Role of membrane trafficking in plasma membrane solute transport

Cells can rapidly and reversibly alter solute transport rates by changing the kinetics of transport proteins resident within the plasma membrane. Most notably, this can be brought about by reversible phosphorylation of the transporter. An additional mechanism for acute regulation of plasma membrane transport rates is by the regulated exocytic insertion of transport proteins from intracellular vesicles into the plasma membrane and their subsequent regulated endocytic retrieval. Over the past few years, the number of transporters undergoing this regulated trafficking has increased dramatically, such that what was once an interesting translocation of a few transporters has now become a widespread modality for regulating plasma membrane solute permeabilities. The aim of this article is to review the models proposed for the regulated trafficking of transport proteins and what lines of evidence should be obtained to document regulated exocytic insertion and endocytic retrieval of transport proteins. We highlight four transporters, the insulin-responsive glucose transporter, the antidiuretic hormone-responsive water channel, the urinary bladder H(+)-ATPase, and the cystic fibrosis transmembrane conductance regulator Cl- channel, and discuss the various approaches taken to document their regulated trafficking. Finally, we discuss areas of uncertainty that remain to be investigated concerning the molecular mechanisms involved in regulating the trafficking of proteins.

A conformationally constrained competitive inhibitor of the sodium-dependent glutamate transporter in forebrain synaptosomes: L-anti-endo-3,4-methanopyrrolidine dicarboxylate

A series of L-3,4-methanopyrrolidine dicarboxylate isomers were investigated as potential inhibitors of the high affinity, sodium-dependent glutamate transporter in rat forebrain synaptosomes. Of the isomers tested, only L-anti-endo-3,4-methanopyrrolidine dicarboxylate (L-anti-endo-MPDC) blocked the uptake of [3H]D-aspartate, a non-metabolized substrate. Kinetic analysis demonstrated that L-anti-endo-MPDC is a potent competitive inhibitor (Ki = 5 microM) comparable to that of L-glutamate and L-trans-2,4-pyrrolidine dicarboxylate (L-trans-2,4-PDC). Conformational analysis of L-glutamate, L-trans-2,4-PDC and L-anti-endo-MPDC are used to refine the pharmacophore model of the transporter binding site.

Biochemical and biophysical identification of cystic fibrosis transmembrane conductance regulator chloride channels as components of endocytic clathrin-coated vesicles

Cystic fibrosis results from mutations in the gene encoding the CFTR Cl- channel. Although CFTR occurs as an integral component of the plasma membrane, recent studies implicate CFTR in endocytic recycling and suggest that the protein may also exist in intracellular vesicular compartments. To test this, we analyzed CFTR in clathrin-coated vesicles (CCV) purified from cells constitutively expressing CFTR at high levels. CFTR immunoreactivity was detected in CCV by immunoblot and was identified as CFTR based on labeling of immunoprecipitates with protein kinase A and by tryptic phosphopeptide mapping. Fusion of uncoated CCV with planar lipid bilayers resulted in the incorporation of kinase- and ATP-activated Cl- channel activity (7.8 pS at 20 degrees C; 11.9 pS at 37 degrees C), with a linear current-voltage relation under symmetrical conditions. Thus, functional CFTR occurs in CCV. Moreover, CFTR interacts with the plasma membrane specific adaptor complex during endocytosis through clathrin-coated pits. Therefore, the abundance of CFTR in the plasma membrane may be regulated by exocytic insertion and endocytic recycling, and these processes may provide an augmentation to protein kinase A activation as a mechanism for regulating CFTR Cl channels in the plasma membrane.

Forskolin- but not ionomycin-evoked Cl- secretion in colonic epithelia depends on intact microtubules

Several transport proteins are known to be trafficked to the cell membrane in response to appropriate secretagogues. In several cases, the response has been shown to be dependent on the cytoskeleton. We tested the hypothesis that the forskolin- and/or ionomycin-sensitive Cl- secretory response in colonic epithelia is dependent on an intact cytoskeleton. Using 125I- efflux as an assay for Cl- transport in the colonic epithelial cell line T84, we found that preincubation of the tissue for 3 h with either of two inhibitors of microtubule polymerization, nocodazole or colchicine, disrupted the cellular tubulin architecture and also reduced the forskolin- but not the ionomycin-evoked I- efflux. In contrast, brief exposure (4 min) to nocodazole was without effect on the forskolin-sensitive efflux, suggesting that the drug is not acting to block the stimulus-response pathway. An inactive structural analogue of colchicine, beta-lumicolchicine, had no inhibitory effect on either the forskolin-sensitive efflux or on microtubular structure. In a second model of Cl- secretion, the stripped rat colon, both colchicine and nocodazole reduced the forskolin-dependent short-circuit current by an average of 30-40%, suggesting a similar mechanism for insertion of Cl- channels into the plasma membrane. These findings suggest that the Cl- secretory response is dependent on microtubules and has a physiological role in the adenosine 3',5'-cyclic monophosphate-dependent, but not the Ca(2+)-dependent, Cl- secretion in colonic epithelia.

Stimulation of phosphoinositide hydrolysis by trans-(+/-)-ACPD is greatly enhanced when astrocytes are cultured in a serum-free defined medium

Recent studies have demonstrated that astrocytes have much greater abilities to produce and respond to signalling molecules in the CNS than had been previously estimated. We now report a dramatic enhancement in the ability of a glutamate metabotropic receptor agonist, 1-aminocyclopentane-trans-(+/-)-1,3-dicarboxylic acid (trans-(+/-)-ACPD, to stimulate phosphoinositide hydrolysis in astrocytes cultured in a serum-free defined medium compared with astrocytes cultured in conventional serum-containing medium (43.2 +/- 3.6 vs. 3.2 +/- 0.48-fold of basal, respectively). This enhancement was selective to trans-(+/-)-ACPD as little or no difference in the response to carbachol or norepinephrine was seen between the two culture conditions. These results indicate a great potential for the phosphoinositide pathway in astrocyte glutamatergic signal transduction.

Trinitrophenyl-ATP blocks colonic Cl- channels in planar phospholipid bilayers. Evidence for two nucleotide binding sites

Outwardly rectifying 30-50-pS Cl- channels mediate cell volume regulation and transepithelial transport. Several recent reports indicate that rectifying Cl- channels are blocked after addition of ATP to the extracellular bath (Alton, E. W. F. W., S. D. Manning, P. J. Schlatter, D. M. Geddes, and A. J. Williams. 1991. Journal of Physiology. 443:137-159; Paulmichl, M., Y. Li, K. Wickman, M. Ackerman, E. Peralta, and D. Clapham. 1992. Nature. 356:238-241). Therefore, we decided to conduct a more detailed study of the ATP binding site using a higher affinity probe. We tested the ATP derivative, 2',3',O-(2,4,6-trinitrocyclohexadienylidene) adenosine 5'-triphosphate (TNP-ATP), which has a high affinity for certain nucleotide binding sites. Here we report that TNP-ATP blocked colonic Cl- channels when added to either bath and that blockade was consistent with the closed-open-blocked kinetic model. The TNP-ATP concentration required for a 50% decrease in open probability was 0.27 microM from the extracellular (cis) side and 20 microM from the cytoplasmic (trans) side. Comparison of the off rate constants revealed that TNP-ATP remained bound 28 times longer when added to the extracellular side compared with the cytoplasmic side. We performed competition studies to determine if TNP-ATP binds to the same sites as ATP. Addition of ATP to the same bath containing TNP-ATP reduced channel amplitude and increased the time the channel spent in the open and fast-blocked states (i.e., burst duration). This is the result expected if TNP-ATP and ATP compete for block, presumably by binding to common sites. In contrast, addition of ATP to the bath opposite to the side containing TNP-ATP reduced amplitude but did not alter burst duration. This is the result expected if opposite-sided TNP-ATP and ATP bind to different sites. In summary, we have identified an ATP derivative that has a nearly 10-fold higher affinity for reconstituted rectifying colonic Cl- channels than any previously reported blocker (Singh, A. K., G. B. Afink, C. J. Venglarik, R. Wang, and R. J. Bridges. 1991. American Journal of Physiology. 260 [Cell Physiology. 30]:C51-C63). Thus, TNP-ATP should be useful in future studies of ion channel nucleotide binding sites and possibly in preliminary steps of ion channel protein purification. In addition, we have obtained good evidence that there are at least two nucleotide binding sites located on opposite sides of the colonic Cl- channel and that occupancy of either site produces a blocked state.

Induction of astrocyte glutamine synthetase activity by the Lathyrus toxin beta-N-oxalyl-L-alpha,beta-diaminopropionic acid (beta-L-ODAP)

beta-N-Oxalyl-L-alpha,beta-diaminopropionic acid (beta-L-ODAP) is thought to be the causative agent in lathyrism due to its neuroexcitatory and neurotoxic properties. We have recently reported that beta-L-ODAP is also gliotoxic at high concentrations (Bridges et al.: Brain Res 561:262, 1991). Evidence is now presented that low, subgliotoxic concentrations of beta-L-ODAP may alter the ability of astrocytes to regulate glutamate concentrations in the CNS by increasing astrocyte glutamine synthetase activity. When astrocytes cultured from rat cortex were exposed to 100 microM beta-L-ODAP for 24 h, the resulting glutamine synthetase activity was 155% of control levels. This effect was enantiomer- and isomer-specific, dose-dependent, and required protein translation as the induction was blocked with cycloheximide. The effect of beta-L-ODAP on glutamine synthetase was not mimicked by alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) or kainate, suggesting that the induction was not transduced solely through activation of cell surface non-N-methyl-D-aspartate (NMDA) glutamate receptors. An intracellular site of action of beta-L-ODAP is proposed because its effect on glutamine synthetase activity could be blocked by the amino acid uptake blocker dihydrokainate.

Gliotoxic actions of excitatory amino acids

Cultures of neonatal Type I astrocytes of the rat were exposed to a series of excitatory amino acid analogs to identify those compounds that were gliotoxic. In addition to L-alpha-aminoadipate, a previously identified gliotoxin, L-homocysteate, L-serine-O-sulfate, L-alpha-amino-4-phosphonobutyrate and L-alpha-amino-3-phosphono-propionate were also found to induce a sequence of degenerative events that led to the lysis of the astrocytes. Cellular injury was assessed by quantifying the activity of lactate dehydrogenase present in the surviving astrocytes. Prior to lysis, the cells went through a succession of distinctive morphological changes, the most prominent of which involved nuclear alterations. The nuclei appeared swollen, contained "pale" or "watery" nucleoplasm and exhibited a very prominent nuclear membrane and obvious nucleoli. These astrocytes appeared quite similar in appearance to the Alzheimer's Type II astrocytes, principally associated with the pathology of hepatic encephalopathy. The nuclear anomalies, which are thought to be indicative of cellular damage and compromised function, were also produced by the endogenous transmitters L-glutamate and L-aspartate, although with time, the affected astrocytes appeared to recover and return to normal morphology, without lyzing. These findings suggest that excessive levels of excitatory amino acids may induce cellular damage to astrocytes, as well as neurons. Once damaged, the resulting reductions in astrocyte function may further contribute to CNS losses and the overall pathology attributed to the excitatory amino acids.

Nucleotide-activated chloride channels in lysosomal membranes

Lysosomal membrane vesicles purified from rat liver contain a basal chloride conductance that was enhanced in the presence of ATP, non-hydrolysable ATP-analogs and, to a lesser extent, GTP. Other nucleotides, including AMP, ADP and cAMP, as well as CTP and UTP were not effective. Following fusion of the vesicles with an artificial phosphatidylethanolamine/phosphatidylserine bilayer, we found that ATP gamma S dramatically increased the incidence of 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS)-sensitive chloride channels with a unitary slope conductance of approx. 40 pS in 300 mM/50 mM KCl buffers and 120 pS in symmetrical 300 mM KCl buffers. Since similar results were obtained with AMP-PNP, the results indicate that lysosomes contain a chloride permeable ion channel that is activated by ATP through allosteric interaction.

Endocytosis is regulated by protein kinase A, but not protein kinase C in a secretory epithelial cell line

Endocytosis in the chloride secreting epithelial cell line T84 was monitored by uptake of the fluid-phase markers FITC-dextran and horseradish peroxidase (HRP). Uptake of marker was inhibited by incubation of cells at 4 degrees C, consistent with an endocytic uptake. Although activation of the cAMP-dependent second messenger pathway has been shown to stimulate exocytosis in this cell line, it caused a 63% reduction in endocytosis as measured by uptake of fluid-phase markers. In contrast, the presence of the protein kinase C activator phorbol-myristate acetate (PMA) caused no significant reduction in the level of endocytosis compared to control, nor did it reverse the inhibitory effect of PKA activation. The data thus suggest that endocytosis in T84 cells is regulated through activation of protein kinase A, but not through activation of protein kinase C.

1-Aminocyclopentane-trans-1,3-Dicarboxylic Acid Induces Glutamine Synthetase Activity in Cultured Astrocytes

In this study we have investigated the effect of excitatory amino acids on the activity of glutamine synthetase, a glial-specific enzyme that plays a key role in the regulation of glutamate concentration in the CNS. We found that of L-glutamate, N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate, kainate, and 1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD), only the metabotropic glutamate receptor agonist trans-ACPD had an effect on glutamine synthetase specific activity in cultures of rat type I cortical astrocytes. Exposure of astrocytes to 1.0 mM trans-ACPD for 24 h resulted in an increase in glutamine synthetase activity to 149 +/- 11% of that in control cultures. This effect was concentration dependent, stereoselective, and blocked by cycloheximide. In addition, the increase in glutamine synthetase activity occurred at lower concentrations of trans-ACPD that did not produce morphological alterations or lysis of the astrocytes as measured by the lactate dehydrogenase content. These findings are consistent with the hypothesis that activation of the metabotropic excitatory amino acid receptor in astrocytes is coupled to the regulation of an enzyme essential to the metabolism and recycling of the excitatory transmitter L-glutamate.

Regulation of plasma membrane recycling by CFTR

The gene that encodes the cystic fibrosis transmembrane conductance regulator (CFTR) is defective in patients with cystic fibrosis. Although the protein product of the CFTR gene has been proposed to function as a chloride ion channel, certain aspects of its function remain unclear. The role of CFTR in the adenosine 3',5'-monophosphate (cAMP)-dependent regulation of plasma membrane recycling was examined. Adenosine 3',5'-monophosphate is known to regulate endocytosis and exocytosis in chloride-secreting epithelial cells that express CFTR. However, mutant epithelial cells derived from a patient with cystic fibrosis exhibited no cAMP-dependent regulation of endocytosis and exocytosis until they were transfected with complementary DNA encoding wild-type CFTR. Thus, CFTR is critical for cAMP-dependent regulation of membrane recycling in epithelial tissues, and this function of CFTR could explain in part the pleiotropic nature of cystic fibrosis.

Purification and functional reconstitution of the cystic fibrosis transmembrane conductance regulator (CFTR)

Circumstantial evidence has accumulated suggesting that CFTR is a regulated low-conductance Cl- channel. To test this postulate directly, we have purified to homogeneity a recombinant CFTR protein from a high-level baculovirus-infected insect cell line. Evidence of purity included one- and two-dimensional gel electrophoresis, N-terminal peptide sequence, and quantitative amino acid analysis. Reconstitution into proteoliposomes at less than one molecule per vesicle was accomplished by established procedures. Nystatin and ergosterol were included in these vesicles, so that nystatin conductance could serve as a quantitative marker of vesicle fusion with a planar lipid bilayer. Upon incorporation, purified CFTR exhibited regulated chloride channel activity, providing evidence that the protein itself is the channel. This activity exhibited the basic biophysical and regulatory properties of the type of Cl- channel found exclusively in CFTR-expressing cell types and believed to underlie cAMP-evoked secretion in epithelial cells.

Regulated endocytosis in a chloride secretory epithelial cell line

The colonic epithelial cell line T84 has been shown to be a good model to investigate the regulation of Cl- secretion by the adenosine 3',5'-cyclic monophosphate (cAMP)-mediated second messenger cascade. Regulated exocytic insertion and endocytic retrieval of transport proteins, or proteins that regulate transport proteins, is one mechanism proposed to regulate plasma membrane solute permeabilities. The aims of our studies were to characterize endocytic processes in T84 cells and to investigate their regulation by known activators of Cl- secretion that are mediated by the cAMP second messenger cascade. Forskolin, an activator of adenylate cyclase, caused a marked inhibition of endocytic uptake of the fluid-phase marker horseradish peroxidase (HRP) and the adsorptive marker wheat germ agglutinin conjugated to HRP. Similar inhibition was obtained with vasoactive intestinal peptide, a secretagogue whose receptor is coupled to adenylate cyclase, and 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate, a membrane-permeable cAMP analogue. 1,9-Dideoxy-forskolin, a forskolin analogue that fails to activate adenylate cyclase, was without effect on endocytosis. Our data show that the net rate of endocytosis, as measured by fluid-phase uptake, is decreased by a cAMP-mediated mechanism. Because the number of Cl- channels or associated regulatory proteins in the plasma membrane reflects a balance between their exocytic insertion and endocytic retrieval, we propose that the cAMP-mediated decrease in endocytosis could contribute to the concomitant increase in plasma membrane Cl- permeability.

Peroxynitrite inhibits sodium uptake in rat colonic membrane vesicles

Peroxynitrite (ONOO-) is a potent oxidizing agent that initiates lipid peroxidation and sulfhydryl oxidation and may be responsible for a portion of the cytotoxicity attributed to superoxide anion (.O2-). We quantified the extent to which ONOO-, xanthine plus xanthine oxidase (XO) and hydrogen peroxide (H2O2), decreased sodium (Na+) uptake into membrane vesicles derived from colonic cells of dexamethasone-treated rats. Carrier-free 22Na+ uptake into vesicles was measured in the presence of an inside-negative membrane potential, produced by the addition of the potassium ionophore valinomycin (10 microM) after removal of all external potassium by cation exchange chromatography. Preincubation of vesicles with either 100 microM or 1 mM ONOO- for 30 s decreased the amiloride-blockable fraction of Na+ uptake by 27 +/- 7% and 65 +/- 2%, respectively (means +/- S.E.; n greater than or equal to 5; P less than 0.05 from control). However, the amiloride-insensitive part of Na+ uptake was not affected, indicating that there was no overt destruction of these vesicles by these ONOO- concentrations. Decomposed ONOO-, hydrogen peroxide (1 microM-10 mM), or xanthine (500 microM) plus XO (10-30 mU/ml), either in the absence or in the presence of 100 microM FeEDTA, did not decrease Na+ uptake. These data suggest that ONOO- is a potent injurious agent that can compromise Na+ uptake across epithelial cells, possibly by damaging Na+ channels.

Identification of a membrane protein from T84 cells using antibodies made against a DIDS-binding peptide

The outwardly rectified chloride channel of secretory epithelial cells is inhibited by disulfonic stilbene (DS) compounds such as 4,4'-diisothiostilbene-2,2'-disulfonic acid (DIDS) [R. J. Bridges, R. T. Worrell, R. A. Frizzell, and D. J. Benos, Am. J. Physiol. 256 (Cell Physiol. 25): C902-C912, 1989]. A 13-amino acid peptide (P49) corresponding to the putative DS binding site region of the murine anion exchange protein was synthesized, and polyclonal antibodies were generated against it and then purified over a P49 affinity column. The resulting monospecific antibodies reacted on Western blots with a 95- to 100-kDa protein from human erythrocytes and a 55- to 60-kDa protein from the human colonic tumor cell line, T84. The reaction with T84 protein did not appear to represent recognition of an anion exchanger because anion efflux from T84 cells was independent of external Cl-. In addition, monoclonal antibodies raised against human band 3 recognized the band 3 protein in human red cell ghost preparations but recognized nothing in T84 cell membrane preparations. In T84 cells, DIDS protected the 60-kDa protein from antibody binding. The anti-P49 antibody blocked outwardly rectified Cl- channels incorporated into planar lipid bilayer membranes from rat colon. Immunocytochemical data reveal specific binding of the anti-P49 antibody to perinuclear cytoplasmic vesicles. Forskolin caused these antibody-labeled vesicles to migrate from the perinuclear region to the plasma membrane under conditions and with a time course identical to that seen for stimulation of Cl- transport in these cells. Our results suggest that the protein may be a part of a chloride channel complex of secretory epithelial cells.

Increased density of excitatory amino acid transport sites in the hippocampal formation following an entorhinal lesion

High affinity transport of excitatory amino acids such as L-glutamate into astrocytes is necessary for the termination of its excitatory signal and the prevention of its excitotoxic effects. The removal of glutamate from the synaptic cleft is carried out by both sodium- and chloride-dependent systems. Both sodium-dependent D-[3H]aspartate and chloride-dependent L-[3H]glutamate binding were found to increase in the dentate gyrus molecular layer of rats following an entorhinal lesion. The increased binding reached a maximum at 5 and 7 days postlesion and returned to normal by 12 days postlesion. No changes in binding were observed at long time points postlesion. This increased ability to transport glutamate may be a compensatory response to protect the remaining neurons from the excitotoxic conditions that accompany neuronal degeneration.

Gliotoxic properties of the Lathyrus excitotoxin beta-N-oxalyl-L-alpha,beta-diaminopropionic acid (beta-L-ODAP)

beta-N-Oxalyl-L-alpha,beta-diaminopropionic acid (beta-L-ODAP) is an excitatory amino acid agonist found in the seeds of Lathyrus sativus that is believed to be the major causative agent in the pathology of human lathyrism. We have found that in addition to its previously recognized neurotoxic properties, beta-L-ODAP is also gliotoxic. When added to cultures of neonatal rat astrocytes, beta-L-ODAP induced a series of morphological changes (e.g., extensive vacuole formation, pale and swollen nuclei with obvious nucleoli, and cellular swelling) that led to the eventual lysis of the glial cells. If the beta-L-ODAP was removed prior to the lysis of the astrocytes, many of the early morphological changes appeared to be reversible. When quantitated by a loss of the lactate dehydrogenase activity, beta-L-ODAP lysed the astrocytes with an LD50 of 2.1 +/- 0.2 mM following 48 h of exposure. Lower concentrations of beta-L-ODAP were found to be more toxic if the duration of the exposure was increased. The results suggest that the overall impact of the toxin on the CNS may represent the cumulative action of beta-L-ODAP at a number of distinct points on both neurons and astrocytes. The potential that these multiple sites of action may affect the normal regulation of extracellular glutamate and, consequently, disturb the balance between its normal and pathological roles is discussed.

Colonic Cl channel blockade by three classes of compounds

We compared the potency and inhibitory actions of three different classes of organic acids on a Cl channel derived from colonic enterocyte plasma membrane vesicles. Chloride channels were incorporated into planar lipid bilayer membranes to examine the effects of the anthranilic acids, diphenylamine 2-carboxylic acid (DPC) and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB), the indanyl alkanoic acids, 2-[(2-cyclopentyl-6,7-dichloro-2,3-dihydro-2-methyl-1-oxo-1H-inden -5-yl)oxy] acetic acid (IAA-94) and its stereoenantiomer IAA-95, and the disulfonic stilbene, 4,4'-dinitro-stilbene-2,2'-disulfonic acid (DNDS). Except for DNDS, each of the blockers was equipotent from both the outer membrane and the cytoplasmic side of the channel protein. The potency order from the outmembrane side was DNDS greater than IAA-94 = IAA-95 greater than NPPB much greater than DPC. In contrast, the potency order from the cytoplasmic side was IAA-94 = IAA-95 greater than NPPB greater than DNDS much greater than DPC. DPC and NPPB caused a concentration-dependent decrease in the single-channel conductance (fast block). DNDS, IAA-94, and IAA-95 caused a flickery-type block and a concentration-dependent decrease in open-channel probability. Kinetic analysis revealed that blockade could be explained by a linear closed-opened-blocked kinetic scheme. Similarities in the electrostatic potential maps of these open-channel blockers suggest they may bind to a single shared binding site within the channel protein.

Glutathione levels in olfactory and non-olfactory neural structures of rats

Olfactory receptor neurons are a CNS entry point for a wide variety of airborne substances. Therefore, it is probable that detoxification mechanisms are present in these neurons to neutralize such agents. Glutathione (GSH) is an essential component of several detoxification schemes, and in this study we examined the distribution and levels of GSH in the olfactory epithelium, olfactory bulb, cortex, hippocampus and cerebellum in neonatal, weanling, adult and aged rats. We report that GSH is primarily localized to the olfactory receptor neurons and their oxons within the olfactory epithelium. It is also localized within the glomerular neuropil and granule cells of the olfactory bulb. Levels of GSH in the olfactory epithelium and hippocampus do not change as a function of age, although GSH levels decrease in several brain regions, including the olfactory bulb, cerebellum and cortex.

Amiloride-inhibitable Na+ conductive pathways in alveolar type II pneumocytes

The purpose of these studies was to document the existence of electrogenic Na+ uptake by membrane vesicles of rabbit alveolar type II (ATII) cells and the extent to which this process was inhibited by amiloride. ATII cells (greater than 85% pure) were obtained by elastase digestion of lung tissue followed by Percoll centrifugation, and an enriched plasma membrane vesicle fraction was obtained by differential centrifugation. 22Na+ uptake into these vesicles was measured in the presence of a negative inside membrane potential, produced by the addition of the K+ ionophore valinomycin (10 microM) after all external K+ was removed. Electrogenic (valinomycin-sensitive) Na+ uptake (ELNa) was defined as the difference in uptake in the presence and absence of valinomycin. ELNa, normalized per milligram protein, was twice as high across ATII cells than alveolar macrophage membrane vesicles, was inhibited by amiloride (50% inhibitory concentration = 10 microM), and was decreased in the presence of an outwardly directed proton gradient (pHin 6.8; pHout 7.8), suggesting that it was not mediated by Na(+)-H+ antiport. Furthermore, ELNa was equally inhibited by increasing concentrations of amiloride and benzamil but was more sensitive to 5-(N-ethyl-N-isopropyl)-2'-4'-amiloride in concentrations of 10-1,000 microM. These findings indicate that a fraction of Na+ transport across ATII membrane vesicles occurs through a conductive pathway, probably a channel, that has different sensitivity to amiloride and its analogues than the previously described epithelial high amiloride-affinity Na+ channel.

Conformationally defined neurotransmitter analogues. Selective inhibition of glutamate uptake by one pyrrolidine-2,4-dicarboxylate diastereomer

In order to determine the conformational requirements for binding of L-glutamate to the proteins involved in the process of neurotransmission, rigid analogues containing an embedded glutamate moiety have been prepared. These "conformer mimics", the pyrrolidine-2,4-dicarboxylates 4, 7, 11, and 14, were synthesized from commercially available trans-4-hydroxy-L-proline and cis-4-hydroxy-D-proline, and then were tested for their ability to inhibit the high-affinity transport of [3H]-L-glutamate into synaptosomes and to block the binding of radioligands to the NMDA (N-methyl-D-aspartate), KA (kainate), and QA (quisqualate) glutamate neurotransmitter receptor sites. While none of the four analogues binds effectively to the excitatory receptors, the L-trans-isomer 7 is a potent and selective competitive inhibitor of L-glutamate transport. These results delineate a specific structural/conformational preference for binding to the uptake system that is distinct from that required for binding to the NMDA, KA, and QA receptors.

A simple assay for agonist-regulated Cl and K conductances in salt-secreting epithelial cells

We developed a convenient flux assay that permits simultaneous measurement of Cl and K conductance pathways in Cl-secreting epithelial cells. Monolayers of the colonic tumor cell line T84 were preloaded with 125I and 86Rb, and isotope effluxes were monitored by a sample-replace procedure. The adenosine 3',5'-cyclic monophosphate (cAMP)-mediated agonists forskolin and prostaglandin E2 increased I efflux with little effect on Rb efflux, whereas the Ca-mediated agonists ionomycin, A23187, and carbachol increased both I and Rb effluxes. Simultaneous determinations of I and Cl or Rb and K effluxes indicated that I and Rb provide good measures of the effluxes of Cl and K, respectively. Forskolin- and ionomycin-stimulated I effluxes were inhibited by the Cl-channel blockers diphenylamine-2-dicarboxylate (DPC), 5-nitro-2-(3-phenylpropyl-amino)benzoic acid (NPPB), and 2-[cyclopentyl-6,7-dichloro-2,3-dihydro-2-methyl-1-oxo-1H- inden-5-yl)oxy]acetic acid (IAA-94) and by high external K. The Rb efflux evoked by ionomycin was inhibited by the K-channel blockers Ba and charybdotoxin. These findings suggest that I and Rb effluxes provide qualitative estimates of agonist-stimulated Cl and K conductance pathways. Thus this method can provide a simple and relatively inexpensive screening assay for Cl and K conductances in cultured cells to assess the effects of agonist, blockers, or genetic manipulations.

Autoradiographic characterization of putative excitatory amino acid transport sites

Removal of excitatory amino acids from the extracellular space is now postulated to occur through at least two distinct transport systems that are distinguished by their ionic dependency. Thus, both sodium-dependent and chloride-dependent systems have been described in the mammalian central nervous system. In this report we attempt to characterize these sites by autoradiography, using D-[3H]aspartate and L-[3H]glutamate as ligands. Previous studies have shown that sequestration of radioligand into membrane vesicles can be a potential artifact when examining transport sites. We have found that sequestration can be alleviated by incubation of tissue sections in xylenes prior to incubation with radioligand. Using in vitro autoradiography we have characterized the two binding sites with respect to their distribution, kinetics and pharmacology. Both appeared to have a single, saturable binding site with Kds in the low micromolar range. Sodium-dependent D-aspartate binding predominated, having a Bmax that was five times greater than chloride-dependent L-glutamate binding in whole brain. The levels of binding to the two sites varied between brain regions. Sodium-dependent D-aspartate binding was highest in the cerebellar molecular layer greater than dentate gyrus molecular layer greater than entorhinal cortex. Chloride-dependent L-glutamate binding was highest in the outer layers of cerebral cortex greater than dentate gyrus molecular layer greater than entorhinal cortex greater than striatum. Pharmacological characterization of these sites also showed major differences. Sodium-dependent D-aspartate binding was most potently inhibited by L-aspartate greater than threo-beta-hydroxyaspartate greater than L-cysteine sulfinic acid greater than L-cysteic acid. Chloride-dependent glutamate binding was most potently inhibited by L-glutamate greater than L-alpha-amino adipic acid greater than quisqualate greater than L-serine-o-sulfate. The differences in distribution, ligand binding properties and pharmacology of these sites suggest that a significant variable in excitatory amino acid circuitry may include heterogeneity in transporters associated with excitatory pathways.

Glutathione metabolism at the blood-cerebrospinal fluid barrier

Glutathione metabolism and transport in the choroid plexus were probed by determining the effects of administration to rats of several compounds (buthionine sulfoximine, L-2-oxothiazolidine-4-carboxylate, L-(alpha 5,5S)-alpha-amino-3-chloro-4,5-dihydro-5-isoxazole acetic acid, gamma-glutamyl alanine, and glutathione monoethyl ester) on the levels of glutathione and cysteine in the cerebrospinal fluid. The findings indicate that glutathione is actively metabolized in the choroid plexus by pathways similar to those in kidney and other tissues. The level of glutathione in the cerebrospinal fluid can be decreased or increased by giving compounds that do not, under similar conditions, appreciably alter total brain levels of glutathione. Glutathione monoethyl ester is effectively transported into the cerebrospinal fluid.

Cholinergic-mediated secretion in the rat colon: neuronal and epithelial muscarinic responses

The acetylcholine receptor agonists, acetylcholine (10(-5)-10(-4 M), carbachol (5 x 10(-6)-5 x 10(-5) M), bethanechol (5 x 10(-5)-5 x 10(-4) M) and dimethylphenylpiperazinium (DMPP, 10(-5) M) increased the short-circuit current (Isc) in the rat colon descendens by a tetrodotoxin (TTX)-sensitive mechanism. Blockade by TTX was still observed after removal of the submucosa, indicating the involvement of neurons of the mucosal plexus. Hexamethonium (10(-5) M) and atropine (10(-6) M) were used to distinguish between nicotinic and muscarinic neuronally mediated effects. The inhibitor of choline uptake, hemicholinium-3 (1 mM), reversibly inhibited the effect of repeated electric field stimulation (EFS). The EFS response was only inhibited by high concentrations of atropine (greater than or equal to 10(-5) M). In mucosa-submucosa preparations 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) was more effective than telenzepine whereas pirenzepine was ineffective. Pirenzepine inhibited the EFS response in mucosa preparations as did telenzepine and 4-DAMP. It was not possible to differentiate between the muscarinic receptors involved in the different parts of the enteric nervous system on the basis of our results.

N-methyl-D-aspartate receptors and Alzheimer's disease

The results of several studies now suggest that the density of N-methyl-D-aspartate (NMDA) receptors is maintained in many Alzheimer's disease (AD) cases, although loss of these receptors can occur in specific regions as a consequence of severe neuronal loss. Recent findings demonstrate that there are at least two subtypes of the NMDA receptor which are allosterically regulated. To determine the status of the NMDA receptor in AD, studies are required which will examine the activation state of the NMDA receptor and the properties of subtypes in relation to neuronal density and structure.

Structure-function studies on N-oxalyl-diamino-dicarboxylic acids and excitatory amino acid receptors: evidence that beta-L-ODAP is a selective non-NMDA agonist

Excitatory amino acids and their receptors play an important role in both normal synaptic transmission and excitotoxic-mediated neuronal death. In the present investigation we have prepared a series of glutamate analogs and examined the pharmacological specificity with which they interact with excitatory amino acid receptors. Included within this group of compounds is a potent excitotoxic amino acid, beta-N-oxalyl-L-alpha, beta-diaminopropionic acid (beta-L-ODAP). This excitotoxin is of particular interest because it has been identified as a major causative agent of human neurolathyrism, a disease characterized by permanent spastic paralysis. The site of action of beta-L-ODAP was delineated with both electrophysiological recordings in hippocampal slices and radioligand binding assays in synaptic plasma membranes. We report that beta-L-ODAP is a potent agonist at the non-N-methyl-D-aspartate (NMDA) type of excitatory amino acid receptor. beta-L-ODAP interacts most potently with the quisqualate class of non-NMDA receptors (IC50 = 1.3 microM), less potently with the kainate receptor (IC50 = 17 microM), and very weakly with NMDA receptors. The specificity of this binding was consistent with physiological experiments that demonstrated that beta-L-ODAP-induced depolarizations were potently blocked by the newly identified non-NMDA receptor antagonist, CNQX, but were not affected by the NMDA antagonist D-AP5. These results extend recent studies that have focused on the contribution of NMDA receptors to excitotoxicity and highlight the potential involvement of non-NMDA receptors in excitotoxic-mediated cell death.

Stilbene disulfonate blockade of colonic secretory Cl- channels in planar lipid bilayers

We studied blockade by 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS) of a secretory Cl- channel from colonic enterocyte plasma membrane vesicles incorporated into planar lipid bilayer membranes. Except for intermittent long-lived closed periods (100 ms to several min), the control channel open probability (Po) was greater than 90%. DNDS, added to the cis or vesicle-containing side, which corresponds to the outer membrane side of the channel, caused a dramatic increase in the number of current transitions from the open-to-closed state. DNDS caused a concentration-dependent decrease in Po with a maximum inhibition of 95 +/- 2.0% and a half-maximal inhibitory concentration of 3.3 +/- 1.4 microM. DNDS added to the trans side of the channel had no effect on either the single-channel conductance or kinetic behavior of the channel. Kinetic analysis revealed that DNDS blockade from the cis side could be explained by a linear, closed-open-blocked, kinetic scheme. The estimated DNDS block rate constants were kon = 3.2 X 10(7) M-1.s-1 and koff = 52 s-1, yielding an equilibrium dissociation constant (KD) of 2.1 +/- 0.38 microM, similar to the Ki for inhibition of Po. The effects of DNDS were fully reversible after perfusion of the cis compartment with DNDS-free solution. In contrast, the covalently reactive 4,4'-diisothiocyano-substituted stilbene disulfonate caused an irreversible blockade of the Cl- channel.

Inhibition of colonic Na+ transport by amiloride analogues

The potency of several amiloride analogues to inhibit electrogenic Na+ transport in colon from dexamethasone-treated rats was compared. Short-circuit current (Isc) across the colonic mucosa and 22Na+ uptake into membrane vesicles derived from colonic enterocytes was determined in dexamethasone-treated rats. Kinetic analysis of inhibition of Isc and 22Na+ uptake revealed the presence of a high- and low-affinity amiloride pathway. One pathway had a high affinity [(Ki-Isc; Ki uptake] to benzamil (15.5 nM; 5.4 nM), phenamil (19.4 nM; 7.0 nM), 3',4'-dichlorobenzamil (29.0 nM; 25.2 nM), and amiloride (115 nM; 12.4 nM) but a much lower affinity to 5-(N-ethyl-N-isopropyl)amiloride (EIPA) (greater than 100 microM; greater than 9.9 microM) and 5-(N-propyl-N-butyl)-2'-4'-dichlorobenzamil (PBDCB) (greater than microM; greater than 32.8 microM). The high-affinity pathway accounted for 75-83% of the transport of Na+. The second pathway had nearly the same low affinity for each of the analogues (e.g., amiloride Ki-Isc 1 microM; Ki uptake 4 microM) and accounted for only 15-25% of the transport of Na+. The results demonstrate that the structure-inhibitory pattern of these amiloride analogues for the high-affinity pathway is the pattern observed in other electrogenic Na+-transporting epithelia and that this pharmacological profile is preserved in membrane vesicles derived from colonic enterocytes. In addition, the potency of EIPA and benzamil to inhibit electroneutral Na+ transport across the colon from normal rats (i.e., not treated with dexamethasone) was also investigated.(ABSTRACT TRUNCATED AT 250 WORDS)

Na+ uptake into colonic enterocyte membrane vesicles

Na+ uptake was studied in colonic enterocyte membrane vesicles prepared from normal and dexamethasone-treated rats. Vesicles from rats treated with dexamethasone demonstrated a fivefold greater 22Na+ uptake compared with vesicles from normal rats. Most of the tracer uptake in membranes derived from treated rats occurred through a conductive, amiloride-blockable pathway located in vesicles with low native K+ permeability and high Cl- permeability. Kinetic analysis of the amiloride inhibition curve revealed the presence of two amiloride-blockable pathways, one with a high affinity (Ki = 9 +/- 1.8 nM), accounting for 85% of the uptake, and one with a low affinity (Ki = 2.2 +/- 0.71 microM), accounting for only 12% of the uptake. Only the low-affinity pathway was detected with vesicles from normal rats. The high sensitivity to amiloride, the dependence on dexamethasone pretreatment, and the relative permeabilities to K+ and Cl- indicate that most of the 22Na+ uptake in membranes derived from treated rats is through a Na+-specific channel located in apical membrane vesicles. Preincubation of the isolated cells from dexamethasone-treated rats at 37 degrees C in Ca2+-free solutions before homogenization and membrane vesicle purification caused a 5- to 10-fold increase in amiloride-blockable 22Na+ uptake compared with vesicles derived from cells maintained at 0 degrees C. The addition of Ca2+, but not of Mg2+, to the incubation solution markedly reduced this temperature-dependent enhancement in 22Na+ uptake. The uptake of 22Na+ into vesicles from normal rats was unaffected by preincubation at 37 degrees C or the addition of Ca+ to the incubation solutions.(ABSTRACT TRUNCATED AT 250 WORDS)

Screening for colorectal cancer: sample techniques

Colorectal cancer is a major health problem. Despite therapeutic advances, the overall five-year survival rate remains at 45%. At the present time the greatest potential for improved survival lies with early detection. The authors of this article have assessed a number of colorectal surveillance programs in an attempt to identify those individuals with potentially pre-malignant disease or early localized cancer. Differences in individual risks and the availability of numerous "screening tests" make it important to provide each patient with the most appropriate investigation. Preliminary evidence suggests that fecal occult blood tests, sigmoidoscopy, and colonoscopy with polypectomy can be effective in reducing the incidence of colon cancer.

Indirect effects of bradykinin on ion transport in rat colon descendens: mediated by prostaglandins and enteric neurons

The effect of bradykinin on two preparations of rat colon descendens was examined. In a mucosa-submucosa preparation consisting of the submucosal plexus, the mucosal plexus and the epithelium bradykinin (10(-10)-5 X 10(-9) mol.l-1) caused an increase in Isc, Gt and Pd which was to more than 70% diminished by TTX. However, in a mucosa preparation consisting of only the mucosal plexus and the epithelium bradykinin caused an increase in Isc, Gt and Pd, which was not affected by TTX. Ten times higher concentrations of bradykinin were needed in the mucosa preparation to reach the same effects as in the mucosa-submucosa preparation. All effects of bradykinin were markedly reduced in the presence of indomethacin indicating that they were mediated by prostaglandins in both preparations. The bradykinin effect in the mucosa-submucosa preparation but not in the mucosa preparation was reduced about 50% by atropine. The results suggest that bradykinin activates prostaglandin synthesis. Prostaglandins subsequently stimulate neurons in the submucosal plexus which induce a secretory response on the epithelium partially mediated by a muscarinic receptor. In a high concentration bradykinin due to the induction of prostaglandin synthesis can also activate directly the mucosal epithelium.

Neuronally mediated and direct effects of prostaglandins on ion transport in rat colon descendens

Two preparations of rat colon descendens were used in order to localize the action sites of iloprost and prostaglandin E2 (PGE2). One preparation, the mucosa-submucosa preparation contained the submucosal and mucosal plexus whereas for the mucosa preparation in addition the submucosa with the submucosal plexus was removed. Iloprost (10(-6) mol.l-1) caused an increase in short-circuit current (Isc), potential difference (Pd) and tissue conductance (Gt) of the mucosa-submucosa preparation reflecting net Cl- secretion as confirmed by unidirectional ion flux measurements. The Cl- secretion was due to an increase in JClsm and a decrease in JClms. These effects were completely abolished by addition of 5 X 10(-5) mol.l-1 atropine. Iloprost had only small and inconsistent effects in the mucosa preparation. In contrast PGE2 (10(-6) mol.l-1) increased Isc, Pd and Gt due to Cl- secretion in both preparations. The Cl- secretion was caused by an increase in JClsm and a decrease in JClms. Only the PGE2 effect in the mucosa-submucosa preparation but not in the mucosa preparation was inhibited by about 50% by atropine. The results suggest that the prostacyclin derivative iloprost induces a Cl- secretion only by an activation of submucosal neurons whereas PGE2 acts both on the epithelium and the submucosal plexus. The neuronal effects of prostaglandins appear to be, at least in part, mediated by muscarinic receptors.

A L-[3H]glutamate binding site on glia: an autoradiographic study on implanted astrocytes

In the present study cultured astrocytes were implanted into the inferior colliculus of rats to create an astrocyte-enriched field that could be examined autoradiographically. The presence of the astrocytes was confirmed with anti-glial fibrillary acidic protein (GFA) immunocytochemistry. We report the presence of a chloride-dependent glutamate binding site on the implanted astrocytes. In the presence of chloride, the specific glutamate binding detected in the implant area was 5-fold greater than that found in a corresponding contralateral region. When the chloride was replaced with acetate, glutamate binding to the astrocytes decreased by more than 80%. The chloride-dependent binding to the astrocytes was insensitive to inhibition by kainic acid (KA) and N-methyl-D-aspartate (NMDA) and sensitive to quisqualate, L-aspartate, L-2-amino-4-phosphonobutyrate, and L-alpha-aminoadipate. The pharmacology of the binding was very similar to that of the in vitro glutamate binding to membranes from cultured astrocytes and to that of a chloride-dependent transport system identified in a glioma cell line. We conclude that the interaction of glutamate with astrocytes is an important component of the total glutamate binding observed in brain slices.

A novel chloride-dependent L-[3H]glutamate binding site in astrocyte membranes

Membrane fractions prepared from astrocytes grown in culture exhibit a specific binding site for L-[3H]glutamate that is Cl--dependent and Na+-independent. The binding site is a single saturable site with a KD of about 0.5 microM, is inhibited by L-aspartate, L-cysteate, and quisqualate, and is insensitive to kainate, N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate, and 2-amino-4-phosphonobutyrate. The pharmacological characteristics of the binding site indicate that it is distinct from any site previously described in synaptic membrane preparations. Comparisons of ionic requirements, ligand specificity, and inhibitor sensitivities, however, suggest the described binding is the first step in a Cl--dependent high-affinity glutamate uptake system. Such binding studies provide a useful model system in which to investigate the close association between excitatory amino acids, astrocytes, the termination of glutamate's excitatory action by high-affinity uptake, and the excitotoxic action of acidic amino acids in membranes of a single cell type.

Properties of an anion-selective channel from rat colonic enterocyte plasma membranes reconstituted into planar phospholipid bilayers

Vesicles derived from epithelial cells of the colonic mucosa of the rat were fused to planar phospholipid bilayer membranes, revealing spontaneously switching anion-conducting channels of 50 pS conductance (at -30 mV with 200 mM Cl- each side). The equilibrium selectivity series was I- (1.7)/Br- (1.3)/Cl- (1.0)/F- (0.4)/HCO3- (0.4)/Na (less than 0.11). Only one dominant open-state conductance could be resolved, which responded linearly to Cl- concentrations up to 600 mM. The single-channel current-voltage curve was weakly rectifying with symmetrical solutions. When 50 mV were exceeded at the high-conductance branch of the curve, switching was arrested in the closed state. At more moderate voltages (+/- 40 mV) kinetics were dominated by one open state of about 35-msec lifetime and two closed states of about 2 and 9-msec lifetime. Of these, the more stable closed state occurred less often. At these voltages one additional closed state of significantly longer lifetime (greater than 0.5 sec) was observed.

In vitro effects of dexamethasone on sodium transport across rat colon

1. The in vitro effects of dexamethasone on Na+ transport across the colon descendens from normal rats was investigated. Amiloride was used at two concentrations, 10 microM and 1 mM, to differentially inhibit the transport of Na+ across the colon. The colon descendens from each rat was divided into four segments and Na+ unidirectional fluxes before and 7 h after the addition of dexamethasone (10(-6) M) were determined under short-circuit conditions. 2. Base-line JnetNa (net flux of Na+) was twice as high in the proximal segment as in the distal segment. The two middle segments had intermediate rates of Na+ transport. JnetNa in control tissue was unaffected by 10 microM-amiloride but was completely inhibited by 1 mM-amiloride. In control tissue, amiloride at either 10 microM or 1 mM had no effect on the transmural potential difference (p.d.), the transmural conductance (Gt) or the short-circuit current (Isc). 3. Dexamethasone caused a time-dependent increase in the p.d. and in the Isc in all four segments of the colon. The increase in the p.d. and Isc was greatest in the most distal segment and less in each of the successive more proximal segments. This segmental difference along the colon was observed in tissue from all animals studied (n greater than 30). 4. The increase in p.d. and Isc caused by dexamethasone was accompanied by an increase in JnetNa to the same maximum rate of 14 mu equiv cm-2 h-1 in each segment.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of the paracellular pathway in the net transport of calcium across the colonic mucosa

Concentration dependent calcium fluxes across the colon descendens of the rat were measured in a modified Ussing chamber. Mucosa (m) to serosa (s) calcium flux showed a saturable component, whereas s to m calcium flux was linearly related to the calcium concentration. At low calcium concentrations net absorption and at concentration above 2.5 mmol/l net secretion of calcium was observed. The results obtained from the unidirectional calcium fluxes when clamping the transepithelial electrical potential agree well with those of the concentration dependence of the calcium fluxes: Only m to s flux has a voltage independent component. Calcium s to m movement is totally voltage dependent. Diffusional s to m calcium flux is greater than the diffusional fraction of the m to s calcium flow. Dexamethasone, known to stimulate water absorption in the colon descendens by an activation of sodium transport, had no effect on the cellular mediated m to s calcium transport but significantly increased paracellular s to m flux parallel to that of the extracellular marker mannitol. This increase in paracellular s to m calcium and mannitol flux was completely abolished by amiloride, which is known to suppress the dexamethasone-induced stimulation in sodium and water absorption. The results demonstrate that the increased paracellular s to m calcium and mannitol flow is oppositely directed to the dexamethasone-induced net fluid movement as it could be expected on the basis of Ussing's "anomalous solvent drag" effect.

The effect of aniline derivatives on absorption of fluid, glucose and sodium in isolated duodenal segments from rats

Paracetamol (5-15 mmol X l-1), phenacetin (1-3 mmol X l-1) and acetanilide (5-20 mmol X l-1) enhanced fluid, glucose and sodium absorption of isolated duodenal segments from rats. In a high concentration paracetamol (30 mmol X l-1) and acetanilide (25 mmol X l-1) inhibited these parameters. The coupling coefficient of 2:1 in sodium-glucose cotransport was not changed under the influence of the aniline derivatives. Phlorizin (10(-5) mol X l-1) completely abolished the stimulatory effect of these drugs. Also in presence of 3-O-methylglucose instead of glucose in the perfusion medium a paracetamol dependent increase in fluid absorption was seen, whereas the absorption of mannitol was unchanged. The results suggest, that the increase in sodium and fluid absorption caused by aniline derivatives is due to the stimulation of active glucose transport. A cytotoxic effect may explain the decrease of absorption at high concentrations of these substances.

Inhibition of the intestinal transport of uracil by hexoses and amino acids

Various hexoses and amino acids were tested as potential inhibitors of the active mucosal to serosal transport of uracil across the everted rat jejunum. Uracil transport displayed Michaelis-Menten type kinetics with a Vmax of 10.4 +/- 0.2 mumol X g-1 X h-1 and an apparent Km of 0.047 +/- 0.002 mM (means +/- S.D.). Scilliroside, an inhibitor of the basolateral (Na+ + K+)-ATPase, dose-dependently inhibited the transport of uracil consistent with the Na+ dependency of uracil transport. Thymine was a full competitive inhibitor (Ki = 0.021 +/- 0.002 mM) of uracil transport. All actively transported substances tested including L-phenylalanine, L-leucine, D-galactose, D-glucose, and 3-O-methylglucose inhibited the transport of uracil. In contrast, L-glucose and fructose, substances which are not actively transported, were without effect on uracil transport. Further studies with D-galactose indicated that it acts as a partial noncompetitive inhibitor (Ki = 6.0 +/- 1.4 mM) of uracil transport. This Ki is in good agreement with the apparent Kt (5.8 +/- 1.1 mM) for D-galactose transport. Phlorizin (0.1 mM), an inhibitor of galactose transport, blocked the inhibitory effect of galactose on uracil transport. In the ileum D-galactose had no effect on uracil transport but thymine caused the same degree of inhibition as in the jejunum. The results demonstrate that heterologous inhibition is a more general phenomenon than had previously been realized.

Mechanistic basis of alterations in mucosal water and electrolyte transport

Diarrhoea can, in principle, result from the stimulation of secretion, the inhibition of absorption, or both. In practice, it is the simultaneous stimulation of secretion and inhibition of absorption that has most frequently been observed. The majority of substances affecting the intestine fall into two categories, those stimulating secretion and inhibiting absorption and those having the opposite effects. It is therefore important to understand both the mechanisms of secretion and the mechanisms of absorption. In this chapter, the basic cellular mechanisms by which water and electrolytes are actively absorbed and secreted in the small and large intestine have been presented. The steps at which these mechanisms are thought to be regulated by one or all of the three potential intracellular mediators, cAMP, cGMP and Ca++, have been indicated. It is hoped that an understanding of these basic cellular mechanisms will aid in understanding the pathophysiological conditions of diarrhoea described in the following chapters.

Mucosal plexus and electrolyte transport across the rat colonic mucosa

Histological and functional studies were performed on a preparation of rat colonic mucosa from which the myenteric and submucosal plexus were removed. This preparation, referred to as the mucosa preparation, was used to investigate the potential influence of the mucosal plexus on electrolyte transport. Two neuropharmacologically active agents were used: sea anemone toxin (ATX II) to stimulate the fibres of the mucosal plexus and tetrodotoxin (TTX) to block the fibres of the mucosal plexus. The morphology of the neuronal network of the mucosal plexus was visualized after the epithelium was removed and whole mount preparations of the lamina propria and circular muscle layer of muscularis mucosae were stained histochemically for acetylcholinesterase activity. Several levels of organization within the mucosal plexus were seen. Each crypt is encircled by a thin bundle of fibres near the top. These thin fibres connect with thicker bundles of fibres that encircle groups of two to five crypts in a broad band. These bundles of fibres are in turn connected to larger bundles of fibres which lie in a flat plane just below the crypts along the circular muscle layer of muscularis mucosae. In addition perikarya and ganglia were revealed within the mucosal plexus. The base-line net transport of Na+ and Cl- across the mucosa preparation was completely inhibited by ATX II (10(-6) M). This effect of ATX II on net Na+ and Cl- transport was accompanied with an increase in the short-circuit current (Isc), transmural conductance, and open-circuit potential difference across the mucosa preparation. The effect of ATX II on Isc was dose dependent with a half-maximal effective concentration at 5 X 10(-8) M-ATX II and a maximal effective concentration of 10(-7) M. ATX II was effective only when added to the serosal solution. Net Na+ and Cl- transport was restored by TTX (10(-6) M) to base-line values in ATX II-treated tissue. In addition the value of all three electrical parameters rapidly returned to the values measured before the addition of ATX II. TTX was effective in antagonizing the effects of ATX II only when added to the serosal solution. The results suggest that the regulation of electrolyte transport across the epithelium is at least one function of the mucosal plexus. Stimulation of the neurones within the mucosal plexus leads to the inhibition of electrolyte absorption.(ABSTRACT TRUNCATED AT 400 WORDS)

A comparison of 2-amino-4-phosphonobutyric acid (AP4) receptors and [3H]AP4 binding sites in the rat brain

The glutamate analogue 2-amino-4-phosphonobutyric acid (AP4) is a potent antagonist at several synapses where an excitatory amino acid appears to be the neurotransmitter. Previous studies identified a Cl-/Ca2+ dependent [3H]glutamate binding site in synaptic plasma membrane (SPM) preparations that was also labeled by [3H]AP4 and exhibited a pharmacology similar to the AP4 receptor. This report examines the pharmacological specificity in both biochemical and electrophysiological preparations in greater detail. Several compounds are identified which readily interact with the apparent binding site in membranes, but neither mimic nor inhibit the action of AP4 in electrophysiological studies. The rate of dissociation of [3H]AP4 from SPMs is shown to increase in the presence of added AP4 and increasing the osmolarity in the SPM binding assay decreases the level of observed [3H]AP4 binding. These findings indicate both a heterogeneous population of binding sites and the occurrence of transport. It is concluded that much of the AP4 binding observed in SPM preparations is to a site other than the AP4 receptor. The results provide a further pharmacological description of AP4 receptors which should facilitate the identification of the receptor in biochemical preparations.

Glutathione specifically labeled with isotopes

A procedure for synthesis of glutathione selectivity labeled with isotopes is described. A strain of Escherichia coli enriched in its content of gamma-glutamylcysteine synthetase and glutathione synthetase by recombinant DNA techniques is immobilized in a carrageenan matrix and treated with toluene to render the cells more permeable to the substrates. The immobilized cell matrix is incubated with a mixture containing the appropriately labeled amino acid, the other amino acid constituents of glutathione, ATP, and acetylphosphate. The radiolabeled product is isolated by column chromatography.