The nature of the neutral Na+−Cl−-coupled entry at the apical membrane of rabbit gallbladder epithelium: II. Na+−Cl− symport is independent of K+

Summary In the epithelium of rabbit gallbladder, in the nominal absence of bicarbonate, intracellular Cl^– activity is about 25mm, about 4 times higher than intracellular Cl^– activity at the electrochemical equilibrium. It is essentially not affected by 10^–4 m acetazolamide and 10^–4 m 4-acetamido-4-isothiocyanostilbene-2,2-disulfonate (SITS) even during prolonged exposures; it falls to the equilibrium value by removal of Na⁺ from the lumen without significant changes of the apical membrane potential difference. Both intracellular Cl^– and Na⁺ activities are decreased by luminal treatment with 25mm SCN^–; the initial rates of change are not significantly different. In addition, the initial rates of change of intracellular Cl^– activity are not significantly different upon Na⁺ or Cl^– entry block by the appropriate reduction of the concentration of either ion in the luminal solution. Luminal K⁺ removal or 10^–5 m bumetanide do not affect intracellular Cl^– and Na⁺ activities or Cl^– influx through the apical membrane. It is concluded that in the absence of bicarbonate NaCl entry is entirely due to a Na⁺–Cl^– symport on a single carrier which, at least under the conditions tested, does not cotransport K⁺.     

Stimulation of duodenal epithelial HCO3− transport in the guinea pig and cat by luminal prostaglandin E2

Segments of guinea pig or cat duodenum distal to the Brunner gland containing area and devoid of bile or pancreatic secretions were cannulated in situ. The unbuffered luminal solution was gassed with 100% O₂ or N₂ and HCO₃⁻ transport titrated at pH 7.40 or 8.00 with solutions containing HCl. Cat duodenum transported HCO₃⁻ at a greater rate (∼17μeq, cm⁻¹, h⁻¹) than did jejunum in the same animals (∼5μeq, cm⁻¹, h⁻¹) and also developed a greater transmucosal electrical potential difference. Luminal application of PGE₂ (1 – 12 μM) in cat duodenum increased HCO₃⁻ transport and the potential difference. HCO₃⁻ transport by guinea pig duodenum (∼27 μeq, cm⁻¹, h⁻¹) was increased by luminal PGE₂ only in animals where transport had been inhibited by pretreatment with aspirin (30 mg/kg intravenously). Exposure of the cat duodenal lumen to HCl (1 – 25 mM, 5 min) stimulated HCO₃⁻ transport and continuous exposure of duodenum in the guinea pig to acid discharged from the stomach may increase endogenous prostaglandin concentrations, resulting in an apparent lack of effect of exogenous prostaglandins. The present results and previous similar findings in amphibians in vitro suggest that surface epithelial transport of HCO₃⁻ protects duodenal mucosa against acid.     

Vasopressin alters the mechanism of apical Cl− entry from Na+:Cl− to Na+:K+:2Cl− cotransport in mouse medullary thick ascending limb

Summary Experiments were performed usingin vitro perfused medullary thick ascending limbs of Henle (MTAL) and in suspensions of MTAL tubules isolated from mouse kidney to evaluate the effects of arginine vasopressin (AVP) on the K⁺ dependence of the apical, furosemide-sensitive Na⁺:Cl^– cotransporter and on transport-related oxygen consumption (QO₂). In isolated perfused MTAL segments, the rate of cell swelling induced by removing K⁺ from, and adding onemm ouabain to, the basolateral solution [ouabain(zero-K⁺)] provided an index to apical cotransporter activity and was used to evaluated the ionic requirements of the apical cotransporter in the presence and absence of AVP. In the absence of AVP cotransporter activity required Na⁺ and Cl^–, but not K⁺, while in the presence of AVP the apical cotransporter required all three ions.⁸⁶Rb⁺ uptake into MTAL tubules in suspension was significant only after exposure of tubules to AVP. Moreover,²²Na⁺ uptake was unaffected by extracellular K⁺ in the absence of AVP while after AVP exposure²²Na⁺ uptake was strictly K⁺-dependent. The AVP-induced coupling of K⁺ to the Na⁺:Cl^– cotransporter resulted in a doubling in the rate of NaCl absorption without a parallel increase in the rate of cellular²²Na⁺ uptake or transport-related oxygen consumption. These results indicate that arginine vasopressin alters the mode of a loop diuretic-sensitive transporter from Na⁺:Cl^– cotransport to Na⁺:K⁺:2Cl^– cotransport in the mouse MTAL with the latter providing a distinct metabolic advantage for sodium transport. A model for AVP action on NaCl absorption by the MTAL is presented and the physiological significance of the coupling of K⁺ to the apical Na⁺:Cl^– cotransporter in the MTAL and of the enhanced metabolic efficiency are discussed.     

Stimulation by HCO 3 − of Na+ transport in rabbit gallbladder

Summary Bicarbonate presence in the bathing media doubles Na⁺ and fluid transepithelial transport and in parallel significantly increases Na⁺ and Cl^– intracellular concentrations and contents, decreases K⁺ cell concentration without changing its amount, and causes a large cell swelling. Na⁺ and Cl^– lumen-to-cell influxes are significantly enhanced, Na⁺ more so than Cl^–. The stimulation does not raise any immediate change in luminal membrane potential and cannot be due to a HCO ₃ ^– -ATPase in the brush border. The stimulation goes together with a large increase in a Na⁺-dependent H⁺ secretion into the lumen. All of these data suggests that HCO ₃ ^– both activates Na⁺–Cl^– cotransport and H⁺–Na⁺ countertransport at the luminal barrier.Thiocyanate inhibits Na⁺ and fluid transepithelial transport without affecting H⁺ secretion and HCO ₃ ^– -dependent Na⁺ influx. It reduces Na⁺ and Cl^– concentrations and contents, increases the same parameters for K⁺, causes a cell shrinking, and abolishes the lumen-to-cell Cl^– influx. It enters the cell and is accumulated in the cytoplasm with a process which is Na⁺-dependent and HCO ₃ ^– -activated. Thus, SCN^– is likely to compete for the Cl^– site on the cotransport carrier and to be slowly transferred by the cotransport system itself.     

The nature of the neutral Na+−Cl−-coupled entry at the apical membrane of rabbit gallbladder epithelium: I. Na+/H+, Cl−/HCO 3 − double exchange and Na+−Cl− symport

Summary Cl^– influx at the luminal border of the epithelium of rabbit gallbladder was measured by 45-sec exposures to³⁶Cl^– and³H-sucrose (as extracellular marker). Its paracellular component was evaluated by the use of 25mm SCN^– which immediately and completely inhibits Cl^– entry into the cell. Cellular influx was equal to 16.7eq cm^–2 hr^–1 and decreased to 8.5eq cm^–2 hr^–1 upon removal of HCO ₃ ^– from the bathing media and by bubbling 100% O₂ for 45 min. When HCO ₃ ^– was present, cellular influx was again about halved by the action of 10^–4 m acetazolamide, 10^–5 to 10^–4 m furosemide, 10^–5 to 10^–4 m 4-acetamido-4-isothiocyanostilbene-2,2-disulfonate (SITS), 10^–3 m amiloride. The effects of furosemide and SITS were tested at different concentrations of the inhibitor and with different exposure times: they were maximal at the concentrations reported above and nonadditive. In turn, the effects of amiloride and SITS were not additive. Acetazolamide reached its maximal action after an exposure of about 2 min. When exogenous HCO ₃ ^– was absent, the residual cellular influx was insensitive to acetazolamide, furosemide and SITS. When exogenous HCO ₃ ^– was present in the salines, Na⁺ removal from the mucosal side caused a slow decline of cellular Cl^– influx; conversely, it immediately abolished cellular Cl^– influx in the absence of HCO ₃ ^– . In conclusion, about 50% of cellular influx is sensitive to HCO ₃ ^– , inhibitable by SCN^–, acetazolamide, furosemide, SITS and amiloride and furthermore slowly dependent on Na⁺. The residual cellular influx is insensitive to bicarbonate, inhibitable by SCN^–, resistant to acetazolamide, furosemide, SITS and amiloride, and immediately dependent on Na⁺. Thus, about 50% of apical membrane NaCl influx appears to result from a Na⁺/H⁺ and Cl^–/HCO ₃ ^– exchange, whereas the residual influx seems to be due to Na⁺–Cl^– contranport on a single carrier. Whether both components are simultaneously present or the latter represents a cellular homeostatic counterreaction to the inhibition of the former is not clear.     

Na+ and Cl− conductances are controlled by cytosolic Cl− concentration in the intralobular duct cells of mouse mandibular glands

Our previously published whole-cell patch-clamp studies on the cells of the intralobular (granular) ducts of the mandibular glands of male mice revealed the presence of an amiloride-sensitive Na⁺ conductance in the plasma membrane. In this study we demonstrate the presence also of a Cl^– conductance and we show that the sizes of both conductances vary with the Cl^– concentration of the fluid bathing the cytosolic surface of the plasma membrane. As the cytosolic Cl^– concentration rises from 5 to 150 mmol/liter, the size of the inward Na⁺ current declines, the decline being half-maximal when the Cl^– concentration is approximately 50 mmol/liter. In contrast, as cytosolic Cl^– concentration increases, the inward Cl^– current remains at a constant low level until the Cl^– concentration exceeds 80 mmol/liter, when it begins to increase. Studies in which Cl^– in the pipette solution was replaced by other anions indicate that the Na⁺ current is suppressed by intracellular Br^-, Cl^– and NO ₃ ^- but not by intracellular I^-, glutamate or gluconate. Our studies also show that the Cl^– conductance allows passage of Cl^– and Br^- equally well, I-less well, and NO ₃ ^- , glutamate and gluconate poorly, if at all. The findings with NO ₃ ^- are of particular interest because they show that suppression of the Na⁺ current by a high intracellular concentration of a particular anion does not depend on actual passage of that anion through the Cl^– conductance. In mouse granular duct cells there is, thus, a reciprocal regulation of Na⁺ and Cl^– conductances by the cytosolic Cl^– concentration. Since the cytosolic Cl^– concentration is closely correlated with cell volume in many epithelia, this reciprocal regulation of Na⁺ and Cl^– conductances may provide a mechanism by which ductal Na⁺ and Cl transport rates are adjusted so as to maintain a stable cell volume.This project was supported by the National Health and Medical Research Council of Australia. We thank Professor P. Barry (University of New South Wales) for assistance with the junction potential measurements.     

HCO 3 − -coupled Na+ influx is a major determinant of Na+ turnover and Na+/K+ pump activity in rat hepatocytes

Summary Recent studies in hepatocytes indicate that Na⁺-coupled HCO ₃ ^– transport contributes importantly, to regulation of intracellular pH and membrane HCO ₃ ^– transport. However, the direction of net coupled Na⁺ and HCO ₃ ^– movement and the effect of HCO ₃ ^– on Na⁺ turnover and Na⁺/K⁺ pump activity are not known. In these studies, the effect of HCO ₃ ^– on Na⁺ influx and turnover were measured in primary rat hepatocyte cultures with²²Na⁺, and [Na⁺] _i was measured in single hepatocytes using the Na⁺-sensitive fluorochrome SBFI. Na⁺/K⁺ pump activity was measured in intact perfused rat liver and hepatocyte monolayers as Na⁺-dependent or ouabain-suppressible⁸⁶Rb uptake, and was measured in single hepatocytes as the effect of transient pump inhibition by removal of extracellular K⁺ on membrane potential difference (PD) and [Na⁺] _i . In hepatocyte monolayers, HCO ₃ ^– increased²²Na⁺ entry and turnover rates by 50–65%, without measurably altering²²Na⁺ pool size or cell volume, and HCO ₃ ^– also increased Na⁺/K⁺ pump activity by 70%. In single cells, exposure to HCO ₃ ^– produced an abrupt and sustained rise in [Na⁺] _i , from 8 to 12mm. Na⁺/K⁺ pump activity assessed in single cells by PD excursions during transient K⁺ removal increased 2.5-fold in the presence of HCO ₃ ^– , and the rise in [Na⁺] _i produced by inhibition of the Na⁺/K⁺ pump was similarly increased 2.5-fold in the presence of HCO ₃ ^– . In intact perfused rat liver, HCO ₃ ^– increased both Na⁺/K⁺ pump activity and O₂ consumption. These findings indicate that, in hepatocytes, net coupled Na⁺ and HCO ₃ ^– movement is inward and represents a major determinant of Na⁺ influx and Na⁺/K⁺ pump activity. About half of hepatic Na⁺/K⁺ pump activity appears dedicated to recycling Na⁺ entering in conjunction with HCO ₃ ^– to maintain [Na⁺] _i within the physiologic range.     

Short-chain fatty acids and CO2 as regulators of Na+ and Cl− absorption in isolated sheep rumen mucosa

Summary Unidirectional ²²Na⁺ and ³⁶Cl^– fluxes were determined in short-circuited, stripped rumen mucosa from sheep by using the Ussing chamber technique. In both CO₂/HCO _– ³ -containing and CO₂/HCO _– ³ -free solutions, replacement of gluconate by short-chain fatty acids (SCFA, 39 mM) significantly enhanced mucosal-toserosal Na⁺ absorption without affecting the Cl^– transport in the same direction. Short-chain fatty acid stimulation of Na⁺ transport was at least partly independent of Cl^– and could almost completely be abolished by 1 mM mucosal amiloride, while stimulation of Na⁺ transport was enhanced by lowering the mucosal pH from 7.3 to 6.5. Similar to the SCFA action, raising the PCO₂ in the mucosal bathing solution led to an increase in the amiloride-sensitive mucosal-to-serosal Na⁺ flux. Along with its effect on sodium transport, raising the PCO₂ also stimulated chloride transport. The results are best explained by a model in which undissociated SCFA and/or CO₂ permeate the cell membrane and produce a raise in intracellular H⁺ concentration. This stimulates an apical Na⁺/H⁺ exchange, leading to increased Na⁺ transport. The stimulatory effect of CO₂ on Cl^– transport is probably mediated by a Cl^–/HCO _– ³ exchange mechanism in the apical membrane. Binding of SCFA anions to that exchange as described for the rat distal colon (Binder and Mehta 1989) probably does not play a major role in the rumen.Abbreviations DIDS 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid - G _t transepithelial conductance (mS·cm^-2) - HSCFA undissociated short-chain fatty acids - J _ms mucosal-to-serosal flux (Eq · cm^-2 · h^-1) - J _net net flux (Eq · cm^-2 · h^-1) - J _sm serosal-to-mucosal flux (Eq · cm^-2 · h^-1) - PD transepithelial potential difference (mV) - SCFA dissociated short-chain fatty acids - SCFA short-chain fatty acids     

Cl− transport in basolateral renal medullary vesicles: I. Cl− transport in intact vesicles

Summary This paper provides the results of studies which characterized conductive³⁶Cl^– flux in basolaterally enriched membrane vesicles prepared from rabbit renal outer medulla. Conductive³⁶Cl^– uptake was studied under two different experimental conditions. In the first,³⁶Cl^– flux was driven by an inside positive voltage created with oppositely directed Cl^– and gluconate gradients. In the second, an inwardly direct K⁺ gradient was used to drive³⁶Cl^– uptake. By these two methods, voltage-sensitive³⁶Cl^– uptake was shown to comprise about 45 and 65%, respectively, of the initial rates of total³⁶Cl^– flux. Separate paired studies demonstrated that the conductive³⁶Cl^– uptake was inhibited by the Cl^– channel blocker diphenylamine-2-carboxylate (DPC) with an IC₅₀ for DPC of 154 m. The voltagedependent³⁶Cl^– uptake had an activation energy of 6.4 kcal/mole. This³⁶Cl^– conductance had an anion selectivity sequence of I^–>Cl^–NO ₃ ^– gluconate.     

Requirement of Cl− and Na+ for the ouabain-resistant control of cell volume in slices of rat liver

Summary The ability of liver cells to control their volume in the presence of ouabain has been studied in tissue slices that were recovering at 38°C from a period of swelling at 1°C. Morphological observations were made in conjunction with measurements of the net movements of water and ions. Extrusion of water in the presence of ouabain (2mm) was accompanied by a net loss of Na⁺ and Cl^– and by the formation of characteristic, rounded vesicles in the peri-canalicular regions of the hepatocytes; bile canaliculi were patent. When incubation was carried out in a medium in which either NO ₃ ^– or SO ₄ ^2– replaced Cl^–, ouabain-resistant water extrusion was prevented and the cytoplasmic vesicles normally found with ouabain were almost totally absent. When these slices were subsequently transferred to Cl^– medium with oubain, extrusion of intracellular water was initiated and cytoplasmic vesicles reappeared. Replacement of medium Na⁺ by Li⁺ mimicked the effects of ouabain on water and ion movements and ultrastructure. In addition, the ouabain-resistant extrusion of water and Cl^– was reduced and there was some diminution in the number of vesicles induced by ouabain. Furosemide (2mm) had little effect on water movement or ultrastructure in the absence of ouabain, but it slowed the net water loss and substantially reduced the formation of cytoplasmic vesicles in the presence of ouabain. The results show a close relationship between ouabain-resistant water extrusion and the formation of the cytoplasmic vesicles that are characteristic of treatment with ouabain. They further suggest that a cotransport of Na⁺ and Cl^– forms an important part of the mechanism underlying ouabain-resistant water extrusion and, specifically, that this cotransport may take place across the membranes of the cytoplasmic vesicles.     

β2-Adrenoreceptor immunoreactivity in cardiac ganglia of the guinea pig

Summary Previous pharmacological studies in co-culture systems have indicated, the presence of β-adrenoreceptors on intrinsic cardiac neurons of the guinea pig (Horackovaet al., 1993) but radiologand binding studies of tissue sections failed to provide a definite answer as to the presence of such receptors on cardiac neuronsin situ, due to the iodine-binding properties of cardiac nerve bundles and ganglia (Molenaaret al., 1992). We therefore addressed this question by immunohistochemistry, using antisera raised against synthetic peptides of the β2-adrenoreceptor. For comparison, cholinergic and catecholaminergic neurons were identified immunohistochemically by means of antibodies against the enzymes involved in the synthesis of acetylcholine (choline acetyltransferase), and of catecholamines (tyrosine hydroxylase). Virtually all intrinsic cardiac neurons contained both β2-adrenoreceptor- and choline acetyltransferase-immunoreactivities. In addition, some nerve fibre bundles exhibited β2-adrenoreceptor-immunoreactivity. Several ganglia were innervated by tyrosine hydroxylase-immunoreactive axons, but the majority of ganglia did not receive tyrosine hydroxylase-immunoreactive nerve terminals, and additional intraganglionic sources of catecholamine synthesis could not be identified. Thus, the results are in favour of β-adrenergic modulation of guinea pig cardiac ganglia by humorally and, partially, by locally released catecholamines.     

Ammonium transport in medullary thick ascending limb of rabbit kidney: Involvement of the Na+, K+, Cl−-cotransporter

Summary In order to investigate the question whether ammonium reabsorption in the thick ascending limb of Henle's loop (TALH) proceeds via the Na⁺,K⁺,Cl^–-cotransporter, plasma membrane vesicles were prepared from TALH cells isolated from rabbit kidney outer medulla and the effect of NH ₄ ⁺ on their transport properties was investigated. It was found that, in the presence of a 78-mmol/liter NaCl gradient, 5 mmol/liter NH ₄ ⁺ inhibited bumetanide-sensitive rubidium flux by 86%; a similar decrease was observed for 5 mmol/liter, K⁺. Inhibition of bumetanide-sensitive rubidium uptake by NH ₄ ⁺ was competitive and an apparentK _i of 1.9 mmol/liter was found Bumetanide-sensitive sodium uptake measured in the presence of a 83 mmol/liter KCl gradient was not inhibited by 5 mmol/liter NH ₄ ⁺ . A 100-mmol/liter NH₄Cl gradient was, however, capable of stimulating bumetanide-sensitive sodium uptake to the same extent as a KCl gradient. These data suggest that NH ₄ ⁺ is accepted by the K⁺ site of the Na⁺,K⁺,Cl-cotransport system and that the transporter can function in a Na⁺, NH ₄ ⁺ ,2Cl mode. Since the affinity of the transporter for NH ₄ ⁺ lies in the concentration range found in the TALH lumen in vivo, it is concluded that Na⁺, NH ₄ ⁺ 2Cl-cotransport can contribute to the NH ₄ ⁺ reabsorption in this tubular segment.     

Studies on chloride permeability of the skin ofLeptodactylus ocellatus: II. Na+ and Cl− effect of inward movements of Cl−

Summary At low concentration (1mm) of Cl^– in the outer solution, the influx of chloride through the isolated skin (J ₁₃ ^Cl ) of the South American frogLeptodactylus ocellatus (L.) seems to be carried by two mechanisms: (i) a passive one that exhibits the characteristics of an exchange diffusion process, and (ii) an active penetration. Studies of the influx and efflux of chloride (J ₁₃ ^Cl andJ ₃₁ ^Cl ) indicate, that the presence of a high (107mm) concentration of Cl^– in the outer solution activates the translocation of this ion through the cells. Studies of the unidirectional flux of Cl^– across the outer barrier (J ₁₂ ^Cl ) indicate that Na⁺ out stimulates the penetration of Cl^– at this level. Cl^– out, in turn, stimulates, theJ ₁₂ ^Na , but this effect is only detected at low concentrations of Na⁺ out.     

The energetics of Cl− active transport inChara

Summary The rate of Cl^– influx in intactChara was inhibited whenever the ATP concentration was reduced by application of metabolic inhibitors. In perfused cells, however, a net influx of Cl^– against its electrochemical gradient could be observed in the absence of ATP. Addition of ATP to the perfusion medium slightly stimulated Cl^– influx in one experiment but had no effect in another. Addition of ADP, NADH or metabolic inhibitors did not alter the influx rate. Consideration of the potential energy gradients across theChara plasmalemma in the perfused state leads to the conclusion that Cl^– influx occurs by cotransport with H⁺ or OH^–.     

Cellular mechanism of HCO 3 − and Cl− transport in insect salt gland

Summary Active HCO ₃ ^t- secretion in the anterior rectal salt gland of the mosquito larva,Aedes dorsalis, is mediated by a 11 Cl^–/HCO ₃ ^– exchanger. The cellular mechanisms of HCO ₃ ^– and Cl^– transport are examined using ion- and voltage-sensitive microelectrodes in conjunction with a microperfused preparation which allowed rapid saline changes. Addition of DIDS or acetazolamide to, or removal of CO₂ and HCO ₃ ^– from, the serosal bath caused large (20 to 50 mV) hyperpolarizations of apical membrane potential (V _a) and had little effect on basolateral potential (V _bl). Changes in luminal Cl^– concentration alteredV _a in a repid, linear manner with a slope of 42.2 mV/decaloga _Cl ^l –. Intracellular Cl^– activity was 23.5mm and was approximately 10mm lower than that predicted for a passive distribution across the apical membrane. Changes in serosal Cl^– concentration had no effect onV _bl, indicating an electrically silent basolateral Cl^– exit step. Intracellular pH in anterior rectal cells was 7.67 and the calculated was 14.4mm. These results show that under control conditions HCO₃ enters the anterior rectal cell by an active mechanism against an electrochemical gradient of 77.1 mV and exits the cell at the apical membrane down a favorable electrochemical gradient of 27.6 mV. A tentative cellular model is proposed in which Cl enters the apical membrane of the anterior rectal cells by passive, electrodiffusive movement through a Cl^–-selective channel, and HCO ₃ ^– exits the cell by an active or passive electrogenic transport mechanism. The electrically silent nature of basolateral Cl^– exit and HCO₃ entry, and the effects of serosal addition of the Cl^–/HCO₃ exchange inhibitor, DIDS, on and transepithelial potential (V _ic) suggest strongly that the basolateral membrane is the site of a direct coupling between Cl^– and HCO ₃ ^– movements.     

Na+, Li+ and Cl− transport by brush border membranes from rabbit jejunum

Na+, Li+, K+, Rb+, Br-, Cl- and SO4(2-) transport were studied in brush border membrane vesicles isolated from rabbit jejunum. Li+ uptakes were measured by flameless atomic absorption spectroscopy, and all others were measured using isotopic flux and liquid scintillation counting. All uptakes were performed with a rapid filtration procedure. A method is presented for separating various components of ion uptake: 1) passive diffusion, 2) mediated transport and 3) binding. It was concluded that a Na+/H+ exchange mechanism exists in the jejunal brush border. The exchanger was inhibited with 300 microM amiloride or harmaline. The kinetic parameters for sodium transport by this mechanism depend on the pH of the intravesicular solution. The application of a pH gradient (pHin = 5.5, pHout = 7.5) causes an increase in Jmax (50 to 125 pmol/mg protein . sec) with no change in Kt (congruent to 4.5 nM). Competition experiments show that other monovalent cations, e.g. Li+ and NH4+, share the Na+/H+ exchanger. This was confirmed with direct measurements of Li+ uptakes. Saturable uptake mechanisms were also observed for K+, Rb+ and SO4(2-), but not for Br-. The Jmax for K+ and Rb+ are similar to the Jmax for Na+, suggesting that they may share a transporter. The SO4(2-) system appears to be a Na+/SO4(2-) cotransport system. There does not appear to be either a Cl-/OH- transport mechanism of the type observed in ileum or a specific Na+/Cl- symporter.     

Elektronenmikroskopische Lokalisation von Na+ und Cl− in Zellen und Geweben

Ohne Zusammenfassung     

Is there a gloxylate cycle in the liver of the fetal guinea pig?

Studies on the effect of the inhibitor of fatty acid oxidation (+)-octanoylcarnitine on the perfused liver of the 48–51 days fetal guinea pig indicate that the oxidation of endogenous fatty acids is a major source of carbon for the citric acid cycle and for synthesis of hexose. Consistent with this the liver can convert isocitrate to glyoxylate and glyoxylate to malate and may therefore operate a glyoxylate cycle allowing the net production of sugars from acetyl-CoA.     

Nature of the neutral Na+-Cl− coupled entry at the apical membrane of rabbit gallbladder epithelium: IV. Na+/H+, Cl−/HCO 3 − double exchange,hydrochlorothiazide-sensitive Na+-Cl− symport and Na+-K+-2Cl− cotransport are all involved

Transepithelial fluid transport was measured gravimetrically in rabbit gallbladder (and net Na+ transport was calculated from it), at 27 degrees C, in HCO(3-)-free bathing media containing 10(-4) M acetazolamide. Whereas luminal 10(-4) M bumetanide or 10(-4) M 4-acetamido-4'-iso-thiocyanostilbene-2,2'-disulfonate (SITS) did not affect fluid absorption, 25 mM SCN- abolished it; hydrochlorothiazide (HCTZ) in the luminal medium reduced fluid absorption from 28.3 +/- 1.6 (n = 21) to 8.6 +/- 1.6 microliters cm-2 hr-1 (n = 10), i.e., to about 30%. This maximum effect was already obtained at 10(-3) M concentration; the apparent IC50 was about 2 x 10(-4) M. The residual fluid absorption, again insensitive to SITS, was completely inhibited by SCN- or bumetanide. Cl- influx at the luminal border of the epithelium, measured under the same conditions and corrected for the extracellular space and paracellular influx, proved insensitive to 10(-4) M bumetanide, but was slowly inhibited by 10(-3) M HCTZ, with maximum inhibition (about 54%) reached after a 10-min treatment; it subsequently rose again, in spite of the presence of HCTZ. However, if the epithelium, treated with HCTZ, was exposed to 10(-4) M bumetanide during the measuring time (45 sec), inhibition was completed and the subsequent rise of Cl- influx eliminated. Intracellular Cl- accumulation with respect to the predicted activity value at equilibrium decreased significantly upon exposure to 10(-3) M HCTZ, reached a minimum within 15-30 min of treatment, then rose again significantly at 60 min. Simultaneous exposure to HCTZ and bumetanide decreased the accumulation to a significantly larger extent as compared to HCTZ alone, already in 15 min, and impeded the subsequent rise. Intracellular K+ activity rose significantly within 30 min treatment with HCTZ; the increase proved bumetanide dependent. The results obtained show that Na(+)-Cl- symport, previously detected under control conditions, is the HCTZ-sensitive type; its inhibition elicits bumetanide-sensitive Na(+)-K(+)-2Cl- cotransport. Thus, the three forms of neutral Na(+)-Cl(-)-coupled transport so far evidenced in epithelia, Na+/H+, Cl-/HCO3- double exchange (in the presence of exogenous bicarbonate), HCTZ-sensitive Na(+)-Cl- symport and bumetanide-sensitive Na(+)-K(+)-2Cl- cotransport, are all present in the apical membrane of rabbit gallbladder.