Biomedical subjects

E A Schwartz

Publications and source records attributed to E A Schwartz.

At least 19 recordsLinked to original sources

PubMed2002

Transport-mediated synapses in the retina.

Most synapses rely on regulated exocytosis for determining the concentration of transmitter in the synaptic cleft. However, this mechanism may not be universal. Several synapses in the retina appear to use a synaptic machinery in which transmitter transporters play an essential role. Two types of transport-mediated synapses have been proposed. These synapses have been best observed in horizontal cells and cones of nonmammalian retinas. Horizontal cells use a transporter to mediate a bidirectional shuttle, whose balance point is set by ion concentrations and voltage. Nonmammalian cones combine exocytosis and the activity of a transporter. Because exocytosis is voltage independent over most of a cone's physiological voltage range, a voltage-dependent transporter determines the concentration of transmitter in the synaptic cleft. These two synapses may be models for transport-mediated synapses that operate in other parts of the brain.

Animals↗

PubMed1999

Exposure of human vascular endothelial cells to sustained hydrostatic pressure stimulates proliferation. Involvement of the alphaV integrins.

The present study investigated the effects of sustained hydrostatic pressure (SHP; up to 4 cm H2O) on human umbilical vein endothelial cell (HUVEC) proliferation, focal adhesion plaque (FAP) organization, and integrin expression. Exposure of HUVECs to SHP stimulated cell proliferation and a selective increase in the expression of integrin subunit alphaV. The increase in alphaV was observed as early as 4 hours after exposure to pressure and preceded detectable increases in the bromodeoxyuridine labeling index. Laser confocal microscopy studies demonstrated colocalization of the alphaV integrin to FAPs. The individual FAPs in pressure-treated cells demonstrated a reduced area and increased aspect ratio and were localized to both peripheral and more central regions of the cells, in contrast to the predilection for the cell periphery in cells maintained under control pressure conditions. The pressure-induced changes in alphaV distribution had functional consequences on the cells: adhesivity of the cells to vitronectin was increased, and alphaV antagonists blocked the pressure-induced proliferative response. Thus, the present study suggests a role for alphaV integrins in the mechanotransduction of pressure by endothelial cells.

Antigens, CD↗

PubMed1999

Kainate receptors mediate synaptic transmission between cones and 'Off' bipolar cells in a mammalian retina.

Light produces a graded hyperpolarization in retinal photoreceptors that decreases their release of synaptic neurotransmitter. Cone photoreceptors use glutamate as a neurotransmitter with which to communicate with two types of bipolar cell. Activation of metabotropic glutamate receptors in 'On' bipolar cells initiates a second-messenger cascade that can amplify small synaptic inputs from cones. In contrast, it is not known how the ionotropic glutamate receptors that are activated in 'Off' bipolar cells are optimized for transmitting small, graded signals. Here we show, by recording from a cone and a synaptically connected 'Off' bipolar cell in slices of retina from the ground squirrel, that transmission is mediated by glutamate receptors of the kainate-preferring subtype. In the dark, a cone releases sufficient neurotransmitter to desensitize most postsynaptic kainate receptors. The small postsynaptic current that persists (<5% of maximum) is quickly modulated by changes in presynaptic voltage. Since recovery from desensitization is slow (the decay time constant is roughly 500 milliseconds), little recovery can occur during the brief (roughly 100-millisecond) hyperpolarization that is produced in cones by a flash of light. By limiting the postsynaptic current, receptor desensitization prevents saturation of the 'Off' bipolar cell's voltage response and allows the synapse to operate over the cone's entire physiological voltage range.

Animals↗

PubMed1999

Functional roles for PECAM-1 (CD31) and VE-cadherin (CD144) in tube assembly and lumen formation in three-dimensional collagen gels.

Various in vitro models have been described that emulate one or more of the processes involved in angiogenesis in vivo. In the present study endothelial cells were cultured in three-dimensional type I collagen lattices in the presence of a mixture of basic fibroblast growth factor, vascular endothelial cell growth factor, and phorbol myristate acetate. Under these conditions, the endothelial cells rapidly assemble into an interconnected network of tube-like structures with a high frequency of intercellular canals or lumens. The formation of the networks and lumens was completely blocked by cycloheximide and by actinomycin D. Monoclonal antibodies directed against CD31 or vascular endothelial cadherin (VE-cadherin) inhibited the formation of endothelial tubes. A subtle difference in the morphology of cells treated with anti-CD31 versus anti-VE-cadherin was noted; namely, cells incubated in the presence of CD31 antibodies were rounded or formed attenuated tube-like structures, both of which were characterized by a single, large intra- or intercellular vacuole. In contrast, tube formation by cells incubated in the presence of VE-cadherin antibodies was also impaired and, most notably, demonstrated a reduction in either vacuole formation or vacuole fusion, depending upon the monoclonal antibody used. We suggest that the two endothelial-junction-associated proteins, CD31 and VE-cadherin, play different roles in the process of tube formation. CD31 appears to be required for cell elongation, migration, and/or invasion in the gels as well as for cell-cell association to form the network structures. VE-cadherin also appears to be required for cell-cell association, but additionally appears to play some role in the process of vacuolization or vacuole fusion leading to intercellular lumen formation.

Antibodies, Monoclonal↗

PubMed1998

Continuous and transient vesicle cycling at a ribbon synapse.

Optical methods were used to study the Ca2+ dependence of vesicle cycling in bipolar cells isolated from goldfish retinas. Uniformly raising the Ca2+ concentration to between 0.8 and 20 microM produced a continuous vesicle cycle of balanced exocytosis and endocytosis with a maximum rate equivalent to the turnover of the entire surface membrane of a terminal every 2 min (or approximately 900 vesicles sec-1). Increasing the Ca2+ concentration above 20 microM inhibited continuous vesicle cycling. In contrast, influx of Ca2+ through voltage-gated channels produced a transient burst of exocytosis that increased the surface area of a terminal by a maximum of 12% (equivalent to the addition of 13,000 vesicles). Endocytosis was delayed until after Ca2+ influx stopped and the average Ca2+ concentration in the terminal declined. Hence, a single terminal has mechanisms for both continuous and transient vesicle cycling.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗

PubMed1997

Analysis and modeling of the primary cilium bending response to fluid shear.

Since a nonmotile, primary (9 + 0) cilium projects from most mammalian kidney epithelial cells into the tubule lumen, where it is exposed to fluid motion, the present study examined primary cilium response to fluid shear stress. The reversible, large-angle bending of the primary cilium upon exposure to fluid shear forces (10(-11)-10(-10) N.m2 = 10(-8)-10(-7) dyn/cm) was characterized in vitro using videomicroscopic side views of PtK1 cells, and the cilium was then mathematically modeled as a cantilevered beam. The flexural rigidity of the primary cilium was calculated to be 3.1 +/- 0.8 x 10(-23) N.m2 with a corrected quadruple integration approach and 1.4-1.6 x 10(-23) N.m2 with the "heavy elastica" theory. Comparison of theoretical profiles to the experimental bending responses of cilia established the validity of the "heavy elastica" model; this model, in turn, was used to predict primary cilium bending behavior under representative conditions in the rat nephron. The results of the study are consistent with the hypothesis that primary cilia serve a mechanosensory function in kidney epithelial cells.

Animals↗

PubMed1996

Asynchronous transmitter release: control of exocytosis and endocytosis at the salamander rod synapse.

1. We have studied exocytosis and endocytosis in the synaptic terminal of salamander rods using a combination of Ca2+ imaging, capacitance measurement and the photolysis of Ca2+ buffers. 2. The average cytoplasmic Ca2+ concentration at the dark resting potential was 2-4 microM. 3. An average cytoplasmic Ca2+ concentration of 2-4 microM maintained a high rate of continuous exocytosis and endocytosis. 4. Changes in the rate of exocytosis were followed in less than 0.7 s by compensatory changes in the rate of endocytosis. 5. Vesicle cycling in the rod synapse is specialized for graded transmission and differs from that previously described for synapses that release synchronized bursts of transmitter.

Acetates↗

PubMed1996

Channel behavior in a gamma-aminobutyrate transporter.

Current produced by a gamma-aminobutyrate (GABA) transporter stably transfected into a mammalian cell line was observed in cell-attached and excised membrane patches. When GABA was absent, a fraction of the transporters produced cation-permeable channels. When GABA plus Na+ was on either side of the membrane, the majority of transporters produced a high-frequency current noise attributed to the movement of ions in an occluded pore.

Biological Transport↗

PubMed1995

Optical analyses of eyeglass lens fragments and the unexpected detection of oral sperm in a homicide case.

A homicide case in which intact spermatozoa were found in the oral cavity of the deceased forty days after his disappearance is reported. The victim's partially frozen body was found outdoors in a wooded area of upstate New York during the month of January. During a subsequent investigation, pieces of eyeglass lens fragments and bloodstains were found in the suspect's house and vehicle. Chemical and optical analyses of the lens fragments are presented as well as results of the serological tests.

Adult↗

PubMed1994

A cGMP-gated current can control exocytosis at cone synapses.

The voltage-gated Ca2+ current in cone photoreceptors operates over only a small part of the physiological voltage range produced by light and, consequently, appears insufficient for controlling transmitter release. We have used a whole-cell voltage clamp to measure membrane current and the capacitance change produced by exocytosis in solitary cone and rod photoreceptors isolated from the salamander retina. In both types of photoreceptor, Ca2+ influx through voltage-gated Ca2+ channels initiated exocytosis. In addition, Ca2+ influx through a cGMP-gated channel in the inner segment and synaptic processes of cones also initiated exocytosis. The cGMP-gated current sustained exocytosis over the entire physiological voltage range.

Ambystoma↗

PubMed1994

A GABA transporter operates asymmetrically and with variable stoichiometry.

Membrane currents produced by the expression of a rat GABA transporter (GAT-1) stably transfected into HEK293 cells were characterized with a whole-cell voltage clamp. Three modes of function were identified: ex-gated currents produced by extracellular GABA, in-gated currents produced by intracellular GABA, and uncoupled currents produced in the absence of GABA. The ex-gated current was not the reversal of the in-gated current; moreover, the stoichiometry between GABA and co-ions was not always fixed. Each mode of function required a different set of ions on the two sides of the membrane. We made rapid solution changes and observed an allosteric effect of Na+ that only occurred at the extracellular surface. Thus, the GAT-1 transporter does not behave like a recirculating carrier but may be described as a pore with ion gates at either end that are controlled in part by allosteric sites.

Allosteric Site↗

PubMed1993

L-glutamate conditionally modulates the K+ current of Müller glial cells.

L-Glutamate inhibits the K+ conductance that dominates the electrical behavior of a Müller glial cell. The effect of glutamate is enhanced by simultaneous exposure to dopamine. L-Glutamate acts at a metabotropic receptor that controls the K+ conductance through two pathways. A rapid pathway produces a partial inhibition in less than 2 s. Thereafter, a slow pathway progressively inhibits the conductance with a half-time of minutes. Pathways initiated by L-glutamate and dopamine appear to converge on and stimulate adenylyl cyclase. A subsequent step is the activation of a cAMP-dependent protein kinase, PKA. The local overflow of L-glutamate from active synapses may functionally remove K+ channels from nearby glial membranes. A uniform rise in extracellular L-glutamate concentration, as might occur during pathological conditions, should suppress a glial cell's K+ conductance and allow other voltage-dependent processes to be influenced by depolarization.

Adenylate Cyclase Toxin↗

PubMed1993

Ions required for the electrogenic transport of GABA by horizontal cells of the catfish retina.

1. Solitary horizontal cells were isolated from catfish retinas. Membrane currents activated by extracellular and intracellular GABA were characterized during a whole-cell voltage clamp. 2. Extracellular GABA activated two currents: a GABAA current, and an 'influx' current mediated by a GABA transporter. The influx current was studied after the GABAA current was blocked with 0.5 mM picrotoxin. The influx current required extracellular Na+ and Cl-. Extracellular Na+ could not be replaced by another alkali metal cation. 3. The influx current also depended upon the identity of ions in the intracellular solution. Either an intracellular alkali metal cation or Cl- was required to produce an influx current. 4. The influx current was inward at -75 mV and decreased as the membrane was depolarized towards +20 mV. When the membrane was depolarized beyond +25 mV, the polarity of the current depended upon the ion composition of the intracellular solution and could be inward, zero or outward. 5. The introduction of GABA into a cell during the course of an experiment produced an outward current. This 'efflux' current was small at -75 mV and increased with depolarization. The efflux current required intracellular Na+ and Cl-. Intracellular Na+ could not be replaced by another alkali metal cation. 6. The efflux current also depended upon the identity of ions in the extracellular solution. An extracellular alkali metal cation was required to produce an efflux current. Removing extracellular Cl- did not affect the efflux current. 7. The outward movement of GABA produced a local accumulation in extracellular GABA concentration that could be detected by the activation of the GABAA current. GABA efflux only occurred during conditions that produced an efflux current. Electroneutral efflux did not occur. 8. In the absence of GABA, extracellular alkali metal cations produced a 'leakage' current. The leakage current was inward at -75 mV and decreased as the membrane was depolarized towards +20 mV. When the membrane was depolarized beyond +25 mV, the polarity of the leakage current depended, like the GABA influx current, upon the ion composition of the intracellular solution and could be inward, zero or outward. The addition of GABA to the intracellular solution produced an efflux current and suppressed the leakage current. 9. We conclude that the transporter mediates electrogenic influx, efflux and leakage. Each mode of operation depends upon ions on both sides of the membrane. Influx and efflux are not symmetrical.

Animals↗

PubMed1992

Hemi-gap-junction channels in solitary horizontal cells of the catfish retina.

1. Solitary horizontal cells were isolated from catfish retinas and their membrane current was recorded with a whole-cell voltage clamp. Reducing the extracellular Ca2+ concentration produced a current that could be suppressed by dopamine. This Ca(2+)- and dopamine-sensitive current is hereafter termed I gamma. The voltage dependence, cytoplasmic regulation, and permeability of the I gamma channel suggest that it is half of a gap-junction channel. 2. I gamma was voltage and time dependent. In the steady state, the current-voltage relation displayed outward rectification at voltages more depolarized than 0 mV and a negative resistance region at voltages more hyperpolarized than -15 mV. The reversal potential was 3.3 +/- 1.5 mV when NaCl was the predominant extracellular salt and potassium-D-aspartate was the predominant intracellular salt. 3. The size of I gamma depended on the extracellular Ca2+ concentration. I gamma was maximal at external Ca2+ concentrations below 10 microM, half-maximal at 220 microM-Ca2+, and reduced to less than 4% of its maximum amplitude at external Ca2+ concentrations above 1 mM. Increasing the extracellular Ca2+ concentration reduced the amplitude of I gamma without changing the shape of the current-voltage relation or the kinetics of inactivation. Thus, rectification does not result from a voltage-dependent block by extracellular Ca2+. 4. Patches of cell membrane were voltage clamped in both the cell-attached and excised-patch configurations. In the cell-attached configuration, the addition of dopamine to the solution outside the patch pipette blocked the opening of channels within the membrane patch. Thus, dopamine closes I gamma channels by initiating an intracellular messenger cascade. In the excised-patch configuration, a maximum conductance of 145 pS was measured while Cs+ and tetraethylammonium+ (TEA+) were the only monovalent cations on both sides of the membrane. 5. The ability of dopamine to suppress I gamma was blocked by introducing an inhibitor of the cyclic AMP-dependent protein kinase, PKI5-24, into the cytoplasm. Thus, the action of dopamine is mediated by a pathway that includes the activation of a cyclic AMP-dependent kinase. 6. I gamma was suppressed by nitroprusside, an agent which activates guanylate cyclase and increases the intracellular cyclic GMP concentration. The effect of nitroprusside was not altered by the intracellular application of PKI5-24. Thus, nitroprusside suppresses I gamma through a pathway that does not include the activation of a cyclic AMP-dependent kinase.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗

PubMed1991

Characterization of deoxyribonucleic acid (DNA) obtained from teeth subjected to various environmental conditions.

This study was designed to determine the effects of various environmental factors on the deoxyribonucleic acid (DNA) obtained from dental pulp. Extracted teeth were subjected to the following conditions: varying pH (3,7,10); temperature (4 degrees C, 25 degrees C, 37 degrees C, incineration); humidity (20%, 66%, 98%); various types of soil (sand, potting soil, garden soil); seawater; burying the teeth outdoors, and aging (one week to six months). In addition, teeth that had been extracted and held at room temperature for 16 and 19 years were also examined. Following isolation of DNA, the samples were analyzed on yield gels to determine the concentration and integrity of the recovered DNA. Restriction digestion with Pst I was followed by electrophoresis of the generated fragments, Southern transfer to nylon membranes, and hybridization to both human and bacterial probes. It was determined that, aside from soil, the environmental conditions examined did not affect the ability to obtain high-molecular-weight human DNA from dental pulp. Restriction fragment length polymorphic (RFLP) analysis of selected samples was performed. Dental pulp patterns were compared with bloodstain exemplars, revealing matching patterns, although an increase in band-shifting was observed with extended exposure to elevated temperatures.

Autoradiography↗

PubMed1990

Electrophysiology of glutamate and sodium co-transport in a glial cell of the salamander retina.

1. Müller cells were isolated from salamander retinas and their membrane voltage was controlled with a whole-cell voltage clamp. External D-aspartate, L-aspartate and L-glutamate each induced a membrane current. D-Glutamate, kainate, quisqualate and N-methyl-D-aspartate were more than 100x less effective than L-aspartate. Kynurenic acid had no effect on the current produced by L-glutamate, L-aspartate or D-aspartate. 2. The current induced by an acidic amino acid (AAA) was completely dependent on the presence of external Na+. Neither Li+, Cs+, choline nor TEA+ were able to substitute for Na+. The relationship between external Na+ concentration and current amplitude can be explained if the binding of three Na+ ions enabled transport. The apparent affinity constant for Na+ binding was 41 mM. Altering K+, H+ and Cl- concentrations demonstrated that these ions are not required for transport. 3. The shape of the current-voltage relation did not depend on the external amino acid concentration. The relationship between D-aspartate concentration and current amplitude can be described by the binding of D-aspartate to a single site with an apparent affinity constant of 20 microM. 4. Influx and efflux of AAA were not symmetric. Although influx was electrogenic, efflux did not produce a current. Moreover, influx stimulated efflux; but efflux inhibited influx. 5. Removing external Na+ demonstrated that Na+ carried a current in the absence of an AAA. Li+ was a very poor substitute for Na+. This current may be due to the uncoupled movement of Na+ through the transporter. The relationship between the external Na+ concentration and the amplitude of the uncoupled current can be explained if the binding of two or three Na+ ions enabled the translocation of Na+ in the absence of an AAA. The apparent affinity constant for Na+ binding was approximately 90 mM. 6. The temperature dependence of the AAA-induced current had a Q10 between 8 and 18 degrees C of 1.95. The Q10 is consistent with a rate constant for influx of 10(4) s-1 (at -70 mV and 20 degrees C). The maximum rate of influx was measured following a concentration jump produced by the photolysis of 'caged' L-glutamate. The onset of the observed current was limited by the 1.3 ms resolution of the recording system. Hence, the rate constant for influx must be faster than 10(3) s-1.(ABSTRACT TRUNCATED AT 400 WORDS)

Ambystoma↗

PubMed1989

Modulation of an electrical synapse between solitary pairs of catfish horizontal cells by dopamine and second messengers.

1. Retinas from channel catfish were dissociated and the cells maintained in culture. Horizontal cells that normally receive input from cone photoreceptors were identified. The conductance of the electrical junction formed between a pair of 'cone' horizontal cells was measured by controlling the membrane voltage of each cell with a voltage clamp maintained through either a micropipette or a patch pipette. The two techniques yielded similar results. 2. Transjunctional current was measured while transjunctional voltage was stepped to values between +/- 60 mV. The current (measured 5 ms after a step) was proportional to voltage over the range tested. For steps to voltages greater than +/- 45 mV, the current exhibited a slight time-dependent decline. 3. Dopamine decreased junctional conductance in a dose-dependent fashion. A 50% reduction was obtained with 10 nM-dopamine. The D1 agonist fenoldopam (100 nM) also decreased junctional conductance. The uncoupling produced by either agent was rapid and reversible. 4. The introduction of 100 microM-cyclic AMP into one cell of a pair decreased junctional conductance by, on average, 40%. Forskolin (1-10 microM), an activator of adenylate cyclase, decreased junctional conductance 50-90%. 5. The introduction of 80 microM-cyclic GMP into one cell of a pair decreased junctional conductance by, on average, 40%. Nitroprusside (1-10 microM), an activator of guanylate cyclase, reduced junctional conductance 40-65%. 6. The introduction of a peptide inhibitor specific for the cyclic AMP-dependent protein kinase reversed a decrease in junctional conductance produced by superfusion with either dopamine (1 microM), fenoldopam (100 nM) or forskolin (5-10 microM). 7. Intracellular Ca2+ concentration was measured with the fluorescent indicator Fura-2. The intracellular Ca2+ concentration was increased by activation of a Ca2+ current. Junctional conductance remained constant as the internal Ca2+ concentration changed from 100 to 700 nM. 8. Intracellular pH was measured with the fluorescent indicator bis-carboxyethylcarboxyfluorescein. The application of acetate (2.5 mM) reduced intracellular pH by 0.2-0.3 units and decreased junctional conductance by approximately 50%. A subsequent application of fenoldopam did not alter intracellular pH, but decreased junctional conductance by more than 50%. 9. The sensitivity of the junctional conductance between isolated horizontal cells to dopamine is consistent with dopamine having a direct effect on coupling in intact retina. Dopamine regulates the activity of a cyclic AMP-dependent protein kinase which in turn modulates junctional conductance. Changes in intracellular pH and Ca2+ concentration are not involved in mediating the effect of dopamine on coupling. Cyclic GMP and intracellular pH may participate in regulatory pathways independent of that used by cyclic AMP.

Animals↗

PubMed1987

Depolarization without calcium can release gamma-aminobutyric acid from a retinal neuron.

Calcium influx is often an essential intermediate step for the release of neurotransmitter. However, some retinal neurons appear to release transmitter by a mechanism that does not require calcium influx. It was uncertain whether depolarization released calcium from an intracellular store or released transmitter by a mechanism that does not require calcium. The possibility that voltage, and not calcium, can regulate the release of transmitter was studied with pairs of solitary retinal neurons. Horizontal and bipolar cells were isolated from fish retinas and juxtaposed in culture. Communication between them was studied with electrophysiological methods. A horizontal cell released its neurotransmitter, gamma-aminobutyric acid, when depolarized during conditions that buffered the internal calcium concentration and prohibited calcium entry. The speed and amount of material released were sufficient for a contribution to synaptic transmission.

Animals↗