Effects of bicarbonate on fluid and electrolyte transport by guinea pig and rabbit gallbladder: Stimulation of absorption

Summary The effect of bicarbonate (HCO₃) on fluid absorption by guinea pig gallbladder was investigatedin vitro. Stimulation of fluid absorption was concentration dependent resulting in a fourfold increase in transport over the range 1 to 50mm. Phosphate, Tris, glycodiazine and glutamine buffers failed to substitutte for HCO₃ in stimulating absorption. Unidirectional²²Na fluxes were measured across short-circuited sheets of guinea pig and rabbit gallbladders mounted in Ussing-type chambers. In both species the net Na flux was unaffected by serosal HCO₃ alone but was stimulated by addition of HCO₃ to the mucosal bathing solution. Transepithelial electrical potential difference in rabbit gallbladder was about 1.4 mV (lumen positive) when HCO₃ was present in the mucosal or in both compartments. This fell to 0.2 mV under HCO₃-free conditions or when HCO₃ was present only in the serosal solution. The respective values for guinea pig gallbladder were –1.6 and –0.6 mV (lumen negative). HCO₃ stimulation of Na absorption by guinea pig gallbladder was abolished by increasing the bathing pH from 7.4 to 7.8, an effect resulting mainly from a reduction inJ _mis ^Na . Tris buffer (25mm) inhibited HCO₃-dependent fluid absorption in this species completely at pH 8.5 and partially at 7.5. These results indicate that HCO₃ stimulates gallbladder transport in both species by an action from the mucosal side. This effect cannot be attributed to simple buffering of H⁺ but may be explained by the participation of HCO₃ in the maintenance of intracellular H⁺ for a Na/H-exchange.     

Effects of bicarbonate on fluid and electrolyte transport by the guinea pig gallbladder: A bicarbonate-chloride exchange

Summary Fluid transport and net fluxes of Na, K, Cl and HCO₃ by guinea pig gallbladder were investigatedin vitro. A perfused gallbladder preparation was devised to simultaneously study unidirectional fluxes of²²Na and³⁶Cl. The net Cl flux exceeded the net Na flux during fluid absorption in the presence of HCO₃. This Cl excess was counter-balanced by a net HCO₃ secretion: a HCO₃–Cl exchange. PGE₁ reversed the direction of fluid transport and abolished the net Cl flux. The magnitude of the HCO₃ secretion remained unchanged, but shifted from a HCO₃–Cl exchange to a net secretion of NaHCO₃ and KHCO₃. Furosemide inhibited both the HCO₃–Cl exchange and HCO₃ secretion after PGE₁ without influencing fluid absorption. Ouabain inhibited the HCO₃–Cl exchange as well as fluid absorption; only the effect on the HCO₃ secretion was entirely reversible. Secreted HCO₃ appeared not to be derived from metabolic sources since HCO₃ secretion was abolished in a HCO₃-free bathing medium. HCO₃ secretion was also dependent on the Na concentration of the bathing fluid. Three lines of evidence are presented in favor of an active HCO₃ secretion in guinea pig gallbladder. HCO₃ is secreted against: (i) a chemical gradient, (ii) an electrical gradient and (iii) the direction of fluid movement under control conditions.     

Salt and water transport by rabbit and guinea pig gallbladder: Effect of amphotericin B on NaCl influx

Summary Previous studies have led to the suggestion that salt and water absorption by rabbit and guinea pig gallbladders exposed to Amphotericin B proceeds by a rheogenic Na pump at the basolateral cell membrane. The present studyin vitro was designed to further characterize transport properties of rabbit and guinea pig gallbladders under control conditions and to identify the properties of gallbladder mucosa which are altered by Amphotericin B to allow for the induced serosa-positive electrical potential differences (PD). Potassium is required in the bathing solution at a low concentration to maintain normal tissue O₂ consumption, fluid absorption and the ability of the tissue to develop the maximum Amphotericin B-induced PD; the relative effectiveness of alkali metal cations in substituting for K is KRb>Cs>Li>Na. The carrier mechanism for coupled influx of Na and Cl across the mucosal border of gallbladder appears to be functional in the presence of Amphotericin B; in addition, the diffusional influx of chloride is not significantly altered by the antibiotic. The primary action of Amphotericin B which appears to modify rabbit and guinea pig gallbladders from having transmural PD's of less than ±1 mV to having serosa-positive PD's of 5–30 mV is an increase in the mucosal cell membrane permeability to Na. This permeability change has the effect of partially uncoupling NaCl influx. A rheogenic Na pump mechanism at the basolateral membrane, presumably in operation under control conditions also, may account for the PD.     

Rat jejunal basolateral membrane Cl/HCO3 exchanger is modulated by a Na-sensitive modifier site

A Cl/HCO₃ exchanger mediates HCO₃ extrusion across rat jejunal basolateral membrane. Previous studies demonstrated that anion antiport activity is positively affected by Na, but evidence was given that this cation is not translocated by the carrier protein. Basolateral membranes isolated from rat jejunum were used to give more insight on Na effect. Uptake studies, performed together with vesicle sidedness determinations, indicated that the greatest stimulation of Cl-dependent HCO₃ uptake occurs when Na is present at both vesicle surfaces. The kinetic dependence of Cl/HCO₃ exchange on equal intra- and extravesicular Na concentration showed a hyperbolic relationship, and the calculated kinetic parameters were V _max=0.153 ± 0.006 nmol mg protein^-1 sec^-1, K _m =23.0 Mm. Ion replacement studies indicated that Na can be partially substituted only by Li and not by other monovalent cations. Results of this study suggest that Na could act as a nonessential activator of the Cl/HCO₃ exchanger. A possible role of the Na-sensitive modifier site in the physiology of jejunal enterocyte is suggested.     

Cl/HCO3 exchange in the basolateral membrane domain of rat jejunal enterocyte

Summary Basolateral membrane vesicles isolated from rat jejunal enterocyte and well purified from brush border contamination were tested to examine Cl and HCO₃ movements. Uptake experiments provided no evidence for a coupling between Na and HCO₃ fluxes; K–HCO₃ and K–Cl cotransports also could be excluded. Transport studies revealed the presence of a Cl/HCO₃ exchanger accepting other anions and inhibitable by the disulfonic stilbenes SITS and DIDS. We can exclude that the evidenced HCO₃-dependent Cl uptake is due to brush border contamination, since in jejunal brush border membranes this mechanism, if present, has a very low transport rate. Besides the Cl/HCO₃ antiporter, a Cl-conductive pathway seems to exist in jejunal basolateral membranes.     

Basolateral Cl/HCO3 exchange in rat jejunum: The effect of sodium

Basolateral membrane vesicles isolated from rat jejunum were used to characterize a Cl/HCO₃ exchange mechanism previously evidenced. Cl uptake experiments provided no evidence for Cl/OH countertransport, confirming anyhow the presence of Cl/HCO₃ antiport, which was inhibited by 2 mm furosemide and unaffected by 2 mm amiloride. An outwardly directed Na gradient stimulated Cl uptake and this effect was increased if Na was present at both vesicle surfaces. To investigate the mechanism of coupling between Na and the transport protein, we performed Na uptake experiments. Na uptake was unaffected by cis-bicarbonate and trans-Cl gradients; the reversal of anion gradients was still ineffective. Similar results were obtained when a pH difference across the membrane vesicles was imposed. This study seems to suggest that Na is not transported by the Cl/HCO₃ exchanger and that another mode of Na dependence must be taken into account.     

Electrolyte transport by gallbladders of rabbit and guinea pig: Effect of amphotericin B and evidence of rheogenic Na transport

Summary Ion transport and electrical properties of rabbit and guinea pig gallbladders were investigated to gain further information about the active transport mechanism that mediates fluid absorption. The intracellular and transepithelial electrical potentials were measured simultaneously using the microelectrode technique. Exposure of the mucosal surface to Amphotericin B resulted in the prompt development of a serosa-positive electrical potential difference (PD) which could not be attributed to an alteration in ion diffusion potentials across either the cell membrane or across the tight junction. Because the Amphotericin B-inducedPD was immediately dependent on warm temperatures and O₂, and was independent of NA and K concentration gradients across the cell membrane, it is suggested that active ion transport is directly responsible for thePD. Since thePD was abolished in the absence of Na in the bathing solutions, a rheogenic Na pump is postulated; this pump also appears to be operative in tissue not exposed to Amphotericin B. The specific tissue properties altered by Amphotericin B to produce a serosa-positivePD remain incompletely defined. The results of the present study indicate that ion transport by rabbit gallbladderin vitro is a consequence of a rheogenic active Na transport mechanism at the basolateral membranes which, in conjunction with a coupled NaCl influx process at the mucosal border, ultimately results in absorption of NaCl and water.     

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

Summary 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 (I_sc) ranged from-15 to 220 A·cm^-2. Variations in basal I_sc resulted from differences in the amount of amiloride (10M mucosal addition)-sensitive Na⁺ absorption. Ion substitution and transepithelial flux experiments showed that 10 M amiloride produced a decrease in the mucosal-to-serosal (M-S) and net Na flux, and that this effect on I_sc was independent of Cl^- and HCO ₃ ^- replacement. When the concentration of mucosal amiloride was increased from 10 to 100 M, little change in I_sc was observed. However, increasing the concentration to 1 mM produced a further inhibition, which often reversed the polarity of the I_sc. The decrease in I_sc due to 1 mM amiloride was dependent on both Cl^- and HCO ₃ ^- , 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 HCO ₃ ^- 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 HCO ₃ ^- -dependent (presumably mediated by Cl^-/HCO ₃ ^- exchange) and occurs independently of Na⁺ absorption.Abbreviations G_t tissue conductance - HEPES tris (hydroxymethyl) aminomethane - (tris) N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid - I_sc short-circuit current - J_r residual flux - M-S mucosal-to-scrosal - S-M serosal-to-mucosal - TTX tetrodotoxin     

CFTR mediates bicarbonate-dependent activation of miR-125b in preimplantation embryo development

Although HCO₃⁻ is known to be required for early embryo development, its exact role remains elusive. Here we report that HCO₃⁻ acts as an environmental cue in regulating miR-125b expression through CFTR-mediated influx during preimplantation embryo development. The results show that the effect of HCO₃⁻ on preimplantation embryo development can be suppressed by interfering the function of a HCO₃⁻-conducting channel, CFTR, by a specific inhibitor or gene knockout. Removal of extracellular HCO₃⁻ or inhibition of CFTR reduces miR-125b expression in 2 cell-stage mouse embryos. Knockdown of miR-125b mimics the effect of HCO₃⁻ removal and CFTR inhibition, while injection of miR-125b precursor reverses it. Downregulation of miR-125b upregulates p53 cascade in both human and mouse embryos. The activation of miR-125b is shown to be mediated by sAC/PKA-dependent nuclear shuttling of NF-κB. These results have revealed a critical role of CFTR in signal transduction linking the environmental HCO₃⁻ to activation of miR-125b during preimplantation embryo development and indicated the importance of ion channels in regulation of miRNAs.     

A putative HCO3 transporter in the cyanobacterium Synechococcus sp. strain PCC 7942

Cyanobacteria possess an inducible mechanism which enables them to concentrate inorganic carbon (Ci) within the cells. An inactivation library was used to raise the high-CO₂-requiring mutant of Synechococcus PCC 7942, IL-2, impaired in HCO⁻₃ transport. Analysis of the relevant genomic DNA detected several modifications, probably due to the single crossover recombination, leading to inactivation of ORF467 (designated ictB) in IL-2. IctB contains 10 trans-membrane regions and is homologous to several transport-related proteins from various organisms. Kinetic analyses of HCO⁻₃ uptake in the wild type and IL-2 suggested the presence of two or three HCO⁻₃ carriers exhibiting different affinities to HCO⁻₃.     

Further evidence for local osmotic coupling in the rabbit cornea

1. 1. The present experiments measure net fluxes of fluid, Cl⁻ and HCO₃⁻ across de-epithelialised rabbit corneas clamped between half chambers and bathed in Ringer solutions.

2. 2. Net fluxes of HCO₃⁻ and fluid occurred together across the cornea from stroma to aqueous when HCO₃⁻ and CO₂ were present in the bathing solution.

3. 3. No net trans-corneal Cl⁻ flux was found

4. 4. The initiation of fluid flow in the presence of HCO₃⁻ and CO₂ cannot be accounted for by bulk-phase osmotic flow across the cornea.

Renal cortical basolateral Na+/HCO 3 − cotransporter III. Evidence for a regulatory protein in the inhibitory effect of protein kinase A

The activity of the Na-H antiporter is inhibited by cyclic AMP-dependent protein kinase A (cAMP.PKA). The inhibitory effect of PKA on the Na-H antiporter is mediated through a regulatory protein that can be dissociated from the antiporter by limited protein digestion. PKA also inhibits the activity of the Na⁺/ HCO ₃ ^? cotransporter. We investigated whether the activity of Na⁺/HCO ₃ ^? cotransporter and the effect of PKA on this transporter may also be regulated by limited protein digestion. In rabbit renal cortical basolateral membranes (BLM) and in solubilized BLM reconstituted in liposomes (proteoliposomes), trypsin (100 μg) increased ²²Na uptake in the presence of HCO₃ but not in the presence of gluconate, indicating that trypsin does not alter diffusive ²²Na uptake but directly stimulates the Na⁺/HCO ₃ ^? cotransporter activity. In proteoliposomes phosphorylated with ATP, the catalytic subunit (CSU) of cAMP-PKA decreased the activity of the Na⁺/HCO ₃ ^? cotransporter (expressed as nanomoles/mg protein/3s) from 23 ± 10 to 14 ± 6 (P < 0.01). In the presence of trypsin, the inhibitory effect of CSU of cAMP-PKA on the activity of Na⁺/HCO ₃ ^? cotransporter was blunted. To identify a fraction that was responsible for the inhibitory effect of the CSU on the Na⁺/HCO ₃ ^? cotransporter activity, solubilized proteins were separated by size exclusion chromatography. The effect of CSU of cAMP-PKA on the Na⁺/HCO ₃ ^? cotransporter activity was assayed in proteoliposomes digested with trypsin with the addition of a fraction containing the 42 kDa protein (fraction S+) or without the 42 kDa protein (fraction S?). With the addition of fraction S?, the CSU of cAMP-PKA failed to inhibit the Na⁺/HCO ₃ ^? cotransporter activity (control 27 ± 6, CSU 27 ± 3) while the addition of fraction S+ restored the inhibitory effect of CSU (27 ± 6 to 3 ± 0.3 P < 0.01). The CSU of cAMP-PKA phosphorylated several proteins in solubilized protein including a 42 kDa protein. Fluorescein isothiocyanate (FITC) labels components of the Na⁺/HCO ₃ ^? cotransporter including the 56 kDa and 42 kDa proteins. In trypsin-treated solubilized protein the 42 kDa protein was not identified with FITC labeling. The results demonstrate that the activity of the Na⁺/HCO ₃ ^? cotransporter is regulated by protein(s) which mediates the inhibitory effect of PKA. Limited protein digestion can dissociate this protein from the cotransporter.     

Photosynthetic metabolism of cyanate by the cyanobacterium Synechococcus UTEX 625

Intact cells of the unicellular cyanobacterium Synechococcus UTEX 625 degraded exogenously supplied cyanate (as KOCN) to CO₂ and NH₃ in a light-dependent reaction. NH₃ release to the medium was as high as 80 mol(mgChl)^-1h^-1 and increased 1.7-fold in the presence of methionine sulfoximine, a glutamine synthetase inhibitor. Cyanate also supporte photosynthetic O₂ evolution to a maximum rate of 188 mol O₂(mgChl)^-1h^-1 at pH 8 and 30°C. Cyanate decomposition in cell-free extracts, measured by mass spectrometry as ¹³CO₂ production from KO¹³CN, occurred in the soluble enzyme fraction, but not in the thylakoid/carboxysome fraction, and was enhanced by HCO₃ ^– and inhibited by the dianion oxalate. CO₂, rather, than HCO₃ ^–, was a product of cyanate decomposition. The ability to decompose cyanate was not dependent upon pre-exposure of cells to cyanate to induce activity. The collective results indicate that Synechococcus UTEX 625 possesses a constitutive, cytosolic cyanase (EC 4.3.99.1), similar in mechanism to that found in some species of heterotrophic bacteria. The reaction catalyzed was: OCN+HCO₃+2H⁺2CO₂+NH₃. In intact cells, the CO₂ produced by the action of cyanase on OCN^- was either directly fixed by the Calvin cycle enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase, leading to O₂ evolution, or leaked into the medium where it was returned to the cell by the active CO₂/HCO₃ ^– transport systems for fixation. However, leakage of CO₂ from air-grown cells was only observed when the active CO₂ transport system was inhibited by darkness or the CO₂ analogue carbon oxysulfide.Abbreviations BTP bistrispropane - C _i inorganic carbon (=CO₂+HCO₃ ^-+CO₃ ^2-) - CA carbonic anhydrase - Chl chlorophyll - COS carbon oxysulfide - MSX methionine sulfoximine - PAR photosynthetically active radiation - Rubisco ribulose bisphosphate carboxylase/oxygenase     

Response of lupins (Lupinus angustifolius L.) and peas (Pisum sativum L.) to Fe deficiency induced by low concentrations of Fe in solution or by addition of HCO3 -

Lupins appear to be more sensitive than peas to Fe deficiency. However, when grown in nutrient solutions between pH 5–6, little difference existed between them in their ability to acidify the solution or to release Fe^III reducing compounds. This experiment was aimed at determining whether differences between species which occurred when Fe deficiency was induced by withholding Fe from an acid solution, are maintained when Fe deficiency is induced by addition of HCO₃ ^-. Lupins and peas were grown in nutrient solutions at 0, 2 and 6 μM of Fe^III EDDHA and either with or without HCO₃ ^- (6 mM). Bicarbonate induced symptoms of Fe deficiency (chlorosis) in both lupins and peas, and markedly decreased the growth of shoots. Symptoms appeared sooner and were more severe in lupins than in peas. Growing plants without HCO₃ ^-, but at the lowest Fe level, decreased the growth and Fe concentration of shoots of lupins but did not induce chlorosis. Growing peas in this treatment, decreased Fe concentrations, but to a lesser extent than in lupins, and did not decrease growth. H⁺-ion extrusion and release of Fe^III reducing compounds was greater in lupins than in peas. Bicarbonate also decreased the growth of roots of lupins but increased the growth of roots of peas. Results indicate that when Fe deficiency is induced by HCO₃ ^-, then the response of lupins and peas are similar to their response in acid solution culture. Differences between species therefore could not be explained by their relative abilities to acidify or release Fe^III reducing compounds. Greater control of the distribution of Fe within the shoots, the presence of a pool of Fe within the roots, a lower threshold for Fe uptake, or a higher content of seed-Fe, may therefore be the reason for the lower sensitivity of peas than lupins to Fe deficiency.     

Content and binding characteristics of the mitochondrial ATPase inhibitor,IF1 in the tissues of several slow and fast heart-rate homeothermic species and in two poikilotherms

We determined the IF₁ contents of pig, rabbit, rat, mouse, guinea pig, pigeon, turtle, and frog heart mitochondria and the effects of varying ionic strength upon the IF₁-mediated inhibition of the ATPase activity of IF₁-depleted rabbit heart mitochondrial particles (RHMP) by IF₁-containing extracts from these same eight species. The IF₁ binding experiments were run at both species-endogenous IF₁ levels and at an IF₁ level normalized to that present in rabbit heart mitochondria. When species-endogenous levels of rabbit heart IF₁ or either speciesendogenous or normalized levels of pig heart IF₁ were incubated with RHMP over a range of KCl concentrations, increasing the [KCl] to 150 mM had relatively little effect on IF₁-mediated ATPase inhibition. When either species-endogenous or normalized levels of guinea pig, pigeon, turtle, or frog heart IF₁ were incubated with RHMP under the same conditions, increasing [KCl] to 150 mM nearly completely blocked IF₁-mediated ATPase inhibition. While species-endogenous levels of rat and mouse heart IF₁ inhibited the ATPase activity of RHMP virtually not at all at any [KCl] examined, normalized levels of rat and mouse IF₁ inhibited the ATPase activity of RHMP to the same extents as species-endogenous levels of pig and rabbit heart IF₁, respectively, in the presence of increasing [KCl]. These experiments suggest that, while pig and rabbit heart mitochondria contain a full complement of higher-affinity IF₁, pigeon, guinea pig, turtle, and frog heart mitochondria cell contain essentially a full complement of a lower-affinity form of IF₁. In contrast, rat and mouse heart mitochondria contain only low levels of IF₁ which exhibit binding characteristics similar to those of the pig and rabbit heart inhibitor. The guinea pig is the only mammal thus far examined that contains a loweraffinity form of IF₁. In the present study we also determined the IF₁ contents and IF₁-to-F₁ ATPase activity ratios of cardiac muscle, skeletal muscle, liver, and brain mitochondria of rabbit, pigeon, and rat, species representative of the three homeothermic regulatory classes.     

Charasomes are not essential for photosynthetic utilization of exogenous HCO3 − inChara corallina

Summary Cells ofChara corallina grown under high CO₂ culture conditions were able to utilize exogenous HCO₃ ^– to give appreciable rates of net photosynthesis. Since these rates of photosynthesis could be detected within 10 min of being transferred from high-CO₂ to normal HCO₃ ^– (pH 8.2) culture conditions, it would appear that the HCO₃ ^–-accumulating system ofChara is not fully repressed under these high CO₂ culture conditions. The membrane potential of these cells also responded to light/dark treatments in a manner consistent with the operation of a HCO₃ ^– acquisition system. With prolonged exposure (2–6 days) to CPW/B, net photosynthesis continued to increase towards the expected control rate and, in parallel, the electrical responses elicited by light/dark treatments converged towards those obtained on control (CPW/B-grown)Chara cells. Charasomes were absent in CPW/CO₂-grownChara, but redeveloped in mature cells once the culture was returned to CPW/B conditions; a minimum period of 7 days in CPW/B was required before charasomes were detected in tissue examined in the transmission electron microscope. As the above-detailed physiological and electrophysiological features were observed with both axial and whorl cells ofChara in which charasomes were completely absent, we conclude that this specialized organelle is not an essential component for photosynthetic utilization of exogenous HCO₃ ^– in this species.Abbreviations CPW/B Chara pond water containing 1.0 mM NaHCO₃, pH8.2 - CPW/CO₂ Chara pond water containing dissolved CO₂, pH 5.5 - DIC dissolved in organic carbon - D.H. dark-induced membrane hyperpolarization - L.H. light-induced membrane hyperpolarization - TEM transmission electron microscopy     

Comparative properties of sensitive to GABA<Subscript>A</Subscript>-ergic ligands Cl<Superscript>−</Superscript>, HCO<Subscript>3</Subscript><Superscript>−</Superscript>-activated Mg<Superscript>2+</Superscript>-ATPase from brain plasma membranes of fish and rats

Action of Cl^? + HCO₃ ^?1 ions on Mg²⁺-ATPase from brain plasma membranes of fish and rats has been studied. Maximal effect of the anions on the “basal” Mg²⁺-ATPase activity is revealed in the presence of 10 mM Cl^? and 3 mM HCO₃ ^?1 at physiological values of pH of incubation medium. The studied Cl^?, HCO₃ ^?-activated Mg²⁺-ATPases of both animal species, by their sensitivity to SH-reagents (5,5-dithio-bis-nitrobenzoic acid, N-ethylmaleimide), oligomycin, and orthovanadate, are similar to transport ATPase of the P-type, but differ from them by molecular properties and by sensitivity to ligands of GABA_A-receptors. It has been established that the sensitive to GABA_A-ergic ligands, Cl^?, HCO₃ ^?-activated Mg²⁺-ATPase from brain of the both animal species is protein of molecular mass around 300 kDa and of Stock’s radius 5.4 nm. In fish the enzyme is composed of one major unit of molecular mass approximately 56 kDa, while in rats-of three subunits of molecular masses about 57, 53, and 45 kDa. A functional and structural coupling of the ATP-hydrolyzing areas of the studied enzyme to sites of binding of GABA_A-receptor ligands is suggested.     

Prostaglandins lack a direct inhibitory action on electrolyte and water transport in the kidney and the erythrocyte

The possibility of a direct effect of prostaglandins of the E, A, and F series upon renal electrolyte and water transport was assessed using in vitro preparations of rabbit cortical and medullary tubular suspensions as well as cortical renal slices from rat and guinea pig and medullary renal slices from rabbit. Net fluxes of Na, K, Cl and H₂O between the intracellular compartment and the extracellular fluid were measured in the presence of PGE₁, PGE₂, PGA₁, PGA₂ and PGF_2α in concentrations ranging from 1 × 10^?5 to 1 × 10^?10M. No inhibitory action was observed with any of these prostaglandins and in fact a slight stimulation of Na transport was seen under some circumstances. We conclude that the natriuresis which follows in vivo administration of some prostaglandins is not the result of a direct inhibition of Na reabsorption at the contraluminal pump site and is most likely secondary to renal vasodilation.We also studied net and isotopic Na fluxes in human erythrocytes. Na transport was not affected by prostaglandins of the E, A or F series using both normal and high sodium erythrocytes. Our results emphasize the need for caution in extrapolating the effects of prostaglandins upon Na transport from one tissue to another since their actions appear to be tissue-specific.     

Characterisation of CO2 and HCO3 − uptake in the cyanobacterium Synechocystis sp. PCC6803

The availability of a complete genome database for the cyanobacterium Synechocystissp. PCC6803 (glucose-tolerant strain) has raised expectations that this organism would become a reference strain for work aimed at understanding the CO₂-concentrating mechanism (CCM) in cyanobacteria. However, the amount of physiological data available has been relatively limited. In this report we provide data on the relative contributions of net HCO₃ ⁻ uptake and CO₂ uptake under steady state photosynthetic conditions. Cells were compared after growth at high CO₂ (2% v/v in air) or limiting CO₂ conditions (20 ppm CO₂). Synechocystishas a very high dependence on net HCO₃ ⁻ uptake at low to medium concentrations of inorganic carbon (Ci). At high Ci concentrations net CO₂ uptake became more important but did not contribute more than 40% to the rate of photosynthetic O₂ evolution. The data also confirm that high Ci cells of Synechocystissp. PCC6803 possess a strong capacity for net HCO₃ ⁻ uptake under steady state photosynthetic conditions. Time course experiments show that induction of maximal Ci uptake capacity on a shift from high CO₂ to low CO₂ conditions was near completion by four hours. By contrast, relaxation of the induced state on return of cells to high CO₂, takes in excess of 230 h. Experiments were conducted to determine if Synechocystissp. PCC6803 is able to exhibit a `fast induction' response under severe Ci limitation and whether glucose was capable of causing a rapid inactivation in Ci uptake capacity. Clear evidence for either response was not found. This revised version was published online in August 2006 with corrections to the Cover Date.