Short-circuit current (Isc), total conductance (Gt) and Cl- influx (JCl) were measured in conditions such that there was net Cl- movement in absence of Na+ transport. In salt-deprived animals compared to salt-adapted ones, there was a significant increase in JCl (563 vs 200 pmol cm-2 s-1), aldosteronaemia (4.2 vs 1.1 nmol/l), as well as MRC density (1458 vs 851 mm-2). After adaptation to dilute Na2SO4 compared to MgCl2, JCl (631 vs 313 pmol cm-2 s-1) as well as the density of mitochondria-rich cells (MRC) (1306 vs 710 mm-2) practically doubled, while the toads' aldosteronaemia was lower (2.4 vs 10.8 mmol/l). In all groups of toads, JCl was matched by Isc, and there was a close correlation between Gt and JCl (r = 0.96), which confirms the conductive nature of transepithelial Cl- movement. Furthermore, the relationship between JCl and MRC density (r = 0.75) argues in favour of a role played by MRC on Cl- conductance of epithelial such as amphibian skin. As aldosterone injected for 1 week into NaCl-adapted toads did not influence MRC density and as aldosteronaemia was not correlated with Cl- conductance, this hormone does not emerge as the determinant of these parameters.

L-glutamine stimulates jejunal sodium and chloride absorption in pig rotavirus enteritis

Rotavirus enteritis is the leading cause of diarrhea in infants worldwide. A research priority of the World Health Organization is to develop oral rehydration solutions containing amino acids or other additives that will stimulate intestinal absorption more efficiently than the current glucose-based oral rehydration solutions. Glutamine is the principal metabolic fuel of the small bowel and a putative stimulator of mucosal repair. This report describes the transport response to mucosal l-glutamine following intestinal injury caused by porcine rotavirus. Peak symptoms and mucosal damage were observed 2–7 days after oral rotavirus inoculation. In vitro transport studies of the maximally injured region, the midjejunum (80% reduction in lactase), surprisingly, showed transport responses to l-glutamine (30 mmol/L) Sand l-alanine (30 mmol/L) that were similar qualitatively and quantitatively to those observed in control tissue. Subsequent application of mucosal d-glucose (30 mmol/L) caused additional stimulation of electrogenic Na+ transport, but the response to glucose was blunted (P < 0.05) in the infected tissues. Glutamine and alanine enhanced Na+ absorption to a similar degree (2–2.5 μEq · cm−2 · h−1), but glutamine stimulated equal amounts of electrogenic and electroneutral NaC1 absorption, whereas alanine had no significant effect on net Cl− flux. Glutamine is a potentially useful substrate for investigation in oral rehydration solutions for infant diarrhea.

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

1. 1. Prairie dog gallbladders mounted in a Ussing-type chamber and bathed with symmetrical Ringer's solutions exhibited a transepithelial resistance ( R t ) of 51 ± 51Ω cm 2 , a lumen negative potential difference (V ms) of 11.5 ± 0.7 mV and a short-circuit current (I sc) of 6.9 ± 0.3 μEq/hr/cm 2. 2. 2. Radioisotopic ion flux experiments revealed that the basal I sc of 6.9 ± 0.3 μEq/hr/cm 2 was mostly accounted for by net Na + absorption of 3.2 ± 0.5 μEq/hr/cm 2 and net Cl − secretion of 2.9 ± 0.3 μEq/hr/cm 2. 3. 3. In HCO 3 − free Ringer's, net Na + flux was virtually abolished, net Cl − flux decreased by 50% and I sc was reduced by 77%. 4. 4. 10 −3 M mucosal amiloride and DIDS reduced I sc by 28 and 24%, respectively. 5. 5. Mucosal NaCl diffusion potentials indicated that the paracellular pathway was cation selective. 6. 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. 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 −.