Biomedical subjects

S H Chung

Publications and source records attributed to S H Chung.

At least 73 records · Page 4Linked to original sources

PubMed1996

Coupled Markov chain model: characterization of membrane channel currents with multiple conductance sublevels as partially coupled elementary pores.

A parameterized Markov chain model is developed to represent the characteristics of channel currents that either are the superposition of many single channels or show multiple conductance sublevels. The simplified model takes the form of a set of binary chains that are interdependent according to a simple lumped coupling parameter. When varied, this parameter realizes a range of behaviors from tight coupling to complete independence. Other model parameters describe the intrinsic characteristics of the binary chains. An identification procedure for the model parameters is developed based on hidden Markov modeling ideas but incorporating a novel parameter estimation. The usefulness of the model in analyzing certain types of data is demonstrated with examples of real channel currents.

Animals↗

PubMed1996

Treatment of avascular ulcers with cytokine-induced tissue generation and skin grafting.

BACKGROUND: Recombinant platelet-derived growth factor (rPDGF-BB) stimulates migration and proliferation of fibroblasts and smooth muscle cells and induces the rapid development of granulation tissue. This study investigated the use of rPDGF-BB and skin grafting to heal avascular ulcers using a rabbit model. We further investigated the effect of a hyaluronic acid carrier and a vascular pedicle on rPDGF-induced tissue generation in this model. METHODS: Large avascular ulcers were created on both ears of New Zealand white rabbits. One ulcer was treated with topical rPDGF-BB, the other with control buffer. After 5 or 7 days, the amount of granulation tissue migration from the wound margin was measured, and the wounds were skin grafted. In another group of ulcers, rPDGF-BB treatment was combined with a hyaluronic acid disk or an intact central axial ear artery and vein. RESULTS: Whereas control wounds generated 1.00 +/- 0.27 mm and 1.75 +/- 0.25 mm of granulated tissue migration from the wound margin by days 5 and 7, respectively, rPDGF-BB treatment increased the amount of granulation tissue migration to 1.88 +/- 0.23 mm and 3.00 +/- 0.86 mm by days 5 and 7 after treatment, respectively (P <0.05 at both time points). Hyaluronic acid disks stimulated 2.50 +/- 0.22 mm of granulation tissue migration by day 7 in controls; when rPDGF-BB was added to the carrier, the migration increased to 4.50 +/- 0.29 mm from the wound margin (P<0.05). Granulation tissue migration was further increased with an intact vascular pedicle: 5.75 +/- 0.48 mm in controls versus 7.75 +/- 0.25 mm with rPDGF-BB treatment (P<0.01). Skin grafts applied to the treated ulcers failed to survive in all groups except those with intact vascular pedicles. CONCLUSIONS. This study demonstrates that rPDGF-BB can increase the amount of granulation tissue generated over an avascular wound. This tissue generation is enhanced by both a hyaluronic acid carrier and a vascular pedicle. A vascular pedicle was required for skin graft survival. This study supports the role of hyaluronic acid in rPDGF-BB induced tissue generation and the requirement of a direct blood supply for functional cytokine-induced tissue generation.

Animals↗

PubMed1996

Energy barrier presented to ions by the vestibule of the biological membrane channel.

The role of the vestibule in influencing the permeation of ions through biological ion channels is investigated. We derive analytical expressions for the electric potential satisfying Poisson's equation with prolate spheroidal boundary conditions. To allow more realistic geometries we devise an iterative method to calculate the electric potential arising from a fixed charge and an arbitrary dielectric boundary, and confirm that the analytical expressions and iterative method give similar potential values. We then investigate the size of the potential barrier presented to an ion by model vestibules of conical and catenary shapes. The height of the potential barrier increases steeply as an ion enters the vestibule and moves toward the constricted region of the channel. We show that the barrier presented by, for example, a 15 degrees conical vestibule can be canceled by placing dipoles with a total moment of about 50 Debyes near the constricted region of the pore. The selectivity of cations and anions can result from the polarity of charge groups or the orientation of dipoles located near the constricted region of the channel.

Biophysical Phenomena↗

PubMed1996

Prevention of microvascular thrombosis by topical application of recombinant tissue factor pathway inhibitor.

Tissue factor pathway inhibitor is a naturally occurring protein inhibitor of factor X and the tissue factor-factor VII complex of the extrinsic pathway of coagulation. The potential of tissue factor pathway inhibitor as a topical antithrombotic agent was evaluated in a rabbit model of thrombosis that combined intimal injury, anastomosis, and a twisted pedicle. In 207 rabbit ears, a near-complete amputation was performed, preserving the central ear artery and vein. The central ear artery was transected, the intima was removed mechanically over a 1-cm length, the artery was anastomosed, and the ear was twisted 360 degrees, wrapping the intact vein around the artery. Before recirculation, the lumen was irrigated on a blinded, randomized basis with either hirudin (100 or 500 units/ml), heparin (50 or 100 units/ml), tissue factor pathway inhibitor (10, 40, 125, or 250 microgram/ml), heparin and tissue factor pathway inhibitor together, or vehicle (control). Upon arterial reflow, the ears were observed for 7 days. Patency rates after 7 days were as follows: hirudin, 30 and 55 percent; heparin, 43 and 50 percent; tissue factor pathway inhibitor, 75 and 90 percent; heparin and tissue factor pathway inhibitor, 75 percent; and vehicle, 6 percent. The higher concentrations of tissue factor pathway inhibitor led to significantly higher patency rates than heparin, hirudin, or control solutions. Electron microscopic evaluation of specimens irrigated with gold- labeled tissue factor pathway inhibitor revealed the inhibitor bound to the injured intimal surface for at least 3 days postoperatively. Coagulation studies showed no change in the clotting profile upon intravascular infusion with tissue factor pathway inhibitor even at the highest dose used topically. We conclude that tissue factor pathway inhibitor is a more effective topical antithrombotic agent than either heparin or hirudin.

Administration, Topical↗

PubMed1996

MR imaging of cerebral activation performed with a gradient-echo technique at 1.5 T: sources of activation signals.

OBJECTIVE: MR imaging of cerebral activation has been successfully performed at 1.5 T for functional maps of the brain. However, major sources of activation signals in such imaging remain controversial. The purpose of this study is to investigate anatomic and physiologic sources of activation signals in MR imaging of cerebral activation performed with a gradient-echo technique at 1.5 T. SUBJECTS AND METHODS: Motor cortex activation studies (n = 8) were conducted using a gradient-echo technique (80/60 [TR/TE], 40 degrees flip angle). MR venograms were then obtained at the same imaging plane to visualize the cortical veins, which were then compared with the shape and location of the activation signals. To investigate the physiologic sources of activation signal, the activation studies were repeated with different TEs (15, 30, and 60 msec), which allowed us to evaluate the blood oxygen level-dependent effect; with different flip angles (40 degrees and 10 degrees); and without and with presaturation of adjacent sections, all of which allowed us to evaluate inflow effect. RESULTS: All activation signals were detected in the sulcus just posterior (n = 7) or lateral (n = 1) to the motor cortex. In seven of eight studies, shape and location of these signals corresponded well with those of the cortical veins. In the eighth study, the correspondence was partial. Activation signals significantly increased at a TE of 60 msec (p < .01), suggesting enhancement of the blood oxygen level-dependent effect at a long TE. Activation signals significantly decreased with a 10 degrees flip angle (p < .01) and with presaturation of adjacent sections (p < .01), indicating that the inflow effect was suppressed by a small flip angle and the elimination of unsaturated inflowing protons. CONCLUSION: Our results suggest that signals in cerebral activation obtained by MR imaging with a gradient-echo technique at 1.5 T arise mainly from the cortical veins draining the activated cortex. Physiologically, both blood oxygen level-dependent and inflow effects contribute to signal generation.

Adult↗

PubMed1995

Evidence that the Rab3a-binding protein, rabphilin3a, enhances regulated secretion. Studies in adrenal chromaffin cells.

Rabphilin3a had been identified in brain as a Rab3a-binding protein and may serve as an effector for Rab3a function. We have cloned a splice variant of brain-Rabphilin3a from a bovine adrenal chromaffin cell cDNA library and investigated the function of the protein in regulated exocytosis in bovine chromaffin cells. The predicted amino acid sequence of chromaffin cell (c-) Rabphilin3a was identical with that of brain (b-) Rabphilin3a except for a 6-amino-acid insert VFSLSA in the amino-terminal half of the protein. An antibody directed against a carboxyl-terminal peptide recognized an 85-kDa protein in COS7 cells transfected with the cDNA in a mammalian expression vector. A band of similar mobility was enriched in a fraction of highly purified chromaffin granule membranes, consistent with the Rabphilin3a being associated with chromaffin granule membranes. Overexpression of either chromaffin cell or brain Rabphilin3a by transfection with the corresponding cDNAs in mammalian expression vectors enhanced DMPP-induced secretion of co-expressed human growth hormone (GH) approximately 30%. Chromaffin cells transfected with a plasmid with the entire coding sequence of c-Rabphilin3a inserted in the antisense orientation inhibited secretion of co-expressed GH by approximately 30%. Rabphilin3a mutants lacking one or both of the carboxyl-terminal C2 domains strongly inhibited DMPP-stimulated exocytosis. The single C2 domain deletion also strongly inhibited Ca(2+)-dependent secretion from digitonin-permeabilized cells. These data indicate that Rabphilin3a is a positive regulator of exocytosis. Because the C2 deletion mutants contain the amino-terminal Rab3a-GTP binding domain, they may inhibit secretion by competing with endogenous Rabphilin3a for interaction with Rab3a-GTP without being able to mimic the functional effects of full-length Rabphilin3a.

Adaptor Proteins, Signal Transducing↗

PubMed1995

Activation of K+ channels by stimulation of metabotropic glutamate receptors.

Single channel potassium currents activated indirectly via stimulation of metabotropic receptors were recorded in cell-attached patches on hippocampal neurones in slices from neonatal rats. When metabotropic receptors in the area outside of the patch are stimulated by glutamate, quisqualate and trans-ACPD, single channel currents appear with a predominant conductance level of about 20 pS. From the reversal potential and substitution of ionic species, we established that the currents are carried by potassium ions. Using selective blockers of ionotropic receptors in conjunction with specific agonists for the metabotropic receptor, it was ascertained that the channel observed is linked to metabotropic receptors. The currents display submaximal conductance states and fluctuate rapidly between evenly-spaced conductance sublevels.

6-Cyano-7-nitroquinoxaline-2,3-dione↗

PubMed1995

Changes in the kinetics and conductance of N-methyl-D-aspartate (NMDA)-receptor activated single channels with temperature.

Single channel currents activated by N-methyl-D-aspartate (NMDA) were recorded in inside-out membrane patches from hippocampal pyramidal cells. The effects of temperature on the conductance level and the kinetics of single channels were accurately determined by using a digital signal processing technique based on hidden Markov models. With increasing temperature, the probability of the channel being in the open state increases steeply, as does the amplitude of channel currents. The proportional change in channel conductance with increasing temperature is greater than the corresponding change in bulk conductivity of 150 mM electrolyte solutions. From the temperature dependence of channel conductance and kinetics, we derived several thermodynamical properties of the NMDA-receptor activated channel. Among these are the height of the energy barrier V presented to an ion traversing the pore, the activation energy Ea and the enthalpy delta H of the channel.

Animals↗

PubMed1995

Localization of intracellular monoclonal antibody specific for mycobacteria in experimentally induced pulmonary tuberculous lesion.

To determine the intracellular localization of intravenously injected infection-specific monoclonal antibodies (mAbs) in the infected cells, immunohistochemical staining was carried out in an animal model having pulmonary tuberculous lesions induced by inoculation of heat-killed Mycobacterium tuberculosis H37Rv. One milligram of intact mouse mAb against mycobacteria (group I, n = 10) and F(ab')2 (group II, n = 6) was intravenously injected to the rabbits of each group. Immunohistochemical staining using an antimouse Ab was performed at days 1, 3, 5, 7 and 8 in group I and at days 1, 2 and 3 in group II by the streptavidin-biotin method. For the control study, 1 ml of nonspecific polyclonal human IgG (group III, n = 10) and 100 micrograms of normal rabbit IgG F(ab')2 (group IV, n = 6) was injected to rabbits and guinea pigs having tuberculous inflammation, respectively. Both groups (group I and II) showed a positive antigen (Ag)-Ab reaction within the cytoplasm of monocytes. A weak but positive reaction was observed intracellularly in group III; however, no positive reaction was seen in group IV. Our results suggest that an intracellular Ag-Ab reaction plays an important role in the localization of infection by immunoscintigraphy using specific mAb fragments.

Animals↗

PubMed1995

Stage specific identification of the expression of GnRH mRNA and localization of the GnRH receptor in mature rat and adult human testis.

PURPOSE: To identify the expression of gonadotropin-releasing hormone (GnRH) messenger ribonuclueic acid (mRNA) and localize GnRH receptors in mature rat and adult human testes. MATERIALS AND METHODS: In situ hybridization and enzymatic receptor binding localization were performed. RESULTS: GnRH mRNA was expressed within the seminiferous tubules in both mature rat and adult human testis. In rats, expression of GnRH mRNA was identified in the Sertoli cells and spermatogenic cells of some seminiferous tubules, but the other tubules did not express any hybridization signal. In humans, expression of GnRH mRNA was identified only in some spermatogenic cells in some seminiferous tubules. The receptors for GnRH were localized to cells in the interstitial tissues of the testis, probably Leydig cells. CONCLUSION: The authors believe that the mature rat and adult human seminiferous tubular cells produce GnRH at the same specific stage of the spermatogenic cycle and that GnRH produced within seminiferous tubules, including Sertoli cells and spermatogenic cells, reacts with neighboring GnRH receptors in interstitial cells, including Leydig cells. The GnRH produced from the Sertoli cells in seminiferous tubules would react with GnRH receptors in interstitial cells as a paracrine hormone.

Adult↗

PubMed1995

The temperature dependence of conductance of the sodium channel: implications for mechanisms of ion permeation.

Voltage-gated sodium channel currents were recorded in cell-attached and inside-out membrane patches from rat ventricular myocytes at temperatures ranging from 4 degrees C to 36 degrees C. The effects of changes in temperature on channel conductance were accurately determined using a digital signal processing technique based on hidden Markov models. We show that the cardiac sodium channel has multiple conductance levels, with the most frequently observed sublevel being either 11 pS or 22 pS at room temperature in 280 mM Na+, depending on the dissociation procedures adopted. The single channel current-voltage relationship is ohmic at all of the temperatures studied. The conductance increases steeply with temperature, with Q10 ranging from 1.4 to 1.5. The proportional change in channel conductance with increasing temperature was greater than the corresponding change in bulk conductivity of electrolyte solutions, suggesting that an ion traversing the channel needs to surmount a small additional energy barrier. An activation energy deduced from a plot of the logarithm of single channel conductance against the inverse of temperature is about 28 kJ mole-1. We provide one possible interpretation of the observed conductance-temperature relationship in terms of the details of the microscopic interactions operating between the protein wall, ions and water molecules.

Animals↗

PubMed1994

An analysis of cardiac sodium channel properties using digital signal processing techniques.

The properties of single sodium channels in membrane patches from isolated rat ventricular myocytes were analyzed. Both cell-attached and inside-out patch configurations were examined. A digital signal processing method was used which allowed accurate determination of single-channel conductance and mean open time, even in situations where many channels coexisted in the patch. The results show that the cardiac sodium channel has a conductance of 5.3 pS at room temperature, and shows no rectification or saturation in the physiological ranges of ion concentration and membrane potential. The active channels contained in the membrane patch opened and closed independently of each other. A transition probability matrix, which describes transitions between all current levels, can also be generated, analysis of which permits an estimate of individual mean channel open time as well as the degree of coupling between channels.

Animals↗

PubMed1994

Zinc induces hyperexcitability in the hippocampus.

The effects of zinc on neuronal excitability in rodent hippocampal slices were examined. In a paired-pulse protocol, the second population spike increased appreciably in the presence of zinc, whereas the first spike and the size of both population excitatory post-synaptic potentials remained unaffected. Changes in the second population spike produced by zinc were most pronounced when the afferents were stimulated with paired-pulses separated by 8-40 ms. The magnitude of altered excitability increased with the concentration of zinc in the perfusate. A long exposure to zinc in physiological concentration caused an epileptiform discharge followed by a period of depression. The effects of zinc could be mimicked with 1-3 microM bicuculline. We conclude that the integrity of the hippocampal inhibitory system is particularly vulnerable to zinc.

Action Potentials↗

PubMed1994

Influence of membrane potential on conductance sublevels of chloride channels activated by GABA.

Single-channel chloride currents activated by 0.5 microM GABA were recorded in cell-attached and inside-out membrane patches from rat cultured hippocampal neurons. The currents displayed multiple conductance states and outward rectification. The number and amplitude of conductance levels were determined over a range of potentials by using digital signal-processing techniques. It was found that, except for a level close to zero, subconductance levels were regularly spaced. There were fewer sublevels at hyperpolarized than at depolarized potentials, and the spacing between levels varied linearly with potential giving an incremental conductance of 8-10 pS that was independent of membrane potential. Outward rectification is related to the change in the number of conductance levels with potential. One hypothesis that is consistent with these observations is that a channel is composed of a number of synchronized, non-rectifying, conducting pores, and that the number of pores activated changes with membrane potential.

Animals↗

PubMed1994

Angiotensin II-mediated stimulation of phospholipase D in rabbit kidney proximal tubule cells.

The present study was undertaken to demonstrate whether or not angiotensin II activates a phospholipase D in rabbit kidney proximal tubule cells. By measuring the formation of [3H]phosphatidic acid and [3H]phosphatidylethanol, we elucidate the direct stimulation of phospholipase D by angiotensin II. Angiotensin II leads to a rapid increase in [3H]phosphatidic acid and [3H]diacylglycerol, and [3H]phosphatidic acid formation preceded the formation of [3H]diacylglycerol. This result suggests that some phosphatidic acid seems to be formed directly from phosphatidylcholine by the action of phospholipase D, not from the action of diacylglycerol kinase on the diacylglycerol. In addition, the other mechanisms by which phospholipase D is activated was examined. We have found that phospholipase D was activated by extracellular calcium ion. It has also been shown that angiotensin II may activate phospholipase D through protein kinase C-independent pathway.

Angiotensin II↗

PubMed1994

Analyzing expression of the calmodulin and ubiquitin-fusion genes of Trypanosoma cruzi using simultaneous, independent dual gene replacements.

We describe here a strategy for introducing simultaneous, independent gene replacements into the Trypanosoma cruzi chromosome. The goal of this study was to use two linear DNA fragments to simultaneously replace the CalA2 calmodulin and FUS1 ubiquitin-fusion genes with the neomycin resistance (neo(r)) and chloramphenicol acetyltransferase (CAT) genes, respectively. One clone (D6), of thirty G418-resistant clones analyzed, carried the desired dual gene replacement. CDNA sequence analysis indicated that the CAT mRNA was accurately trans-spliced using the previously identified FUS1 mini-exon addition site. However, DNA sequence analysis of the intergenic sequence immediately upstream of the neo(r) gene in clone D6 identified a mutation which altered the pattern of trans-splicing of the neo(r) mRNA. Possible effects of this mutation on 3' splice acceptor site selection are discussed.

Animals↗

PubMed1994

Temperature dependence of conductivity in electrolyte solutions and ionic channels of biological membranes.

Temperature is a key parameter in the description of any physical system. Experimental study of the temperature dependence of conductivity is very valuable in building and testing theoretical models. This fact does not appear to be fully appreciated and exploited in the study of ionic channels of biological membranes owing in part to the lack of an adequate theory for the temperature dependence of conductivity in electrolyte solutions. To redress this imbalance, and to encourage further temperature-dependence studies in ionic channels, we first give explicit expressions for the conductivity of ions in electrolyte solutions in terms of the microscopic parameters of the liquid. We then propose that the dynamics of ion transport in membrane channels are similar to that in bulk electrolyte solutions, except that ions permeating the pore need to surmount a potential barrier, the height of which can be deduced experimentally. Finally, we use our model to analyze the conductance-temperature relationships obtained in two types of single ionic channels.

Cell Membrane↗

PubMed1993

Gait-related motor patterns and hindlimb kinetics for the cat trot and gallop.

To assess speed- and gait-related changes in semitendinosus (ST) activity, EMG was recorded from three cats during treadmill locomotion. Selected step cycles were filmed, and hip and knee joint kinematics were synchronized with EMG records. Swing-phase kinetics for trot and gallop steps at 2.25 m/s were compared for gait-related differences. Also, swing kinetics for different gallop forms were compared. With few exceptions, ST-EMG was characterized by two bursts for each step cycle; the first preceded paw off (STpo), and the second preceded paw contact (STpc). The two-burst pattern for the walk was defined by a high-amplitude STpo burst and a brief, low-amplitude STpc burst; at the slowest walk speeds, the STpc burst was occasionally absent. For the trot, the STpo burst was biphasic, with a brief pause just after paw off. With increasing walk-trot speeds, the duration of both bursts (STpo, STpc) remained relatively constant, but recruitment increased. Also, the onset latency of the STpo burst shifted, and a greater proportion of the burst was coincident with knee flexion during early swing. At the trot-gallop transition, there was an abrupt change in the two-burst pattern, and galloping was characterized by a high-amplitude STpc burst and a brief, low-amplitude STpo burst. At the fastest gallop speeds, the STpo burst was often absent, and the reduction in or elimination of the burst was associated with a unique pattern of swing phase kinetics at the knee. Knee flexion during the gallop swing was sustained by two inertial torques related to hip linear acceleration (HLA) and leg angular acceleration (LAA); correspondingly, muscle contraction was unnecessary. Conversely, knee flexion at the onset of the trot swing relied on a flexor muscle torque at the knee acting with an inertial flexor torque (LAA). Rotatory and transverse gallops at 4.0 m/s had similar swing phase kinetics and ST-EMG. Gait-related changes in ST-EMG, particularly at the trot-gallop transition, are not congruent with neural models assuming that details of the ST motor pattern are produced by a spinal CPG. We suggest that motor patterns programmed by the spinal CPG are modulated by input from supraspinal centers and/or motion-related feedback from the hindlimbs to provide appropriate gait-specific activation of the ST.

Animals↗