Active transport of iodide and other anions across the choroid plexus

Expression of the sodium iodide symporter (NIS) in reproductive and neural tissues of teleost fish

Iodine, an essential component of thyroid hormones, can only be obtained through the diet. The sodium iodide symporter (NIS) transports iodide across mammalian intestinal and thyroid epithelia to deliver iodide for thyroid hormone production. Using reverse transcription-polymerase chain reaction (RT-PCR) we confirmed that mRNA for a homolog of mammalian NIS is expressed in comparable locations, both sub-pharyngeal thyroid tissue and intestine, in multiple teleost fish species, supporting a conserved mechanism for intestinal-thyroid iodine transport across vertebrates. To determine when in embryogenesis NIS expression is initiated we utilized in situ hybridization (ISH) during development of zebrafish (Danio rerio) embryos. This revealed expression of nis as early as 2 days post fertilization (dpf) along the dorsal surface of the yolk sac, suggesting a function to import iodine from yolk. To evaluate the potential for maternal deposition of iodine in yolk, RT-PCR and further in situ staining of ovarian tissue in gravid female zebrafish confirmed NIS mRNA presence in the ooplasm and granulosa layer of early stage follicles. This further suggests that maternally-deposited NIS mRNA may be available for early embryogenesis. Unexpectedly, ISH in embryos revealed robust nis expression in the central nervous system throughout days 2-5 days post fertilization, with adult whole brain ISH localizing expression in the hypothalamus, cerebellum, and optic tectum. RT-PCR on whole brain tissue from five species of adult fish representing three taxonomic orders likewise revealed robust CNS expression. These unexpected locations of nis expression suggest novel, as yet undescribed reproductive and neural functions of NIS in teleost species.

SLC26A7 constitutes the thiocyanate-selective anion conductance of the basolateral membrane of the retinal pigment epithelium

Anion channels in the retinal pigment epithelium (RPE) play an essential role in the transport of Cl^- between the outer retina and the choroidal blood to regulate the ionic composition and volume of the subretinal fluid that surrounds the photoreceptor outer segments. Recently, we reported that the anion conductance of the mouse RPE basolateral membrane is highly selective for the biologically active anion thiocyanate (SCN^-), a property that does not correspond with any of the Cl^- channels that have been found to be expressed in the RPE to date. The purpose of this study was to determine the extent to which SLC26A7, a SCN^- permeable-anion exchanger/channel that was reported to be expressed in human RPE, contributes to the RPE basolateral anion conductance. We show by quantitative RT-PCR that Slc26a7 is highly expressed in mouse RPE compared with other members of the Slc26 gene family and Cl^- channel genes known to be expressed in the RPE. By applying immunofluorescence microscopy to mouse retinal sections and isolated cells, we localized SLC26A7 to the RPE basolateral membrane. Finally, we performed whole cell and excised patch recordings from RPE cells acutely isolated from Slc26a7 knockout mice to show that the SCN^- conductance and permeability of its basolateral membrane are dramatically smaller relative to wild-type mouse RPE cells. These findings establish SLC26A7 as the SCN^--selective conductance of the RPE basolateral membrane and provide new insight into the physiology of an anion channel that may participate in anion transport and pH regulation by the RPE.

Inter-species variation in monovalent anion substrate selectivity and inhibitor sensitivity in the sodium iodide symporter (NIS)

The sodium iodide symporter (NIS) transports iodide, which is necessary for thyroid hormone production. NIS also transports other monovalent anions such as tetrafluoroborate (BF4-), pertechnetate (TcO4-), and thiocyanate (SCN-), and is competitively inhibited by perchlorate (ClO4-). However, the mechanisms of substrate selectivity and inhibitor sensitivity are poorly understood. Here, a comparative approach was taken to determine whether naturally evolved NIS proteins exhibit variability in their substrate transport properties. The NIS proteins of thirteen animal species were initially assessed, and three species from environments with differing iodide availability, freshwater species Danio rerio (zebrafish), saltwater species Balaenoptera acutorostrata scammoni (minke whale), and non-aquatic mammalian species Homo sapiens (human) were studied in detail. NIS genes from each of these species were lentivirally transduced into HeLa cells, which were then characterized using radioisotope uptake assays, 125I- competitive substrate uptake assays, and kinetic assays. Homology models of human, minke whale and zebrafish NIS were used to evaluate sequence-dependent impact on the organization of Na+ and I- binding pockets. Whereas each of the three proteins that were analyzed in detail concentrated iodide to a similar degree, their sensitivity to perchlorate inhibition varied significantly: minke whale NIS was the least impacted by perchlorate inhibition (IC50 = 4.599 μM), zebrafish NIS was highly sensitive (IC50 = 0.081 μM), and human NIS showed intermediate sensitivity (IC50 = 1.566 μM). Further studies with fifteen additional substrates and inhibitors revealed similar patterns of iodide uptake inhibition, though the degree of 125I- uptake inhibition varied with each compound. Kinetic analysis revealed whale NIS had the lowest Km-I and the highest Vmax-I. Conversely, zebrafish NIS had the highest Km and lowest Vmax. Again, human NIS was intermediate. Molecular modeling revealed a high degree of conservation in the putative ion binding pockets of NIS proteins from different species, which suggests the residues responsible for the observed differences in substrate selectivity lie elsewhere in the protein. Ongoing studies are focusing on residues in the extracellular loops of NIS as determinants of anion specificity. These data demonstrate significant transport differences between the NIS proteins of different species, which may be influenced by the unique physiological needs of each organism. Our results also identify naturally-existing NIS proteins with significant variability in substrate transport kinetics and inhibitor sensitivity, which suggest that the affinity and selectivity of NIS for certain substrates can be altered for biotechnological and clinical applications. Further examination of interspecies differences may improve understanding of the substrate transport mechanism.

Prognostic Significance of Sodium Iodide Symporter and Deiodinase Enzymes mRNA Expression in Gastric Cancer

Context

Thyroid hormones (THs) are critically important for development, homeostasis, and metabolic regulation in mammals. Iodine, one of the constituents of TH, is actively supplied by sodium iodide symporter (NIS) into the thyroid gland. TH is subsequently transported to distant organs where its activation and deactivation is catalyzed by isoforms of deiodinases (DIOs). NIS protein has been known to overexpress in cancer cases of the breast and gastrointestinal organs. Recent studies show a possible role of DIOs in various cancers.

Aims

In the present investigation, the prognostic significance of NIS and DIO-1, 2 and 3 was studied in gastric cancer using a data mining bioinformatic approach.

Methods

"The Kaplan-Meier plotter" database was used for direct in silico validation in clinically relevant 876 gastric cancer patients with >15 years of follow-up information. After obtaining KM survival plots, hazard ratio and log-rank P value were calculated.

Results

Increased expression of NIS and DIO 1-3 is significantly associated with worsen overall survival of gastric cancer patients followed for 20 years. Prognostic roles of NIS and individual DIOs were assessed in different types of gastric cancer classified based on morphologies, human epidermal growth factor receptor-2 receptor status, treatment choices, and different clinicopathological features.

Conclusions

Based on these analyses, the present study found the indication of prognostic values of these genes. This information will contribute to better understanding of managing complex and heterogeneous gastric cancer. Further, these findings may be beneficial as a companion diagnostic tool predicting more accurate gastric cancer prognosis.

Electrophysiological impact of thiocyanate on isolated mouse retinal pigment epithelial cells

Our recent electrophysiological analysis of mouse retinal pigment epithelial (RPE) cells revealed that in the presence of 10 mM external thiocyanate (SCN^-), voltage steps generated large transient currents whose time-dependent decay most likely results from the accumulation or depletion of SCN^- intracellularly. In the present study, we investigated the effects of more physiologically relevant concentrations of this biologically active anion. In whole cell recordings of C57BL/6J mouse RPE cells, we found that, over the range of 50 to 500 µM SCN^-, the amplitude of transient currents evoked by voltage steps was proportional to the extracellular SCN^- concentration. Transient currents were also produced in RPE cells when the membrane potential was held constant and the external SCN^- concentration was rapidly increased by pressure-ejecting 500 µM SCN^- from a second pipette. Other results indicate that the time dependence of currents produced by both approaches results from a change in driving force due to intracellular SCN^- accumulation or depletion. Finally, by applying fluorescence imaging and voltage-clamping techniques to BALB/c mouse RPE cells loaded with the anion-sensitive dye MQAE, we demonstrated that in the presence of 200 or 500 µM extracellular SCN^-, depolarizing voltage steps increased the cytoplasmic SCN^- concentration to an elevated steady state within several seconds. Collectively, these results indicate that, in the presence of physiological concentrations of SCN^- outside the RPE, the conductance and permeability of the RPE cell membranes for SCN^- are sufficiently large that SCN^- rapidly approaches electrochemical equilibrium within the cytoplasm when the membrane voltage or external SCN^- concentration is perturbed.

Mouse retinal pigment epithelial cells exhibit a thiocyanate-selective conductance

The basolateral membrane anion conductance of the retinal pigment epithelium (RPE) is a key component of the transepithelial Cl^- transport pathway. Although multiple Cl^- channels have been found to be expressed in the RPE, the components of the resting Cl^- conductance have not been identified. In this study, we used the patch-clamp method to characterize the ion selectivity of the anion conductance in isolated mouse RPE cells and in excised patches of RPE basolateral and apical membranes. Relative permeabilities ( P_A/ P_Cl) calculated from reversal potentials measured in intact cells under bi-ionic conditions were as follows: SCN^- >> ClO₄^- > [Formula: see text] > I^- > Br^- > Cl^- >> gluconate. Relative conductances ( G_A/ G_Cl) followed a similar trend of SCN^- >> ClO₄^- > [Formula: see text] > I^- > Br^- ≈Cl^- >> gluconate. Whole cell currents were highly time-dependent in 10 mM external SCN^-, reflecting collapse of the electrochemical potential gradient due to SCN^- accumulation or depletion intracellularly. When the membrane potential was held at -120 mV to minimize SCN^- accumulation in cells exposed to 10 mM SCN^-, the instantaneous current reversed at -90 mV, revealing that P_SCN/ P_Cl is approximately 500. Macroscopic current recordings from outside-out patches demonstrated that both the basolateral and apical membranes exhibit SCN^- conductances, with the basolateral membrane having a larger SCN^- current density and higher relative permeability for SCN^-. Our results suggest that the RPE basolateral and apical membranes contain previously unappreciated anion channels or electrogenic transporters that may mediate the transmembrane fluxes of SCN^- and Cl^-.

Sodium iodide symporter (NIS) in extrathyroidal malignancies: focus on breast and urological cancer

Background

Expression and function of sodium iodide symporter (NIS) is requisite for efficient iodide transport in thyrocytes, and its presence in cancer cells allows the use of radioiodine as a diagnostic and therapeutic tool in thyroid neoplasia. Discovery of NIS expression in extrathyroidal tissues, including transformed cells, has opened a novel field of research regarding NIS-expressing extrathyroidal neoplasia. Indeed, expression of NIS may be used as a biomarker for diagnostic, prognostic, and therapeutic purposes. Moreover, stimulation of endogenous NIS expression may permit the radioiodine treatment of extrathyroidal lesions by concentrating this radioisotope.

Results

This review describes recent findings in NIS research in extrathyroidal malignancies, focusing on breast and urological cancer, emphasizing the most relevant developments that may have clinical impact.

Conclusions

Given the recent progress in the study of NIS regulation as molecular basis for new therapeutic approaches in extrathyroidal cancers, particular attention is given to studies regarding the relationship between NIS and clinical-pathological aspects of the tumors and the regulation of NIS expression in the experimental models.

The Na+/I- symporter (NIS): mechanism and medical impact

The Na(+)/I(-) symporter (NIS) is the plasma membrane glycoprotein that mediates active I(-) transport in the thyroid and other tissues, such as salivary glands, stomach, lactating breast, and small intestine. In the thyroid, NIS-mediated I(-) uptake plays a key role as the first step in the biosynthesis of the thyroid hormones, of which iodine is an essential constituent. These hormones are crucial for the development of the central nervous system and the lungs in the fetus and the newborn and for intermediary metabolism at all ages. Since the cloning of NIS in 1996, NIS research has become a major field of inquiry, with considerable impact on many basic and translational areas. In this article, we review the most recent findings on NIS, I(-) homeostasis, and related topics and place them in historical context. Among many other issues, we discuss the current outlook on iodide deficiency disorders, the present stage of understanding of the structure/function properties of NIS, information gleaned from the characterization of I(-) transport deficiency-causing NIS mutations, insights derived from the newly reported crystal structures of prokaryotic transporters and 3-dimensional homology modeling, and the novel discovery that NIS transports different substrates with different stoichiometries. A review of NIS regulatory mechanisms is provided, including a newly discovered one involving a K(+) channel that is required for NIS function in the thyroid. We also cover current and potential clinical applications of NIS, such as its central role in the treatment of thyroid cancer, its promising use as a reporter gene in imaging and diagnostic procedures, and the latest studies on NIS gene transfer aimed at extending radioiodide treatment to extrathyroidal cancers, including those involving specially engineered NIS molecules.

Thyroid Na+/I- symporter. Mechanism, stoichiometry, and specificity

The rat thyroid Na+/I- symporter (NIS) was expressed in Xenopus laevis oocytes and characterized using electrophysiological, tracer uptake, and electron microscopic methods. NIS activity was found to be electrogenic and Na+-dependent (Na+ >> Li+ >> H+). The apparent affinity constants for Na+ and I- were 28 +/- 3 mM and 33 +/- 9 microM, respectively. Stoichiometry of Na+/anion cotransport was 2:1. NIS was capable of transporting a wide variety of anions (I-, ClO3-, SCN-, SeCN-, NO3-, Br-, BF4-, IO4-, BrO3-, but perchlorate (ClO4-) was not transported. In the absence of anion substrate, NIS exhibited a Na+-dependent leak current (approximately 35% of maximum substrate-induced current) with an apparent Na+ affinity of 74 +/- 14 mM and a Hill coefficient (n) of 1. In response to step voltage changes, NIS exhibited current transients that relaxed with a time constant of 8-14 ms. Presteady-state charge movements (integral of the current transients) versus voltage relations obey a Boltzmann relation. The voltage for half-maximal charge translocation (V0.5) was -15 +/- 3 mV, and the apparent valence of the movable charge was 1. Total charge was insensitive to [Na+]o, but V0.5 shifted to more negative potentials as [Na+]o was reduced. NIS charge movements are attributed to the conformational changes of the empty transporter within the membrane electric field. The turnover rate of NIS was >/=22 s-1 in the Na+ uniport mode and >/=36 s-1 in the Na+/I- cotransport mode. Transporter density in the plasma membrane was determined using freeze-fracture electron microscopy. Expression of NIS in oocytes led to a approximately 2. 5-fold increase in the density of plasma membrane protoplasmic face intramembrane particles. On the basis of the kinetic results, we propose an ordered simultaneous transport mechanism in which the binding of Na+ to NIS occurs first.

Saturable efflux of the peptides RC-160 and Tyr-MIF-1 by different parts of the blood-brain barrier

Peptides have been shown to be transported in the direction of both blood to brain and brain to blood. Although blood to brain transport is known to occur at both the choroid plexus and the capillary bed of the brain, comprising the two major components of the blood-brain barrier, the location of efflux systems for peptides remains largely unstudied. We adapted established methodologies to study this question for two peptides known to be transported out of the brain after injection into the cerebrospinal fluid (CSF): Tyr-MIF-1, transported by peptide transport system (PTS)-1 and RC-160, a somatostatin analog transported by PTS-5. Radioactive iodide, known to be transported out of the brain primarily by the capillaries, also was studied. We found that after injection into brain tissue, RC-160 and iodide were rapidly transported out of the brain by saturable mechanisms. By contrast, efflux of Tyr-MIF-1 was slow and nonsaturable after injection into brain tissue, but rapid and saturable after injection into the lateral ventricle of the brain. Autoradiography confirmed that peptide injected into brain tissue did not diffuse far from the site of injection during the study period. The results indicate that the efflux system for RC-160 is located at least partly at the capillaries and suggest that the major location for the efflux system of Tyr-MIF-1 is at the choroid plexus.

Desmethylimipramine, a potent inhibitor of synaptosomal norepinephrine uptake, has diverse effects on thyroid hormone processing in rat brain. I. Effects on in vivo uptake of 125I-labeled thyroid hormones in rat brain

Several lines of evidence point to an interaction between amine uptake inhibitors (tricyclic antidepressants) and thyroid hormones. To examine this issue under conditions which would minimize secondary effects of drug treatment, desmethylimipramine (DMI), a highly specific norepinephrine uptake inhibitor, was given acutely as a single i.p. dose one hour before i.v. [125I]triiodothyronine (T3*) or [125I]thyroxine (T4*). Tissues were analysed after rat decapitation at 3, 5, 10, and 20 min intervals thereafter. DMI had a small but significant inhibitory effect on the brain uptake of both T3* (7.4%) and T4* (19%) over their respective 20-min time courses as indicated by two-way ANOVA. To examine the drug response further and to determine the effect of thyroid status on the response, hypothyroid (HYPO) and T4-induced hyperthyroid (HYPER) rats, were given i.v. T3* and, 5 min later, i.p. DMI or saline. They were killed 3 h later and tissue analysed. Because DMI effects on T4* uptake could not be evaluated over a 3 h period without blocking T4* to T3* conversion, sodium ipodate (60 mg/kg) was given in 2 doses before i.v. T4*. Under these conditions, DMI significantly reduced brain concentrations of the administered T3* and T4* in HYPO (15% and 19%) and in HYPER rats (13% and 25%). These results suggest that, as it does in the case of norepinephrine, DMI blocks the uptake site for T3 and T4 in rat brain. No information is available regarding the relationship, if any, between the thyroid hormone and norepinephrine uptake sites.

The effects of outward forced convective flow on inward diffusion in human dentine in vitro

In vitro experiments were conducted to evaluate the influence of outward forced convective flow on the inward diffusion of radioactive iodide. When the smear layer was present, application of 15 cmH2O (1.47 kPa) outward-directed filtration pressure reduced the inward flux of iodide by about 10-20% depending upon the hydraulic conductance of each specimen. When the smear layer was removed by acid etching, the same 1.47 kPa pressure lowered the inward iodide flux by as much as 60%, depending on the hydraulic conductance. The results demonstrate the importance of the balance between inward diffusion and outward bulk-fluid movement on the rate of permeation of exogenous solutes.

Cyclic AMP alteration of chloride transport into the choroid plexus-cerebrospinal fluid system

The permeation of 36Cl from blood into choroid plexus (CP) and cerebrospinal fluid (CSF) was studied in adult rats intraventricularly injected with either dibutyryl (db)-cAMP, theophylline with db-cAMP, or forskolin. All 3 treatments significantly enhanced the 15-min distribution (Vd) of 36Cl into CSF, by an average of 17-33%. In contrast, ouabain and acetazolamide (inhibitors of CSF production) reduced 36Cl uptake into CSF by 34% and 13%, respectively. A correlative analysis of the various treatments revealed that the volume of distribution of 36Cl in CSF varied inversely with that in the choroid plexus. This inverse relationship suggests drug effects on Cl movement across the apical (CSF-facing) membrane of CP.

Transport of iodothyronines from bloodstream to brain: contributions by blood:brain and choroid plexus:cerebrospinal fluid barriers

Thyroid hormone entering the brain from the cerebral circulation must first cross barriers at the the blood:brain and choroid plexus:cerebrospinal fluid interfaces. The route taken after entry through those barriers might bring about selective delivery of hormone to different regions of the brain and those differences might be crucial for the ultimate functional effects of the hormone. To determine whether and how distribution of hormone in the brain might vary according to the route of entry, film autoradiograms of serially sectioned brains were prepared after delivery of a pulse of 125I-labeled thyroid hormone into either the right lateral cerebral ventricle or the femoral vein. The results after intrathecal injection, reflecting the penetration of hormone into brain after crossing the choroid plexus:cerebrospinal fluid barrier, revealed a markedly limited, essentially periventricular distribution of radioactivity at both 3 and 48 h after hormone administration. Results after i.v. administration, which allows hormone access across both barriers, revealed an initial distribution pattern (at 3 h) generally similar to that seen after administration of markers of cerebral blood flow; at 48 h there was strong resolution in selected brain regions never noted to be labeled after intrathecal hormone injection. The functional implications of the differences in results produced by the two different routes of hormone entry are not known. However, ready access to circumventricular organs would appear to be favored by hormone crossing the choroid plexus:cerebrospinal fluid barrier whereas access to the panoply of nuclear triiodothyronine receptors would be favored by hormone crossing the blood:brain barrier. Therefore both routes of barrier transport should be taken into account in assessing the kinetics and actions of thyroid hormones in the central nervous system.

Stilbenes inhibit exchange of chloride between blood, choroid plexus and cerebrospinal fluid

Disulfonic stilbenes (DIDS and SITS) were used to probe the nature of Cl transport in choroid plexus (CP) of adult rats. DIDS was more effective than SITS in reducing in vitro choroid cell [Cl]. Intravenously- or intraventricularly-administered DIDS substantially decreased 36Cl transport from blood to CP to CSF. Rate of CSF renewal was diminished nearly 30% by DIDS. These inhibitory findings implicate an anion antiporter for mediating Cl flux (and thus water) between blood and CSF.

Nonexclusive solute transport thru protein channels. Model of the Na,K ATPase complex and similar channels as general transport routes

In earlier work this author put forward a model of the Na,K ATPase complex as a general transport channel. Detailed treatment was limited to anion and monovalent cation transport. Here the functional mechanisms of the Na,K ATPase and similar protein channels as transport routes for all ionic fluxes and also amino acid, sugar and other solutes are presented. Anions, monosaccharide -OH groups and amino acid carboxyls bind to common arginyls and lose hydration water. They combine with cations which bind to adjacent side chain carboxyls, forming neutral ion pairs or positively charged complexes which have minimums in size, hydration and free polar groups. The smaller size and polarity facilitate entry into the tight, structured water channel of some 8-10 A outer bore. Solute fluxes depend on membrane redox activity which maintains channel sulfhydryls in reduced state required for proper transport. ATP binding at channels contributes to transport conformation while ATP hydrolysis gives high efflux of Na+, H+ and Ca2+ as phosphate ion pairs. This cation efflux current clears cations from inner membrane sites, increases negative potential and provides Na+ and H+ about the outer combining sites, while maintaining their inward gradients. Binding of many agents widens the outer bore to give larger, less selective influx.

Saturable transport of peptides across the blood-brain barrier

Peptides can be transported across the blood-brain barrier by saturable transport systems. One system, characterized with radioactively labeled Tyr-MIF-1 (Tyr-Pro-Leu-Gly-amide), is specific for some of the small peptides with an N-terminal tyrosine, including Tyr-MIF-1, the enkephalins, beta-casomorphin, and dynorphin (1-8). Another separate system transports vasopressin-like peptides. The choroid plexus has at least one system distinguishable from those above that is capable of uptake and possibly transport of opiate-like peptides. The possibility of saturable transport of other peptides has been investigated to a varying degree. Specificity, stereo-specificity, saturability, allosteric regulation, modulation by physiologic and pharmacologic manipulations, and noncompetitive inhibition have been demonstrated to occur in peptide transport systems and suggest a role for them in physiology and disease.

The electrogenic, Na+-dependent I- transport system in plasma membrane vesicles from thyroid glands

Using vesicles from the plasma membrane of hog thyroid, we have characterized its Na+-dependent I- transport system. We have found it to be totally Na+ dependent; K+ cannot substitute and Li+ can partially substitute for Na+; the Na+:I- flux ratio is larger than one; the system is electrogenic, being stimulated by a delta psi negative inside the vesicles. A number of large, lipophilic anions are fully-competitive inhibitors of Na+-dependent I- uptake; the closer their atomic radii are to that of iodine, the smaller their Ki values.

Substrate utilization in the isolated perfused cortical thick ascending limb of rabbit nephron

Isolated segments of cortical thick ascending limbs (cTAL) of rabbit kidney were perfused in vitro and the equivalent short circuit current (Isc) was measured. In a first series all substrates were removed on either side. Isc fell rapidly to 50 +/- 12% after 3 min and to 27 +/- 6% (n = 5) after 10 min. This indicates that in cTAL segments Isc is strictly dependent on the presence of substrates. In series two it was tested what substrates can be utilized by the cTAL segment, and from which epithelial side [bath (b) or lumen (1)] the substrates are taken up. From the 1-side only butyrate (10 mmol X 1(-1) sustained the Isc at 95 +/- 2% (n = 7). All other tested substrates (10 mmol X 1(-1): pyruvate, acetate, beta-OH-butyrate, D-glucose, and L-lactate lead to a marked decline in Isc. From the b-side several substrates (5--10 mmol X 1(-1) sustained the Isc: D-glucose, D-mannose, butyrate, beta-OH-butyrate, acetoacetate, L-lactate, acetate and pyruvate. Other compounds (1--10 mmol X 1(-1): citrate, alpha-ketoglutarate, succinate, glutamate, glutamine, propionate, caprylate and oleate did not sustain Isc. In the third series the mechanism of substrate utilization from the basolateral cell side was studied. It was shown that the Isc is a saturable function of the D-glucose, L-lactate, acetate, pyruvate or beta-OH-butyrate concentration with apparent Km's between 0.05--1.0 mmol X 1(-1). Several known inhibitors of sugar and of anion transport were tested at the bath side: phlorrhizin was without effect. Phloretin (500 mumol X 1(-1) inhibited Isc by 96%, yet its effect was not dependent on the presence of substrates on the b-side since inhibition occurred also if the b-perfusate contained no substrate and Isc was driven by luminal butyrate. Also SITS (5 mmol X 1(-1) exerted only a small inhibitory effect which was not specific since it was also observed with luminal butyrate. alpha-Cyano-m-OH-cinnamate (10 mmol X 1(-1) inhibited the Isc specifically when L-lactate was the bath substrate. Probenecid (1 mmol X 1(-1) had a similar yet less marked inhibitory effect. The D-glucose uptake from the b-side was specifically inhibited by cytochalasin B at 5 X 10(-6) mol X 1(-1). We conclude that the cTAL segment of the rabbit utilizes D-glucose and/or small anions such as pyruvate or L-lactate or acetate to energize salt reabsorption.(ABSTRACT TRUNCATED AT 400 WORDS)

Recent research into the nature of cerebrospinal fluid formation and absorption

Recent information regarding the nature of bulk cerebrospinal fluid formation and absorption is reviewed, integrated with previous knowledge, and applied to the clinical setting.

Bicarbonate transport across the frog choroid plexus and its control by cyclic nucleotides

We have studied the effects of ions, hormones, diuretics and cyclic nucleotides on the short-circuit current (Isc) across the frog choroid plexus. In normal HCO3 Ringer solution, the trans-epithelial potential difference (p.d.), Isc and resistance (Rt) were -0.8 mV (ventricular side negative with respect to to the blood (serosal) side), 6 microA/cm2 and 170 omega cm2, respectively. Removal of Na, Cl or HCO3 from the solution decreased the Isc to 1.9, 2.3 or -4.6 microA/cm2, respectively. Theophylline, dibutyryl cyclic AMP, isoproterenol, prostaglandin E1, ACTH, cholera toxin and forskolin all significantly increased the Isc. The theophylline-induced change in Isc (delta Itheosc) was reduced by 50% upon Cl substitution with gluconate, and was abolished to less than 12% by Na-free and HCO3-free solutions. pH monitoring of the bathing solutions showed that acidification of the serosal bathing fluid was enhanced by theophylline while that of the ventricular solution was retarded. Ouabain, acetazolamide, SITS, DIDS and furosemide inhibited both Isc and delta Itheosc. We conclude that HCO3 secretion by the choroid plexus into the c.s.f. is controlled by hormones which stimulate the adenylate cyclase system, and propose a model of ion transport across the choroidal epithelium.

Altered anion handling by choroid plexus in renal insufficiency

Abnormal localization of 99mTc pertechnetate was detected in the choroid plexus of patients with renal insufficiency despite the administration of potassium perchlorate. This was found in 8 of 21 studies on patients on conservative medical treatment, in 35 of 41 studies in patients on hemodialysis, in 12 of 22 studies in renal transplant recipients, and in 1 of 5 with acute renal insufficiency, but not in any of 33 persons free of renal disease. Abnormal retention of pertechnetate was also observed in the salivary glands of 21 renal patients. The increased localization of 99mTc pertechnetate could not be related to high blood levels of radioactivity, altered protein binding, increased erythrocyte labeling or to impaired gastrointestinal absorption of potassium perchlorate. It is proposed that this finding represents a manifestation of choroid plexus dysfunction resulting in altered handling of anions by this organ. A possible relation to neurological disease in chronic renal insufficiency is suggested.

Kinetics of the sodium pump in the frog choroid plexus

1. Active Na+ transport across the choroid plexus is mediated by Na-K pumps in the brush border membrane of the epithelium. We have studied the kinetics of Na+ pumping across the brush border membrane using a 22Na efflux procedure. 2. Frog choroid plexuses were loaded with 22Na in K+ -free, cold Ringer solution containing various Na+ concentrations. The efflux of 22Na from the tissues was monitored at 22 degrees C and the rate constant (k) was estimated for each flux interval. The pump component was obtained from the change in k(delta k) induced by the addition of KCl to the superfusate. 3. Ouabain blocked this K+ -sensitive Na+ efflux and other monovalent cations substituted for K+ in the sequence Tl+, Rb+, K+, greater than NH+4 greater than Cs+ much greater than Li+. Active sodium transport (delta k x Nac) increased in a sigmoidal and saturable way with intracellular Na+ and with extracellular K+ concentrations. 4. Kinetic parameters were estimated using the model of Garay & Garrahan (1973). The data indicate that there were two ligand binding sites for K+ on the outside face and three ligand binding sites for Na+ on the cytoplasmic face of the pump. The Kms for potassium and sodium were 1 . 1 and 7 mM. 5. The results also suggest that the Na-K pump has a coupling ratio of 1 . 5. We conclude that the Na-K pump in this epithelium is very similar to those in single cells such as erythrocytes, nerve and muscle.

The effect of severe hypoglycemia upon cerebrospinal fluid formation, ventricular iodide clearance, and brain electrolytes in rabbits

Severe insulin-induced hypoglycemia in rabbits reduces cerebrospinal fluid (CSF) formation, but not ventricular iodide clearance as measured by ventriculocisternal perfusion. This indicates that CSF production is ultimately glucose-dependent but that ventricular iodide clearance is not. The data suggest that severe hypoglycemia results in intracellular potassium loss within the brain and show that extracellular sodium replaces lost intracellular potassium. Hypoglycemia probably results in cellular adenosine triphosphate (ATP) reduction which affects membrane Na/K ATPase and the ability of the brain cell to maintain a potassium gradient. Potassium levels in the CSF also rise consequent to hypoglycemia. Homeostatic mechanisms that maintain a constant CSF potassium, therefore, are also affected by hypoglycemia.

Autoregulation of thyroid iodide transport: possible mediation by modification in sodium cotransport

Cat thyroid slices were employed to determine whether preincubation with excess iodide, a procedure previously reported to depress subsequently measured iodide transport activity, is associated with any alteration in the relationship between sodium and iodide transport. Kinetic analysis revealed that the inhibition of iodide-concentrating activity by iodide pretreatment was accompanied by a reduction in the apparent affinity of the iodide transport for iodide as reflected by increase in the value of KA, the concentration of iodide required to achieve half-maximal transport activity. A reduction in extracellular sodium concentration was also accompanied by an increase in the value of KA for iodide transport. Thus, the KA of iodide transport in control thyroid tissue was increased from a value of 8.6 +/- 1.2 to 16.5 +/- 2.4 microM, by a reduction in extracellular sodium from 144 to 52 mM. In contrast, in thyroid tissue subjected to a 2-h preincubation with 30 microM NaI, a similar reduction in extracellular sodium concentration was associated with only a 20% increase in KA from a value of 17.7 +/- 2.4 to 21.7 +/- 2.2 microM. Analysis of the kinetic data suggests that the autoregulatory effect of excess iodide results in a decline in the sodium dependency of iodide transport. This loss of sodium dependency is accompanied by an increase in the affinity of the process for sodium, as well as a reduction in the affinity of the transport system for iodide.

Mode of action of furosemide on the chloride-dependent short-circuit current across the ciliary body epithelium of toad eyes

The effects of furosemide on the chloride-dependent short-circuit current across the toad ciliary epithelium were examined. Under control conditions, the short-circuit current obeyed Michaelis-Menten kinetics against medium chloride concentration, the Michaelis constant (Km) for chloride being 90 mM and the maximal short-circuit current (Vmax) 128 mu A/cm2. Furosemide added to the aqueous side of the epithelium rapidly reduced the short-circuit current; the effect was reversible. The effect of furosemide addition to the stromal side was much smaller and slower than that from the aqueous side. The dose-dependent range of furosemide action was from 0.1 micro M to 1 mM with 50% inhibition occurring at about 3 micro M. Lineweaver-Burk plot of the short-circuit current against the chloride concentration showed that furosemide decreased the value of Vmax and increased the Km; the inhibition being of mixed type. A Hill plot of the dose-response curve yielding a slope of unity suggested one furosemide molecule combines with one chloride transport site. Probenecid, a competitive inhibitor of organic acid transport reduced the effects of furosemide significantly when added simultaneously. The involvement of organic acid transport system in the mechanism of furosemide action on chloride transport was suggested.

Accumulation of 125I in the oocytes, thyroids, and plasma of laying Japanese quail treated with cyclic AMP, theophylline, or prostaglandins E1 or E2

Dibutyryl cyclic AMP, theophyline, or prostaglandins E1 or E2 were injected intraperitoneally into laying Japanese quail in an attempt to stimulate increased transference of iodide by the ovarian membranes. Theophyline induced increased 18-hr 125I accumulations (by 7 to 18% of the injected dose) of borderline significance in the growing oocytes of some test groups. Theophyline plus DBcAMP treatment lead to a slight increase in 125I accumulation in the thyroid. Other treatments had no apparent effect on 125I transference. The prostaglandins induced oviposition within minutes of injection, as has been reported for uterine application of prostaglandins by other investigators. Means for plasma 125I levels for treated quail were 1.7 to 12.5 times greater than control levels. Values for oocyte to plasma and thyroid to plasma 125I ratios, it appeared that DBcAMP, theophyline, and the prostaglandins inhibited rather than stimulated the accumulation of 125I by the ovary and thyroid of the quail.

Synaptosomal [125I]triiodothyronine after intravenous [125I]thyroxine

We administered [125I]thyroxine intravenously to adult male rats and measured uptake and subcellular distribution of the hormone and its metabolites in brain. Fractional brain uptake decreased after a large dose of iodothyronine, providing evidence for saturability of the uptake mechanism. Well-defined patterns of regional and subcellular labeling were noted within 1 h after [125I]thyroxine injection. Radioactivity in synaptosomes was always greater than in any other particle separated per gram of brain, increasing linearly relative to radioactivity in brain cytosol during the 1st h. Although [125I]triiodothyronine derived from [125I]thyroxine was not identified in serum at any time interval, it was measurable in synaptosomes within 20 min and in brain cytosol within 1 h after labeled hormone administration. Concentrations of the radioactive metabolite were twofold greater and ratios of [125I]triiodothyronine to [125I]thyroxine concentration were threefold greater in synaptosomes than in cytosol. Therefore, thyroxine may be converted to triiodothyronine within nerve terminals. Synaptosomal localization of iodothyronines and their metabolites may be relevant to the marked central and peripheral adrenergic nervous system effects of these aromatic amino acid hormones.

Comparison of the transfer of 125I, 82Br, and 26Cl into the growing oocytes of the Japanese quail

Laying Japanese quail were doubly-labeled with either 125I and 36Cl or 82Br and 36Cl. Eighteen-hour accumulations, expressed as percent of tracer doses, were: growing oocytes, 125I 48.2 vs. 36Cl 2.07, 82Br 4.01 vs. 36Cl 1.99; thyroids, 125I 10.6 vs. 36Cl .030, 82Br .027 vs. 36Cl .020. Concentrations (expressed as percent per gram) of the radionuclides in the growing oocytes and the thyroids as compared with concentrations in the plasma were: for oocytes, the mean 125I concentration was 199 times the mean plasma concentration, 82Br was .34 x and 36Cl was .13 x; for thyroids, 125I was 13,500 x, 82Br was .48 x, and 36Cl was .41 x the mean plasma concentration. Eight day totals for yolks of eggs laid were 125I 29% vs. 36Cl 3.7%.

Effect of bicarbonate and other buffers on choroid plexus Na+/K+pump

The Na+/K+ pump on the apical membrane of the choroid plexus epithelium was found to be sensitive to bicarbonate. Glycodiazine, a non-volatile, lipid soluble buffer with a pK of 5.7, mimicked the effect of bicarbonate, and was transported across the epithelium in the same direction as sodium. These results are explained in terms of a Na+/H+ exchange mechanism on the basal-lateral membrane.

Lithium-induced inhibition of 125I accumulation by thyroids and growing oocytes of Japanese quail

A single dose of 975 mumol. Li Cl suppressed thyroidal accumulation of iodide in laying Japanese quail as measured at 1, 6, and 18 hr. after injecting 125 I and in mature male quail at 1 hr. Thyroidal accumulation was above control values in males by 18 hr. Levels of 125 I were unchanged in growing oocytes but in testes were 3 to 60 times the control levels. When accumulations were expressed relative to the concentration of 125I in the plasma, in lithium-treated quail levels in thyroids were 0.02 to 0.5 times, in oocytes 0.2 to 0.9 and in testes 0.7 to 0.8 times control values. Results indicate that a single large dose of LiCl caused retention of 125I in the extracellular fluid of Japanese quail and thereby in the testes but inhibited transport of the tracer into the thyroid and growing oocytes.

The significance of 125I as a tracer for lymphocytes, liver cells and erythrocytes after iodination by the lactoperoxidase-H2O2 technique

Human erythrocytes, porcine and rat liver cells, porcine spleen lymphocytes and cultured human lymphoma cells (266 Bl) have been labelled with 125I by the lactoperoxidase-H2O2 method. Large amounts of radioactivity were released when the iodinated cells were incubated in different buffers, and the rate of the release varied considerably between the different cells. Incubation at a higher temperature increased the release rate, while metabolic inhibitors such as iodoacetamide, trasylol or sodium azide did not. When collagenase was used during the preparation of spleen lymphocytes, the rate of the radioactivity release was decreased about 50%. Several findings indicated that the released radioactivity originated from free iodide. When the labelled lymphocytes were treated with a nonionic detergent, Nonidet P-40, 90% of the total radioactivity was solubilized. Only 10-15% of the radioactivity was stably bound in macromolecular material. The remaining part, corresponding to the amount of radioactivity released during incubation, was shown to be free iodide. It is concluded that the significance of the 125I-label in living cells has to be studied in each case at the specific experimental conditions used. After the unspecifically trapped iodide is released--normally after about 2 h--the label is considered to be useful for studies with intact cells.

Developmental studies of the compartmentalization of water and electrolytes in the choroid plexus of the neonatal rat brain

Rats of various postnatal ages were utilized to study developmental changes in the distribution of Na, K and H2O between the various compartments of the lateral ventricular plexus (LVP). During the 3 weeks after birth, as the LVP grows from 0.5 to 0.8 mg, there is a significant increase in plexus K which is accompanied by a progressive decrease in Na and H2O. Also, during this postnatal period the decrease in [3h]inulin space in the plexus is proportional to the decrease in the Na space. Between 3 weeks and adulthood, the [3h]inulin and Na spaces are both augmented to a similar extent; moreover, during this same period of development there is a trebling of the residual [51cr]erythrocyte volume. Despite the substantial changes in the volume of the extracellular fluid and of the residual blood in the plexus with age, the calculated concentrations (mEquiv./kg H2O) of choroid cell Na (30-35) and K (145-155) are similar for all ages investigated. The derived data for cellular ionic concentration, together with the analysis of the ionic concentration gradients (cerebrospinal fluid/plasma H2O), suggest that the transport mechanism which translocates Na and K across the choroidal membrane is operative as early as 3-4 days postnatal. The important role of the choroid plexus in central nervous system homeostasis is discussed in relation to the developing brain.

Ovarian 125I transference in the laying Japanese quail: apparent stimulation by FSH and lack of stimulation by TSH

Six-hour accumulations of 125I were measured in the growing oocytes and thyroids of laying Japanese quail injected with 0.002 to 2 mg. of FSH or 0.004 to 4 I.U. of TSH per day for six days. FSH at 0.2 and 2 mg./day increased the 125I content of the growing oocytes to 1.4 times the control values. The numbers and the total weights of these growing oocytes doubled. The egg-laying rate remained unchanged. The FSH-induced apparent stimulation of 125 I transference was seen as a consequence of the increase in oocyte material rather than to a direct effect of FSH on the membranes of the avarian follicular cells. TSH did not increase the transference of 125I.

Activation of tubulo-glomerular feedback by chloride transport

To define the luminal agent(s) responsible for the reduction of nephron filtration rate following increases of loop of Henle flow rate early proximal flow rate (EPFR) during loop perfusion with 17 different salt solutions were compared to the non-perfused tubules. During orthograde microperfusions a reduction of EPFR as indication of a feedback response was noted with a number of monovalent Cl- and Br- salts (LiCl, KCl, NaCl, RbCl, CsCl, NH4Cl, choline Cl, NaBr, KBr), with Na+ salts except Na acetate (NaHCO3, NaNO3, NaF, NaI, NaSCN), and with CaCl2 and MgCl2. These latter 2 solutions where used in a concentration of 70 mM while all other solutions had a concentration of 140 mM. During retrograde perfusion from the distal to the proximal end of the loop of Henle EPFR fell significantly with Cl- and Br- salts with percentage changes of EPFR ranging from -8.0 to -44.3%. In contrast, Cl- free salts and Cl- salts of divalent cations were associated with percentage changes of EPFR ranging from +7.1 to -6.2%, significance being reached only during perfusion with NaSCN. When furosemide (5 x 10(-4) M) was added to NaBr or KBr a feedback response was not observed. During orthograde perfusion with NaNO3 distal Cl- concentrations were 44.2 +/- 5.08, mM (mean +/- S.E.) at a perfusion rate of 10 nl/min and 59.1 +/- 3.93 mM at a rate of 40 nl/min. CaCl2 perfusion induced a marked elevation of distal Cl- concentrations to levels higher than 140 mM. Loop chloride handling was normal during RbCl perfusion. The magnitude of the feedback response during retrograde perfusion was not changed by lowering NaCl concentration from 140 to 60 mM, but fell when NaCl concentration was further reduced. In contrast to orthograde perfusions it was insensitive to changes in flow rate. Our results are compatible with the thesis that feedback responses depend critically upon the rate of Cl- transport probably across the macula densa cells. Br- ions can replace Cl- because they appear to share a common transport pathway which can be inhibited with furosemide. Unspecificity of feedback responses during orthograde microperfusions is due to presence of Cl- ions in the macula densa region even when solutions are initially Cl- free. Cl- salts of divalent cations do not elicit a feedback response because Cl- transport is severely curtailed.