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At least 757 records · Page 42Linked to original sources
PubMed2003

Channels as taste receptors in vertebrates.

Taste reception is fundamental for proper selection of food and beverages. Chemicals detected as taste stimuli by vertebrates include a large variety of substances, ranging from inorganic ions (e.g., Na(+), H(+)) to more complex molecules (e.g., sucrose, amino acids, alkaloids). Specialized epithelial cells, called taste receptor cells (TRCs), express specific membrane proteins that function as receptors for taste stimuli. Classical view of the early events in chemical detection was based on the assumption that taste substances bind to membrane receptors in TRCs without permeating the tissue. Although this model is still valid for some chemicals, such as sucrose, it does not hold for small ions, such as Na(+), that actually diffuse inside the taste tissue through ion channels. Electrophysiological, pharmacological, biochemical, and molecular biological studies have provided evidence that indeed TRCs use ion channels to reveal the presence of certain substances in foodstuff. In this review, we focus on the functional and molecular properties of ion channels that serve as receptors in taste transduction.

Animals↗
PubMed2002

Coordination and localization in spinal motor systems.

We review here experiments examining the hypothesis that vertebrate spinal motor systems produce movement through the flexible combination of a small number of units of motor output. Using a variety of preparations and techniques, these experiments provide evidence for such spinally generated units and for the localization of the networks responsible for producing them within different regions of the spinal cord. Such an organization might help to simplify the production of movement, reducing the degrees of freedom that need to be specified by providing a set of units involved in regulating features common to a range of behaviors.

Animals↗
PubMedDate not supplied

Salt and water transport by epithelial cells.

Epithelial cell volume is a sensitive indicator of the balance between solute entry into the cell and solute exit. Solute accumulation in the cell leads to cellular swelling because the water permeability of the cell membranes is high. Similarly, solute depletion leads to cellular shrinkage. The rate of volume change under a variety of experimental conditions may be utilized to study the rate and direction of solute transport by an epithelial cell. The pathways of water movement across an epithelium may also be deduced from the changes in cellular volume. A technique for the measurement of the volume of living epithelial cells is described and a number of experiments are discussed in which cellular volume determinations provided significant new information about the dynamic behavior of epithelia.

Animals↗
PubMed1990

Thin-layer chromatographic analysis of myelin lipids, their differential O-deacylation by primary alkylamines and their selective staining by thionine. A limited phylogenetic study.

A silica gel thin-layer chromatographic procedure is described for the study of the myelin lipid patterns in a small phylogenetic series of nerve tissue specimens. It involves the selective staining by the thiazine dye thionine and the interpretations were facilitated by a preceding primary alkylamine O-deacylation step. Glycolipids, including sulfatides, and ethanolamine plasmalogens were the principal characterizing lipids.

Acylation↗
PubMed1988

The cellular basis for suppressive rod-cone interaction.

The response to spatially focal flicker is enhanced by dim, spatially diffuse, rod-stimulating backgrounds. This effect is called suppressive rod-cone interaction (SRCI) as it reflects a tonic, suppressive influence of dark-adapted rods upon cone pathways which is removed by selective rod-light adaptation. SRCI is observed in amphibian retina with intracellular recordings from most cone-driven cells including the cones themselves, and is most obvious using stimuli flickering at frequencies too rapid for rods to follow. SRCI is blocked by glutamate analogs which selectively block the photic response of horizontal cells (HCs). In the presence of these agents, flicker responses from bipolar cells and cones are enhanced to levels normally seen only with selective rod-light adaptation. In the HCs themselves, SRCI is similarly blocked by lead chloride which blocks rod-, but not cone-related activity. In amphibian and cat HCs and in human observers, SRCI is limited by a space constant of very similar value (between 100 and 150 microns). We suggest that SRCI in all three species is mediated by HCs: in amphibians, SRCI must at least partially reflect rod-modulation of HC feedback onto cones.

Animals↗
PubMedDate not supplied

Serial inhibitory synapses in retina.

Whole-cell voltage clamp in the retinal slice and intracellular current clamp in the intact retina were used to study inhibitory interactions in the inner plexiform layer. Picrotoxin or strychnine reduced inhibitory, light-evoked currents in a majority of ganglion cells. However, in nearly a third of the ganglion cells, each of these antagonists enhanced the inhibitory synaptic current. All inhibitory current was blocked by the addition of the other antagonist. This indicates a cross-inhibition between GABAergic and glycinergic feedforward pathways. Blocking of GABAARs with SR95531 shortened the time course of both excitatory and inhibitory synaptic currents in ganglion cells. Application of picrotoxin, which blocked both GABAARs and GABACRs, produced the opposite effect. Recordings in the intact retina indicated that the light responses of ON bipolar cells, sustained ON, and transient ON-OFF third-order neurons were all made more transient by SR95531 and made more sustained by picrotoxin. The data suggest that a GABAC feedback pathway to bipolar cells makes light responses more phasic and that this feedback is inhibited through a GABAAR pathway. Consequently, the balance between GABAAR and GABACR inhibition regulates the time course of inputs to ganglion cells.

Ambystoma↗
PubMed1980

Intracellular study of ionic events underlying intestinal membrane transport of oligopeptides.

It is now clear that after a protein meal much of the alpha-amino nitrogen which is absorbed from the small intestinal tract is transported into the intestinal epithelium across the brush border membrane of the enterocyte in the form of oligopeptides rather than free amino acids. We report here the results of electrophysiological experiments using intracellular microelectrodes which allow us to add important information concerning the mechanism of membrane transport which must underlie oligopeptide absorption, in particular with respect to the role of Na+ in this peptide transport. Such experiments not only reveal the energetics involved in the major pathway for protein absorption from the gastrointestinal tract but may also be important for understanding the mechanism of action of small peptides in quite different cells, such as the Na+-sensitive binding of the neurohormone oligopeptide enkephalin to the enkephalin receptors in the central nervous system.

Animals↗
PubMedDate not supplied

Bipolar cells in the mudpuppy retina use an excitatory amino acid neurotransmitter.

The bipolar cells of the vertebrate retina are the principal neuronal elements which transmit photoreceptor activity from the outer to the inner retina. An important function of the bipolars is to segregate photoreceptor input into independent ON and OFF channels which are subserved, respectively, by the depolarizing and hyperpolarizing bipolar subtypes. Ultrastructural and physiological observations suggest that chemical neurotransmission is the predominant means of bipolar input to the inner retina. Both ON and OFF bipolars apparently release excitatory transmitters. Histological studies with cytotoxic agents and physiological studies indicate that third-order neurones have excitatory amino acid receptors. In ON-OFF amacrine and ganglion cells, which receive input from both bipolars, ON and OFF excitation have a similar ionic basis, suggesting that the same transmitter may be released by both types of bipolars. We have now found that (+/-)cis-2,3-piperidine dicarboxylic acid (PDA), a new excitatory amino acid antagonist, blocks bipolar input to the inner retina and thus suggests that an excitatory amino acid is a bipolar cell transmitter.

Amino Acids↗
PubMedDate not supplied

Identification of a distinct synaptic glutamate receptor on horizontal cells in mudpuppy retina.

The separation of ON and OFF channels and the development of an antagonistic surround occur at the first synapse in the vertebrate retina. This functional differentiation is mediated by the action of the photoreceptor neurotransmitter on the ON bipolar, OFF bipolar and horizontal cells, respectively. Glutamate mimics the action of the photoreceptor transmitter on all second-order neurones in fish, amphibian and mammalian retinas. The diversity of cellular responses produced by one neurotransmitter raises the possibility of multiple postsynaptic receptor-ionophore complexes. We reported previously that one glutamate analogue, 2-amino-4-phosphonobutyrate, reveals that the ON bipolar synaptic receptor is pharmacologically different from those of other second-order neurones. The results presented here demonstrate that another glutamate analogue, D-O-phosphoserine, selectively antagonizes the synaptic responses of horizontal cells. Taken together, these findings indicate that there are three glutamate-like receptor subtypes in the outer retina and suggest a correlation between receptor subtype and the physiological properties of second-order neurones.

Action Potentials↗
PubMedDate not supplied

Mediation of cell volume regulation by Ca2+ influx through stretch-activated channels.

Animal cells initially swell in hypotonic media by osmotic water equilibration, but their volume is subsequently regulated by a net loss of KCl and amino acids with concomitant loss of cell water. Mechanisms for regulating cell volume are important in allowing cells to adapt to variations in external tonicity and metabolic load. In red cells the KCl loss is mediated by electroneutral ion transport mechanisms. In contrast, conductive K+ and Cl- transport pathways are activated during regulatory volume decrease in several cell types including epithelia. The activation seems to be mediated by internal Ca2+, but the detailed mechanism is not known. In a leaky epithelium, the choroid plexus epithelium, we have found a cation-selective, Ca2+-permeable channel which opens with membrane stretch. The epithelium also contains a high density of the large (approximately 200 pS) type of Ca2+- voltage-activated K+ channel. Both channels are normally closed. I propose that in hypotonic media, the stretching of the cell membrane produced by the initial swelling causes influx of Ca2+ through the stretch-activated channels, which activates the neighbouring large K+ channels to produce increased K+ outflux with associated loss of cell water.

Animals↗
PubMed1981

Effect of parathyroid hormone on bicarbonate secretion in the guinea-pig stomach and the amphibian isolated gastric mucosa.

1. The effect of parathyroid hormone on gastric bicarbonate secretion was determined in the anaesthetized guinea pig. Subcutaneous injections of bovine parathyroid hormone (75 U.S.P units day-1 kg-1) for 7 days caused a significant increase in HCO3- output. There was also a rise in K+ output and a slight elevation of H+ secretion. A similar increase in HCO3- output occurred after acute intravenous injection of the hormones (75 U.S.P. units/kg). 2. Both chronic and acute administration of parathyroid hormone caused a significant increase in serum calcium concentration and it is likely that the changes in gastric ion outputs reflect raised calcium levels. Given alone intravenous calcium (1.5 mg/kg body wt.) stimulated gastric secretion of both HCO3- and H+. 3. To determine whether parathyroid hormone had a direct action on gastric ion transport experiments were performed in the amphibian isolate mucosa. Antrum transports HCO3- spontaneously while HCO3- transport in fundus was studied after inhibition of the greater H+ secretion by the histamine H2-receptor antagonist metiamide. Parathyroid hormone at a concentration of 0.2 United States Pharmacopea (U.S.P.) unit/ml in the nutrient-side bathing solution inhibited both antral and fundic HCO3- transport. A higher concentration (2.0 units/ml) had no effect on fundic H+ secretion. 4. The inhibitory effect in vitro was greater in the antrum and parathyroid hormone may almost abolish the active component of HCO3- transport in this tissue. It is likely that any similar inhibition of gastric HCO3- secretion by parathyroid hormone in vivo is masked by the stimulatory effects of released calcium.

Animals↗
PubMed1993

A chromatin folding model that incorporates linker variability generates fibers resembling the native structures.

The "30-nm" chromatin fibers, as observed in eukaryotic nuclei, are considered a discrete level in a hierarchy of DNA folding. At present, there is considerable debate as to how the nucleosomes and linker DNA are organized within chromatin fibers, and a number of models have been proposed, many of which are based on helical symmetry and imply specific contacts between nucleosomes. However, when observed in nuclei or after isolation, chromatin fibers show considerable structural irregularity. In the present study, chromatin folding is considered solely in terms of the known properties of the nucleosome-linker unit, taking into account the relative rotation between consecutive nucleosomes that results from the helical twist of DNA. Model building based on this premise, and with a constant length of linker DNA between consecutive nucleosomes, results in a family of fiber- and ribbon-like structures. When the linker length between nucleosomes is allowed to vary, as occurs in nature, fibers showing the types of irregularity observed in nuclei and in isolated chromatin are created. The potential application of the model in determining the three-dimensional organization of chromatin in which nucleosome positions are known is discussed.

Animals↗
PubMed1995

Conserved nucleoprotein structure at the ends of vertebrate and invertebrate chromosomes.

Eukaryotic chromosomes terminate with telomeres, nucleoprotein structures that are essential for chromosome stability. Vertebrate telomeres consist of terminal DNA tracts of sequence (TTAGGG)n, which in rat are predominantly organized into nucleosomes regularly spaced by 157 bp. To test the hypothesis that telomeres of other animals have nucleosomes, we compared telomeres from eight vertebrate tissues and cell cultures, as well as two tissues from an invertebrate. All telomeres have substantial tracts of (TTAGGG)n comprising 0.01-0.2% of the genome. All telomeres are long (20-100 kb), except for those of sea urchin, human, and some chicken chromosomes, which are 3-10 kb in length. All of the animal telomeres contained nucleosome arrays, consistent with the original hypothesis. The telomere repeat lengths vary from 151 to 205 bp, seemingly uncorrelated with telomere size, regularity of nucleosome spacing, species, or state of differentiation but surprisingly correlated with the repeat of bulk chromatin within the same cells. The telomere nucleosomes were consistently approximately 40 bp smaller than bulk nucleosomes. Thus, animal telomeres have highly conserved sequences and unusually short nucleosomes with cell-specific structure.

Animals↗
PubMed1999

Stretch-activated single K+ channels account for whole-cell currents elicited by swelling.

Functionally significant stretch-activated ion channels have been clearly identified in excitable cells. Although single-channel studies suggest their expression in other cell types, their activity in the whole-cell configuration has not been shown. This discrepancy makes their physiological significance doubtful and suggests that their mechanical activation is artifactual. Possible roles for these molecules in nonexcitable cells are acute cell-volume regulation and, in epithelial cells, the complex adjustment of ion fluxes across individual cell membranes when the rate of transepithelial transport changes. We report the results of experiments on isolated epithelial cells expressing in the basolateral membrane stretch-activated K+ channels demonstrable by the cell-attached patch-clamp technique. In these cells, reversible whole-cell currents were elicited by both isosmotic and hyposmotic cell swelling. Cation selectivity and block by inorganic agents were the same for single-channel and whole-cell currents, indicating that the same entity underlies single-channel and whole-cell currents and that the single-channel events are not artifactual. In these cells, when the rate of apical-membrane NaCl entry increases, the cell Na+ content and volume also increase, stimulating the Na+,K+-ATPase at the basolateral membrane, i.e., both Na+ extrusion and K+ uptake increase. We speculate that, under these conditions, the parallel activation of basolateral K+ channels (by the swelling) elevates conductive K+ loss, tending to maintain the cell K+ content constant ("pump-leak parallelism"). This study describes a physiologically relevant stretch-activated channel, at both the single-channel and whole-cell levels, in a nonneural cell type.

Animals↗
PubMed1989

Protein import through the nuclear pore complex is a multistep process.

The transport of macromolecules across the nuclear envelope is mediated by the nuclear pore complex (NPC). Using cryo-electron microscopy and image processing we have mapped the interaction of three specific gold probes with the NPC and obtained projection maps of two possible intermediates in nuclear import. The probes used in these experiments were (a) mAb-414, which cross-reacts with Xenopus nucleoporins containing O-linked N-acetyl glucosamines; (b) wheat germ agglutinin, a transport inhibitor; and (c) nucleoplasmin, a transport substrate. Strong binding sites of the three probes are circularly arrayed on NPCs between radii of 100 and 125 A and may be coextensive. These results suggest that nucleoplasmin-gold (NP-gold) can form at least three distinct complexes with a central transport assembly of the NPC, which may represent intermediates of a multistep protein import pathway. Initially, NP-gold appears to bind at multiple sites located around the periphery of the closed NPC transporter and also directly over the center where it can dock. In a subsequent step NP-gold is translocated through the nuclear pore.

Acetylglucosamine↗
PubMed1994

Actions of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) in retinal ON bipolar cells indicate that it is an agonist at L-AP4 receptors.

Metabotropic glutamate receptors (mGluRs) include receptors sensitive to L-2-amino-4-phosphonobutyrate (L-AP4) and 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD). To determine whether 1S,3R-ACPD is an agonist at retinal L-AP4 receptors, whole cell voltage clamp recordings were obtained from mudpuppy ON bipolar cells in a superfused retinal slice and L-AP4 and 1S,3R-ACPD were bath applied. Both compounds evoked similar outward currents which reversed near 0 mV and were accompanied by an increased input resistance. Responses to both agonists washed out in parallel suggesting they act through the same second messenger pathway(s). Inhibitors of cGMP-PDE activity suppressed responses to both L-AP4 and 1SR,3RS-ACPD, suggesting that both compounds activate cGMP-PDE. Responses to 1S,3R-ACPD were occluded by prior activation of L-AP4 receptors, but not blocked by the non-AP4, mGluR antagonists, L-aminophosphonopropionic acid (L-AP3) or 4-carboxy-3-hydroxyphenylglycine (4C3H-PG). These results indicate that 1S,3R-ACPD is an agonist at L-AP4 receptors. 1S,3S-ACPD and 4C3H-PG evoked outward currents similar to L-AP4 suggesting they may also be L-AP4 receptor agonists. Using the b-wave of the ERG as an assay for ON bipolar cell responses, concentration/response curves were obtained for ACPD enantiomers. The rank-order potency of ACPD enantiomers at L-AP4 receptors in ON bipolar cells is similar to their rank-order potency at non-AP4, mGluRs in brain which suggests that the receptors possess similar binding sites and may be members of a common receptor family.

Aminobutyrates↗