Electrophysiology of flounder intestinal mucosa. I. Conductance properties of the cellular and paracellular pathways

Guanylin activates Cl(-) secretion into the lumen of seawater eel intestine via apical Cl(-) channel under simulated in vivo conditions

Guanylin (GN) action on seawater eel intestine was examined under simulated in vivo conditions, where isotonic luminal fluid has low NaCl and high MgSO4 (MgSO4 Ringer). In Ussing chamber, MgSO4 Ringer induced serosa-negative potential difference (PD) even after bumetanide treatment, which is due to the higher paracellular Na(+) permeability over Cl(-), as confirmed by the replacement by MgCl2 (no Cl(-) gradient) or Na2SO4 Ringer (no Na(+) gradient). Luminal GN reversed serosa-negative PD, probably by enhancing Cl(-) secretion into the lumen, as the GN effect was blocked by apical Cl(-) channel blockers [diphenylamine-2-carboxylic acid (DPC), 5-nitro-2-(3-phenylpropylamino) benzoic acid, glibenclamide but not cystic fibrosis transmembrane regulator (CFTR)inh-172] or replacement of luminal fluid by MgCl2 Ringer. The blockers' effect was undetectable when normal Ringer was on both sides. In the sac preparation, NaCl secretion occurred into the lumen (Na(+) > Cl(-)), and GN further enhanced Cl(-) secretion (Cl(-) > Na(+)), resulting in water secretion. These GN effects were also blocked by DPC. Quantitative analyses showed that isotonic NaCl is absorbed when luminal fluid is normal Ringer, but, when luminal fluid is MgSO4 Ringer, hypertonic NaCl, almost equivalent to seawater, is secreted into the lumen after GN. These results indicate that GN stimulates the secretion of hypertonic NaCl into the lumen of seawater eel intestine, like rectal gland of marine elasmobranchs, to get rid of excess NaCl although marine teleost intestine is thought to have only absorptive-type cells with a unique Na-K-Cl cotransport system. The secreted NaCl may activate the cotransport system and further help absorb water in the final segment of seawater eel intestine.

Mechanisms of guanylin action on water and ion absorption at different regions of seawater eel intestine

Guanylin (GN) inhibited water absorption and short-circuit current (Isc) in seawater eel intestine. Similar inhibition was observed after bumetanide, and the effect of bumetanide was abolished by GN or vice versa, suggesting that both act on the same target, Na(+)-K(+)-2Cl(-) cotransporter (NKCC), which is a key player for the Na(+)-K(+)-Cl(-) transport system responsible for water absorption in marine teleost intestine. However, effect of GN was always greater than that of bumetanide: 10% greater in middle intestine (MI) and 40% in posterior intestine (PI) for Isc, and 25% greater in MI and 34% in PI for water absorption. After treatment with GN, Isc decreased to zero, but 20-30% water absorption still remained. The remainder may be due to the Cl(-)/HCO3 (-) exchanger and Na(+)-Cl(-) cotransporter (NCC), since inhibitors for these transporters almost nullified the remaining water absorption. Quantitative PCR analysis revealed the presence of major proteins involved in water absorption; the NKCC2β and AQP1 genes whose expression was markedly upregulated after seawater acclimation. The SLC26A6 (anion exchanger) and NCCβ genes were also expressed in small amounts. Consistent with the inhibitors' effect, expression of NKCC2β was MI > PI, and that of NCCβ was MI << PI. The present study showed that GN not only inhibits the bumetanide-sensitive Na(+)-K(+)-Cl(-) transport system governed by NKCC2β, but also regulates unknown ion transporters different from GN-insensitive SLC26A6 and NCC. A candidate is cystic fibrosis transmembrane conductance regulator Cl(-) channel, as demonstrated in mammals, but its expression is low in eel intestine, and its role may be minor, as indicated by the small effect of its inhibitors.

Esophageal desalination is mediated by Na⁺, H⁺ exchanger-2 in the gulf toadfish (Opsanus beta)

Esophageal desalination is a crucial step in the gastrointestinal water absorption pathway, as this pre-intestinal processing establishes the osmotic conditions necessary for water absorption. Previous work has shown that esophageal Na(+) absorption is amiloride sensitive; however, it is as yet unclear if Na(+), H(+) exchangers (NHE) or Na(+) channels (ENaC) are responsible. The purpose of the current study was therefore to investigate the roles that NHE isoforms may play in this process in a marine teleost, the gulf toadfish (Opsanus beta), as well as what role NHE isoforms may play in the downstream intestinal Na(+) transport. A combination of symmetrical current clamp and asymmetrical voltage clamp experiments showed the esophagus to contain both an ion absorptive current (I(sc)=0.83±0.68) and serosal side negative transepithelial potential (TEP=-4.9±0.6). (22)Na uptake (J(Na)(m→s)) was inhibited by 0.5 mM EIPA, with no effect of 0.1 mM amiloride, 1 mM furosemide or 1 mM thiazide. A Cl(-) free saline reduced J(Na)(m→s) by 40% while also reducing conductance and reversing TEP. These results suggest that both transcellular and paracellular components contribute to esophageal Na(+) transport, with transcellular transport mediated by NHE. The NHE1, NHE2 and NHE3 genes were amplified and tissue distribution analysis by real-time PCR showed high NHE2 expression levels in the esophagus and stomach. Little NHE3 expression was observed throughout the gastrointestinal tract, and NHE2 expression was absent from the intestine. Hypersalinity (60 ppt) had no effect on the expression profile of NHE2, slc4a2, scl26a6, CAc or V-type ATPase (β-subunit), suggesting that esophageal desalination is less flexible in response to osmotic stress than the intestine.

Intestinal fluid absorption in anadromous salmonids: importance of tight junctions and aquaporins

The anadromous salmonid life cycle includes both fresh water (FW) and seawater (SW) stages. The parr-smolt transformation (smoltification) pre-adapt the fish to SW while still in FW. The osmoregulatory organs change their mode of action from a role of preventing water inflow in FW, to absorb ions to replace water lost by osmosis in SW. During smoltification, the drinking rate increases, in the intestine the ion and fluid transport increases and is further elevated after SW entry. In SW, the intestine absorbs ions to create an inwardly directed water flow which is accomplished by increased Na⁺, K⁺-ATPase (NKA) activity in the basolateral membrane, driving ion absorption via ion channels and/or co-transporters. This review will aim at discussing the expression patterns of the ion transporting proteins involved in intestinal fluid absorption in the FW stage, during smoltification and after SW entry. Of equal importance for intestinal fluid absorption as the active absorption of ions is the permeability of the epithelium to ions and water. During the smoltification the increase in NKA activity and water uptake in SW is accompanied by decreased paracellular permeability suggesting a redirection of the fluid movement from a paracellular route in FW, to a transcellular route in SW. Increased transcellular fluid absorption could be achieved by incorporation of aquaporins (AQPs) into the enterocyte membranes and/or by a change in fatty acid profile of the enterocyte lipid bilayer. An increased incorporation of unsaturated fatty acids into the membrane phospholipids will increase water permeability by enhancing the fluidity of the membrane. A second aim of the present review is therefore to discuss the presence and regulation of expression of AQPs in the enterocyte membrane as well as to discuss the profile of fatty acids present in the membrane phospholipids during different stages of the salmonid lifecycle.

Intestinal anion exchange in marine teleosts is involved in osmoregulation and contributes to the oceanic inorganic carbon cycle

Marine teleost fish osmoregulation involves seawater ingestion and intestinal fluid absorption. Solute coupled fluid absorption by the marine teleost fish intestine has long been believed to be the product of Na(+) and Cl(-) absorption via the Na(+) :K(+) :2Cl(-) co-transporter (NKCC2). However, the past decade has revealed that intestinal anion exchange contributes significantly to Cl(-) absorption, in exchange for HCO(3) (-) secretion, and that this process is important for intestinal water absorption. In addition to contributing to solute coupled water absorption intestinal anion exchange results in luminal precipitation of CaCO(3) which acts to reduce luminal osmotic pressure and thus assist water absorption. Most recently, activity of apical H(+) -pumps, especially in distal segments of the intestine have been suggested to not only promote anion exchange, but also to reduce luminal osmotic pressure by preventing excess HCO(3)(-) concentrations from accumulating in intestinal fluids, thereby aiding water absorption. The present review summarizes and synthesizes the most recent advances in our view of marine teleosts osmoregulation, including our emerging understanding of epithelial transport of acid-base equivalents in the intestine, the consequences for whole organism acid-base balance and finally the impact of piscine CaCO(3) formation on the global oceanic carbon cycle.

A brief history of the study of fish osmoregulation: the central role of the Mt. Desert Island Biological Laboratory

The Mt. Desert Island Biological Laboratory (MDIBL) has played a central role in the study of fish osmoregulation for the past 80 years. In particular, scientists at the MDIBL have made significant discoveries in the basic pattern of fish osmoregulation, the function of aglomerular kidneys and proximal tubular secretion, the roles of NaCl cotransporters in intestinal uptake and gill and rectal gland secretion, the role of the shark rectal gland in osmoregulation, the mechanisms of salt secretion by the teleost fish gill epithelium, and the evolution of the ionic uptake mechanisms in fish gills. This short review presents the history of these discoveries and their relationships to the study of epithelial transport in general.

Application of the Na+ recirculation theory to ion coupled water transport in low- and high resistance osmoregulatory epithelia

The theory of Na+ recirculation for isosmotic fluid absorption follows logically from Hertz's convection-diffusion equation applied to the exit of water and solutes from the lateral intercellular space. Experimental evidence is discussed indicating Na+ recirculation based upon the following approaches: (i) An isotope tracer method in small intestine. Simultaneous measurement of water flow and ion transport in toad skin epithelium demonstrating, (ii) occasional hyposmotic absorbates, and (iii) reduced fluid absorption in the presence of serosal bumetanide. (iv) Studies of the metabolic cost of net Na+ absorption demonstrating an efficiency that is lower than the 18 Na+ per O2 consumed given by the stoichiometry of the Na+/K+-pump. Mathematical modeling predicts a significant range of observations such as isosmotic transport, hyposmotic transport, solvent drag, anomalous solvent drag, the residual hydraulic permeability in proximal tubule of AQP1(-/-) mice, the adverse relationship between hydraulic permeability and the concentration difference needed to reverse transepithelial water flow, and in a non-contradictory way the wide range of metabolic efficiencies from above to below 18 Na+/O2. Certain types of observations are poorly or not at all reproduced by the model. It is discussed that such lack of agreement between model and experiment is due to cellular regulations of ion permeabilities that are not incorporated in the modeling. Clarification of these problems requires further experimental studies.

Intestinal carbonic anhydrase, bicarbonate, and proton carriers play a role in the acclimation of rainbow trout to seawater

Abrupt transfer of rainbow trout from freshwater to 65% seawater caused transient disturbances in extracellular fluid ionic composition, but homeostasis was reestablished 48 h posttransfer. Intestinal fluid chemistry revealed early onset of drinking and slightly delayed intestinal water absorption that coincided with initiation of NaCl absorption and HCO(3)(-) secretion. Suggestive of involvement in osmoregulation, relative mRNA levels for vacuolar H(+)-ATPase (V-ATPase), Na(+)-K(+)-ATPase, Na(+)/H(+) exchanger 3 (NHE3), Na(+)-HCO(3)(-) cotransporter 1, and two carbonic anhydrase (CA) isoforms [a general cytosolic isoform trout cytoplasmic CA (tCAc) and an extracellular isoform trout membrane-bound CA type IV (tCAIV)], were increased transiently in the intestine following exposure to 65% seawater. Both tCAc and tCAIV proteins were localized to apical regions of the intestinal epithelium and exhibited elevated enzymatic activity after acclimation to 65% seawater. The V-ATPase was localized to both basolateral and apical regions and exhibited a 10-fold increase in enzymatic activity in fish acclimated to 65% seawater, suggesting a role in marine osmoregulation. The intestinal epithelium of rainbow trout acclimated to 65% seawater appears to be capable of both basolateral and apical H(+) extrusion, likely depending on osmoregulatory status and intestinal fluid chemistry.

The intestinal guanylin system and seawater adaptation in eels

Guanylin and uroguanylin are principal intestinal hormones secreted into the lumen to regulate ion and water absorption via a specific receptor, guanylyl cyclase-C (GC-C). As the intestine is an essential organ for seawater (SW) adaptation in teleost fishes, the intestinal guanylin system may play a critical role in SW adaptation. Molecular biological studies identified multiple guanylins (guanylin, uroguanylin and renoguanylin) and their receptors (GC-C1 and GC-C2) in eels. The relative potency of the three ligands on cGMP production in transiently expressed receptors was uroguanylin > guanylin >or= renoguanylin for CG-C1 and guanylin >or= renoguanylin > uroguanylin for GC-C2. Eel guanylin and GC-C genes are expressed exclusively in the intestine and kidney, and the level of expression is greater in SW eels than in freshwater (FW) eels except for renoguanylin. Physiological studies using Ussing chambers showed that the middle and posterior intestine are major sites of action of guanylins, where they act on the mucosal side to decrease short circuit current (I(sc)) in a dose-dependent manner. The ID(50) of guanylins for transport inhibition was 50-fold greater than that of atrial natriuretic peptide that acts from the serosal side as an endocrine hormone. However, only guanylins reversed I(sc) to levels below zero. Pharmacological analyses using various blockers showed that among transporters and channels localized on the intestinal cells of SW teleost fish, the cystic fibrosis transmembrane conductance regulator Cl(-) channel (CFTR) on the apical membrane is the major target of guanylins. Collectively, guanylins are synthesized locally in the intestine and secreted into the lumen to act on the GC-Cs in the apical membrane of eel intestinal cells. Then, intracellular cGMP production after ligand-receptor interaction activates CFTR and probably induces Cl(-) and/or HCO3- secretion into the lumen as suggested in mammals. The physiological significance of the anion secretion induced by the luminal guanylin/GC-C system on SW adaptation may rival or exceed that of the serosally derived natriuretic peptides in the euryhaline eel.

Gastrointestinal transport of Ca2+ and Mg2+ during the digestion of a single meal in the freshwater rainbow trout

A diet containing an inert marker (ballotini beads, quantified by X-radiography) was used to quantify the transport of two essential minerals, Ca(2+) and Mg(2+) from the diet during the digestion and absorption of a single meal of commercial trout food (3% ration). Initially, net uptake of Ca(2+) was observed in the stomach followed by subsequent Ca(2+) fluxes along the intestine which were variable, but for the most part secretory. This indicated a net secretion of Ca(2+) along the intestinal tract resulting in a net assimilation of dietary Ca(2+) of 28%. Similar handling of Ca(2+) and Mg(2+) was observed along the gastrointestinal tract (GI), although net assimilation differed substantially between the cations, with Mg(2+) assimilation being close to 60%, mostly a result of greater uptake by the stomach. The stomach displayed the highest net uptake rates for both cations (1.5 and 1.3 mmol kg(-1) fish body mass for Ca(2+) and Mg(2+), respectively), occurring within 2 h following ingestion of the meal. Substantial secretions of both Ca(2+) and Mg(2+) were observed in the anterior intestine, which were attributed to bile and other intestinal secretions, while fluxes in the mid and posterior intestine were small and variable. The overall patterns of Ca(2+) and Mg(2+) handling in the GI tract were similar to those observed for Na(+) and K(+) (but not Cl(-)) in a previous study. Overall, these results emphasize the importance of dietary electrolytes in ionoregulatory homeostasis.

Na+ Recirculation and Isosmotic Transport

The Na(+) recirculation theory for solute-coupled fluid absorption is an expansion of the local osmosis concept introduced by Curran and analyzed by Diamond & Bossert. Based on studies on small intestine the theory assumes that the observed recirculation of Na(+) serves regulation of the osmolarity of the absorbate. Mathematical modeling reproducing bioelectric and hydrosmotic properties of small intestine and proximal tubule, respectively, predicts a significant range of observations such as isosmotic transport, hyposmotic transport, solvent drag, anomalous solvent drag, the residual hydraulic permeability in proximal tubule of AQP1 (-/-) mice, and the inverse relationship between hydraulic permeability and the concentration difference needed to reverse transepithelial water flow. The model reproduces the volume responses of cells and lateral intercellular space (lis) following replacement of luminal NaCl by sucrose as well as the linear dependence of volume absorption on luminal NaCl concentration. Analysis of solvent drag on Na(+) in tight junctions provides explanation for the surprisingly high metabolic efficiency of Na(+) reabsorption. The model predicts and explains low metabolic efficiency in diluted external baths. Hyperosmolarity of lis is governed by the hydraulic permeability of the apical plasma membrane and tight junction with 6-7 mOsm in small intestine and < or = 1 mOsm in proximal tubule. Truly isosmotic transport demands a Na(+) recirculation of 50-70% in small intestine but might be barely measurable in proximal tubule. The model fails to reproduce a certain type of observations: The reduced volume absorption at transepithelial osmotic equilibrium in AQP1 knockout mice, and the stimulated water absorption by gallbladder in diluted external solutions. Thus, it indicates cellular regulation of apical Na(+) uptake, which is not included in the mathematical treatment.

Ion transport in the intestine of Gobius niger in both isotonic and hypotonic conditions

Ion transport in the intestine of Gobius niger, a euryhaline teleost, was studied in both isotonic and hypotonic conditions. Isolated tissues, mounted in Ussing chambers and bilaterally perfused with isotonic Ringer solution, developed a serosa negative transepithelial voltage and a short circuit current indicating a net negative current in absorptive direction. Bilateral removal of Cl- and Na+ from the bathing solutions as well as the luminal removal of K+in the presence of Ba2+(10(-3) M) almost abolished both Vt and Isc. Similar results were obtained by adding bumetanide (10(-5)M) to the luminal bath while other inhibitors of Cl- transport mechanisms were ineffective. These observations suggest that salt absorption begins with a coupled entry of Na+, Cl-, and K+ across the apical membrane; a Ba2+inhibitable K+ conductance, demonstrated also by micropuncture experiments, recycles the ion into the lumen. Salt entry into the cell is driven by the operation of the basolateral Na+/K(+)-ATPase since serosal ouabain (10(-4)M) completely abolished both Vt and Isc; this pump also completes the Na(+) absorption. The inhibitory effect of both serosal bumetanide (10(-4)M) and SITS (5 x 10(-4)M) suggests that Cl- would leave the cell via the KCl cotransport, the Cl/HCO3- antiport and/or conductive pathways. Bilateral exposure of tissues to hypotonic media produced a reduction of both the transepithelial voltage and the short circuit current probably due to the activation of homeostatic ionic fluxes involved in cell volume regulation. The results of experiments with both isolated enterocytes and intestine exposed to hypotonic solution suggested that the recovery of cell volume, after the initial cell swelling, involves a parallel opening of K+ and Cl- channels to facilitate net solute and water effluxes from the cell. J. Exp. Zool. 301A:49-62, 2004.

Water metabolism in the eel acclimated to sea water: from mouth to intestine

Eels seem to be a suitable model system for analysing regulatory mechanisms of drinking behavior in vertebrates, since most dipsogens and antidipsogens in mammals influence the drinking rate in the seawater eels similarly. The drinking behavior in fishes consists of swallowing alone, since they live in water and water is constantly held in the mouth for respiration. Therefore, contraction of the upper esophageal sphincter (UES) muscle limits the drinking rate in fishes. The UES of the eel was innervated by the glossopharyngeal-vagal motor complex (GVC) in the medulla oblongata (MO). The GVC neurons were immunoreactive to an antibody raised against choline acetyltransferase (ChAT), an acetylcholine (ACh) synthesizing enzyme, indicating that the eel UES muscle is controlled cholinergically by the GVC. The neuronal activity of the GVC was inhibited by adrenaline or dopamine, suggesting catecholaminergic innervation to the GVC. The AP and the commissural nucleus of Cajal (NCC) in the MO projected to the GVC and were immunoreactive to an antibody raised against tyrosine hydroxylase (TH), rate limiting enzyme to produce catecholamines from tyrosine. Therefore, it is likely that activation in the AP or the NCC may inhibit the GVC and thus relaxes the UES muscle, which allows for water to enter into the esophagus. During passing through the esophagus, the imbibed sea water (SW) was desalted to approximately 1/2 SW, which was further diluted in the stomach and arrived at the intestine as approximately 1/3 SW, almost isotonic to the plasma. Finally, from the diluted SW, the eel intestine absorbed water following the Na(+)-K(+)-2Cl(-) cotransport (NKCC2) system. The NaCl and water absorption across the intestine was regulated by various factors, especially by peptides such as atrial natriuretic peptide (ANP) and somatostatin (SS-25 II). During desalination in the esophagus, however, excess salt enters into the blood circulation, which is liable to raise the plasma osmolarity. However, the eel heart was constricted powerfully by the hyperosmolarity, suggesting that the hyperosmolarity enhances the stroke volume to the gill, where excess salt was extruded powerfully via Na(+)-K(+)-2Cl(-) cotransport (NKCC1) system.

Metabolic zonation in teleost gastrointestinal tract. Effects of fasting and cortisol in tilapia

Activities of several metabolic enzymes show distinct patterns of zonation along the intestinal tract of tilapia (Oreochromis niloticus), rainbow trout (Oncorhynchus mykiss) and copper rockfish (Sebastes caurinus). Zonation is species and enzyme specific, with different metabolic activities concentrated in specific areas, and few generalizations can be made. The rockfish show the smallest degree of zonation, with highest activities in the third quarter of the intestine, and shallow gradients to either side, and a general upswing in activity towards the distal end. In the trout, mitochondrial enzyme activities (citrate synthase, glutamate dehydrogenase, malate dehydrogenase) are highest in the pyloric caeca and decrease along the length of the small intestine. This pattern is accentuated for malic enzyme and glucose 6-phosphate dehydrogenase. These enzymes drop precipitously in activity after the first few sections of the small intestine, while other NADP-linked dehydrogenases (isocitrate dehydrogenase, and 6-phosphogluconate dehydrogenase) show moderate activity in pyloric caeca and peak toward the distal section of the small intestine. In tilapia, glutamate dehydrogenase shows a similar decrease as in trout, but citrate synthase peaks towards the distal sections. NADP-dependent dehydrogenases reveal distinct patterns, peaking in different sections of the intestine-malic enzyme in the proximal midsection, glucose 6-phosphate dehydrogenase in the distal mid-section, and isocitrate dehydrogenase in the anal section. Enzyme activities in the stomach of trout and tilapia also show zonation, with the midsection generally displaying the highest activities. A 5-day treatment of tilapia with an intraperitoneal cortisol deposit (25 mg kg(-1) wet mass) drastically alters metabolic performance along the gut in enzyme specific patterns, generally increasing enzyme activities in site-specific arrangements. Cortisol treatment also leads to the expected increases in activities of phosphoenolpyruvate carboxykinase, pyruvate kinase and aspartate aminotransferase in liver, but not in kidney. Aspartate aminotransferase is the only enzyme in brain significantly increased by cortisol treatment. Short-term food deprivation changes enzyme patterns, often resembling those observed after cortisol administration. We conclude that brain, liver and intestinal amino acid metabolism is an important target for cortisol action in fish and that metabolic zonation is a key factor to be reckoned with when analyzing physiological phenomena in the fish intestine.

Secretory activation of basolateral membrane Cl- channels in guinea pig distal colonic crypts

Cell-attached recordings revealed Cl(-) channel activity in basolateral membrane of guinea pig distal colonic crypts isolated from basement membrane. Outwardly rectified currents ((gp)Cl(or)) were apparent with a single-channel conductance (gamma) of 29 pS at resting membrane electrical potential; another outward rectifier with gamma of 24 pS was also observed ( approximately 25% of (gp)Cl(or)). At a holding potential of -80 mV gamma was 18 pS for both (gp)Cl(or) currents, and at +80 mV gamma was 67 and 40 pS, respectively. Identity as Cl(-) channels was confirmed in excised patches by changing bath ion composition. From reversal potentials, relative permeability of K(+) over Cl(-) (P(K)/P(Cl)) was 0.07 +/- 0.03, with relative permeability of Na(+) over Cl(-) (P(Na)/P(Cl)) = 0.08 +/- 0.04. A second type of Cl(-) channel was seen with linear current-voltage (I-V) relations ((gp)Cl(L)), having subtypes with gamma of 21, 13, and 8 pS. Epinephrine or forskolin increased the number of open (gp)Cl(or) and (gp)Cl(L). Open probabilities (P(o)) of (gp)Cl(or), (gp)Cl(L21), and (gp)Cl(L13) were voltage dependent in cell-attached patches, higher at more positive potentials. Kinetics of (gp)Cl(or) were more rapid with epinephrine activation than with forskolin activation. Epinephrine increased P(o) at the resting membrane potential for (gp)Cl(L13). Secretagogue activation of these Cl(-) channels may contribute to stimulation of electrogenic K(+) secretion across colonic epithelium by increasing basolateral membrane Cl(-) conductance that permits Cl(-) exit after uptake via Na(+)-K(+)-2Cl(-) cotransport.

Active sulfate secretion by the intestine of winter flounder is through exchange for luminal chloride

SO4(2-) transport by winter flounder intestine in Ussing chambers was characterized. With 50 mM SO4(2-) (physiological level) bathing the lumen, net absorption (lumen to blood) dominated. Under short-circuited conditions, 1 mM SO4(2-) on both sides, net active SO4(2-) secretion occurred (8.55 +/- 0.96 nmol. cm(-2). h(-1)). NaCN (10 mM), ouabain (10(-4) M), and luminal DIDS (0.2 mM) inhibited net secretion. Removal of luminal Cl- and HCO3- together (Cl--HCO3-) or Cl- alone blocked net secretion, whereas removal of luminal HCO3- alone increased net secretion. SO4(2-) uptake into foregut brush-border membrane vesicles was stimulated by a trans-Cl- gradient (in > out) and unaffected by a trans-HCO3- gradient (in > out). Short-circuiting with K+ (in = out) and valinomycin had no effect on Cl--stimulated SO4(2-) uptake, suggesting electroneutral exchange. Satiety (i.e., full stomach) stimulated the unidirectional absorptive flux, eliminating net secretion. It was concluded that the intestine is a site of SO4(2-) absorption in marine teleosts and that active SO4(2-) secretion is in exchange for luminal Cl-.

General models for water transport across leaky epithelia

The group of leaky epithelia, such as proximal tubule and small intestine, have several common properties in regard to salt and water transport. The fluid transport is isotonic, the transport rate increases in dilute solutions, and water can be transported uphill. Yet, it is difficult to find common features that could form the basis for a general transport model. The direction of transepithelial water transport does not correlate with the direction of the primary active Na+ transport, or with the ultrastucture as defined by the location of apical and basolateral membranes, of the junctional complex and the lateral intercellular spaces. The presence of specific water channels, aquaporins, increases the water permeability of the epithelial cell membranes, i.e., the kidney proximal tubule. Yet other leaky epithelia, for example, the retinal pigment epithelium, have no known aquaporins. There is, however, a general correlation between the direction of transepithelial transport and the direction of transport via cotransporters of the symport type. A simple epithelial model based on water permeabilities, a hyperosmolar compartment and restricted salt diffusion, is unable to explain epithelial transport phenomena, in particular the ability for uphill water transport. The inclusion of cotransporters as molecular water pumps in these models alleviates this problem.

Analysis of the sodium recirculation theory of solute-coupled water transport in small intestine

Our previous mathematical model of solute-coupled water transport through the intestinal epithelium is extended for dealing with electrolytes rather than electroneutral solutes. A 3Na+-2K+ pump in the lateral membranes provides the energy-requiring step for driving transjunctional and translateral flows of water across the epithelium with recirculation of the diffusible ions maintained by a 1Na+-1K+-2Cl- cotransporter in the plasma membrane facing the serosal compartment. With intracellular non-diffusible anions and compliant plasma membranes, the model describes the dependence on membrane permeabilities and pump constants of fluxes of water and electrolytes, volumes and ion concentrations of cell and lateral intercellular space (lis), and membrane potentials and conductances. Simulating physiological bioelectrical features together with cellular and paracellular fluxes of the sodium ion, computations predict that the concentration differences between lis and bathing solutions are small for all three ions. Nevertheless, the diffusion fluxes of the ions out of lis significantly exceed their mass transports. It is concluded that isotonic transport requires recirculation of all three ions. The computed sodium recirculation flux that is required for isotonic transport corresponds to that estimated in experiments on toad small intestine. This result is shown to be robust and independent of whether the apical entrance mechanism for the sodium ion is a channel, a SGLT1 transporter driving inward uphill water flux, or an electroneutral Na+-K+-2Cl- cotransporter.

Molecular water pumps

There is good evidence that cotransporters of the symport type behave as molecular water pumps, in which a water flux is coupled to the substrate fluxes. The free energy stored in the substrate gradients is utilized, by a mechanism within the protein, for the transport of water. Accordingly, the water flux is secondary active and can proceed uphill against the water chemical potential difference. The effect has been recognized in all symports studied so far (Table 1). It has been studied in details for the K+/Cl- cotransporter in the choroid plexus epithelium, the H+/lactate cotransporter in the retinal pigment epithelium, the intestinal Na+/glucose cotransporter (SGLT1) and the renal Na+/dicarboxylate cotransporter both expressed in Xenopus oocytes. The generality of the phenomenon among symports with widely different primary structures suggests that the property of molecular water pumps derives from a pattern of conformational changes common for this type of membrane proteins. Most of the data on molecular water pumps are derived from fluxes initiated by rapid changes in the composition of the external solution. There was no experimental evidence for unstirred layers in such experiments, in accordance with theoretical evaluations. Even the experimental introduction of unstirred layers did not lead to any measurable water fluxes. The majority of the experimental data supports a molecular model where water is cotransported: A well defined number of water molecules act as a substrate on equal footing with the non-aqueous substrates. The ratio of any two of the fluxes is constant, given by the properties of the protein, and is independent of the driving forces or other external parameters. The detailed mechanism behind the molecular water pumps is as yet unknown. It is, however, possible to combine well established phenomena for enzymes into a working model. For example, uptake and release of water is associated with conformational changes during enzymatic action; a specific sequence of allosteric conformations in a membrane bound enzyme would give rise to vectorial transport of water across the membrane. In addition to their recognized functions, cotransporters have the additional property of water channels. Compared to aquaporins, the unitary water permeability is about two orders of magnitude lower. It is suggested that the water permeability is determined from chemical associations between the water molecule and sites within the pore, probably in the form of hydrogen-bonds. The existence of a passive water permeability suggests an alternative model for the molecular water pump: The water flux couples to the flux of non-aqueous substrates in a hyperosmolar compartment within the protein. Molecular water pumps allow cellular water homeostasis to be viewed as a balance between pumps and leaks. This enables cells to maintain their intracellular osmolarity despite external variations. Molecular water pumps could be relevant for a wide range of physiological functions, from volume regulation in contractile vacuoles in amoeba to phloem transport in plants (Zeuthen 1992, 1996). They could be important building blocks in a general model for vectorial water transport across epithelia. A simplified model of a leaky epithelium incorporating K+/Cl-/H2O and Na+/glucose/H2O cotransport in combination with channels and primary active transport gives good quantitative predictions of several properties. In particular of how epithelial cell layers can transport water uphill.

A rapid method for the evaluation of the ionic permeabilities across epithelial cell membranes

This short note presents a recipe for the calculation of the ionic permeabilities across epithelial cell membranes. The method requires the Goldman-Hodgkin-Katz formalism as well as the consideration of the equivalent electrical circuit for an epithelial cell. The equivalent electrical circuit is solved in terms of the equivalent electromotive forces coupled in series with the ionic resistances of both cell membranes (apical and basolateral). The present procedure is feasible for any leaky epithelial cell membrane with the condition that this membrane (apical or basolateral) does not contain primary or secondary mechanisms for active transport.

Characteristics of ionic transport processes in fish intestinal epithelial cells

A general mathematical version of the cell model of a leaky epithelium for the NaCl absorption is presented, analysed and integrated numerically. The model consists in the adequate differential equations that describe the rate of change of the intracellular ion concentrations and are expressed in strict accordance with the law of mass conservation. The model includes many state variables representing ion concentrations, the cell volume, and membrane potentials. Ion movements are described by the Michaelis-Menten kinetics or by the constant field flux equation (Goldman-Hodgkin-Katz). In this paper, we model the intracellular ion concentrations, change in the cell volume, the transmembrane flux and membrane potentials of intestinal epithelium of both fresh water and sea water fish, and generate several simulations (in both the steady state and the transient state analysis) that appear to accord with prior experimental data in this area. For the ion movements of the sea water fish intestine, there were included a Na+/K+ pump, a K(+)-Cl- symport system, the K+ and Cl- channels in the basolateral membrane, whereas a Na(+)-K(+)-2Cl- cotransporter for NaCl absorption and K+ channels are located in the apical membrane. In the fresh water fish intestinal cells, the NaCl absorption is performed by two coupled antiporters Na+/H+ and Cl-/HCO3- presumably responsible for the intracellular pH regulation. In this type of cells, Na+ and K+ channels are located within the apical membrane, whereas Cl- channels are located within the basolateral membrane. The osmotically induced water transport across the apical and basolateral membranes has been taken into account as well. The simulations plot the steady state values for membrane potential difference, short-circuit current and intracellular ionic concentrations using the magnitude of the transmembrane flux through the Na+/K+ pump and Na(+)-K(+)-2Cl- cotransporter, or the basolateral Cl- permeability as dependent variables. The model behaves appropriately with regard to several experimental studies regarding the hyperpolarization (sea water fish intestine) and depolarization (fresh water fish intestine) of the apical membrane potential and inhibition of the short-circuit flux with reduced NaCl absorption. The model is also used to make several analytical predictions regarding the response of the membrane potential and ionic concentrations to variations in the basolateral Cl- flux. Furthermore, maintaining conservation of both mass and electroneutrality and taking into account the osmolar forces is an important advantage, because it allows a rigorous analysis of the relationship between membrane potential difference, volume and flux. The model can be used in the analysis and planning of the experiments and is capable of predicting the instantaneous values of ionic fluxes and intracellular concentrations and of cell volume.

Active potassium transport across guinea-pig distal colon: action of secretagogues

1. Adrenaline (5 microM) stimulated a K+ secretory current by 2.2 mu equiv h-1 cm-2 in isolated guinea-pig distal colonic epithelium. This secretory activity was inhibited entirely by addition of the loop diuretic bumetanide to the serosal solution. On-going K+ uptake via the absorptive pathway was unaltered by these changes. 2. Prostaglandin E2 (PGE2, 2 microM) stimulated electrogenic K+ secretion and Cl- secretion by 3.0 and 3.6 mu equiv h-1 cm-2, respectively. Serosal addition of bumetanide completely inhibited this K+ secretion but blocked only approximately 70% of Cl- secretion. The bumetanide-insensitive Cl- secretory current was dependent on the presence of Cl- and HCO3- in the bathing solutions. 3. Stimulation of electrogenic K+ secretion by PGE2 occurred with a half-maximal concentration of 4 nM, an affinity approximately 300 times higher than that for stimulation of Cl- secretion by PGE2. 4. Forskolin (10 microM) stimulated Cl- secretion by 4.9 mu equiv h-1 cm-2. The apparent K+ secretory rate was increased by only 1.5 mu equiv h-1 cm-2. A bumetanide-insensitive short-circuit current (ISC) was apparent and of the same size as that stimulated by PGE2. 5. Addition of the Ca2+ ionophore A23187 (10 microM), in the presence of indomethacin (1 microM) to reduce prostaglandin production, inhibited the K+ absorptive pathway by 40% and concurrently stimulated a small rate of electrogenic K+ secretion. 6. Active K+ absorption was inhibited by the addition of ouabain, omeprazole or SCH28080 to the mucosal solution. Both omeprazole and SCH28080 also stimulated a small negative ISC, consistent with electrogenic K+ secretion. 7. Association of K+ absorption, K+ secretion and Cl- secretion is indicated by similarities in transport mechanism and by secretagogue regulation. In particular, maximal rates of K+ secretory current require uptake via apical membrane K+ pumps. Such interrelations support a common cellular locus for these ion transport pathways.

Requirement of HCO3- for Cl(-)-absorption in seawater-adapted eel intestine

The role of HCO3-/CO2 buffer in Cl- absorption was examined in the in vitro perfused eel intestine adapted to seawater. Cl- absorption, expressed as short-circuit current (Isc), was measured in either 20 mM HCO3-/1% CO2 Ringer or HEPES Ringer, pH 8.0. Unilateral (mucosal or serosal) substitution of HCO3-/CO2 with HEPES/O2 was without effect on Isc and transepithelial voltage (Vt), whereas bilateral removal of HCO3-/CO2 reduced Isc and Vt by 50%, indicating that the presence of HCO3-/CO2 buffer at one side of the epithelium is sufficient to keep Cl- absorption at the maximum rate. We examined in further detail the individual components of the HCO3-/CO2 system that stimulates Cl- absorption. We found that, in tissues bathed with HEPES Ringer, addition of 1% CO2 to the luminal or serosal solution (final pH = 7.6 in the chamber) had no effect on Isc and Vt, while both electrical parameters could be restored to control values by unilateral (luminal or serosal) substitution of HEPES Ringer with 20 mM HCO3-/1% CO2 Ringer or 20 mM HCO3- alone. Stimulation of Isc induced by unilateral (luminal or serosal) HCO3-/CO2 was inhibited by luminal or serosal 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid (SITS) (0.25 mM) or by serosal Na+ removal, whereas amiloride (1 mM), luminal or serosal, had no effect. Acetazolamide (0.1 mM, both sides) inhibited stimulation of Isc induced by luminal addition of HCO3-/CO2, whereas it was without effect when HCO3-/CO2 was added serosally or bilaterally.(ABSTRACT TRUNCATED AT 250 WORDS)

Cyclic GMP regulation of a voltage-activated K channel in dissociated enterocytes

Enterocytes from the intestinal epithelium of the winter flounder were isolated by collagenase digestion and incubated in flounder Ringer solution. Conventional whole-cell and amphotericin-perforated whole-cell recording techniques were used to characterize the properties of a voltage-activated K current present in dissociated cells. Resting membrane potentials and series resistances were significantly lower (from -23 to -39 mV and 29 to 13 M omega, respectively) when amphotericin was used to achieve the whole-cell configuration. When cells were placed in flounder Ringer solution, held at -80 mV and subsequently stepped to a series of depolarizing voltages (from -70 to 0 mV), an outward current was observed that exhibited inactivation at voltages above -20 mV. This current was sensitive to holding potential and was not activated when the cells were held at -40 mV or above. When cells were bathed in symmetric K Ringer solution and the same voltage protocol was applied to the cell, inward currents were observed in response to the negative intracellular potentials. Reversal potentials at two different extracellular K concentrations were consistent with K as the current-carrying ion. BaCl2 (2 mM) and CsCl (0.5 mM) both produced voltage-dependent blockade of the current when added to the bathing solution. Charybdotoxin (300 nM extracellular concentration) completely blocked the current. The IC50 for charybdotoxin was 50 nM. Cyclic GMP inhibited the voltage-activated current in flounder Ringer and in symmetric K Ringer solution. The cyclic GMP analog, 8-Br cGMP, lowered the threshold for voltage activation and potentiated inactivation of the current at voltages above -40 mV.(ABSTRACT TRUNCATED AT 250 WORDS)

Cl- secretion by cultured shark rectal gland cells. II. Effects of forskolin on cellular electrophysiology

Employing microelectrode techniques we have assessed the cellular electrophysiological properties of shark rectal gland (SRG) cells in primary culture. In the absence of secretagogues a 10-fold reduction in the Cl- concentration of the apical superfusate shark Ringer solution had little effect on either apical membrane electrical potential difference (Va) or fractional resistance (fRa), indicating little, if any, apical membrane Cl- conductance. Superfusing the basolateral surface with high-K+ shark Ringer solution (K+ increased 10-fold) depolarized the basolateral membrane electrical potential difference (Vb) by 43 mV, indicating that this barrier is largely K+ conductive. In addition, basolateral Ba2+ (5 mM) depolarized Vb by 12 mV and reduced fRa from 0.92 to 0.58, results consistent with a K(+)-conductive basolateral membrane in unstimulated SRG cells. Basolateral forskolin (10(-6) M) depolarized Va by 25 mV and caused a dramatic reduction in fRa from 0.97 to approximately 0.10. Under these conditions, a 10-fold decrease in apical superfusate Cl- concentration depolarized Va by 37 mV, revealing an adenosine 3',5'-cyclic monophosphate-induced apical membrane Cl- conductance. The time course of the forskolin-induced changes in Va and Vb suggests that the basolateral membrane K+ conductance increased and maintained the driving force for apical Cl- exit, as in other Cl(-)-secreting epithelia. These electrophysiological properties compare favorably with those of the perfused SRG tubule and indicate that SRG primary cultures are a suitable model for Cl(-)-secreting epithelia.

Apical and basal membrane ion transport mechanisms in bovine retinal pigment epithelium

1. Intracellular voltage recordings using conventional and double-barrelled chloride-selective microelectrodes have been used to identify several transport mechanisms at the apical and basolateral membranes of the isolated bovine retinal pigment epithelium (RPE)-choroid preparation. Intracellular recordings were obtained from two cell populations, melanotic (pigmented) and amelanotic (non-pigmented). The electrical properties of these two populations are practically identical. For melanotic cells the average apical resting membrane potential (VA) is -61 +/- 2 mV (mean +/- S.E.M., n = 49 cells, thirty-three eyes). For these cells the ratio of apical to basolateral membrane resistance (a) was 0.22 +/- 0.02. The mean transepithelial voltage and resistance were 6 +/- 1 mV and 138 +/- 7 omega cm2, respectively. 2. The apical membrane, which faces the distal retina, contains a Ba(2+)-inhibitable K+ conductance and a ouabain-inhibitable, electrogenic Na(+)-K+ pump. In addition it contains a bumetanide-sensitive mechanism, the putative Na(+)-K(+)-Cl- cotransporter. The basolateral membrane contains a DIDS (4,4'-diisothiocyanostilbene-2,2'-disulphonic acid)-inhibitable chloride channel. The relative conductances of the apical and basolateral membranes to K+ and Cl- are TK approximately 0.9 and TCl approximately 0.7, respectively. 3. The ouabain-induced fast phase of apical membrane depolarization (0-30 s) was used to calculate the equivalent resistances of the apical (RA) and basolateral (RB) cell membranes, as well as the paracellular or shunt resistance (RS). They are: 3190 +/- 400, 17920 +/- 2730 and 2550 +/- 200 omega (mean +/- S.E.M., n = 9 tissues), respectively. From these data the equivalent electromotive forces (EMF) at the apical (EA) and basolateral (EB) membranes were also calculated. They are: -69 +/- 5.0 and -24 +/- 5.0 mV, respectively. 4. Intracellular Cl- activity (aiCl) was measured using double-barreled ion-selective microelectrodes. In the steady state aiCl = 61 +/- 4.0 mM and the Nernst potential ECl = -13.5 +/- 1.5 mV (mean +/- S.E.M., n = 4). 5. In the intact eye or in retina, RPE-choroid preparations it has been shown that the transition between light and dark alters the K+ concentration in the extracellular (or subretinal) space between the photoreceptors and the apical membrane of the RPE. These light-induced changes in subretinal [K+]o were qualitatively simulated in vitro by altering apical K+ between 5 and 2 mM. This produced a sequence of voltage changes at the apical and basolateral membranes that had three operationally distinct phases. Phase 1 is generated by the combination of an apical membrane K+ diffusion potential and inhibition of the electrogenic Na(+)-K+ pump.(ABSTRACT TRUNCATED AT 400 WORDS)

Intracellular potassium activity and the role of potassium in transepithelial salt transport in the human reabsorptive sweat duct

We have measured the intracellular potassium activity, [K+]i and the mechanisms of transcellular K+ transport in reabsorptive sweat duct (RSD) using intracellular ion-sensitive microelectrodes (ISMEs). The mean value of [K+]i in RSD is 79.8 +/- 4.1 mM (n = 39). Under conditions of microperfusion, the [K+]i is above equilibrium across both the basolateral membrane, BLM (5.5 times) and the apical membrane, APM (7.8 times). The Na+/K+ pump inhibitor ouabain reduced [K+]i is insensitive to the Na+/K+/2 Cl- cotransport inhibitor bumetanide in the bath. Cl- substitution in the lumen had no effect on [K+]i. In contrast, Cl- substitution in the bath (basolateral side) depolarized BLM from -26.0 +/- 2.6 mV to -4.7* +/- 2.4 mV (n = 3; *indicates significant difference) and decreased [K+]i from 76.0 +/- 15.2 mM to 57.7* +/- 12.7 mM (n = 3). Removal of K+ in the bath decreased [K+]i from 76.3 +/- 15.0 mM to 32.3 +/- 7.6 mM (n = 4) while depolarizing the BLM from -32.5 +/- 4.1 mV to -28.3* +/- 3.0 mV (n = 4). Raising the [K+] in the bath by 10-fold increased [K+]i from 81.7 +/- 9.0 mM to 95.0* +/- 13.5 mM and depolarized the BLM from -25.7 +/- 2.4 mV to -21.3* +/- 2.9 mV (n = 4). The K+ conductance inhibitor, Ba2+, in the bath also increased [K+]i from 85.8 +/- 6.7 mM to 107.0* +/- 11.5 mM (n = 4) and depolarized BLM from -25.8 +/- 2.2 mV to -17.0* +/- 3.1 mV (n = 4). Amiloride at 10(-6) M increased [K+]i from 77.5 +/- 18.8 mM to 98.8* +/- 21.6 mM (n = 4) and hyperpolarized both the BLM (from -27.5 +/- 1.4 mV to -46.0* +/- 3.5 mV, n = 4). However, amiloride at 10(-4) M decreased [K+]i from 64.5 +/- 0.9 mM to 36.0* +/- 9.9 mM and hyperpolarized both the BLM (from -24.7 +/- 1.4 mV to -43.5* +/- 4.2 mV) and APM (from -18.3 +/- 0.9 mV to -43.5* +/- 4.2 mV, n = 6). In contrast to the observations at the BLM, substitution of K+ or application of Ba2+ in the lumen had no effect on the [K+]i or the electrical properties of RSD, indicating the absence of a K+ conductance in the APM.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanism of active K+ secretion by flounder urinary bladder

We investigated the mechanism of active K+ transport by the urinary bladder of the winter flounder by measuring transepithelial properties in Ussing Chambers and by determining the cellular electrical potential profile using conventional microelectrodes. In the absence of transmural electrochemical potential gradients isolated bladders can exhibit a serosa-to-mucosa short circuit which is due entirely to net K+ secretion. The properties of transcellular K+ movement can be adequately described by a model which provides for active K+ uptake across the basolateral membrane via an electrogenic Na/K ATPase and K+ exit from the cell across the apical membrane down an electrochemical potential gradient via K+ channels which are blocked by mucosal barium. The conductance of the apical membranes of the transporting cells appears to be due almost solely to K+ while that of the basolateral membrane may be due largely to C1-.

Basolateral impalement of intestinal villus cells: electrophysiology of Cl- transport

A method of dissecting the serosal muscle layers is described that transforms the villus of isolated Amphiuma small intestine into a flat sheet one cell layer thick, allowing rapid equilibration of the serosal medium with the basolateral membrane of the villus tip cells and direct access of the basal membrane to microelectrodes. The "villus sheet" preparation was used to examine the luminal and basolateral mechanisms of Cl- transport. The serosal membrane potential (Vs), measured with conventional microelectrodes, averaged -79.7 mV in tissues bathed in Cl- -free medium; the mucosal membrane potential (Vm) averaged -80.9 mV. Fractional resistance measured directly was 0.82 and 0.14 for the mucosal and serosal membranes, respectively. Elevation of bath [K] reduced Vm and Vs by 30.3 and 44.5 mV, respectively. Cl- (20 mM) added to the luminal medium reduced Vm by 23.9 mV and stimulated Cl- transport; luminal addition of furosemide then increased Vm by 5.6 mV and reduced Cl- transport. Addition of Cl- (20 mM) to the Cl- -free serosal fluid increased Vs 2.0 +/- 1.9 mV. On reducing the serosal [Cl] 10-fold Vs decreased 2.0 +/- 2.2 mV. These and other results indicate that basolateral Cl- exit is not over a conductive pathway. The villus sheet affords new opportunities for studying enterocyte function in the intact mucosa.

Cyclic nucleotide-dependent protein kinase inhibition by H-8: effects on ion transport

We explored the potential role of cyclic nucleotide-dependent protein phosphorylation in regulating ion transport across flounder intestinal mucosa by studying the effects of N-[2(methylamino)-ethyl]-s-isoquinolinesulfonamide (H-8), a selective inhibitor of cyclic nucleotide-dependent protein kinase in vitro. Addition of H-8 reversed the inhibitory effects of 8-bromoguanosine 3',5'-cyclic-monophosphate (8-BrcGMP), 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP), atriopeptin III (AP III), and vasoactive intestinal peptide (VIP) on the short-circuit current (Isc) and transepithelial potential difference (PD). Flux measurements established that these changes in Isc and PD directly reflected changes in Na and Cl absorption by the intestine. H-8 was unable, however, to reverse the inhibitory effects on Isc and PD of the Ca ionophore ionomycin and of substance P at dosages exceeding those needed to reverse the effects of AP III, VIP, and the cyclic nucleotides. We conclude that 1) H-8 (100 microM or less) does not exert toxic effects, 2) exogenously added cyclic nucleotide analogues inhibit ion transport through activation of cyclic nucleotide-dependent kinases resulting in protein phosphorylation, 3) activation of these kinases is an essential intermediate step in the inhibitory action of AP III and VIP on ion transport, and 4) the Ca ionophore ionomycin and substance P appear to inhibit ion transport by a mechanism that is independent of cyclic nucleotide-dependent protein phosphorylation.

cAMP stimulates the Na+-K+ pump in frog retinal pigment epithelium

Adenosine 3', 5'-cyclic monophosphate (cAMP) induced increases in active Na+ secretion and K+ absorption that were blocked by apical ouabain (10(-4) M), suggesting stimulation of the Na+-K+ pump. cAMP also produced rapid membrane voltage and resistance changes that could be divided chronologically into three phases. In phase 1, the basolateral membrane depolarized at a faster rate than the apical membrane, probably as a result of an increase in basolateral membrane conductance. In phase 2, the apical membrane repolarized toward control faster than the basal membrane, whereas in phase 3 the basolateral membrane repolarized faster than the apical membrane. Apical ouabain completely inhibited the cAMP-induced repolarization of the apical membrane during phase 2. Thus the stimulation of the Na+-K+ pump occurs within minutes of cAMP elevation. Na+ removal from the basal side did not block the cAMP-induced voltage changes, indicating that the initial conductance increase is not due to Na+. In contrast, Na+ removal from the apical bath inhibited all phases of the cAMP response. This suggests that apical membrane Na+-dependent transport mechanisms mediate the stimulation of the Na+-K+ pump. cAMP also caused a significant drop in intracellular K+ activity (approximately 5 mM) that preceded phase 2. This drop could stimulate the Na+-K+ pump, as suggested by previous experiments.

Characteristics and functions of Na-K-Cl cotransport in epithelial tissues

This review summarizes our present understanding of Na-K-Cl cotransport and its physiological role in absorption and secretion of electrolytes and water in epithelial tissues. In the past several years an extensive literature about this cotransporter has developed due to its widespread distribution in a variety of cell types and its essential role in fluid and electrolyte transport in several epithelial tissues. We summarize this literature and speculate on the future characterization of this transport system. Although this review focuses on cotransport as it relates to absorptive and secretory processes in epithelia, important information concerning the pharmacology, stoichiometry, and regulation of Na-K-Cl cotransport in nonepithelial systems (i.e., erythrocytes, fibroblasts, squid axon, etc.) has been included to supplement areas that are less well established in the epithelial literature.

Active K transport across rabbit distal colon: relation to Na absorption and Cl secretion

We measured isotopic unidirectional fluxes of K to elucidate the mechanisms of active K transport across the distal colon of the rabbit. Separate pathways for active K absorption and active K secretion were detected using various transport inhibitors and stimulators. The rate and direction of net K transport depend on the activities of these two pathways. K absorption was reduced by orthovanadate (both solutions) or serosal Ba, consistent with ATPase-dependent uptake of K across the apical membrane and exit via a Ba-sensitive basolateral K conductance. K secretion was inhibited by serosal ouabain or mucosal Ba, indicating that K secretion involves basolateral uptake via the Na-K pump and apical exit via a Ba-sensitive K conductance. Active K secretion appears to be electrogenic, since inhibition by ouabain produced equivalent changes in the net K flux and short-circuit current. Addition of bumetanide to the serosal solution or the removal of either Na or Cl from the serosal solution inhibited K secretion; mucosal solution amiloride was without effect. These results indicate that this K secretory process is independent of electrogenic Na absorption but is mechanistically similar to Cl secretory processes. Both epinephrine and prostaglandin E2 (PGE2) stimulate K secretion, but only PGE2 also stimulates Cl secretion. The response to these secretogogues suggests that the mechanisms underlying K and Cl secretion are closely linked but can be regulated independently.

Ionic conductance pathways in the mouse medullary thick ascending limb of Henle. The paracellular pathway and electrogenic Cl- absorption

Net Cl- absorption in the mouse medullary thick ascending limb of Henle (mTALH) involves a furosemide-sensitive Na+:K+:2 Cl- apical membrane symport mechanism for salt entry into cells, which occurs in parallel with a Ba++-sensitive apical K+ conductance. The present studies, using the in vitro microperfused mouse mTALH, assessed the concentration dependence of blockade of this apical membrane K+-conductive pathway by Ba++ to provide estimates of the magnitudes of the transcellular (Gc) and paracellular (Gs) electrical conductances (millisiemens per square centimeter). These studies also evaluated the effects of luminal hypertonicity produced by urea on the paracellular electrical conductance, the electrical Na+/Cl- permselectivity ratio, and the morphology of in vitro mTALH segments exposed to peritubular antidiuretic hormone (ADH). Increasing luminal Ba++ concentrations, in the absence of luminal K+, produced a progressive reduction in the transcellular conductance that was maximal at 20 mM Ba++. The Ba++-sensitive transcellular conductance in the presence of ADH was 61.8 +/- 1.7 mS/cm2, or approximately 65% of the total transepithelial conductance. In phenomenological terms, the luminal Ba++-dependent blockade of the transcellular conductance exhibited negative cooperativity. The transepithelial osmotic gradient produced by luminal urea produced blebs on apical surfaces, a striking increase in shunt conductance, and a decrease in the shunt Na+/Cl- permselectivity (PNa/PCl), which approached that of free solution. The transepithelial conductance obtained with luminal 800 mM urea, 20 mM Ba++, and 0 K+ was 950 +/- 150 mS/cm2 and provided an estimate of the maximal diffusion resistance of intercellular spaces, exclusive of junctional complexes. The calculated range for junctional dilution voltages owing to interspace salt accumulation during ADH-dependent net NaCl absorption was 0.7-1.1 mV. Since the Ve accompanying ADH-dependent net NaCl absorption is 10 mV, lumen positive, virtually all of the spontaneous transepithelial voltage in the mouse mTALH is due to transcellular transport processes. Finally, we developed a series of expressions in which the ratio of net Cl- absorption to paracellular Na+ absorption could be expressed in terms of a series of electrical variables. Specifically, an analysis of paired measurement of PNa/PCl and Gs was in agreement with an electroneutral Na+:K+:2 Cl- apical entry step. Thus, for net NaCl absorption, approximately 50% of Na+ was absorbed via a paracellular route.

Stoichiometry and ion affinities of the Na-K-Cl cotransport system in the intestine of the winter flounder (Pseudopleuronectes americanus)

Na-K-Cl cotransport stoichiometry and affinities for Na, K and Cl were determined in flounder intestine. Measurement of simultaneous NaCl and RbCl influxes resulted in ratios of 2.2 for Cl/Na and 1.8 for Cl/Rb. The effect of Na and Rb on Rb influx showed first order kinetics with K1/2 values of 5 and 4.5 mM and Hill coefficients of 0.9 and 1.2, respectively. The effect of Cl on rubidium influx showed a sigmoidal relationship with K1/2 of 20 mM and a Hill coefficient of 2.0. The effects of variations in Na and Cl concentration on short-circuit current (Isc) were also determined. The K1/2 for Na was 7 mM with a Hill coefficient of 0.9 and the K1/2 for Cl was 46 mM with a Hill coefficient of 1.9. Based on the simultaneous influx measurements, a cotransport stoichiometry of 1Na:1K:2Cl is concluded. The Hill coefficients for Cl suggest a high degree of cooperativity between Cl binding sites. Measurements of the ratio of net Na and Cl transepithelial fluxes under short-circuit conditions (using a low Na Ringer solution to minimize the passive Na flux) indicate that the Cl/Na flux ratio is approximately 2:1. Therefore, Na recycling from serosa to mucosa does not significantly contribute to the Isc. Addition of serosal ouabain (100 microM) inhibited Rb influx, indicating that Na-K-Cl cotransport is inhibited by ouabain. This finding suggests that a feedback mechanism exists between the Na-K-ATPase on the basolateral membrane and the apical Na-K-2Cl cotransporter.