Translocation of inhaled ultrafine manganese oxide particles to the central nervous system

BACKGROUND

Studies in monkeys with intranasally instilled gold ultrafine particles (UFPs; <100 nm) and in rats with inhaled carbon UFPs suggested that solid UFPs deposited in the nose travel along the olfactory nerve to the olfactory bulb.

METHODS

To determine if olfactory translocation occurs for other solid metal UFPs and assess potential health effects, we exposed groups of rats to manganese (Mn) oxide UFPs (30 nm; approximately 500 microg/m(superscript)3(/superscript)) with either both nostrils patent or the right nostril occluded. We analyzed Mn in lung, liver, olfactory bulb, and other brain regions, and we performed gene and protein analyses.

RESULTS

After 12 days of exposure with both nostrils patent, Mn concentrations in the olfactory bulb increased 3.5-fold, whereas lung Mn concentrations doubled; there were also increases in striatum, frontal cortex, and cerebellum. Lung lavage analysis showed no indications of lung inflammation, whereas increases in olfactory bulb tumor necrosis factor-alpha mRNA (approximately 8-fold) and protein (approximately 30-fold) were found after 11 days of exposure and, to a lesser degree, in other brain regions with increased Mn levels. Macrophage inflammatory protein-2, glial fibrillary acidic protein, and neuronal cell adhesion molecule mRNA were also increased in olfactory bulb. With the right nostril occluded for a 2-day exposure, Mn accumulated only in the left olfactory bulb. Solubilization of the Mn oxide UFPs was <1.5% per day.

CONCLUSIONS

We conclude that the olfactory neuronal pathway is efficient for translocating inhaled Mn oxide as solid UFPs to the central nervous system and that this can result in inflammatory changes. We suggest that despite differences between human and rodent olfactory systems, this pathway is relevant in humans.

Neurosteroids act on recombinant human GABAA receptors

The endogenous steroid metabolites 3 alpha,21dihydroxy-5 alpha-pregnan-20-one and 3 alpha-hydroxy-5 alpha-pregnan-20-one potentiate GABA-activated Cl- currents recorded from a human cell line transfected with the beta 1, alpha 1 beta 1, and alpha 1 beta 1 gamma 2 combinations of human GABAA receptor subunits. These steroids are active at nanomolar concentrations in potentiating GABA-activated Cl- currents and directly elicit bicuculline-sensitive Cl- currents when applied at micromolar concentrations. The potentiating and direct actions of both steroids were expressed with every combination of subunits tested. However, an examination of single-channel currents recorded from outside-out patches excised from these transfected cells suggests that despite the common minimal structural requirements for expressing steroid and barbiturate actions, the mechanism of GABAA receptor modulation by these pregnane steroids may differ from that of barbiturates.

Characterization of the permselective properties of excised human skin during iontophoresis

The iontophoretic and passive transport of [3H]mannitol, 22Na+, 36Cl-, and 45Ca++ across excised human cadaver skin was studied using diffusion cells. The anode (+) was placed in the side of the diffusion cell facing the epidermis and the cathode (-) was placed in the side facing the dermis, and current densities at 0, 0.078, 0.16, and 0.23 mA.cm-2 were investigated. The results showed that mannitol and Na+ were transported preferentially by anodal (+) iontophoresis, Cl- was transported by cathodal (-) iontophoresis, and all respective fluxes were approximately proportional to the applied current density. When the skin was present as a membrane barrier between the two diffusion cell chambers the voltage induced flux of Na+ was found to be higher than its free solution value, and that of Cl- was lower. Taken together these results suggest that the skin is a permselective membrane and exists with an "apparent" net negative charge at the free solution pH of 7.4. During iontophoresis this permselectivity leads to current-induced volume flow, which provides a primary mechanism for the transport for a polar uncharged molecule such as mannitol. When Ca++ is substituted for Na+ on the side of the diffusion cell facing the epidermis, the Cl- flux from the dermal side is enhanced with a portion of the remaining charge being carried by Ca++. The mannitol flux from the epidermal side was decreased under these conditions. This implies that Ca++ alters the anion/cation flux ratio in the excised tissue, possibly by binding to fixed negative charges in the membrane, with the result that the volume flow is decreased.(ABSTRACT TRUNCATED AT 250 WORDS)

Short-chain fatty acids stimulate active sodium and chloride absorption in vitro in the rat distal colon

Studies were performed to determine the mechanism by which short-chain fatty acids increase colonic Na and Cl absorption by determining unidirectional 22Na and 36Cl fluxes across isolated stripped mucosa from the rat distal colon under voltage clamp conditions. Mucosal butyrate (25 mM, in the absence of bicarbonate) significantly enhanced both net Na and net Cl absorption by 7.0 +/- 1.3 and 6.9 +/- 1.0 microEq/h.cm2, respectively, without increasing the short-circuit current. Net Na and Cl absorption in butyrate-Ringer's solution and HCO3-Ringer's solution were identical. Butyrate stimulation of Na (and Cl) absorption was Cl-dependent and prevented by 1 mM mucosal amiloride, an inhibitor of Na-H exchange, but was HCO3-independent and not inhibited by acetazolamide, a carbonic anhydrase inhibitor. In contrast, bicarbonate-stimulated Na (and Cl) absorption was also Cl-dependent and amiloride-sensitive, but was significantly inhibited by acetazolamide. The effect of mucosal butyrate on net Na and Cl absorption was substantially greater than serosal butyrate, which in the presence of bicarbonate did not alter ion transport. The stimulation of Na and Cl absorption by mucosal butyrate was significantly greater than by propionate and acetate, whereas mucosal formate did not alter Na transport. The results of this study permit the following model: short-chain fatty acid stimulation of active Na and Cl absorption involves uptake of the nonionized form of butyrate and the coupling of Na-H and Cl-butyrate exchanges.

Adaptation of the distal convoluted tubule of the rat. Structural and functional effects of dietary salt intake and chronic diuretic infusion

We studied the effects of dietary NaCl intake on the renal distal tubule by feeding rats high or low NaCl chow or by chronically infusing furosemide. Furosemide-treated animals were offered saline as drinking fluid to replace urinary losses. Effects of naCl intake were evaluated using free-flow micropuncture, in vivo microperfusion, and morphometric techniques. Dietary NaCl restriction did not affect NaCl delivery to the early distal tubule but markedly increased the capacity of the distal convoluted tubule to transport Na and Cl. Chronic furosemide infusion increased NaCl delivery to the early distal tubule and also increased the rates of Na and Cl transport above the rates observed in low NaCl diet rats. When compared with high NaCl intake alone, chronic furosemide infusion with saline ingestion increased the fractional volume of distal convoluted tubule cells by nearly 100%, whereas dietary NaCl restriction had no effect. The results are consistent with the hypotheses that (a) chronic NaCl restriction increases the transport ability of the distal convoluted tubule independent of changes in tubule structure, (b) high rates of ion delivery to the distal nephron cause tubule hypertrophy, and (c) tubule hypertrophy is associated with increases in ion transport capacity. They indicate that the distal tubule adapts functionally and structurally to perturbations in dietary Na and Cl intake.

Acute and chronic ethanol treatments alter GABA receptor-operated chloride channels

The effect of ethanol exposure in vitro on the GABA receptor-operated chloride channel was evaluated by monitoring 36Cl- influx in a membrane vesicle suspension (microsacs) prepared from mouse cerebellum. These experiments directly demonstrate ethanol augmentation of muscimol-stimulated chloride flux. DBA/2J mice were made tolerant to and dependent on ethanol by administration of an ethanol containing liquid diet for 7 days. Exposure to physiologically relevant concentrations of ethanol (10-45 mM) in vitro potentiated muscimol stimulation of 36Cl- uptake in control (pair-fed) membranes, but had no effect on cerebellar microsacs from tolerant/dependent mice. Muscimol stimulation of 36Cl- uptake was not different for pair-fed and ethanol-treated mice. Augmentation of muscimol-induced 36Cl- flux by in vitro ethanol was abolished by a single 4 g/kg injection of ethanol. This "acute tolerance" occurred within 5 min and disappeared within 24 hr after ethanol treatment. The reduced sensitivity of ethanol treated (chronic and acute) mice to ethanol potentiation of muscimol stimulated 36Cl- uptake offers a biochemical correlate to the phenomenon of ethanol tolerance. Moreover, the findings suggest that this biochemical tolerance develops rapidly following a single hypnotic dose of ethanol.

Stretch-activated anion currents of rabbit cardiac myocytes

1. Stretch-activated anion currents were studied in sino-atrial and atrial cells using the whole-cell patch clamp technique. With continuous application of positive pressure (5-15 cmH2O) through the patch clamp electrode, the cell was inflated and the membrane conductance was increased. 2. Voltage clamp steps revealed that the stretch-activated currents had time-independent characteristics. The increase in membrane conductance was reversible on subsequent application of negative pressure to the electrode. 3. The reversal potential of the stretch-activated currents was shifted by 60 mV for a 10-fold change in intracellular Cl- concentration, while it was unaffected by replacement of Na+ in the extracellular solution by N-methyl-D-glucamine. Cell superfusion with Cl(-)-deficient solution (10 mM Cl-) reduced the amplitude of outward current. These findings indicate that the stretch-activated conductance is Cl- selective. 4. The sequence of anion permeability through the stretch-activated conductance was determined to be I-(1.7) > NO3-(1.5) > Br-(1.2) > Cl-(1.0) > and F-(0.6). SCN- appeared to be more permeant than I-. 5. The stretch-activated conductance was reduced by the Cl- channel blockers, 4,4'-dinitrostilbene-2,2'-disulphonic acid disodium salt, 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid or anthracene-9-carboxylate (9-AC). Administration of furosemide or bumetanide had no effect. 6. The stretch-activated Cl- current was recorded even though intracellular Ca2+ ions were chelated by including 10 mM EGTA in the pipette solution. Neither the specific peptide inhibitor of cyclic AMP-dependent protein kinase (50 microM), nor the non-selective blocker of protein kinases, H-7 (20 microM), was effective in reducing the stretch-activated Cl- current, suggesting that the stretch-activated Cl- current is a novel type of cardiac Cl- current, which shows a different modulatory mechanism from that of other cardiac Cl- currents.

Lung hyperpermeability and asthma prevalence in schoolchildren: unexpected associations with the attendance at indoor chlorinated swimming pools

AIMS

To study whether exposure to nitrogen trichloride in indoor chlorinated pools may affect the respiratory epithelium of children and increase the risk of some lung diseases such as asthma.

METHODS

In 226 healthy children, serum surfactant associated proteins A and B (SP-A and SP-B), 16 kDa Clara cell protein (CC16), and IgE were measured. Lung specific proteins were measured in the serum of 16 children and 13 adults before and after exposure to NCl(3) in an indoor chlorinated pool. Relations between pool attendance and asthma prevalence were studied in 1881 children. Asthma was screened with the exercise induced bronchoconstriction test (EIB).

RESULTS

Pool attendance was the most consistent predictor of lung epithelium permeability. A positive dose-effect relation was found with cumulated pool attendance and serum SP-A and SP-B. Serum IgE was unrelated to pool attendance, but correlated positively with lung hyperpermeability as assessed by serum SP-B. Changes in serum levels of lung proteins were reproduced in children and adults attending an indoor pool. Serum SP-A and SP-B were already significantly increased after one hour on the pool side without swimming. Positive EIB and total asthma prevalence were significantly correlated with cumulated pool attendance indices.

CONCLUSIONS

Regular attendance at chlorinated pools by young children is associated with an exposure dependent increase in lung epithelium permeability and increase in the risk of developing asthma, especially in association with other risk factors. We therefore postulate that the increasing exposure of children to chlorination products in indoor pools might be an important cause of the rising incidence of childhood asthma and allergic diseases in industrialised countries. Further epidemiological studies should be undertaken to test this hypothesis.

Anion and cation permeability of a chloride channel in rat hippocampal neurons

The ionic permeability of a voltage-dependent Cl channel of rat hippocampal neurons was studied with the patch-clamp method. The unitary conductance of this channel was approximately 30 pS in symmetrical 150 mM NaCl saline. Reversal potentials interpreted in terms of the Goldman-Hodgkin-Katz voltage equation indicate a Cl:Na permeability ratio of approximately 5:1 for conditions where there is a salt gradient. Many anions are permeant; permeability generally follows a lyotropic sequence. Permeant cations include Li, Na, K, and Cs. The unitary conductance does not saturate for NaCl concentrations up to 1 M. No Na current is observed when the anion Cl is replaced by the impermeant anion SO4. Unitary conductance depends on the cation species present. The channel is reversibly blocked by extracellular Zn or 9-anthracene carboxylic acid. Physiological concentrations of Ca or Mg do not affect the Na:Cl permeability ratio. The permeability properties of the channel are consistent with a permeation mechanism that involves an activated complex of an anionic site, an extrinsic cation, and an extrinsic anion.

Macula densa cells sense luminal NaCl concentration via furosemide sensitive Na+2Cl-K+ cotransport

The macula densa cells of the juxtaglomerular apparatus probably serve as the sensor cells for the signal which leads to the appropriate tubuloglomerular feedback response. The present study reports basolateral membrane voltage (PDbl) measurements in macula densa cells. We isolated and perfused in vitro thick ascending limb segments with the glomerulus, and therefore the macula densa cells, and the early distal tubule still attached. Macula densa cells were impaled with microelectrodes under visual control. PDbl was recorded in order to examine how these cells sense changes in luminal NaCl concentrations. The addition of furosemide, a specific inhibitor of the Na+2Cl-K+ cotransporter in the thick ascending limb, to the lumen of the perfused thick ascending limb hyperpolarized PDbl from -55 +/- 5 mV to -79 +/- 4 mV (n = 7). Reduction of NaCl in the lumen perfusate from 150 mmol/l to 30 mmol/l also hyperpolarized PDbl from -48 +/- 3 mV to -66 +/- 5 mV (n = 4). A Cl- concentration step in the bath from 150 mmol/l to 30 mmol/l resulted in a 24 +/- 4 mV (n = 4) depolarization of PDbl. This depolarization of PDbl was absent when furosemide was present during the Cl- concentration step. These data suggest that the macula densa cells sense changes in luminal NaCl concentration via coupled uptake of Na+ and Cl-.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhancement of gamma-aminobutyric acid-activated Cl- currents in cultured rat hippocampal neurones by three volatile anaesthetics

1. The effects of the volatile anaesthetics enflurane, halothane and isoflurane on gamma-aminobutyric acid (GABA)A receptor-mediated chloride currents were studied in cultured rat hippocampal neurones. Transient current responses were obtained by brief pressure application of GABA to the cell body of neurones under voltage clamp. 2. All three anaesthetics increased the peak amplitude and duration of current 2. All three anaesthetics increased the peak amplitude and duration of current responses to brief applications of GABA. These effects were fully reversible, and did not involve alterations in the reversal potential for GABA responses. 3. The experimental concentrations of anaesthetics were measured directly using gas chromatography. The enhancement of GABA currents increased with increasing anaesthetic concentration. Clinically effective concentrations of anaesthetics (between 1 and 1.5 times MAC (minimum alveolar concentration) produced significant enhancement of GABA currents. 4. These results demonstrate that the changes in the time course of synaptic inhibition reported in the presence of the volatile anaesthetics are likely to result from modification of the function of postsynaptic GABAA receptor-channel complexes. These findings also support the hypothesis that GABAA receptor complexes serve as common molecular target sites for a variety of structurally diverse anaesthetic molecules.

Convective solvent flow across the skin during iontophoresis

Enhanced flux of neutral solutes during transdermal iontophoresis is attributed largely to electroosmotic volume flow. In this study, the iontophoretic fluxes of tritiated water (3H2O) and 14C-labeled mannitol through hairless mouse skin (HMS) were determined. The following questions were addressed: (i) What is the variability of water flux during iontophoresis? (ii) To what extent is the iontophoretic flux of a neutral solute correlated with water flux? (iii) Does the intrinsic permeability of the skin to neutral solutes change following iontophoresis? (iv) What is the effect of low pH on electroosmotic volume flow? and (v) Does the skin remain permselective after removal of the stratum corneum? Transport of both water and mannitol reached steady-state levels during 10 hr of constant-current iontophoresis (0.36 mA/cm2). Anodal fluxes exceeded cathodal values. Cathodal mannitol flux was retarded, relative to passive transport, by net volume flow in the opposite direction, such that transport of this molecule increased significantly after the termination of current passage. Anodal equivalent volume flows for water and mannitol, respectively, were 2.7 (+/- 1.3) and 1.23 (+/- 0.59) microL/hr cm2, indicating that only approximately 50% of the water flux participated in the electroosmosis of mannitol. The passive permeability of water and mannitol after 10 hr of iontophoresis was, respectively, 6 and 30 times greater than the pretreatment values. At pH 7, the cationic permselectivity of HMS was marginal [the Na+ transport number (tNa+) was determined to be 0.46] and less than that reported for human skin.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional contribution of the alpha7 subunit to multiple subtypes of nicotinic receptors in embryonic chick sympathetic neurones

1. Many studies of the alpha7 subunit of the neuronal nicotinic acetylcholine receptor (nAChR) family have demonstrated that this alpha-bungarotoxin (alpha-BgTx)-binding neuronal receptor can participate in ACh-gated channels. Heterologous expression studies reveal that alpha7 subunits form homomeric channels of unusually high Ca2+ permeability. However, the physiological role of the alpha7 subunit in native neuronal nAChR channels is less clear. 2. We present evidence that the alpha7 subunit contributes to the function of at least three subtypes of native nAChR expressed by embryonic chick sympathetic neurones. These subtypes are functionally distinct from heterologously expressed homomeric alpha7 nAChRs as well as homomeric-like currents described in studies of hippocampal and parasympathetic neurones. 3. The proposed nAChRs differ from one another and from homomeric alpha7 nAChRs in their sensitivity to block by alpha7 subunit-specific antagonists: alpha-BgTx and methyllycaconitine (MLA). While MLA blocks 60 % of the macroscopic ACh response, alpha-BgTx inhibits a small component of the macroscopic current described by slow-on and slow-off kinetics. 4. Functional deletion of the alpha7 subunit by antisense oligonucleotide treatment eliminates the susceptibility of the nAChRs to block by both MLA and alpha-BgTx. 5. Single channel recordings combined with pharmacological and antisense-mediated 'deletion' techniques reveal that alpha-BgTx-sensitive alpha7-containing nAChRs have a small unitary conductance (18 pS), brief open time kinetics and relatively low open probability (Po). MLA-sensitive alpha7 nAChRs are characterized by a conductance of approximately 35 pS, intermediate burst duration, and a relatively high Po. 6. The third nAChR subtype deleted by alpha7 antisense treatment is characterized by a unitary conductance of 50 pS and prolonged opening duration. 7. We propose that these three populations of native alpha7-containing nAChRs are distinct heteromeric complexes that include other alpha and/or beta nAChR subunits.

Relative contributions of passive equilibrium and active transport to the distribution of chloride in mammalian cortical neurons

1. Active and passive factors affecting the chloride gradient of cortical neurons were assessed using intracellular recordings from neurons in slices of cingulate cortex maintained in vitro. The chloride equilibrium potential (ECl-) was estimated indirectly from the reversal potentials of responses to perisomatic gamma-aminobutyric acid (GABA) application and the Cl(-)-dependent inhibitory postsynaptic potential (IPSP). Under control conditions the mean resting potential (Vm; -69.7 mV) was not significantly different than the mean IPSP reversal potential (EIPSP; -70.1 mV). 2. Increasing the external potassium concentration ([K+]o) from 1 to 10 mM shifted the mean EIPSP from -80.4 to -61.8 mV. The mean EIPSP was approximately equal to the mean Vm at all [K+]oS. The conditions of Donnan equilibrium are not met in [K+]o less than 10 mM. 3. Polarization of Vm up to 20 mV away from EIPSP for 4 min with maintained current injection had no significant effect on EIPSP. 4. The GABA reversal potential was maintained 37-52 mV less negative than Vm after equilibration in saline in which the external chloride concentration had been reduced from 133 to 5 mM by substitution with isethionate. Vm and input resistance were not significantly different from control values in cells recorded under these conditions. 5. We conclude that Cl- is not passively distributed in cortical neurons, perhaps due to a low resting Cl- permeability. 6. Impalement with electrodes containing 2 M KCl resulted in a rapid 10 mV depolarizing shift in EIPSP that then remained relatively constant. Intracellular iontophoresis of Cl- resulted in a further depolarizing shift of EIPSP of 5-10 mV that returned to control in less than 1 min. The time course of recovery of IPSP amplitude could be fit with a single exponential having a mean time constant of 6.9 +/- 1.5 s and was independent of the amount of Cl- injected or stimulation frequency. 7. Reductions in temperature from 37 to 32 degrees C significantly increased the mean time constant of IPSP recovery from Cl- injection to 11.1 +/- 3.3 s, corresponding to Q10 = 2.6.(ABSTRACT TRUNCATED AT 400 WORDS)

Human pendrin expressed in Xenopus laevis oocytes mediates chloride/formate exchange

Pendred syndrome, characterized by congenital sensorineural hearing loss and goiter, is one of the most common forms of syndromic deafness. The gene causing Pendred syndrome (PDS) encodes a protein designated pendrin, which is expressed in the thyroid, kidney, and fetal cochlea. Pendrin functions as an iodide and chloride transporter, but its role in the development of hearing loss and goiter is unknown. In this study, we examined the mechanism of pendrin-mediated anion transport in Xenopus laevis oocytes. Unlabeled formate added to the uptake medium inhibited pendrin-mediated (36)Cl uptake in X. laevis oocytes. In addition, the uptake of [(14)C]formate was stimulated in oocytes injected with PDS cRNA compared with water-injected controls. These results indicate that formate is a substrate for pendrin. Furthermore, chloride stimulated the efflux of [(14)C]formate and formate stimulated the efflux of (36)Cl in oocytes expressing pendrin, results consistent with pendrin-mediated chloride/formate exchange. These data demonstrate that pendrin is functionally similar to the renal chloride/formate exchanger, which serves as an important mechanism of chloride transport in the proximal tubule. A similar process could participate in the development of ion gradients within the inner ear.

Stress-induced behavioral depression in the rat is associated with a decrease in GABA receptor-mediated chloride ion flux and brain benzodiazepine receptor occupancy

Rats exposed to inescapable tailshock exhibit deficits in learning a simple shuttlebox escape task 24 h later. This syndrome has been termed 'behavioral depression' or 'learned helplessness', and is a model of stress-induced depression. In the present study a significant (25%) decrease in GABA receptor-mediated chloride ion flux as measured by muscimol-stimulated 36Cl- uptake in synaptoneurosomes was found in the cerebral cortices of rats that failed the shuttlebox task as compared to naive control rats. Rats which were exposed to tailshock and subsequently learned the escape task did not show a significant difference in muscimol-stimulated 36Cl- uptake as compared to naive control rats. Similarly, rats that failed to learn the shuttlebox escape task had significantly lower in vivo [3H]Ro15-1788 specific binding in cerebral cortex (43%), hippocampus (35%) and striatum (33%) as compared to naive control rats. In cerebellum and hypothalamus, there were significant reductions in specific [3H]Ro15-1788 binding in both animals that failed and animals that learned the shuttlebox escape task as compared to naive controls. To control the stress of the footshock associated with the shuttlebox escape task, we investigated the effect of gridshock in which total footshock received was equivalent to that received by rats who failed the shuttlebox task. There were no differences in muscimol-stimulated 36Cl- uptake or in vivo [3H]Ro15-1788 specific binding between naive controls and rats administered footshock independent of a learning task. These data suggest that the development of stress-induced behavioral depression may be associated with a decrease in GABA receptor-mediated chloride channel function.

Influence of the route of administration and the chemical form (MnCl2, MnO2) on the absorption and cerebral distribution of manganese in rats

The absorption and cerebral distribution of manganese (Mn) have been studied with respect to the route of administration and the chemical form of the Mn compound. Different groups of adult male rats received either MnCl2, 4H2O or MnO2 once a week for 4 weeks at a dose of 24.3 mg Mn/kg body wt. (b.w.) by oral gavage (g.) or 1.22 mg Mn/kg b.w. by intraperitoneal injection (i.p.) or intratracheal instillation (i.t.). Control rats were treated with 0.9% saline. Four days after the last administration the rats were killed and the concentration of Mn measured in blood, hepatic and cerebral tissues (cortex, cerebellum, and striatum). The liver Mn concentration was not affected by the treatments whatever the chemical form or the route of administration of the Mn compound. Administration of MnCl2 by g., i.p., and i.t. routes produced equivalent steady-state blood Mn concentrations (about 1000 ng Mn/100 ml), representing increases of 68, 59, and 68% compared with controls, respectively. Mn concentrations were significantly increased in the cortex but to a lesser extent (g., 22%; i.p., 36%; i.t., 48%) and were higher in the cerebellum after i.p. and i.t. administrations than after oral gavage. Rats treated i.t. with MnCl2 showed an elective increase of the striatal Mn concentration (205%). In contrast, MnO2 given orally did not significantly increase blood and cerebral tissue Mn concentrations; the low bioavailability is most likely due to the lack of intestinal resorption. Administration of MnO2 i.p. and i.t., however, led to significant increases of Mn concentrations in blood and cerebral tissues. These increments were not significantly different from those measured after MnCl2 administration, except for striatal Mn after i.t. which was markedly less (48%) after MnO2 administration. A comparison of the blood Mn kinetics immediately after g. and i.t. treatment with MnCl2 or MnO2 indicated that the higher elevation of blood Mn concentration (> 2000 ng Mn/100 ml) after i.t. administration of MnCl2 could account for the elective uptake of Mn in the striatum observed in repeated dosing experiments. It is concluded that the modulation of Mn distribution in brain regions according to the route of administration and the chemical form of the Mn compound may be explained on the basis of different blood Mn kinetics and regional anatomic specificities of the striatal region.

Cl- flux through a non-selective, stretch-sensitive conductance influences the outer hair cell motor of the guinea-pig

Outer hair cells underlie high frequency cochlear amplification in mammals. Fast somatic motility can be driven by voltage-dependent conformational changes in the motor protein, prestin, which resides exclusively within lateral plasma membrane of the cell. Yet, how a voltage-driven motor could contribute to high frequency amplification, despite the low-pass membrane filter of the cell, remains an enigma. The recent identification of prestin's Cl- sensitivity revealed an alternative mechanism in which intracellular Cl- fluctuations near prestin could influence the motor. We report the existence of a stretch-sensitive conductance within the lateral membrane that passes anions and cations and is gated at acoustic rates. The resultant intracellular Cl- oscillations near prestin may drive motor protein transitions, as evidenced by pronounced shifts in prestin's state-probability function along the voltage axis. The sensitivity of prestin's state probability to intracellular Cl- levels betokens a more complicated role for Cl- than a simple extrinsic voltage sensor. Instead, we suggest an allosteric modulation of prestin by Cl- and other anions. Finally, we hypothesize that prestin sensitivity to anion flux through the mechanically activated lateral membrane can provide a driving force that circumvents the membrane's low-pass filter, thus permitting amplification at high acoustic frequencies.

Blood-brain barrier permeability and brain concentration of sodium, potassium, and chloride during focal ischemia

Brain edema formation during the early stages of focal cerebral ischemia is associated with an increase in both sodium content and blood-brain barrier (BBB) sodium transport. The goals of this study were to determine whether chloride is the principal anion that accumulates in ischemic brain, how the rate of BBB transport of chloride compares with its rate of accumulation, and whether the stimulation seen in BBB sodium transport is also seen with other cations. Focal ischemia was produced by occlusion of the middle cerebral artery (MCAO) in anesthetized rats. Over the first 6 h after MCAO, the amount of brain water in the center of the ischemic cortex increased progressively at a rate of 0.15 +/- 0.02 (SE) g/g dry wt/h. This was accompanied by a net increase in brain sodium (48 +/- 12 mumol/g dry wt/h) and a loss of potassium (34 +/- 7 mumol/g dry wt/h). The net rate of chloride accumulation (16 +/- 1 mumol/g dry wt/h) approximated the net rate of increase of cations. Three hours after MCAO, the BBB permeability to three ions (22Na, 36Cl, and 86Rb) and two passive permeability tracers ([3H]alpha-aminoisobutyric acid ([3H]AIB) and [14C]urea) was determined. Permeability to either passive tracer was not increased, indicating that the BBB was intact. The rate of 36Cl influx was 3 times greater and the rate of 22Na influx 1.8 times greater than their respective net rates of accumulation in ischemic brain.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurotoxicity of manganese chloride in neonatal and adult CD rats following subchronic (21-day) high-dose oral exposure

The purpose of this study was to evaluate the relative sensitivity of neonatal and adult CD rats to manganese-induced neurotoxicity. Identical oral manganese chloride (MnCl(2)) doses (0, 25, or 50 mg kg(-1) body wt. day(-1)) were given to neonatal rats throughout lactation (i.e. from postnatal day (PND) 1 through 21) and to adult male rats for 21 consecutive days. The MnCl(2) doses administered to neonates were ca. 100-fold higher than those resulting from the consumption of an equivalent volume of rat's milk. Rats were assessed using similar behavioral and neurochemical evaluations. Several statistically significant changes occurred in Mn-exposed rats relative to control animals. Neonates given the high dose of MnCl(2) had reduced body weight gain. An increased pulse-elicited acoustic startle response amplitude was observed in neonates from both MnCl(2) treatment groups on PND 21. Increased striatal, hippocampal, hindbrain and cortical Mn concentrations were observed in all Mn-exposed neonates on PND 21. Increased hypothalamic and cerebellar Mn concentrations were also observed on PND 21 in neonates from the high-dose group only. Increased striatal, cerebellar and brain residue Mn concentrations were observed in adult rats from the high-dose group. Increased striatal dopamine and 3,4-dihydroxyphenylacetic acid levels were observed only in PND 21 neonates from the high-dose group. No treatment-related changes were observed in clinical signs, motor activity (assessed in neonates on PND 13, 17, 21 +/- 1 and in adults), passive avoidance (assessed in neonates on PND 20 +/- 1 and in adults) or neuropathology (assessed in PND 21 neonates only). The results of our experiment suggest that neonates may be at greater risk for Mn-induced neurotoxicity when compared to adults receiving similar high oral levels of Mn.

Endogenous expression of type II cGMP-dependent protein kinase mRNA and protein in rat intestine. Implications for cystic fibrosis transmembrane conductance regulator

Certain pathogenic bacteria produce a family of heat stable enterotoxins (STa) which activate intestinal guanylyl cyclases, increase cGMP, and elicit life-threatening secretory diarrhea. The intracellular effector of cGMP actions has not been clarified. Recently we cloned the cDNA for a rat intestinal type II cGMP dependent protein kinase (cGK II) which is highly enriched in intestinal mucosa. Here we show that cGK II mRNA and protein are restricted to the intestinal segments from the duodenum to the proximal colon, with the highest amounts of cGK II protein in duodenum and jejunum. cGK II mRNA and protein decreased along the villus to crypt axis in the small intestine, whereas substantial amounts of both were found in the crypts of cecum. In intestinal epithelia, cGK II was specifically localized in the apical membrane, a major site of ion transport regulation. In contrast to cGK II, cGK I was localized in smooth muscle cells of the villus lamina propria. Short circuit current (ISC), a measure of Cl- secretion, was increased to a similar extent by STa and by 8-Br-cGMP, a selective activator of cGK, except in distal colon and in monolayers of T84 human colon carcinoma cells in which cGK II was not detected. In human and mouse intestine, the cyclic nucleotide-regulated Cl- conductance can be exclusively accounted for by the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel. Viewed collectively, the data suggest that cGK II is the mediator of STa and cGMP effects on Cl- transport in intestinal-epithelia.

Evidence for a morphological component in acid-base regulation during environmental hypercapnia in the brown bullhead (Ictalurus nebulosus)

Exposure of adult brown bullheads Ictalurus nebulosus (120-450 g) to environmental hypercapnia (2% carbon dioxide in air) and subsequent recovery caused transient changes in whole body net sodium flux (JnetNa+) and net chloride flux (JnetCl-) resulting largely from changes in whole body sodium influx (JNa+in) and chloride influx (JinCl-). Scanning electron microscopy (SEM) revealed that the fractional area of chloride cells (CCs) on the interlamellar regions was reduced by 95% during environmental hypercapnia. During post-hypercapnic recovery, gill filament CC fractional area increased. The changes in JinCl- during and after environmental hypercapnia were closely associated with the changes in CC fractional area while the changes in JinNa+ did not correspond to the changes in CC fractional area. Transmission electron microscopy (TEM) supported the SEM observations of CC surface area changes and demonstrated that these changes were caused by covering/uncovering by adjacent pavement cells (PVCs). Lamellar and filament PVC microvilli density increased during hypercapnia while there was a subsequent reduction in the posthypercapnic period. These data suggest that an important mechanism of acid-base regulation during hypercapnic acidosis is modification of the chloride cell-associated Cl-/HCO3- exchange mechanism. We suggest that bullheads vary availability, and thus functional activity, of this transporter via reversible morphological alterations of the gill epithelium. The increase in density of PVC microvilli may be associated with sodium uptake and/or acidic equivalent excretion during acidosis.

Urinary Potassium Excretion and Sodium Sensitivity in Blacks

Based on racial differences in urinary potassium excretion and responses to diuretics, we present a model suggesting that a major cause of sodium sensitivity in blacks is an augmented activity of the Na-K-2Cl cotransport in the thick ascending limb of Henle's loop. This would result in an increased ability to conserve not only sodium but also water, and an upward and rightward shift in the operating point of tubuloglomerular feedback, which may cause an increase in the glomerular capillary hydraulic pressure and predilection to glomerular injury with and without hypertension. In this sense, the biological implication of sodium sensitivity in blacks and in humans in general has ramifications above and beyond salt-evoked increase in blood pressure.