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Biomedical subjects

S H Chung

Publications and source records attributed to S H Chung.

At least 91 records · Page 5Linked to original sources
PubMed1993

Fluctuation analysis of patch-clamp or whole-cell recordings containing many single channels.

A novel analytical method for characterizing single-channel currents from recordings containing many identical, independent channels is described. The method is based on the assumption that the opening and closing of each single channel contributing to the summed current can be represented as a first-order, discrete-time, binary Markov chain and that the variance of the quiescent channel noise is known. Utilizing the first 3 moments of the record, and its power spectrum, all relevant single-channel parameters can be estimated. This includes the number of channels, the open current amplitude of a single channel, the mean open and closed durations and the probability of a channel being in the open state. In addition, the magnitude of the shot noise resulting from the flux of ions across the membrane can be estimated. Using fictitious multi-channel recordings generated by summing 2-990 independent binary Markov chains together with additive white noise, we have tested the reliability of the method in estimating the statistics of single channels. Finally, we discuss how the technique may be extended to cope with data which has been low-pass filtered, and also suggest further experiments which the technique now makes possible.

Electrophysiology↗
PubMed1993

CT diagnosis of superior vena cava syndrome: importance of collateral vessels.

OBJECTIVE: The purpose of this study was to evaluate the importance of collateral vessels in the CT diagnosis of superior vena cava syndrome. MATERIALS AND METHODS: Forty-seven patients with CT evidence of obstruction or compression of the superior vena cava or its major tributaries were included in this study. Contrast-enhanced CT scans had been obtained with a combined bolus and drip-infusion technique. We evaluated the CT scans, with particular attention to the presence of collateral vessels (and if present, their location), without knowing the patients' clinical histories. On reviewing the patients' charts, we found 23 patients with signs or symptoms of superior vena cava syndrome. We then correlated the presence of collateral vessels with the presence of signs and symptoms of superior vena cava syndrome. To verify the frequency of visible collateral vessels in healthy subjects, we also evaluated CT scans of 50 control subjects who did not have mediastinal disease or signs and symptoms of superior vena cava syndrome. RESULTS: Collateral vessels were seen on CT scans in 24 patients. Among the 23 patients who had signs and symptoms of superior vena cava syndrome, 22 patients had CT scans that showed collateral vessels. Two cases were false-positive and one was false-negative. The presence of collateral vessels, regardless of the number and location of the vessels shown on CT scans, was highly accurate as a predictor of superior vena cava syndrome, with a sensitivity of 96% and a specificity of 92%. Collateral vessels were seen in three (6%) of the 50 control subjects. CONCLUSION: We conclude that the presence of collateral vessels is a highly sensitive and specific CT sign of superior vena cava syndrome.

Adolescent↗
PubMed1992

Muscarinic acetylcholine receptor produced in recombinant baculovirus infected Sf9 insect cells couples with endogenous G-proteins to activate ion channels.

Following the infection of insect ovarian cells (Sf9) with recombinant bearing the cDNA coding for the rat muscarinic acetylcholine (ACh) receptor subtype m3, ionic flux across the membrane in response to the application of ACh was examined electrophysiologically. We show that ACh activates potassium currents. The response is abolished when cells are treated with pertussis toxin. No ACh-induced currents are observed from uninfected cells or cells infected with virus which do not contain the cDNA coding for ACh receptors in its genome. The characteristics of single channel currents show time-dependent changes following the application of ACh. Initially, ACh activates brief channel currents with a conductance of about 5 pS. The conductance level of channels gradually increases in steps to 10 pS and then to 20 pS and 40 pS. At the same time, channel open probability also increases. Thereafter, additional channels appear, opening and closing independently of, or at times in synchrony with, the original channel.

Acetylcholine↗
PubMed1992

Subcutaneously implanted catheters reduce the incidence of peritonitis during CAPD by eliminating infection by periluminal route.

UNLABELLED: Recent experiences with Y-connectors suggest that the flush-before-fill effectively reduces intraluminal infection. Periluminal infection, however, remains an important route of peritonitis (P). We have recently reported reduced P incidence with the introduction of a new access technique as described by Moncrief in which the external segment of peritoneal catheter is left implanted subcutaneously for 6 weeks before exteriorization and bag exchanges. P developed once every 14.0 patient-mos with the new access while the incidence was one episode per 10.7 mos with conventional access. Significantly fewer patients with the new access compared to those with conventional access experienced P during the observation period (p < 0.01). Although the overall incidence of exit-site infection (ESI) was not different, there were significantly fewer episodes of simultaneous P and ESI with the new access (2P in 47 episodes of ESI) than with conventional access (36P in 126 ESI). While 10 of the 36 episodes of simultaneous infection in the conventional technique were caused by same organisms, none of the 2 episodes with the new access technique was caused by same organisms. CONCLUSION: The results of this study suggest that the new access technique reduces P incidence by virtually eliminating infection by the periluminal route.

Adolescent↗
PubMed1991

Adaptive processing techniques based on hidden Markov models for characterizing very small channel currents buried in noise and deterministic interferences.

Techniques for characterizing very small single-channel currents buried in background noise are described and tested on simulated data to give confidence when applied to real data. Single channel currents are represented as a discrete-time, finite-state, homogeneous, Markov process, and the noise that obscures the signal is assumed to be white and Gaussian. The various signal model parameters, such as the Markov state levels and transition probabilities, are unknown. In addition to white Gaussian noise, the signal can be corrupted by deterministic interferences of known form but unknown parameters, such as the sinusoidal disturbance stemming from AC interference and a drift of the base line owing to a slow development of liquid-junction potentials. To characterize the signal buried in such stochastic and deterministic interferences, the problem is first formulated in the framework of a Hidden Markov Model and then the Expectation Maximization algorithm is applied to obtain the maximum likelihood estimates of the model parameters (state levels, transition probabilities), signals, and the parameters of the deterministic disturbances. Using fictitious channel currents embedded in the idealized noise, we first show that the signal processing technique is capable of characterizing the signal characteristics quite accurately even when the amplitude of currents is as small as 5-10 fA. The statistics of the signal estimated from the processing technique include the amplitude, mean open and closed duration, open-time and closed-time histograms, probability of dwell-time and the transition probability matrix. With a periodic interference composed, for example, of 50 Hz and 100 Hz components, or a linear drift of the baseline added to the segment containing channel currents and white noise, the parameters of the deterministic interference, such as the amplitude and phase of the sinusoidal wave, or the rate of linear drift, as well as all the relevant statistics of the signal, are accurately estimated with the algorithm we propose. Also, if the frequencies of the periodic interference are unknown, they can be accurately estimated. Finally, we provide a technique by which channel currents originating from the sum of two or more independent single channels are decomposed so that each process can be separately characterized. This process is also formulated as a Hidden Markov Model problem and solved by applying the Expectation Maximization algorithm. The scheme relies on the fact that the transition matrix of the summed Markov process can be construed as a tensor product of the transition matrices of individual processes.

Algorithms↗
PubMed1991

Forward-backward non-linear filtering technique for extracting small biological signals from noise.

A novel and computationally efficient, non-linear signal processing technique for reducing background noise to reveal small biological signals is described. The signal estimate is formed by weighting the outputs of a set of causal (forward) and anti-causal (backward) predictors. The weights used to combine the predictors are adaptively determined at each data point to reflect the performance of the respective predictor within a short analysis window. The method is specifically designed for revealing fast transient signals dominated by noise, such as single-channel or post-synaptic currents. Markovian and exponentially decaying signals embedded in the amplifier noise were extracted using this method and compared with the original signals. The results of such simulations demonstrate the advantage of this non-linear method over low-pass filtering. Brief pulses imbedded in a broad-band amplifier noise can be reliably recovered using our non-linear filtering technique. Moreover, the kinetics of a single channel and the time constant of exponentially decaying signals can be measured with acceptable accuracy even when the signals are dominated by noise.

Electronics↗
PubMed1990

Coupled potassium channels induced by arachidonic acid in cultured neurons.

Exposure of the inside surface of patches of membrane excised from cultured rat hippocampal neurons to arachidonic acid (10-100 microM) caused the appearance of potassium currents of variable amplitude similar to those activated by GABA or baclofen in cell-attached patches. The amplitude of single-channel currents increased with time after exposure to 20 or 50 microM arachidonic acid and also increased when arachidonic acid concentration was increased from 20 to 50 or 100 microM. Current-amplitude probability histograms had peaks at integral multiples of an 'elementary' current. It is proposed that arachidonic acid or its metabolites cause synchronous opening and closing of coupled conducting units (co-channels) in cell membranes.

Animals↗
PubMed1990

Characterization of single channel currents using digital signal processing techniques based on Hidden Markov Models.

Techniques for extracting small, single channel ion currents from background noise are described and tested. It is assumed that single channel currents are generated by a first-order, finite-state, discrete-time, Markov process to which is added 'white' background noise from the recording apparatus (electrode, amplifiers, etc). Given the observations and the statistics of the background noise, the techniques described here yield a posteriori estimates of the most likely signal statistics, including the Markov model state transition probabilities, duration (open- and closed-time) probabilities, histograms, signal levels, and the most likely state sequence. Using variations of several algorithms previously developed for solving digital estimation problems, we have demonstrated that: (1) artificial, small, first-order, finite-state, Markov model signals embedded in simulated noise can be extracted with a high degree of accuracy, (2) processing can detect signals that do not conform to a first-order Markov model but the method is less accurate when the background noise is not white, and (3) the techniques can be used to extract from the baseline noise single channel currents in neuronal membranes. Some studies have been included to test the validity of assuming a first-order Markov model for biological signals. This method can be used to obtain directly from digitized data, channel characteristics such as amplitude distributions, transition matrices and open- and closed-time durations.

Animals↗
PubMed1990

GABA-induced potassium channels in cultured neurons.

When gamma-aminobutyric acid (GABA) or baclofen were applied to cultured rat hippocampal neurons, single-channel potassium currents appeared after a delay of 30 s or more in patches of membrane on the cell surface isolated from the agonists by the recording pipette. The appearance of currents in patches not exposed to agonist, the delay in their appearance and the suppression of currents in cells pre-incubated with pertussis toxin indicate the involvement of an intracellular second messenger system. The channels were associated with a GABAB receptor rather than a GABAA receptor as they were blocked by baclofen, a GABAB antagonist, but were not affected by bicuculline, a GABAA antagonist. A feature of the single channel currents was their variable amplitude: they had a maximum conductance of ca. 70 pS and displayed many lower conductance states that were integral multiples of 5-6 pS. In several cells exposed to GABA or baclofen, first small currents and then progressively larger currents appeared: current amplitude was a multiple of an elementary current. It is suggested that binding of GABA to GABAB receptors activates a second messenger system causing opening of oligomeric potassium channels.

Animals↗
PubMed1990

Linkage of the calmodulin and ubiquitin loci in Trypanosoma cruzi.

We describe here the organization of the calmodulin genes of Trypanosoma cruzi and their linkage to the ubiquitin gene family. The nucleotide sequence of the CalA2 gene has been determined and is 85% homologous to the protein coding sequence of the calmodulin genes of the African trypanosome, Trypanosoma brucei. The proteins encoded by CalA2 and the T. brucei genes contain a single mismatch out of a total of 149 amino acids. The genome of T. cruzi contains eight calmodulin genes present at two distinct loci (2.8 and 2.65) each of which is linked to downstream ubiquitin genes. Within each locus two calmodulin gene families have been defined, CalA and CalB. Each calmodulin locus consist of alternating tandem arrays of the CalA and CalB genes. Both 2.8 and 2.65 calmodulin loci and their respective downstream ubiquitin genes share the same DNA coding strand. Transcription of the calmodulin genes in the epimastigote stage of T. cruzi results in the generation of two stable mRNAs of 1.6Kb and 1.1Kb.

Amino Acid Sequence↗
PubMed1990

Hepatitis C infection in hemodialysis units.

We performed an epidemiological study of the hepatitis C infection on 112 patients of 3 urban hemodialysis units using a recently developed anti-HCV recombinant based assay. Eleven patients (9.8%) were positive for anti-HCV. Among them, 8 (72.7%) were positive for anti-HBc, one of whom was HBsAg positive and 6 of whom were also anti-HBs positive. Surprisingly, all of the anti-HCV (+) patients were normal alanine aminotransferase. The mean age of the anti-HCV (+) patients was 50.7 +/- 3.3 (mean +/- SE) and that of the anti-HCV (-) was 47.6 +/- 1.3. The mean duration (month) of hemodialysis of the anti-HCV (+) and anti-HCV (-)groups were 52.7 +/- 7.2 (mean +/- SE) and 60.9 +/- 9.7, respectively. The prevalence of anti-HCV among anti-HBc positive subjects was 9.5% and that among anti-HBc negative subjects was 17.6%. This didn't have any statistical significance according to the criteria of the study (p = 0.308). The prevalence of anti-HCV among the transfusion positive group was 11.0% and that of the transfusion negative group was 7.7%. This data showed the tendency for a higher prevalence of anti-HCV among the transfusion positive group, but this also didn't reach statistical significance (p = 0.424). Of the 40 normal controls, none were anti-HCV positive. The prevalence of HBsAg in our hemodialysis units was 12.5%. This rate was not so much higher than the average population in Korea. The prevalence of anti-HCV and previous hepatitis B virus infection also had no significant relationship.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1990

Determinants of fertility control in Korea.

The objective of this study is to examine the socioeconomic and intervening determinants of fertility control in Korea. The conceptual framework applied here is based on the synthesis framework of fertility control developed by Easterlin and Crimmins. The data came from the 1974 Korean National Family Survey, which was conducted as part of the World Fertility Survey. The study focuses on the relative importance of the socioeconomic factors and intervening variables in the determination of fertility control. The most interesting finding is that there are only small differentials in fertility control by socioeconomic factors. The analysis emphasizes the importance of examining the determinants of fertility control in terms of the intervening variables, which include the components of natural fertility, desired family size, and costs of fertility control. These intervening variables provide general support for the theoretical predictions. Moreover, most of the coefficients for these variables are statistically significant at the .05 level. The analysis also shows that motivation for fertility control is an important factor in determining use of fertility control.

Age Factors↗
PubMed1990

Peritoneal solute clearances in diabetics.

In order to examine solute transport across the peritoneal membrane and responsiveness of the peritoneal microcirculation to a vasodilator in diabetics on continuous ambulatory peritoneal dialysis (CAPD), we obtained peritoneal clearances of urea (Curea) and creatinine (Ccr), protein concentrations in the drained dialysate (DPC), and percentage of peritoneal glucose absorption (% PGA) before and after nitroprusside (NP) addition to the dialysate in 13 diabetics (DM) and 13 nondiabetics (non-DM) matched for age, sex, body weight, and duration of CAPD. Control (before NP) Curea, Ccr, DPC, and %PGA were not different between DM and non-DM. NP significantly enhanced Curea, Ccr, and DPC in both DM and non-DM. Curea, Ccr, DPC, and %PGA after NP were again not different between DM and non-DM. The findings suggest that peritoneal solute clearances and responsiveness of the peritoneal microcirculation to NP in diabetics are not different from nondiabetics at the beginning of CAPD despite evidence for widespread vascular diseases in diabetic ESRD patients.

Biological Transport, Active↗
PubMed1989

Orotate phosphoribosyltransferase from yeast: studies of the structure of the pyrimidine substrate binding site.

The pH dependencies of both the forward and reverse orotate phosphoribosyltransferase (ORPTase)-catalyzed reactions have been examined and determined to be dissimilar, with maximal activity for the forward reaction near to pH 8. The maximal activity of the reverse pyrophosphorolysis was observed between pH 6.5 and 7.5. Appropriate pK values were determined using computer fitting exercises. One such pK value (equal to 8.6) suggested the presence of lysine residues at the OPRTase active site. Incubations of OPRTase with the substrate analog, uracil 6-aldehyde, in the presence of sodium borohydride, suggested that this compound is a covalent modifier of OPRTase lysine residues, and substrate protection studies provided evidence that the affected lysine residues were located near to both the phosphoribosyl 1-pyrophosphate (PRibPP) and the orotate binding sites. Similar studies with pyridoxal 5-phosphate and labeled sodium borohydride as modifiers have revealed that two modified active site lysine residues per OPRTase subunit account for the loss of 90% of the enzymatic activity with this reagent. We suggest that essential lysine residues, along with divalent metal ions, are located at the OPRTase active site, and form ion-pair bonds with anionic PRibPP and orotate as these substrates bind to the enzyme. We also report that 5-azaorotate is an alternate substrate for OPRTase (Km = 75.5 +/- 0.1 microM) leading to formation of an unstable nucleotide product).

Affinity Labels↗
PubMed1987

Signal processing technique to extract neuronal activity from noise.

A method of extracting extracellularly recorded action potentials from background electronic noise is described. Segments of traces containing stimulus-induced activity are Fourier transformed and the increase in the total power density over that of control noise segments is used as a measure of stimulus-induced neuronal activity. We show first, with observations from the amphibian visual system and mammalian auditory system, that our technique yields similar quantitative information to that obtained from the conventional spike counting method when the recording arrangement is optimal. Moreover, the size and centre of a visual receptive field can be determined even when the evoked action potentials are buried in the background noise. To investigate the potential of this technique further, we have used it to study the auditory responses in the amphibian midbrain. The power spectral density, we demonstrate here, is proportional to the stimulus intensity over a wide range, and varies systematically with stimulus frequency and the direction of sound source. Other possible applications of this technique, together with the theoretical basis for it, are discussed.

Action Potentials↗
PubMed1986

Enzymatic kinetic analyses that employ high-performance liquid chromatography. Competition between orotate- and hypoxanthine/guanine-phosphoribosyltransferases for a common substrate.

Enzymatic assay procedures that employ high-performance liquid chromatography (HPLC) have been proven to be sensitive and versatile methods for accomplishing kinetic analyses of enzyme-catalyzed reactions, with nucleotides as substrates or products. Both orotate phosphoribosyltransferase (OPRTase) and hypoxanthine/guanine phosphoribosyltransferase (HGPRTase) have been purified from Baker's yeast and analyzed kinetically using a modification of published HPLC procedures. Because these two enzymes exist in the cytosol of yeast and might compete for the limiting (approximately equal to 15 microM) concentration of phosphoribosyl alpha-1-pyrophosphate (PRibPP), we elected to examine both equilibrium and steady-state effects of one enzymatic reaction on the other with HPLC. First, under the condition of equivalent mass concentrations of OPRTase and HGPRTase, the initial rate of orotidine monophosphate synthesis and the equilibrium state were greatly affected by the presence of HGPRTase activity. In contrast, the presence of the OPRTase activity had no effect on the HGPRTase-catalyzed reaction under these conditions. Second, to examine a competition by these enzymes for PRibPP in vivo, we have established that the total activities (units/ml) of OPRTase and HGPRTase in yeast cell extracts were 740 units/ml and 450 units/ml, respectively (a 1.7:1 ratio). These relative activities were then employed in an in vitro reaction competition analysis. The results were similar to the those obtained from experiments where equivalent OPRTase and HGPRTase activities were employed and reveal profound initial velocity and equilibrium effects of one reaction on the other. Thus a real competition between these enzymes for PRibPP may occur in the yeast cell cytosol, as determined by this unique HPLC competition assay procedure.

Binding, Competitive↗