Net Cl Transport Research Articles

ION TRANSPORT IN THE INTESTINE OF ANGUILLA ANGUILLA: GRADIENTS AND TRANSLOCATORS

The transport of Na+, K+ and Cl- across the isolated epithelium of the eel intestine was studied using a combination of four experimental strategies: short-circuiting, measurements of intracellular potentials and ion concentrations, application of a variety of transport inhibitors and measurement of unidirectional fluxes with radioactive tracers. When short-circuited, the system performs a net transport of Cl- and Na+ towards the blood side, with a stoichiometry approaching 2, and a much smaller net transport of K+ towards the lumen. The system is totally driven by the sodium pump located in the basolateral barrier and the main coupling between the fluxes of the three ions is through the operation of a furosemide-sensitive transporter in the apical barrier, probably a 2Cl-/Na+/K+ symporter. The inhibitory effect of DIDS and picrylsulphonic acid on the short-circuit current, when added to the serosal side, suggests the presence of a sodium-dependent anionic shuttle located in the basolateral membrane. The short-circuit current is inhibited by H25, a non-specific inhibitor of the K+/Cl- symport, added to the serosal side. This effect occurs after a delay of at least 5 min and may result from the diffusion of the drug to the apical barrier, where it blocks the 2Cl-/Na+/K+ symport with much higher affinity.

Regional adrenergic regulation of ion transport across the isolated porcine gallbladder

The regional transport properties of the porcine gallbladder epithelium were studied using Ussing chambers in which tissues were bathed in porcine Ringer solution. Under basal conditions, tissues from the neck absorbed Na and Cl. Fundic tissues also absorbed Na, but net Cl transport was not different from zero. Serosal norepinephrine (NE; 0.1 microM) stimulated Na and Cl absorption in the neck but only Cl absorption in the fundus. The effects of NE on Na and Cl transport were blocked by pretreatment with yohimbine, suggesting that alpha 2-adrenoceptors mediate the transport-related actions of this neurotransmitter. Serosal isoproterenol (0.1 microM) produced a tetrodotoxin-insensitive, propranolol-sensitive increase in the short-circuit current (Isc) in fundic tissues but not in tissues from the neck. The beta 2-adrenergic agonist salbutamol produced a response similar to that of isoproterenol. However, the beta 1-adrenergic agonist dobutamine had no effect. Isoproterenol was 5.8-fold more potent than salbutamol in increasing the Isc. A possible explanation for the regional effects of beta-agonists is that beta-adrenoceptors are localized to epithelial cells in the fundic region. These results and results obtained previously suggest regional differences in basal Na and Cl transport across the porcine gallbladder epithelium and that both alpha 2- and beta 2-adrenoceptors act to regulate ion transport in this tissue.

Active and passive chloride transport by the rabbit cortical collecting duct

Dietary K intake has been shown to influence solute flux in the cortical collecting duct (CCD). We recently observed active Cl secretion by the CCD when rabbits were fed a K-rich diet. The present studies examined two different methods of K loading (KHCO3 or KCl) on net Cl flux (JNet Cl), active Cl flux (JA Cl), and passive Cl flux (JP Cl) by the rabbit CCD. These two methods of K loading were compared with a low-K diet (control or NaHCO3 diet) that had the same anion composition as the KHCO3 diet (Na for K substitution). In the low-K group there was neither significant net Cl transport nor active Cl secretion. Increasing dietary K content (KHCO3 group) resulted in significant net Cl secretion (JNet Cl = -26.3 +/- 9.3 pmol.mm-1.min-1). Moreover, K secretion was significantly greater (23.8 +/- 4.3 pmol.mm-1.min-1) and transepithelial voltage (VT) was more lumen negative (-14.8 +/- 5.9 mV) compared with the low-K group (P less than 0.05). Consequently, there was both significant passive Cl absorption and significant active Cl secretion. In the KCl group there was significant net Cl absorption (JNet Cl = 17.2 +/- 5.1 pmol.mm-1.min-1, P less than 0.05 vs. low-K diet). However, Na absorption significantly exceeded JNet Cl (JNet Na = 42.0 +/- 7.2 pmol.mm-1.min-1, P less than 0.01), and this group also exhibited the most lumen-negative voltage (VT = -35.4 +/- 5.4 mV).(ABSTRACT TRUNCATED AT 250 WORDS)

83. THYROXIN (T4) AFFECTS THE INTESTINAL TRANSPORT OF ELECTROLYTES

Altered states of thyroidism are associated to alterations in stool patterns. The aim of our study was to see whether T4 has a role in intestinal handling of electrolyte and water transport. In the first set of experiments, adult rats were divided into 3 groups (G-1, 2, 3) and either untreated (G-1), treated with Tapazole+ T4 (G-2) or with Tapazole alone (G-3) for 4 wks. Transepithelial bidirectional fluxes of Na and Cl were measured (and JRnet calculated) across ileal mucosa mounted in Ussing chambers. Serum T4 in G-1 (5.0±1.3μg/dl; mean±SD) and G-2 (5.2±2.2) did not differ but both differed from G-3 (2.7±0.5; p<0.001). Net ileal Cl flux (absorption “+”; secretion “-”) did not differ between G-1 (-1.2±2.4 μEq/cm2hr) and G-2 (-.2±1;6) but both differed from G-3 (+3.1±1.3; p<0.001). T4 correlated with Cl net transport (r=.61;p<0.025), and the latter with JRnet (assumed to be HCO3 net flux) (r=.65;p<0.005). In the 2nd set of experiments, rats were given orally T4 for 4 wks. They were then divided in 3 groups, according to serum T4 levels: G-4, T4 ranging 6.0-7.5 μg/dl (controls), G-5, T4 7.6-9.5 and G-6, T4 >9.6. G-5 rats showed, when compared to G-4, marked shift toward secretion in JClnet (G-5: Cl= -4.5±1.2 vs G-4: Cl= +2.36±.92, p<0.01. Again, serum T4 correlated (r= -.52, p<0.05) with Cl net transport. On the contrary, G-6 rats differed from G-5 in that they paradoxically showed higher rates of absorption for both Na and Cl. He conclude that serum T4 affects intestinal net transport of Cl by shifting it toward secretion, while it affects in the opposite manner the net transport of JR. These effects, no longer seen at the highest T4 serum levels, are consistent with a direct effect of T4 on the C1/HCO3 exchange mechanism, and may explain the alterations in stool patterns seen in dysthyroid states.

Effects of TPA on short-circuit current across frog skin

TPA (12-O-tetradecanoylphorbol-13-acetate) is an effective tumor promoter that affects a variety of ion transport processes. To examine the relationship between effects on transport and growth and differentiation, we have been studying the actions of TPA on frog skin, a particularly well-characterized epithelium. We have reported that high concentrations of TPA stimulate base-line short-circuit current (ISC) and inhibit the subsequent natriferic action of vasopressin. The current study of 89 preparations extends those findings. The Km of the stimulatory effect of TPA is approximately 3 nM; this high affinity indicates that the transport phenomenon does not simply reflect a nonspecific interaction of phorbol ester with the plasma membranes. TPA acts largely or entirely at the mucosal surface of both split and whole skins; thus the sidedness of the effect does not arise from adsorption onto the underlying connective tissue when TPA is applied to the serosal surface of whole skin. Amiloride, an inhibitor of apical Na+ entry, abolishes ISC across frog skins pretreated with TPA. The phorbol ester also increases ISC across split skins, preparations which do not produce net Cl-transport. Indomethacin (1 microM) blocks PGE1 release, but does not alter the response to TPA at a fivefold lower concentration than previously used. NDGA (nordihydroguaretic acid, 10 microM), an inhibitor of the lipoxygenase pathway, partially inhibited the responses of ISC to 8 nM TPA. The present results indicate that frog skin is highly responsive to TPA at concentrations known to activate protein kinase C in broken-cell preparations.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of antidiuretic hormone on the electrical properties of canine tracheal epithelium.

The canine tracheal epithelium causes net transport of Cl- from serosa to mucosa and of Na+ from mucosa to serosa. These processes are manifested by the short-circuit current which represents the algebraic sum of both fluxes. Antidiuretic hormone (ADH) alters the permeability of several epithelia. This work examines the effect and possible mechanism of action of ADH on the potential difference (PD), short-circuit current (ISC) and conductance (G) of canine tracheal epithelium. The epithelium was mounted in Ussing-type chambers containing Krebs-Henseleit solution at 37 degrees C (pH 7.4) and bubbled with 95% O2 and 5% CO2. PD was recorded on an Cole-Parmer recorder by means of the classical technique and ISC was recorded automatically using a voltage clamp. ADH (mucosal surface) significantly decreased PD, ISC and G in a dose-dependent fashion. Similar doses of ADH applied to the serosal side did not produce a significant effect. Angiotensin II (Agt II, serosal side) and amphotericin B (mucosal side) reversed the depressor effect of mucosal ADH. The effect of ADH was less when the Cl- serosa to mucosa movement was blocked by pretreatment with serosal furosemide. It is considered that ADH blocks apical membrane Cl-channels and that angiotensin reverses this effect by increasing apical membrane permeability to Cl-. Amphotericin B reverses the effect of ADH possibly by increasing Na+ absorption.

Steady‐state analysis of ion fluxes in Necturus gall‐bladder epithelial cells.

1. The steady-state fluxes of Na, K and Cl ions have been measured in Necturus gall-bladder epithelium by a technique that involves labelling the cells with tracer ions from the mucosal bath only, whilst the serosa is kept at low specific activity. After removing tracer, the efflux is followed into the serosal bath, revealing two exponential components. 2. The time constant of the fast component lies between 0.03 and 0.04 s-1 and corresponds to that of the extracellular space. The slow component closely matches the cellular efflux, with constants which lie between 0.14 and 0.46 X 10(-2) s-1. 3. Full unstirred-layer calculations have been performed to determine the specific activities in the mucosal solution, the cell and the corium (subepithelium). These involved measuring the diffusion coefficients of Na and Cl in the isolated corium: they are restricted by factors of 0.17 and 0.11. 4. The partial flux equations for this double-membrane system have been solved to obtain the cellular fluxes for all three ions. The results indicate that: (i) the net transcellular Na flux is 190 pmol cm-2 s-1, equivalent to the transepithelial salt flux during fluid secretion; (ii) the net transcellular K flux is effectively zero because this ion recirculates across the serosal membrane; (iii) the net transcellular Cl flux is 27 pmol cm-2 s-1, or 15% of the net transepithelial salt flux. 5. The permeability of the paracellular pathway to Cl is 1.65 X 10(-5) cm s-1 and the available driving forces will allow a maximum net electrodiffusive Cl transport of 10% through the shunt pathway. 6. 1:1 coupling of Na and Cl net fluxes at the mucosal membrane of this epithelium cannot be present, and processes other than simple electrodiffussion are required to effect net Cl transport by another route. 7. The serosal fluxes of K and Cl do not obey the flux-ratio equation. A component of these fluxes must be present which is neither active (pumped) nor passive (electrodiffusive and independent). If they are symmetrical in the steady state then the ratio of these exchange fluxes lies between 2:1 and 3:1 depending upon the size of the pump flux. They support the view that a mode of coupled K and Cl transport may be operating at the basolateral membrane of these cells.

Effect of cigarette smoke on esophageal epithelium of the rabbit

Cigarette smoking is a recognized risk factor for esophageal mucosal disease. For this reason we investigated the effects of smoke on esophageal epithelial electrolyte transport and barrier function in the rabbit. Studies were performed using an extract of cigarette smoke (EOCS) prepared from high-tar, high-nicotine cigarettes. Epithelia were exposed to EOCS in vivo or in vitro in the Ussing chamber. Acute in vivo exposure to EOCS lowered in vivo esophageal potential difference by 61%, and in vitro studies established that this was due to inhibition of active Na transport from mucosa to serosa. Exposure to an EOCS had no effect on net Cl transport or epithelial permeability, the latter reflected by the absence of change in electrical resistance or mannitol flux. The ability of an EOCS to lower potential difference (and inhibit Na transport) was dose-related and equally effective whether contact occurred with the luminal or serosal surface of the tissue. Similar studies performed with an EOCS prepared from filtered smoke established that the component(s) in EOCS responsible for the effects on transport resided in the particulate phase of smoke (i.e., nicotine and “tars”). However, nicotine only inhibited Na transport from the serosal side of the tissue, thus indicating that one or more tars either cause or contribute to the effect of an EOCS on transport. The inhibition by smoke of ion transport in esophageal epithelium may well be an early deleterious link in the pathophysiological chain between cigarette smoking and esophageal mucosal disease.

Regulation of chloride self exchange by cAMP in cortical collecting tubule.

The hormonal control of Cl transport was examined in rabbit cortical collecting tubules using the lumen-to-bath 36Cl tracer rate coefficient (KCl, nm/s). Tracer movement via Cl-HCO3 exchange was minimized by using HCO3-CO2-free solutions. The electrical driving force was minimized by treating with amiloride. Under these conditions, net Cl transport was zero, yet there was a large KCl that fell 88% on removing bath (trans) Cl. These results are consistent with the mechanism of tracer flux being predominantly Cl self exchange. KCl fell spontaneously with time in vitro; after this decline KCl could be stimulated with 8-bromo-cAMP. cAMP present from the onset of perfusion prevented the time-dependent fall in KCl. When tracer movement was restricted to diffusion by eliminating Cl self exchange (0 Cl bath), cAMP had no effect on KCl. Although both isoproterenol and vasopressin are known to stimulate adenylate cyclase in this epithelium, only isoproterenol mimicked the cAMP effect on KCl. The isoproterenol effect was blocked by either propranolol or prostaglandin E2. Lumen addition of the disulfonic stilbene DIDS had no effect on KCl. Lumen addition of furosemide or trichloromethiazide had minimal or no effect. Taken together, these results indicate that Cl self exchange is regulated by beta-adrenergic agents acting via cAMP. The lack of an effect of vasopressin suggests cellular heterogeneity in this response to cAMP.

KCl Transport Across an Insect Epithelium: Characterization of K-Stimulated Cl Absorption and Active K Transport

ABSTRACT The kinetics of 36C1 fluxes across cAMP-stimulated, short-circuited locust rectum were studied. Raising external K+ from 0 to 100 mM increased both Kt and Vmax for net Cl transport by four- to six-fold. Hill plots of indicated non-cooperative Cl interactions. The sequence for cation stimulation of was K &amp;gt; Rb &amp;gt; Cs &amp;gt; Na &amp;gt; NH4. Low levels of K were stimulatory only when added to the mucosal side. Cyclic AMP (cAMP) caused a small active absorption of K, although this was minor compared to the fourfold increase in transepithelial K diffusion (PK ). Neither cAMP stimulation of nor of PK was sensitive to Cl removal, suggesting that K-stimulated Cl absorption and K transport are not mediated by the same co-transport mechanism. Potassium is the counter-ion for electrogenic Cl transport because was less than 10% of the during cAMP exposure under Isc conditions, but equalled at open-circuit.

Effects of bicarbonate and pH on chloride transport by gastric mucosa.

HCO3 and pH dependence of net Cl transport (JClnet) by resting (metiamide-treated) frog gastric mucosa has been investigated in vitro by measuring short-circuit current (Isc = JClnet) and transepithelial conductance (G). With either 100% O2 or 95% O2-5% CO2 gassing, HCO3-free solutions caused large (greater than 50%) reductions in Isc and G. Increases in [HCO3] of the serosal, but not mucosal, solution caused increases in Isc and G. At least part of the effect appeared to be due specifically to the HCO3 moiety, as opposed to the pH changes that also occurred. In HCO3-free solutions (100% O2), increasing serosal solution pH above 7 with either permeable or impermeable buffers caused Isc and G to increase; permeable buffers were somewhat more effective than impermeable buffers. Measurements of intracellular pH (pHc) with [14C]DMO or [14C]methylamine showed that increases in extracellular pH (pHo) caused increases in cellular pH (pHc), and these changes in pHc were independent of buffer type. We conclude that HCO3 and/or high pHo stimulate Isc and G and that buffer permeability and cellular concentration can also affect transport.

Luminal influences on potassium secretion: chloride replacement with sulfate.

Electrolyte transport by the renal distal tubule of rats was studied by in vivo continuous microperfusion to determine whether replacing luminal chloride with sulfate would stimulate net potassium secretion. Results in a first series of experiments showed that replacing all Cl in the perfusion fluid with SO4 reduced net Na absorption, reversed the direction of net Cl transport from net absorption to net secretion, increased the lumen-negative transepithelial voltage, and increased net K secretion. In a second series of experiments adding NaCl to a perfusion fluid containing SO4 increased net Na absorption, reversed the direction of net Cl transport from net secretion to net absorption, decreased the transepithelial voltage, and decreased net K secretion. In a third series of experiments addition of 10(-6) M amiloride to a SO4-containing perfusion fluid reduced the transepithelial voltage but did not prevent the increase in K secretion, indicating that the changes in K secretion cannot be explained solely by changes in electrical driving forces. Since net fluid absorption was controlled, the changes in K secretion cannot be attributed to solvent drag effects. In all three series of experiments the rate of net K secretion was increased when luminal Cl concentration was reduced to less than 10 mM. The results suggest the existence of a cotransport system mediating K and Cl absorption by the distal tubule. Inhibition of K absorption by low lumen Cl may contribute to the increased net K secretion seen when nonchloride anions are present in distal fluid.

Sulfate transport in rabbit ileum: characterization of the serosal border anion exchange process.

The preceding paper [30] shows that transepithelial ileal SO4 transport involves Na-dependent uptake across the ileal brush border, and Cl-dependent efflux across the serosal border. The present study examines more closely the serosal efflux process. Transepithelial mucosa (m)-to-serosa (s) ands-to-m fluxes (J ms,J sm) across rabbit ileal mucosa were determined under short-circuit conditions. SO4 was present at 0.22mm. In standard Cl, HCO3 Ringer's,J SO4 was 81.3±5.3 (1se) andJ SO4 was 2.5±0.2 nmol cm−2 hr−1 (n=20). Serosal addition of 4-acetamido-4′-isothiocyanostilbene-22′-disulfonate (SITS), 44′-diisothiocyanostilbene-22′-disulfonate (DIDS) or 1-anilino-8-naphthalene-sulfonate (ANS) inhibited SO4 transport, SITS being the most potent. Several other inhibitors of anion exchange in erythrocytes and other cells had no effect on ileal SO4 fluxes. In contrast to its effect on SO4 transport, SITS (500 μm) did not detectably alter Cl transport. Replacement of all Cl, HCO3 and PO4 with gluconate reducedJ SO4 by 70% and increasedJ SO4 by 400%. A small but significantJ net SO4 remained.J SO4 could be increased by addition to the serosal side of Cl, Br, I, NO3 or SO4. The stimulatory effect of all these anions was saturable and SITS-inhibitable. The maximalJ SO4 in the presence of Cl was considerably higher than in the presence of SO4 (73.1 and 42.2 nmol. cm−2 hr−1, respectively;p<0.001). TheK 1/2 value for Cl was 7.4mm, 10-fold higher than that for SO4 (0.7mm). Omitting HCO3 and PO4 had no measurable effects on SO4 fluxes. This study shows that (i) SO4 crosses the serosal border of rabbit ileal mucosa by anion exchange; (ii) the exchange process is inhibited by SITS, DIDS and ANS, but not by several other inhibitors of anion exchange in other systems; (iii) SO4 may exchange for Cl, Br, I, NO3 and SO4 itself, but probably not for HCO3 or PO4; (iv) kinetics of the exchange system suggest there is a greater affinity for SO4 than for Cl, although the maximal rate of exchange is higher in the presence of Cl; and, finally (v) SITS has little or no effect on net Cl transport.

Methylprednisolone stimulation of guanylate cyclase activity in rat small intestinal mucosa: Possible role in electrolyte transport

A single, intramuscular dose of the glucocorticoid methylprednisolone (3 mg/100 g) significantly increased rat jejunal and ileal mucosal guanylate cyclase (GTP pyrophosphatelyase [cyclizing]; EC 4.6.1.2) activity and cyclic GMP concentration 6 h after treatment. This increase in guanylate cyclase activity was maximal 6 h after injection, was not associated with changes in mucosal cyclic AMP concentration, and occurred before any increase in NaK-activated adenosine triphosphatase activity could be detected. In contrast to the findings in the small intestine, no alterations in guanylate cyclase activity were present in the rat colon 6 or 72 h after methylprednisolone treatment. The increase in ileal guanylate cyclase activity and cGMP concentration coincided with increases in ileal short-circuit current, potential difference, net Cl secretion, and serosal-to-mucosal unidirectional Cl flux. In contrast, in the colon, where no changes in guanylate cyclase activity were observed, methylprednisolone did not alter electrolyte transport in vitro. A similar correlation was observed between various methylprednisolone doses, the increase in guanylate cyclase activity and the alteration in ileal short-circuit current and net Cl transport. The methylprednisoloneinduced increase in ileal guanylate cyclase activity was not prevented by pretreatment with the mineralocorticoid competitive inhibitor spironolactone, nor did the mineralocorticoid deoxycorticosterone acetate (0.5 mg/100 g) affect ileal or colonic mucosal guanylate cyclase activity. In addition, isolated enterocytes from methylprednisolone-treated animals exhibited significant increases in guanylate cyclase activity with the greatest percentage increase occurring in the crypt cell population. These results suggest that the changes in chloride transport which occurred in the small intestine 6 h after methylprednisolone administration may be due to alterations in guanylate cyclase activity, and further support the role of cGMP as an intracellular mediator of ileal chloride secretion.

Na and Cl Transport across the Isolated Anterior Intestine of the Plaice Pleuronectes Platessa

ABSTRACT The tissue was found to have a serosa negative potential, and shortcircuit currents equivalent to the net Cl transport. A significant part of the Cl uptake was Na dependent and a similar fraction of the Na uptake was Cl dependent. Short-circuit current and uptake of both ions were inhibited by loop diuretics and analogues. 4. Isc and p.d. were abolished by ouabain. The observations are consistent with the idea of a coupled NaCl entry into the cell, using the energy inherent in the Na gradient; Na being pumped out of the cells by the Na pump and followed electrically by Cl−. Net chloride transport and the serosa negative potential would be a consequence of the permselective properties of the junctions allowing Na but not Cl to recycle back to the mucosal solution.

Ion selectivity of the cation transport system of isolated intact cattle rod outer segments. Evidence for a direct communication between the rod plasma membrane and the rod disk membranes

The ion selectivity of cation transport through the plasma membrane of isolated intact cattle rod outer segments (rods) is investigated by means of 45Ca-exchange experiments and light-scattering experiments. These techniques appear to provide complementary information: the 45Ca experiments ( 45Ca fluxes in rods) describe electroneutral antiport, whereas the light-scattering experiments (shrinkage and swelling of rods upon hypertonic shocks with various electrolytes) reveal electrogenic uniport. Electroneutral symport of ions (salt transport) does not take place without addition of external ionophores and application of salts of weak acids. 1. 1. Intact rods recover from a hypertonic shock in the presence of FCCP when lithium, sodium and potassium acetate are applied, but not when ammonium chloride, calcium and magnesium acetate are used. This indicates that the plasma membrane of isolated intact cattle rods is relatively permeable to net transport of Na +, Li + and K +, and relatively impermeable to net transport of Cl −, Mg 2+ and Ca 2+ under conditions that do not give rise to diffusion potentials. 2. 2. Rapid ( t 1 2 < 1 min ) efflux of 45Ca from preloaded intact rods is observed when Na +, Ca 2+, Sr 2+, and under certain conditions also Ba 2+, are added to the external medium. Li +, K +, Rb +, Cs +, Mg 2+ and Mn 2+ are ineffective in this respect as well as protons at pH 7.4. It is concluded that 45Ca efflux reflects electroneutral exchange diffusion of internal 45Ca with external Na +, Ca 2+, Sr 2+ and Ba 2+, respectively. 3. 3. All tested cations lower the rate of 45Ca uptake. The latter can be described by a single rate constant indicating a homogeneous rod preparation and a homogeneous endogenous Ca 2+ pool. However, only those cations which stimulate 45Ca efflux from preloaded rods lower the final equilibrium of 45Ca uptake. Except for the effects of K +, Rb + and Cs + the reduction of the rate of 45Ca uptake by external cations appears to arise from competition for a common site on the plasma membrane. The observed affinities for this site do not correlate with actual transport (as indicated by the ability to stimulate 45Ca efflux). 4. 4. K + increases the affinity of the exchange diffusion system to Ca 2+ from 1 μM to 0.16 μM and changes the relative affinities with respect to Ca 2+ for the other cations (Na +, Mg 2+, Mn 2+, Sr 2+, Ba 2+). Furthermore, the maximal rate of Ba-Ca exchange is strongly stimulated by K +, whereas the maximal rate of Ca-Ca exchange is reduced at saturating Ca 2+ concentrations. 5. 5. The exchange diffusion transport mode can be turned off by external Na + in a process that is not of a stochastic nature, which implies interdependence of individual transport entities and which results in an inhomogeneity of the endogenous Ca 2+ pool. K + acts as antagonist of Na + in this effect. The relevance of these findings is discussed in relation to the generally accepted view, that a diffusable transmitter in the rod cytosol communicates the photochemical event in the disk membrane to the electrical properties of the plasma membrane. It is argued that the exchange diffusion system present in the plasma membrane of isolated cattle rods has a number of properties in common with the system responsible for the dark current through the outer segment of a rod cell in the retina. It is concluded that the exchange diffusion transport mode of the cation transport system in the plasma membrane of isolated cattle rods has access to both the extracellular side of the plasma membrane and the disk interior. Under these conditions it behaves as a single system, which exchanges cations directly from the extracellular space to the disk interior, whereas the disk membranes do not appear to contain a separate Ca 2+ transport system.

Ricinoleic acid stimulation of active anion secretion in colonic mucosa of the rat.

Perfusion of the colon with ricinoleic acid produces fluid and electrolyte accumulation. The mechanism of these changes in water and electrolyte movement is uknown. These studies were designed to determine whether ricinoleic acid effects active ion transport across isolated rat colonic mucosa. 0.5 mM Na ricinoleate produced significant increases in potential difference (3.8 +/- 0.5 mV) and short-circuit current (Isc) (99.2 +/- 10.1 muA/cm2). The increases in Isc produced by Na ricinoleate were inhibited by both removal of bicarbonate and chloride and by the presence of theophylline. The hydroxy fatty acid also resulted in a significant decrease in net Na absorption from 4.7 +/- 0.8 to 0.1 +/- 0.7 mueq/h cm2 and reversed net Cl transport from absorption (+ 4.5 +/- 0.9) to secretion (-2.2 +/- mueq/h cm2). In parallel studies 0.5 mM Na ricinoleate increased mucosal cyclic AMP content by 58%. The concentrations of Na ricinoleate required to produce detectable and maximal increases in both Isc and cyclic AMP were the same. These results provide evidence in support of the concept that hydroxy fatty acid-induced fluid and electrolyte accumulation is driven by an active ion secretory process.

Mouse mammary epithelial cells on floating collagen gels: Transepithelial ion transport and effects of prolactin

Epithelial cells dissociated from midpregnant BALB/c mouse mammary glands were cultured for as long as 20 days as confluent monolayers on floating collagen gels. Detached gels bearing monolayers were placed in lucite Ussing chàmbers for measurement of transepithelial potential difference (PD), short-circuit current (I(sc)), resistance (R), and unidirectional fluxes of Na(+) and Cl(-) during short-circuit current conditions (PD = 0). With Hanks' solution bathing both sides of cultures maintained with insulin and cortisol, PD = -12.8 mV (serosal side ground), I(sc) = 24.6 muA/cm(2), and R = 507 omega.cm(2). Net absorption of Na(+) equaled I(sc), and there was no net Cl(-) transport. PD and I(sc) were reduced 50% by mucosal addition of 10 muM amiloride and to zero by metabolic inhibition with nitrogen gas or by serosal addition of 0.1 mM ouabain. In similar cultures supplemented with prolactin, PD and I(sc) increased to -15.8 mV and 48.0 muA/cm(2), respectively, and R decreased to 374 omega.cm(2). Inhibitor effects were similar to those seen in prolactin-free cultures. Prolactin exposure resulted in a 3-fold increase in net absorption of Na(+). Na(+) absorption was not equivalent to I(sc), and there was little Cl(-) absorption; therefore, prolactin induced active transport of other, as yet unidentified, ions. These effects of prolactin require at least 3 days to occur and cannot be attributed to the known contamination with neurohypophysial hormones. The prolactin-induced increase in Na(+) absorption parallels its Na(+)-retaining ability in lower vertebrates and could be part of the mechanism that keeps milk Na(+) concentration low in intact glands.

Na and Cl transport and short-circuit current in rabbit ileum.

Na and Cl fluxes and short-circuit current (Isc) in rabbit ileum have been studied as a function of ionic concentrations in HCO3-free solutions. Both net Na flux (JNanet) and Isc show similar saturation functions of [Na] at fixed [Cl]. They show no significant difference between zero and 112 mM Na but at 140 mM Na Isc is significantly greater than the JNanet. Net Cl transport, secretion, is observed only at 140 mM Na and is approximately equivalent to the difference between the Isc and JNanet. The transcellular mucosa-to-serosa Na fluxes measured at 140 and 70 mM Na do not differ significantly from the corresponding Isc. The net Cl flux varies with [Cl] at fixed [Na] while Isc is virtually not affected by [Cl]. These results suggest that the absorptive Na transport process is electrogenic and responsible for the Isc and that the secretory fluxes of Na and Cl are coupled, require high [Na], vary with [Cl], and do not contribute to Isc. K-free solution abolishes the Isc after a prolonged lag. Finally, the effect of a low resistance shunt pathway on active Na absorption is examined with a four-compartment model.

Effects of Alcohol on Ion Transport by Isolated Gastric and Esophageal Mucosa

The effects of ethyl alcohol, 20%, were measured on steady state rates of ion transport by isolated mucosa. Luminal, but not serosal, addition of alcohol altered ion transport. Alcohol caused a sustained reduction in apparent basal acid secretion by fundic mucosa. These tissues were resistant to ordinarily effective stimulants of acid secretion (histamine, theophyline, dibutyryl cyclic AMP, and gastrin) by rabbit mucosa. Across antral mucosa, net Na transport was inhibited, and net Cl transport was reduced in the presence of alcohol. The unidirectional fluxes of both ions were increased as was permeability to erythritol. These effects of alcohol did not require the presence of acid in the lumen. The rate of luminal acid loss by antrum was increased by alcohol. Addition of ouabain alone, which inhibits net Na and Cl transport, did not alter the rate of luminal acid loss by antrum. Alcohol, however, still increased the rate of luminal acid loss by antral mucosa treated with ouabain. Alcohol reduced the electrical resistance of fundic and antral mucosa but had no effect on the rate of luminal acid loss or electrical resistance of esophageal mucosa. Luminal application of alcohol inhibits active ion transport in addition to increasing cation and anion permeability. However, inhibition of active transport per se is not necessarily associated with an increase in mucosal permeability.