Net 22Na Research Articles

Effect of the putative Ca2+-receptor agonist Gd3+ on the active transepithelial Na+ transport in frog skin.

In this communication we show that Gd3+ acts as an activator of the apical sodium channel (ENaC) in frog skin epithelia. Application of Gd3+ to the apical solution of frog skin epithelia increased the Na+ absorption measured as the amiloride-inhibitable short-circuit current (Isc). The stimulation was dose dependent with a concentration for half-maximal stimulation (EC50) of 0.023 mM. The change in Isc was found to correlate with the net Na+ flux, confirming that Gd3+ enhances Na+ absorption. By monitoring the cellular potential (Vsc) with microelectrodes during addition of Gd3+, it was found that Vsc depolarized as Isc rose, indicating that Gd3+ affects apical Na+ permeability (PNa). This was confirmed by measuring the I/V relations of the apical membrane. In the presence of benzimidazolylguanidin (BIG), a drug known to abolish the Na+ self-inhibition, Gd3+ had no effect on Isc. The Na+ self-inhibition was investigated using fast changes of the apical Na+ concentration on K+-depolarized epithelia. BIG was found to abolish the Na+ self-inhibition and to activate the basal Na+ transport, whereas Gd3+ only activated the basal Na+ transport but had no effect on the self-inhibition. These results indicate the existence of an alternative nonhormonal mechanism to Na+ self-inhibition, via which both Gd3+ and BIG act, possibly components of the Na+ feedback inhibition system.

Copper ion redox state is critical for its effects on ion transport pathways and methaemoglobin formation in trout erythrocytes

We have studied the mechanism of copper uptake by the cells, its oxidative action and effects on ion transport systems using rainbow trout erythrocytes. Cupric ions enter trout erythrocytes as negatively charged complexes with chloride and hydroxyl anions via the band 3-mediated Cl −/HCO 3 − exchanger. Replacement of Cl − by gluconate, and complexation of cupric ions with histidine abolish rapid Cu 2+ uptake. Within the cell cupric ions interact with haemoglobin, causing methaemoglobin formation by direct electron transfer from heme Fe 2+ to Cu 2+, and consecutive proton release. Ascorbate-mediated reduction of cupric ions to cuprous decreases copper-induced metHb formation and proton release. Moreover, cuprous ions stimulate Na +/H + exchange and residual Na + transport causing net Na + accumulation in the cells. The effect requires copper binding to an externally facing thiol group. Copper-induced Na + accumulation is accompanied by K + loss occurring mainly via K +–Cl − cotransporter. Taurine efflux is also stimulated by copper exposure. However, net loss of osmolytes is not as pronounced as Na + uptake and modest swelling of the cells occurs after 5 min of copper exposure. Taken together the results indicate that copper toxicity, including copper transport into the cells and its interactions with ion transport processes, depend on the valency and complex formation of copper ions.

CFTR contributes to cAMP-mediated inhibition of Na+/H+ exchange in murine jejunum

In small intestinal epithelium of wild-type (WT) mice, increases in intracellular cAMP result in stimulation of CFTR-mediated anion secretion and inhibition of electroneutral Na + absorption (via Na+/H ÷ exchange). However, several reports indicate that anion secretion and inhibition of Na t absorption do not occur following cAMP treatment of the intestine in both cystic fibrosis (CF) patients and CFTR knockout (CFTR(-)) mice. To better understand the role of CFTR during cAMP inhibition of intestinal Na ÷ absorption, we investigated: (1) whether the anion secretory function of CFTR is required for complete inhibition of Na + absorption; and (2) whether electroneutral Na ÷ absorption in the OF intestine can be normally regulated by other physiological mechanisms, e.g. extracellular hyperosmolarity. ~Na flux studies were performed on WT and CFTR(-) murine jejunum mounted in Ussing chambers (voltage-clamped) and bathed in physiological Ringers solution. Under basal conditions (+ vehicle), wr jejunum exhibited net Na ÷ absorption at a rate of 6.1 _+ 1.3/~Eq/cm2.hr (n =6). Basal Na ÷ absorption could be completely inhibited by treatment with 100/~M 5-(N-EthyI-N-isopropyl} amiloride (EIPA), a specific inhibitor of Na÷/H + exchanger activity. Treatment of the W'i jejunum with 10/~M forskolin/100 p,M IBMX to increase intracellular cAMP also completely inhibited net Na ÷ flux in WT mice (0.4 _+ 0.5 ,~Eq/cm2.hr, n=6), but did not affect net Na + absorption in the CFTR(-) jejunum (basal = 4.8 _+ 0.9 vs. treated = 4.5 _+ 0.8/~Eq/em2.hr; n=6#). To examine the role of CFTR-mediated anion secretion, Wl jejuna were simultaneously treated with forskolin/IBMX and humetanide (100/~M serosal) to inhibit CFTR-mediated Clsecretion. Bumetanide treatment recovered 46% of net Na ÷ absorption (2.2 _+ 0.4 ~Eq/cm2.hr, n = 6). To examine whether intestinal Na + absorption can be inhibited in the absence of CFTR, net Na* absorption was measured after addition of 300 mM mannitol to the bath solutions of WT and CFTR(-) jejunua. Extracellular hyperosmolarity completely inhibited Ha + absorption in both WT (-0.2 _+ 0.7/~Eq/cm2.hr, n=6) and CFTR(-) intestine (0.1 _+ 0.2 p.Eq/cm2.hr, n =9). In conclusion, functional CFTR is required for complete inhibition of intestinal Na ÷ absorption during cAMP stimulation but not during extracellular hyperosmolarity. These findings suggest the hypothesis that complete cAMP inhibition of intestinal Na ÷ absorption may require changes in epithelial cell volume resulting from CFrR-mediated anion secretion. Supported by NIH and AHA.

On the mechanism of sodium-proton exchange in crayfish

In salt depleted crayfish net sodium and proton fluxes were coupled 1:1 as required by the frog skin–turtle bladder model. In addition, three proton pump inhibitors produced equal reductions of both fluxes. It is concluded that the model operates in these animals. Net Na + and H + fluxes were very small in tap water adapted animals, but regression analysis clearly showed that they were coupled, though perhaps not 1:1. Proton pump inhibitors, at concentrations that suppressed ++++fluxes in salt-depleted crayfish, had no measureable effect on proton movement in tap water-adapted animals. Two of them (dicyclocarbodiimide and N-ethyl maleimide), caused a small reduction in Na + influx without affecting proton efflux. These experiments provide no support for operation of the frog–turtle system in adult crayfish adapted to tap water. A 2Na + — H + exchanger is considered from an energetic point of view. Such a system might be able to couple Na + and H + fluxes in dilute, near neutral solutions ([Na +]∼1–2 mM; pH 7).

Na+-Dependent Fluid Absorption in Intact Perfused Rat Colonic Crypts

The traditional paradigm of fluid movement in the mammalian colon is that fluid absorption and secretion are present in surface and crypt cells, respectively. We have recently demonstrated Na(+)-dependent fluid absorption in isolated crypts that are devoid of neurohumoral stimulation. We now explore the mechanism of Na(+)-dependent fluid absorption in isolated rat colonic crypts. Net fluid absorption was determined using microperfusion techniques and methoxy[(3)H]inulin with ion substitutions and transport inhibitors. Net fluid absorption was reduced but not abolished by substitution of either N-methyl-D-glucamine- Cl(-) or tetramethylammonium for Na(+) and by lumen addition of 5-ethylisopropyl amiloride, an amiloride analogue that selectively inhibits Na(+)-H(+) exchange. Net fluid absorption was also dependent on lumen Cl(-) because removal of lumen Cl(-) significantly (P < 0.001) reduced net fluid absorption. DIDS at 100 micromol/L, a concentration at which DIDS is an anion exchange inhibitor, minimally reduced net fluid absorption (P < 0.05). In contrast, either 500 micromol/L DIDS, a concentration at which DIDS is known to act as a Cl(-) channel blocker, or 10 micromol/L NPPB, a Cl(-) channel blocker, both substantially inhibited net fluid absorption (P < 0.001). Finally, both the removal of bath Cl(-) and addition of bath bumetanide, an inhibitor of Na-K-2Cl cotransport and Cl(-) secretion, resulted in a significant increase in net fluid absorption. (1) Net Na(+)-dependent net fluid absorption in the isolated colonic crypt represents both a larger Na(+)-dependent absorptive process and a smaller secretory process; and (2) the absorptive process consists of a Na(+)-dependent, HCO(3)(-)-independent process and a Na(+)-independent, Cl(-)-dependent, HCO(3)(-)-dependent process. Fluid movement in situ represents these transport processes plus fluid secretion induced by neurohumoral stimulation.

Intestinal HCO3− secretion in marine teleost fish: evidence for an apical rather than a basolateral Cl−/HCO3− exchanger

Intestinal fluid was collected from 11 marine teleost fish from the Baltic sea and the Pacific ocean. The anterior, mid and posterior segments of the intestine contained 33–110 mM of HCO3 − equivalents (with exception of the Atlantic cod which contained only 5–15 mM). Considering literature values of transepithelial potentials and concentration gradients, these high levels of HCO3 − equivalents are probably the result of active HCO3 − transport. Possible HCO3 − transport mechanisms were studied in the Pacific sanddab (Citharichthys sordidus) in vitro. Measurements of net secretion of HCO3 − equivalents across the intestinal epithelium revealed mucosal DIDS sensitivity (10−4 M) and Cl−-dependence of the HCO3 − equivalent net flux, but no serosal DIDS (10−4 M) sensitivity. Net Na+ uptake was abolished in the absence of Cl−, but some Cl− uptake persisted in the absence of Na+, at a rate similar to that of net HCO3 − secretion. Anterior, mid and posterior segments of the intestine performed similarly. These observations support the presence of an apical rather than a basolateral Cl−/HCO3 − exchanger and thus contrast the currently accepted model for intestinal HCO3 − secretion. This apical Cl−/HCO3 − exchanger alone, however, is not sufficient for maintaining the observed HCO3 − equivalents gradient in vivo. We suggest a coupling of cytosolic carbonic anhydrase, a basolateral proton pump and the apical Cl−/HCO3 − exchanger to explain the intestinal HCO3 − transport.

Interaction of pyrethroids with ion transport pathways present in frog skin

We measured the ability of deltamethrin and permethrin to stimulate the ion transport operated by the frog skin. Ion transport was monitored by measuring the short-circuit current. Deltamethrin and permethrin, added to the fluid bathing the internal surface of the isolated frog skin, showed a seasonal feature and were more effective in increasing short-circuit current in the period between June and October. Transepithelial influxes and outfluxes of 22Na+ and 36C1− across symmetrical parts of the short-circuited skin were then measured. Deltamethrin was found to increase net Na+ absorption and, to a lesser extent, Cl1 secretion. The presence of a Cl− secretory mechanism is supported by two observations, those being that the short-circuit current value, recorded when deltamethrin and amiloride (inhibitor of Na+ channels) were simultaneously present, was higher than that obtained in the presence of amiloride alone and that bumetanide, a classic inhibitor of Cl− secretion, completely inhibited the component of the Cl− outflux that was induced by the deltamethrin stimulation. The stimulations of short-circuit current respectively caused by deltamethrin (type II pyrethroid) and permethrin (type I) were comparable and similarly affected by indomethacin (an inhibitor of cyclooxygenase) and W7 (an inhibitor of the Ca2+/calmodulin system). These findings suggest that, in frog skin, the cellular mechanism of action of type I and type II pyrethroids is similar.

The MDCK cell line is made up of populations of cells with diverse resistive and transport properties

The MDCK cell line has been used as a model for the renal distal tubule. When grown on permeable supports, the cells form tight junctions and transport sodium and calcium. Widely different values have, however, been obtained for resistance and transports by different investigators. Since the MDCK cell line is genetically heterogeneous, one potential reason for observed differences is the use of different populations of cells. Dilutional techniques have been used to obtain eleven MDCK cell clones. The clones exhibit a range of values for resistance (300–4000Ω cm2), net Ca2+transport (3.2–81pmol /cm2/min) and net Na+transport (0.4–6.0nmol/cm2/min). To determine if the characteristics of clonal cell lines changed with increasing passage number, resistance was measured at relatively early and late passage number. Resistance increased with increasing passage number in two of six lines and decreased in one. These studies demonstrate that differences between previous studies may be due to differences in cell population, length of time since passage, and passage number. Clonal cell lines should provide a useful tool to investigate ion transport and mechanisms of regulation of transport in this model of renal distal tubule.

Ionic and osmotic components of salt stress specifically modulate net ion fluxes from bean leaf mesophyll

ABSTRACTIonic mechanisms of salt stress perception were investigated by non‐invasive measurements of net H+, K+, Ca2+, Na+, and Cl− fluxes from leaf mesophyll of broad bean (Vicia faba L.) plants using vibrating ion‐selective microelectrodes (the MIFE technique). Treatment with 90 m M NaCl led to a significant increase in the net K+ efflux and enhanced activity of the plasma membrane H+‐pump. Both these events were effectively prevented by high (10 m M) Ca2+ concentrations in the bath. At the same time, no significant difference in the net Na+ flux has been found between low‐ and high‐calcium treatments. It is likely that plasma membrane K+ and H+ transporters, but not the VIC channels, play the key role in the amelioration of negative salt effects by Ca2+ in the bean mesophyll. Experiments with isotonic mannitol application showed that cell ionic responses to hyperosmotic treatment are highly stress‐specific. The most striking difference in response was shown by K+ fluxes, which varied from an increased net K+ efflux (NaCl treatment) to a net K+ influx (mannitol treatment). It is concluded that different ionic mechanisms are involved in the perception of the ‘ionic’ and ‘osmotic’ components of salt stress.

Effect of Sudden Salt Stress on Ion Fluxes in Intact Wheat Suspension Cells

Although salinity is one of the major problems limiting agricultural production around the world, the underlying mechanisms of high NaCl perception and tolerance are still poorly understood. The effects of different bathing solutions and fusicoccin (FC), a known activator of plasma membrane ATPase, on plasma membrane potential (Em) and net fluxes of Na+, K+and H+were studied in wheat suspension cells (Triticum aestivum) in response to different NaCl treatments. Emof cells in Murashige and Skoog (MS) medium was less negative than in cells exposed to a medium containing 10mM KCl+0.1m M CaCl2(KSM) and to a basic salt medium (BSM), containing 1m M KCl and 0.1m M CaCl2. Multiphasic Na+accumulation in cells was observed, peaking at 13min after addition of 120m M NaCl to MS medium. This time scale was in good agreement with net Na+flux changes measured non-invasively by moving ion-selective microelectrodes (the MIFE system). When 120m M NaCl was added to all media studied, a quick rise of Na+influx was reversed within the first 20min. In both 120 and 20m M NaCl treatments in MS medium, net Na+efflux was observed, indicating that active Na+transporters function in the plant cell response to salt stress. Lower external K+concentrations (KSM and BSM) and FC pre-treatment caused shifts in Na+fluxes towards net influx at 120m M NaCl stress.

Developmental Change in Na,K-ATPase α1 and β1 Expression in Normal and Hypothyroid Rat Renal Cortex

Renal Na,K-ATPase drives active reabsorption of sodium and cotransported solutes along the nephron. There is a large increase in net Na⁺ reabsorption in the postnatal rat kidney. It has previously been established that the postnatal increase in expression of sodium pump isoforms in the brain, but not the heart, is blunted in the hypothyroid neonate. The aims of this study were to establish whether the developmental increase in renal sodium transport is associated with coordinate increases in the abundance of the sodium pump α₁ catalytic and β₁ glycoprotein subunits and Na,K-ATPase activity, and to determine whether thyroid status influences the postnatal increase in renal Na,K-ATPase expression. Pregnant rats were made hypothyroid with low iodine diet, propylthiouracil and perchlorate. Offspring were hypothyroid assessed by triiodothyronine/thyroxine RIA. Renal cortical membranes were prepared from euthyroid and hypothyroid rats from 6 to 24 days of age. There was no change in Na,K-ATPase activity or expression between 6 and 15 days. Between 15 and 24 days, Na,K-ATPase activity increased 1.35-fold while sodium pump α₁ and β₁ subunit abundance increased coordinately to 1.7- and 2-fold over the previous period, respectively. In hypothyroid neonates, kidney weight was less than in euthyroids, and Na,K-ATPase activity, α₁ and β₁ subunit pool sizes did not significantly increase as a function of age between 6 and 24 days. We conclude that the postnatal increase in sodium pump activity can be accounted for by coordinate increases in the pool sizes of α₁ and β₁ subunits and that, like in brain, this increased Na,K-ATPase expression is dependent on normal thyroid status.

Copper Effects on Ion Transport across Lamprey Erythrocyte Membrane: Cl−/OH− Exchange Induced by Cuprous Ions

We studied the effects of prelytic copper concentrations on cell volume, intracellular pH, and ion transport in lamprey erythrocytes. Ion fluxes and pH were measured by radioactive tracer technique, patch clamp, and flame photometry. Prelytic CuSO4 concentration of 100 μM caused anion-dependent intracellular acidification and increase in Cl− influx after 2 min lag-phase. In the presence of ascorbate copper effect was amplified and lag-phase was skipped. Pretreatment of the cells with N-phenyl maleimide abolished copper-induced changes completely. Copper treatment caused an increase in Na+ fluxes in both directions and a net Na+ uptake. Copper-induced Na+ transport was partially amiloride(MIA)-sensitive representing Na+/H+ exchange. The nature of the amiloride-insensitive fraction of copper-activated Na+ influx remains unknown. Cell swelling after 15 min of copper exposure induced regulatory volume decrease response involving KCl extrusion via K+ and Cl− volume-sensitive channels. We suggest that the effects of copper on ion transport fit the following sequence of events: (i) cupric ions are reduced to cuprous state on the membrane surface, (ii) electroneutral pairs CuCl and CuOH mediate chloride/hydroxyl exchange, as shown before for trialkyltin, dissipating transmembrane pH gradient, and (iii) changes in intracellular pH result in the activation of the Na+/H+ exchange and consecutive volume changes cause the RVD response.

Gill lipid metabolism and unidirectional Na + flux in the European eel ( Anguilla anguilla) after transfer to dilute media: the formation of wax alcohols as a primary response

The present study investigated the mechanism involved when fish tighten their gill membrane after transfer to dilute media. Ten individually assayed eels adapted to fresh water (FW) and labelled with radioactive 22 Na in the plasma showed mean exchange rates in equilibrium with ambient radioactive 24 Na in FW at 70 μmol kg −1 h −1. Just after transfer to demineralized water (DW), the mean Na + release rate went up to 120 μmol kg −1 h −1; after 24 h in DW it came down again and levelled off at 40 μmol kg −1 h −1 during a period of up to 2 weeks. During the same period the mean Na + uptake rate was 12 μmol kg −1 h −1, i.e., the general picture in DW was a net Na + loss. In another similar experiment, groups of three FW-adapted eels were incubated in vivo for up to 24 h with ( 14 C ) acetate added as lipid precursor to the ambient water. Incubation in FW showed about 20% of the total 14 C -activity incorporated into gill lipids as ( 14 C ) wax alcohols (WA; octadecanol and eicosanol). This percentage went up to 50% shortly after transfer to DW and came down again to about 20% after 2 weeks in DW. Single eels labelled with 22 Na in the plasma showed a statistically significant positive linear correlation of percentage ( 14 C ) wax alcohols with log [ 22 Na efflux]. Based on the observed parallel between Na + flux and gill lipid metabolism, it is suggested that the eel reacts at first to a loss of Na + by synthesizing wax alcohols that can tighten the gill membrane.

Compartmentalization of extracellular cGMP determines absorptive or secretory responses in the rat jejunum.

We examined potential mechanisms by which angiotensin subtype-2 (AT2) receptor stimulation induces net fluid absorption and serosal guanosine cyclic 3',5'-monophosphate (cGMP) formation in the rat jejunum. L-arginine (L-ARG) given intravenously or interstitially enhanced net fluid absorption and cGMP formation, which were completely blocked by the nitric oxide (NO) synthase inhibitor, N-nitro-L-arginine methylester (L-NAME), but not by the specific AT2 receptor antagonist, PD-123319 (PD). Dietary sodium restriction also increased jejunal interstitial fluid cGMP and fluid absorption. Both could be blocked by PD or L-NAME, suggesting that the effects of sodium restriction occur via ANG II at the AT2 receptor. L-ARG-stimulated fluid absorption was blocked by the soluble guanylyl cyclase inhibitor 1-H-[1,2,4]oxadiazolo[4, 2-alpha]quinoxalin-1-one (ODQ). Cyclic GMP-specific phosphodiesterase in the interstitial space decreased extracellular cGMP content and prevented the absorptive effects of L-ARG. Angiotensin II (ANG II) caused an increase in net Na+ and Cl- ion absorption and 22Na+ unidirectional efflux (absorption) from the jejunal loop. In contrast, intraluminal heat-stable enterotoxin of Escherichia coli (STa) increased loop cGMP and fluid secretion that were not blocked by either L-NAME or ODQ. These findings suggest that ANG II acts at the serosal side via AT2 receptors to stimulate cGMP production via soluble guanylyl cyclase activation and absorption through the generation of NO, but that mucosal STa activation of particulate guanylyl cyclase causes secretion independently of NO, thus demonstrating the opposite effects of cGMP in the mucosal and serosal compartments of the jejunum.

Stimulation of sodium chloride absorption from secreting rat colon by short-chain fatty acids.

Inhibition of electroneutral NaCl absorption by cyclic adenosine monophosphate (cAMP) results in fluid malabsorption in cholera. Short-chain fatty acids (SCFA) stimulate electroneutral NaCl absorption from the colon. The present study investigated effects of elevated cAMP on SCFA-stimulated NaCl absorption in rat distal colon. The effect of SCFA on fluxes of 22Na and 36Cl was studied under voltage-clamp conditions, in the presence and absence of secretagogues inducing mucosal cAMP elevation [ie, theophylline, cholera toxin (CT) and forskolin]. The effect of butyrate concentration on Na absorption in CT- and theophylline-treated mucosa was compared with control normal mucosa. cAMP was measured in isolated colonocytes in the presence of secretagogues with and without SCFA using a radioassay method. All secretagogues were noted to inhibit net Na absorption and to induce net Cl secretion. In the presence of SCFA, net Na absorption was normal, and net Cl secretion was partly reversed. The flux data indicated that NaCl absorption from secreting colon was stimulated by SCFA and that Cl secretion was partially inhibited. The effects of SCFA on NaCl absorption were similar regardless of the secretagogue used. The kinetics of butyrate-stimulated Na absorption were altered by theophylline and CT, which decreased Km (6.87 and 7.17, respectively, compared to 10.75 mM for control) and increased Vmax (4.55 and 8.33 compared to 3.45 mM/eq/cm2/hr for control). cAMP production by colonocytes in response to secretory stimuli was significantly reduced (34.4%) by butyrate but not by acetate or propionate. In conclusion, SCFA-stimulated Na absorption is up-regulated by cAMP and may be an absorptive pathway that can be utilized in the therapy of cholera. Effects of butyrate on cAMP generation are also likely to be useful in secretory diarrhea.

Absorptive apical amiloride-sensitive Na+ conductance in human endometrial epithelium.

1. Human endometrial epithelial cells cultured on porous tissue culture supports formed tight, polarized epithelial monolayers with features characteristic of tight epithelia. Endometrial epithelial layers generated significant transepithelial electrical resistance (750 Omega cm2) and potential difference (15.3 mV), with an inward short-circuit current (Isc; 20.5 microA cm-2). 2. The Isc was linearly proportional to the external Na+ concentration and was abolished in the absence of Na+. The Isc was sensitive to apical amiloride. Net 22Na+ flux was in the absorptive apical to basolateral direction and fully accounted for the inward Isc. In addition, apical to basolateral and net 22Na+ transport were reduced in the presence of amiloride. 3. The Isc was also sensitive to addition of ouabain and Ba2+ to the basal solution, consistent with a role for basolateral Na+-K+-ATPase and K+ channels in generation of the current. 4. These data demonstrate that human endometrial epithelial cells in primary culture produce tight, functional monolayers on permeable supports. We provide the first evidence that human endometrial epithelial cells have an inward Isc accounted for by an amiloride-sensitive Na+ conductance. The Na+-absorptive function of the endometrium may provide an appropriate environment for sperm function and embryo growth.

Ion transport across the skin of the mudskipper Periophthalmus modestus

An amphibious teleost, the mudskipper ( Periophthalmus modestus), showed a serosa-positive transepithelial potential difference (PD) across the skin isolated from several areas of the body. However, the highest PD was observed in skins isolated from the pectoral fin. In skin isolated from the pectoral fin of mudskipper, the PD was due to active Cl − secretion, because net 36Cl − flux was identical with the simultaneously measured short-circuit current ( I sc). No net Na + flux was observed under the short-circuited condition. The I sc in skins isolated from pectoral fin was diminished by omitting Cl − or Na + bilaterally from the bathing media. The I sc was also diminished by serosal ouabain, a specific blocker of Na + K + -ATPase , and by serosal bumetanide, a specific inhibitor of the Na +-K +-Cl − cotransporter. K + channel blockers, Ba 2+ and Cs +, applied to the serosal surface and the Cl − channel blocker, diphenylamine-2-carboxilic acid, applied to the apical membrane reduced I sc and increased tissue resistance, R t. The Na + channel blocker, amiloride, had no effect on either I sc or R t. From these data we proposed a current model for ion transport across the skin of the mudskipper; Na + K + -ATPase , Na +-K +-Cl − cotransporter and K + channels are localized to the basolateral membrane and CI channels are located in the apical membrane of the epithelium. A morphological study demonstrated that skins isolated from the pectoral fin contained a large number of mitochondrion-rich cells (MRC) and many pits on the surface. Beneath the pits, relatively large cells were observed on light micrograph, which contained many cytoplasmic granules, presumably mitochondria. A good correlation was obtained between the area occupied by the MRC and the I sc. These characteristics were similar to those of the operculum, which is considered as a model for gills in seawater teleosts but different from those of frog skins.

SCFA and Electrolyte Absorption in the Colon of Three Rodent Species

The effects of nutritional regime on the colonic absorption of electrolytes, water, and short-chain fatty acids (SCFA) were studied in three rodent species (voles, spiny mice, and laboratory mice) differing in their ecological background. The effects of different levels of SCFA, induced by restriction of food intake, on the capacity of the in situ perfused colon to absorb water and electrolytes was studied. In addition, in vitro experiments were carried out to assess the mechanisms related to electrolyte transport in the proximal and distal colon. Water and solutes absorption from the perfused colon in the food restricted voles were significantly lower compared to voles fed ad libitum. No such effect was recorded in the two murid species. In vitro studies, employing Ussing chambers, were conducted to characterize the sodium and chloride transport mechanisms. In the colon of the voles and spiny mice as well as in the distal colon of the laboratory mice, net Na + flux was not significantly different from that of Cl −, and both were higher than the measured equivalent short circuit current. The results suggest an electroneutral sodium transport mechanism in all three species. A high capacity to adjust the rate of water absorption to the amount of fermentable substance in the intestine was demonstrated only in the vole.

Branchial Ionic Flux Responses in Rainbow Trout to Chloramine-T after Acclimation to Different Levels of Water Hardness

Abstract Whole-body unidirectional and net Na+ and Cl– fluxes were determined in rainbow trout Oncorhynchus mykiss exposed to chloramine-T (9 mg/L) alone or in combination with 17 mmol NaCl/L (0.1 % weight per volume, w/v) after acclimation to either city of Ottawa tap water (TW: Na+ 0.121 ± 0.004 mmol/L; K+ 0.015 ± <0.001 mmol/L; Ca2+ 0.351 ± 0.004 mmol/L; Cl– 0.141 ± 0.002 mmol/L) or artificial soft water (ASW: Na+ 0.043 ± 0.003 mmol/L; K+ 0.005 ± <0.001 mmol/L; Ca2+ 0.063 ± 0.002 mmol/L; Cl– 0.026 ± 0.002 mmol/L). Preexposure fluxes revealed that ASW-acclimated fish had reduced rates of Na+ uptake and efflux compared with TW-acclimated fish. However, rates of Cl– uptake among ASW-acclimated fish were comparable with those of TW-acclimated fish. Exposure to chloramine-T in TW resulted in significant increases in the Cl– efflux, giving rise to a significant and negative net flux. Exposure of ASW-acclimated fish to chloramine-T caused a significant negative increase in the Na+ efflux and net flux but only...

Correlation between transepithelial Na+ transport and transepithelial water movement across isolated frog skin (Rana esculenta).

In the present work the coupling under short-circuited conditions between the net Na(+)-influx across isolated frog skin and the transepithelial transport of water was examined i.e., the short-circuit current (Isc) and the transepithelial water movement (TEWM) were measured simultaneously. It has been shown repeatedly that the Isc across isolated frog skin is equal to the net transepithelial Na+ transport. Furthermore the coupling between transepithelial uptake of NaCl under open-circuit conditions and TEWM was also measured. The addition of antidiuretic hormone (AVT) to skins incubated under short-circuited conditions resulted in an increase in the Isc and TEWM. Under control conditions Isc was 9.14 +/- 2.43 and in the presence of AVT 45.9 +/- 7.3 neq cm-2 min-1 (n = 9) and TEWM changed from 12.45 +/- 4.46 to 132.8 +/- 15.8 nL cm-2 min-1. The addition of the Na+ channel blocking agent amiloride resulted in a reduction both in Isc and TEWM, and a linear correlation between Isc and TEWM was found. The correlation corresponds to that 160 +/- 15 (n = 7) molecules of water follow each Na+ across the skin. In another series of experiments it was found that there was a linear correlation between Isc and the increase in apical osmolarity needed to stop the TEWM. The data presented indicate that the observed coupling between the net transepithelial Na+ transport and TEWM is caused by local osmosis.