Local osmotic coupling to the active trans-endothelial bicarbonate flux in the rabbit cornea

The present experiments investigate HCO₃⁻, Cl⁻ and fluid fluxes across partially destromalised corneas. Although there is no net flux of Cl⁻, there is a net flux of HCO₃⁻ across the endothelium from stromal side to aqueous side which is accompanied by a flux of water in the same direction. Bulk phase osmosis cannot account for the initiation of the flux of fluid. Local osmotic coupling between ions and water is postulated to occur in the preparation. The exudate is hypertonic to the bathing Ringer solution.     

Cl−HCO3 exchange in rat renal basolateral membrane vesicles

Pathways for HCO₃⁻ transport across the basolateral membrane were investigated using membrane vesicles isolated from rat renal cortex. The presence of Cl⁻---HCO₃⁻ exchange was assessed directly by ³⁶Cl⁻ tracer flux measurements and indirectly by determinants of acridine orange absorbance changes. Under 10% CO₂/90% N₂ the imposition of an outwardly directed HCO₃⁻ concentration gradient (pH_o 6/pH_i 7.5) stimulated Cl⁻ uptake compared to Cl⁻ uptake under 100% N₂ in the presence of a pH gradient alone. Mediated exchange of Cl⁻ for HCO₃ was suggested by the HCO₃⁻ gradient-induced concentrative accumulation of intravesicular Cl⁻. Maneuvers designed to offset the development of ion-gradient-induced diffusion potentials had no significant effect on the magnitude of HCO₃⁻ gradient-driven Cl⁻ uptake further suggesting chemical as opposed to electrical Cl−HCO₃⁻ exchange coupling. Although basolateral membrane vesicle Cl⁻ uptake was observed to be voltage sensitive, the DIDS insensitivity of the Cl conductive pathway served to distinguish this mode of Cl⁻ translocation from HCO₃ gradient-driven Cl⁻ uptake. No evidence for cotransport was obtained. As determined by acridine orange absorbance measurements in the presence of an imposed pH gradient (pH_o 7.5/pH_i 6), a HCO₃⁻ dependent increase in the rate of intravesicular alkalinization was observed in response to an outwardly directed Cl⁻ concentration gradient. The basolateral membrane vesicle origin of the observed Cl⁻−HCO₃⁻ exchange activity was verified by experiments performed with purified brush-border membrane vesicles. In contrast to our previous observations of the effect of Cl⁻ on HCO₃⁻ gradient-driven Na⁺ uptake suggesting a basolateral membrane Na⁺−HCO₃⁻ for Cl⁻ exchange mechanism, no effect of Na⁺ on Cl−HCO₃⁻ exchange was observed in the present study.     

Vanadate inhibition of the Ca-ATPase from human red cell membranes

1. (1) VO₃⁻ combines with high affinity to the Ca²⁺-ATPase and fully inhibits Ca²⁺-ATPase and Ca²⁺-phosphatase activities. Inhibition is associated with a parallel decrease in the steady-state level of the Ca²⁺-dependent phosphoenzyme.

2. (2) VO₃⁻ blocks hydrolysis of ATP at the catalytic site. The sites for VO₃⁻ also exhibit negative interactions in affinity with the regulatory sites for ATP of the Ca²⁺-ATPase.

3. (3) The sites for VO₃⁻ show positive interactions in affinity with sites for Mg²⁺ and K⁺. This accounts for the dependence on Mg²⁺ and K⁺ of the inhibition by VO₃⁻. Although, with less effectiveness, Na⁺ substitutes for K⁺ whereas Li⁺ does not. The apparent affinities for Mg²⁺ and K⁺ for inhibition by VO₃⁻ seem to be less than those for activation of the Ca²⁺-ATPase.

4. (4) Inhibition by VO₃⁻ is independent of Ca²⁺ at concentrations up to 50 μM. Higher concentrations of Ca²⁺ lead to a progressive release of the inhibitory effect of VO₃⁻.

Transport of butyrate across the isolated bovine rumen epithelium — interaction with sodium, chloride and bicarbonate

The Ussing chamber technique was used for studying unidirectional fluxes of ¹⁴C-butyrate across the bovine rumen epithelium in vitro. Significant amounts of butyrate were absorbed across the bovine rumen epithelium in vitro, without any external driving force. The paracellular pathway was quantitatively insignificant. The transcellular pathway was predominately voltage-insensitive. The serosal to mucosal (SM) pathway was regulated by mass action, whereas the mucosal to serosal (MS) pathway further includes a saturable process, which accounted for 30 to 55% of the MS flux. The studied transport process for ¹⁴C-butyrate across the epithelium could include metabolic processes and transport of ¹⁴C-labelled butyrate metabolites. The transport of butyrate interacted with Na⁺, Cl⁻ and HCO₃⁻, and there was a linear relationship between butyrate and sodium net transport. Lowering the sodium concentration from 140 to 10 mmol l⁻¹ decreased the butyrate MS flux significantly. Amiloride (1 mmol l⁻¹) did, however, not reduce the butyrate flux significantly. Chloride concentration in itself did not seem to influence the transport of butyrate, but chloride-free conditions tended to increase the MS and SM flux of butyrate by a DIDS-sensitive pathway. DIDS (bilateral 0.5 mmol l⁻¹) did further decrease the butyrate SM flux significantly at all chloride concentrations. Removing bicarbonate from the experimental solutions decreased the MS and increased the SM flux of butyrate significantly, and abolished net butyrate flux. There were no significant effects of the carbonic anhydrase inhibitor Acetazolamide (bilateral 1.0 mmol l⁻¹). The results can be explained by a model where butyrate and butyrate metabolites are transported both by passive diffusion and by an electroneutral anion-exchange with bicarbonate. The model couples sodium and butyrate via CO₂ from metabolism of butyrate, and intracellular pH.     

Bile salt alteration of ion transport across jejunal mucosa

The effect of conjugated dihydroxy and trihydroxy bile salts on electrolyte transport across isolated rabbit jejunal mucosa was studied. Both taurochenodeoxycholic acid and taurocholic acid increased the short-circuit current (I_sc) in bicarbonate-Ringer solution but not in a bicarbonate-free, chloride-free solution. Taurochenodeoxycholic acid was significantly more effective than taurocholic acid in increasing I_sc. The presence of theophylline prevented the taurochenodeoxycholic acid-and taurocholic acid-induced increase in I_sc. Transmural ion fluxes across jejunal mucosa demonstrated that 2 mM taurochenodeoxycholic acid decreased net Na⁺ absorption, increased net Cl⁻ secretion and increased the residual flux (which probably represents HCO₃⁻ secretion). These studies support the hypothesis that cyclic AMP may be a mediator of intestinal electrolyte secretion.     

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.     

Intracellular pH Regulation in Cultured Astrocytes from Rat Hippocampus : II. Electrogenic Na/HCO3 Cotransport

In the preceding paper (Bevensee, M.O., R.A. Weed, and W.F. Boron. 1997. J. Gen. Physiol. 110: 453–465.), we showed that a Na⁺-driven influx of HCO₃ ⁻ causes the increase in intracellular pH (pH_i) observed when astrocytes cultured from rat hippocampus are exposed to 5% CO₂/17 mM HCO₃ ⁻. In the present study, we used the pH-sensitive fluorescent indicator 2′,7′-biscarboxyethyl-5,6-carboxyfluorescein (BCECF) and the perforated patch-clamp technique to determine whether this transporter is a Na⁺-driven Cl-HCO₃ exchanger, an electrogenic Na/HCO₃ cotransporter, or an electroneutral Na/HCO₃ cotransporter. To determine if the transporter is a Na⁺-driven Cl-HCO₃ exchanger, we depleted the cells of intracellular Cl⁻ by incubating them in a Cl⁻-free solution for an average of ∼11 min. We verified the depletion with the Cl⁻-sensitive dye N-(6-methoxyquinolyl)acetoethyl ester (MQAE). In Cl⁻-depleted cells, the pH_i still increases after one or more exposures to CO₂/HCO₃ ⁻. Furthermore, the pH_i decrease elicited by external Na⁺ removal does not require external Cl⁻. Therefore, the transporter cannot be a Na⁺-driven Cl-HCO₃ exchanger. To determine if the transporter is an electrogenic Na/ HCO₃ cotransporter, we measured pH_i and plasma membrane voltage (V_m) while removing external Na⁺, in the presence/absence of CO₂/HCO₃ ⁻ and in the presence/absence of 400 μM 4,4′-diisothiocyanatostilbene-2,2′-disulphonic acid (DIDS). The CO₂/HCO₃ ⁻ solutions contained 20% CO₂ and 68 mM HCO₃ ⁻, pH 7.3, to maximize the HCO₃ ⁻ flux. In pH_i experiments, removing external Na⁺ in the presence of CO₂/HCO₃ ⁻ elicited an equivalent HCO₃ ⁻ efflux of 281 μM s⁻¹. The HCO₃ ⁻ influx elicited by returning external Na⁺ was inhibited 63% by DIDS, so that the predicted DIDS-sensitive V_m change was 3.3 mV. Indeed, we found that removing external Na⁺ elicited a DIDS-sensitive depolarization that was 2.6 mV larger in the presence than in the absence of CO₂/ HCO₃ ⁻. Thus, the Na/HCO₃ cotransporter is electrogenic. Because a cotransporter with a Na⁺:HCO₃ ⁻ stoichiometry of 1:3 or higher would predict a net HCO₃ ⁻ efflux, rather than the required influx, we conclude that rat hippocampal astrocytes have an electrogenic Na/HCO₃ cotransporter with a stoichiometry of 1:2.     

Production of CO2 in Soil Profiles of a California Annual Grassland

Soils play a key role in the global cycling of carbon (C), storing organic C, and releasing CO₂ to the atmosphere. Although a large number of studies have focused on the CO₂ flux at the soil–air interface, relatively few studies have examined the rates of CO₂ production in individual layers of a soil profile. Deeper soil horizons often have high concentrations of CO₂ in the soil air, but the sources of this CO₂ and the spatiotemporal dynamics of CO₂ production throughout the soil profile are poorly understood. We studied CO₂ dynamics in six soil profiles arrayed across a grassland hillslope in coastal southern California. Gas probes were installed in each profile and gas samples were collected weekly or biweekly over a three-year period. Using soil air CO₂ concentration data and a model based on Fick’s law of diffusion, we modeled the rates of CO₂ production with soil profile depth. The CO₂ diffusion constants were checked for accuracy using measured soil air ²²²Rn activities. The modeled net CO₂ production rates were compared with CO₂ fluxes measured at the soil surface. In general, the modeled and measured net CO₂ fluxes were very similar although the model consistently underestimated CO₂ production rates in the surficial soil horizons when the soils were moist. Profile CO₂ production rates were strongly affected by the inter- and intra-annual variability in rainfall; rates were generally 2–10 times higher in the wet season (December to May) than in the dry season (June to November). The El Niño event of 1997–1998, which brought above-average levels of rainfall to the study site, significantly increased CO₂ production in both the surface and subsurface soil horizons. Whole profile CO₂ production rates were approximately three times higher during the El Niño year than in the following years of near-average rainfall. During the dry season, when the net rates of CO₂ flux from the soil profiles are relatively low (4–11 mg C– CO₂ m⁻² h⁻¹), 20%–50% of the CO₂ diffusing out of the profiles appears to originate in the relatively moist soil subsurface (defined here as those horizons below 40 cm in depth). The natural abundance ¹⁴C signatures of the CO₂ and soil organic C suggest that the subsurface CO₂ is derived from the microbial mineralization of recent organic C, possibly dissolved organic C transported to the subsurface horizons during the wet season.     

The effect of copper on frog skin. The role of sulphydryl groups

1. 1. Cu²⁺ at a concentration of 10⁻⁴ M, when applied to the external side of the frog skin produces an increase in the short-circuit current (I_sc).

2. 2. This effect was studied in skins of Rana temporaria adapted to cold (5°C) and room temperature (20°C), skins of Rana pipiens adapted to cold, and the results compared with those obtained previously with Rana ribibunda.

3. 3. The observed effect is less dependent upon the adaptation to cold than upon the functional state of the skin: skins with low short circuit currents have a bigger response to Cu²⁺ than skins with high I_sc.

4. 4. A species difference cannot be ruled out since skins of Rana ribibunda exhibiting high I_sc give good responses to Cu²⁺.

5. 5. 5,5′-dithiobis(2-nitrobenzoic acid), a sulphydryl-oxidizing reagent, produces an effect similar to that of Cu²⁺, and dithiothreitol an SH-reducing agent, reverses the effect of this ion.

6. 6. Cu²⁺ also induces an increase in the unidirectional K⁺ fluxes and unmasks a net outward potassium flux.

7. 7. The outward K⁺ flux induced by Cu²⁺ is sensitive to ouabain.

8. 8. It is concluded that Cu²⁺ increases the permeability of the external barrier of the frog skin to Na⁺ and K⁺, probably by reacting with SH groups.

Effects of tryptamine on active sodium and chloride transport in the isolated bullfrog cornea

The effects of the serotonin analogue, tryptamine, on the active transepithelial transport of Na⁺ and Cl⁻ in the in vitro bullfrog cornea were studied. Tryptamine, 1 mM, inhibited both the short-circuit current (I_sc) and potential difference (PD) of corneas transporting either Na⁺ alone or both Na⁺ and Cl⁻. The electrical resistance, R, increased in all cases. Both unidirectional Na⁺ and Cl⁻ fluxes were decreased by tryptamine and these changes accounted for the inhibitory effects on the I_sc. The effects of tryptamine were considered along with with those of 2 mM theophylline and 0.1 mM ouabain. Tryptamine inhibited the I_sc and both undirectional Cl⁻ fluxes which were previously stimulated by theophylline. Theophyline addition, after tryptamine preincubation, increased the Cl⁻ undirectional fluxes but did not restore the inhibited I_sc. The inhibitory effects of tryptamine on active Na⁺ and Cl⁻ transport were different from those of ouabain. While both drugs inhibited the forward Na⁺ and Cl⁻ fluxes, their backfluxes decreased with tryptamine and increased with ouabain. The addition to the bathing solution of tryptamine after ouabain preincubation reduced the ouabain-increased backward Cl⁻ flux and further increased the electrical resistance. These results are analyzed in terms of an electrical model from which it appears that tryptamine's mechanism of action was to decrease cellular permeability to the transepithelial movement of Na⁺ and Cl⁻.     

Evidence for a simple Cl conductance pathway in nutrient membrane of frog stomach

A decrease in nutrient Cl⁻ results in an increased negativity of the nutrient relative to the secretory side. The possibility emerged that Cl⁻ transport could be attributed to a neutral mechanism involving Cl⁻ in the nutrient membrane coupled to a simple Cl⁻ conductance pathway in the secretory membrane. The decrease in PD (potential difference) with a decrease in nutrient Cl⁻ could arise from a decrease in cellular Cl⁻ so that the ratio of Cl⁻ in cell to Cl⁻ in secretory solution was decreased. Experiments were designed to determine whether there was a need to assume a simple Cl⁻ conductance pathway. A 10-fold decrease in Cl⁻ gave in HCO₃⁻-containing nutrient solutions a PD decrease of 20 mV, in HCO₃⁻-free nutrient solutions, a PD decrease of 13.5 mV, and in HCO₃⁻-free and Na⁺-free solutions, a PD decrease of 6.7 mV. The decrease of 6.7 mV could not be attributed to a neutral Cl⁻HCO₃⁻ exchanger or a NaCl symport. Also there was no evidence for a KCl symport from changes in Cl⁻ in presence and absence of K⁺. It followed that the decrease of 6.7 mV provided evidence for a simple Cl⁻ conductance pathway in the nutrient membrane.     

Seasonal patterns in soil surface CO<Subscript>2</Subscript> flux under snow cover in 50 and 300 year old subalpine forests

Soil CO₂ flux can contribute as much as 60–80% of total ecosystem respiration in forests. Although considerable research has focused on quantifying this flux during the growing season, comparatively little effort has focused on non-growing season fluxes. We measured soil CO₂ efflux through snow in 50 and ~300 year old subalpine forest stands near Fraser CO. Our objectives were to quantify seasonal patterns in wintertime soil CO₂ flux; determine if differences in soil CO₂ flux between the two forest ages during the growing season persist during winter; and to quantify the sample size necessary to discern treatment differences. Soil CO₂ flux during the 2002–2003 and 2003–2004 snow season averaged 0.31 and 0.35 μmols m⁻² s⁻¹ for the young and old forests respectively; similar to the relative difference observed during summer. There was a significant seasonal pattern of soil CO₂ flux during the winter with fluxes averaging 0.22 μmols m⁻² s⁻¹ in December and January and increasing to an average of 0.61 μmols m⁻² s⁻¹ in May. Within-plot variability for measurements used in calculating flux was low. The coefficients of variation (CV) for CO₂ concentration, snowpack density, and snow depth were 17, 8 and 14%, respectively, yielding a CV for flux measurements within-plot of 29%. A within plot CV of 29% requires 8 sub-samples per plot to estimate the mean flux with a standard error of ±10% of the mean. Variability in CO₂ flux estimates among plots (size = 400 m²) was similar to that within plot and was also low (CV = ~28%). With a CV of 28% among plots, ten plots per treatment would have a 50% probability of detecting a 25% difference in treatment means for α = 0.05.     

Microfluorometric analysis of Cl permeability and its relation to oscillatory Ca signalling in glucose-stimulated pancreatic β-cells

The cytoplasmic concentrations of Cl⁻([Cl⁻]_i) and Ca²⁺ ([Ca²⁺]_i) were measured with the fluorescent indicators N-(ethoxycarbonylmethyl)-6-methoxyquinilinum bromide (MQAE) and fura-2 in pancreatic β-cells isolated from ob/ob mice. Steady-state [Cl⁻]_i in unstimulated β-cells was 34 mM, which is higher than expected from a passive distribution. Increase of the glucose concentration from 3 to 20 mM resulted in an accelerated entry of Cl⁻ into β-cells depleted of this ion. The exposure to 20 mM glucose did not affect steady-state [Cl⁻]_i either in the absence or presence of furosemide inhibition of Na⁺, K⁺, 2 Cl⁻ co-transport. Glucose-induced oscillations of [Ca²⁺]_i were transformed into sustained elevation in the presence of 4,4′ diisothiocyanato-dihydrostilbene-2,2′-disulfonic acid (H₂DIDS). A similar effect was noted when replacing 25% of extracellular Cl⁻ with the more easily permeating anions SCN⁻, I⁻, NO₃⁻ or Br⁻. It is concluded that glucose stimulation of the β-cells is coupled to an increase in their Cl⁻ permeability and that the oscillatory Ca²⁺ signalling is critically dependent on transmembrane Cl⁻ fluxes.     

Characterization of carbonic anhydrase and anion exchange in the erythrocytes of bowfin (Amia calva), a primitive air-breathing fish

The purpose of this study was to investigate the characteristics of carbonic anhydrase (CA) and the Cl⁻/HCO₃⁻ exchanger (Band 3; AE1) in the erythrocytes of bowfin (Amia calva), a primitive air-breathing fish, in order to further understand the strategies of blood CO₂ transport in lower vertebrates and gain insights into the evolution of the vertebrate erythrocyte proteins, CA and Band 3. A significant amount of CA activity was measured in the erythrocytes of bowfin (70 mmol CO₂ min⁻¹ ml⁻¹), although it appeared to be lower than that in the erythrocytes of teleost fish. The turnover number (K_cat) of bowfin erythrocyte CA was intermediate between that of the slow type I CA isozyme in agnathans and elasmobranchs and the fast type II CA in the erythrocytes of the more recent teleost fishes, but the inhibition properties of bowfin erythrocyte CA were similar to the fast mammalian CA isozyme, CA II. In contrast to previous findings, a plasma CA inhibitor was found to be present in the blood of bowfin. Bowfin erythrocytes were also found to possess a high rate of Cl⁻/HCO₃⁻ exchange (6 nmol HCO₃⁻ s⁻¹ cm⁻²) that was sensitive to DIDS. Visualization of erythrocyte membrane proteins by SDS-PAGE revealed a major band in the 100 kDa range for the trout, which would be consistent with the anion exchanger. In contrast, the closest major band for the membranes of bowfin erythrocytes was around the 140 kDa range. Taken together, these results suggest that the strategy for blood CO₂ transport in bowfin is probably similar to that in most other vertebrates despite several unique characteristics of erythrocyte CA and Band 3 in these primitive fish.     

Antiabsorptive effects of 16, 16 dimethyl prostaglandin E2 and isolated rat colon

Ulcerative colitis is distinguished by abundant prostaglandin E₂ (PGE₂) in the stools and by severe diarrhea. To determine whether luminal PGE₂ alters normal colonic absorption, Na⁺ and Cl⁻transport across isolated rat proximal colon were studied before and after 16, 16 dimethyl PGE₂ (dmPGE₂) addition to flux chambers. Luminal administration of dmPGE₂ significantly reduced the net mucosal to serosal fluxes of Na⁺ and Cl⁻. These antiabsorptive tive effects of dmPGE₂ on Na⁺ and Cl⁻ active transport were reflected by a reduced metabolic rate of colonic tissue slices incubated with dmPGE₂. Addition of dmPGE₂ significantly reduced oxidation of glucose by the colon. Structurally, dmPGE₂ reduced the length of colonic mucosal microvilli, thereby decreasing absorptive surface area. These results suggest that PGE₂ released into the colonic lumen of patients with ulcerative colitis exerts antiabsorptive effects on the colon and in this way contributes to the associated diarrhea.     

Evidence of multiple operational affinities for d-glucose inside the human erythrocyte membrane

1. 1. The Michaelis-Menten parameters of labelled d-glucose exit from human erythrocytes at 2°C into external solution containing 50 mM d-galactose were obtained. The K_m is 3.4 ± 0.4 mM, V 17.3 ± 1.4 mmol · 1⁻¹ cell water · min⁻¹ for this infinite-trans exit procedure.

2. 2. The kinetic parameters of equilibrium exchange of d-glucose at 2°C are K_m = 25 ± 3.4 mM, V 30 ± 4.1 mmol · 1⁻¹ cell water · min⁻¹.

3. 3. The K_m for net exit of d-glucose into solutions containing zero sugar is 15.8 ± 1.7 mM, V 9.3 ± 3.3 mol 9.3 ± 3.3 mol · 1⁻¹ cell water · min⁻¹.

4. 4. This experimental evidence corroborates the previous finding of Hankin, B.L., Lieb, W.R. and Stein, W.D. [(1972) Biochim. Biophys. Acta 255, 126–132] that there are sites with both high and low operational affinities for d-glucose at the inner surface of the human erythrocyte membrane. This result is inconsistent with current asymmetric carrier models of sugar transport.

The biogeochemistry of chlorine at Hubbard Brook,New Hampshire,USA

Chlorine is a minor constituent of most rocks and a minor (although essential) element in plants, but it cycles rapidly through the hydrosphere and atmosphere. In forest ecosystem studies, chloride ion (Cl⁻) is often thought to be conservative in the sense that the sources and sinks within the ecosystem are assumed negligible compared to inputs and outputs. As such, Cl⁻ is often used as a conservative tracer to assess sources and transformations of other ions. In this paper we summarize research on chloride over the course of 36 years (1964–2000) at the Hubbard Brook Experimental Forest (HBEF) in central New Hampshire, USA. Evidence presented here suggests that in the 1960s and 1970s the dominant source of atmospheric Cl⁻ deposition was from pollutant sources, probably coal burning. In the 1970s the Cl⁻ inputs in bulk deposition declined, and the lower Cl⁻ deposition in the last two decades is dominated by marine sources. Between 1964 and 2000 there was no significant trend in Cl⁻ export in stream flow, thus the net hydrologic flux (NHF = bulk deposition inputs − streamflow outputs) has changed over this period. Early in the record the NHF was on average positive, indicating net retention of Cl⁻ within the system, but since about 1980 the NHF has been consistently negative, indicating an unmeasured input or source within the ecosystem. Dry deposition can account for at least part of that unmeasured source, and it appears that release of Cl⁻ from mineralization of soil organic matter (SOM) may also play an important role. We believe that accumulation of Cl⁻ in vegetation during the 1960s and 1970s offset the unmeasured source and resulted in net ecosystem retention. Accumulation of vegetative biomass has ceased since about 1982, leading to the apparent net export (negative NHF) since that time. Although we have no direct measurements of Cl⁻ accumulation in vegetation, our estimates suggest that an aggrading forest could sequester about 32 mol Cl ha⁻¹ year⁻¹, or about a third of the annual average bulk deposition flux to this ecosystem. Experimental additions of Cl⁻ to the forest floor cause increases in Cl⁻ concentration in foliage, throughfall, and soil solution. Manipulations of vegetation also affect the Cl⁻ cycle. Harvesting or devegetation of watersheds causes an increase in the Cl⁻ concentration and flux in stream water for several years after the disturbance. This period of release is followed by a period of reaccumulation of Cl⁻ that may last more than 15 years. In this respect, the behavior of Cl⁻ after disturbance parallels that of NO₃⁻, for which export increases after disturbance due to reduced plant nitrogen uptake and mineralization of nitrogen from detritus, rather than SO₄²⁻, for which export decreases after disturbance due to pH-dependent adsorption onto mineral soils. The interannual pattern of Cl⁻ export from the system primarily reflects the atmospheric inputs, but the net retention and cycling of Cl⁻ within the system appears to be largely under biological, rather than geochemical, control.     

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     

Effect of the absence of bicarbonate upon intracellular pH and calcium fluxes in pancreatic islet cells

The removal of extracellular HCO⁻₃ together with a decrease in pCO₂, in order to maintain a normal extracellular pH, caused a sustained increase of intracellular pH in rat pancreatic islets. This increase was more marked in glucose-deprived than in glucose-stimulated islets, and was associated with a facilitation of ⁴⁵Ca efflux from the glucose-deprived islets. Such a facilitation was slightly reduced in the absence of extracellular Ca²⁺ and abolished at low extracellular Na⁺ concentration. It failed to occur in glucose-stimulated islets, whether in the presence or absence of extracellular Ca²⁺. The removal of HCO⁻₃ and decrease in the pCO₂ also reduced the magnitude of both the secondary rise in ⁴⁵Ca efflux and stimulation of insulin release normally evoked by an increase in glucose concentration. These findings suggest that changes in intracellular pH affect both the outflow of Ca²⁺ from islet cells as mediated by Na⁺-Ca²⁺ countertransport and the inflow of Ca²⁺ by gated Ca²⁺ channels. The experimental data are also compatible with the view that islet cells are equipped with an active process of bicarbonate-chloride exchange involved in the regulation of intracellular pH.     

Preference of carbon absorption determines the competitive ability of algae along atmospheric CO2 concentration

Although many studies have focused on the effects of elevated atmospheric CO₂ on algal growth, few of them have demonstrated how CO₂ interacts with carbon absorption capacity to determine the algal competition at the population level. We conducted a pairwise competition experiment of Phormidium sp., Scenedesmus quadricauda, Chlorella vulgaris and Synedra ulna. The results showed that when the CO₂ concentration increased from 400 to 760 ppm, the competitiveness of S. quadricauda increased, the competitiveness of Phormidium sp. and C. vulgaris decreased, and the competitiveness of S. ulna was always the lowest. We constructed a model to explore whether interspecific differences in affinity and flux rate for CO₂ and HCO₃ ⁻ could explain changes in competitiveness between algae species along the gradient of atmospheric CO₂ concentration. Affinity and flux rates are the capture capacity and transport capacity of substrate respectively, and are inversely proportional to each other. The simulation results showed that, when the atmospheric CO₂ concentration was low, species with high affinity for both CO₂ and HCO₃ ⁻ (HCHH) had the highest competitiveness, followed by the species with high affinity for CO₂ and low affinity for HCO₃ ⁻ (HCLH), the species with low affinity for CO₂ and high affinity for HCO₃ ⁻ (LCHH) and the species with low affinity for both CO₂ and HCO₃ ⁻ (LCLH); when the CO₂ concentration was high, the species were ranked according to the competitive ability: LCHH > LCLH > HCHH > HCLH. Thus, low resource concentration is beneficial to the growth and reproduction of algae with high affinity. With the increase in atmospheric CO₂ concentration, the competitive advantage changed from HCHH species to LCHH species. These results indicate the important species types contributing to water bloom under the background of increasing global atmospheric CO₂, highlighting the importance of carbon absorption characteristics in understanding, predicting and regulating population dynamics and community composition of algae.