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P Shukla

Publications and source records attributed to P Shukla.

At least 19 recordsLinked to original sources
PubMed2001

Non-Hermitian random matrices and the calogero-sutherland model.

We study the statistical properties of the eigenvalues of non-Hermitian operators associated with the dissipative complex systems. By considering the Gaussian ensembles of such operators, a hierarchical relation between the correlators is obtained. Further, the eigenvalues are found to behave like particles moving on a complex plane under two-body (inverse square) and three-body interactions and there seems to underlie a deep connection and universality in the spectral behavior of different complex systems.

Growth↗
PubMed2001

Persistence in one-dimensional Ising models with parallel dynamics.

We study persistence in one-dimensional ferromagnetic and antiferromagnetic nearest-neighbor Ising models with parallel dynamics. The probability P(t) that a given spin has not flipped up to time t, when the system evolves from an initial random configuration, decays as P(t) approximately 1/t(straight theta(p)) with straight theta(p) approximately 0.75 numerically. A mapping to the dynamics of two decoupled A+A-->0 models yields straight theta(p)=3/4 exactly. A finite size scaling analysis clarifies the nature of dynamical scaling in the distribution of persistent sites obtained under this dynamics.

Journal Article↗
PubMed2000

Electrical approach to study rhodopsin activation in single cells with early receptor current assay.

The ERC is a conformation-dependent charge motion similar to the gating currents of ionic channels. Both the waveforms and bandwidth of ERCs and ionic channel gating currents are similar, providing support to the initial suggestion that the ERP was a kind of gating current. In ionic channels the electrostatic field promotes motion of alpha-helical elements that stimulate large-scale molecular events that promote opening of the ionic pore. In ionic channels gating currents of expressed channel mutants has contributed significantly to understanding the mechanism of activation. Given the known role of electrical processes to rhodopsin activation, the ERC approach applied to mutant and wild-type visual pigments is likely to lead to a fuller understanding of the mechanism of conformational activation. This method is currently well suited to investigate the later phases of rhodopsin activation that are thought to be electrostatic in nature. We anticipate that ERC studies will make significant contributions to understanding how the breakdown of the electrostatic interaction between the PSB and its counterion is initiated and propagated to induce the proton uptake on the cytoplasmic surface of the pigment and the shaping of the transducin docking domain. We encourage collaboration to apply the ERC methodology to interesting mutant pigments and retinal analogs. We expect that the ERC methodology can soon be applied to understand rapid charge displacements associated with photochemistry (i.e., R1), the effects of transduction proteins on R2, and the measurement of electrical processes during cone visual pigment activation.

Cell Fusion↗
PubMed2000

What are the risks of diagnostic and therapeutic endoscopic retrograde cholangiopancreatography?

BACKGROUND: Diagnostic and therapeutic endoscopic retrograde cholangiopancreatography (ERCP) has been practised widely over the last 20 years, and it has revolutionized the diagnosis and management of biliary and pancreatic conditions. More recently newer techniques (magnetic resonance imaging) for diagnosis and therapy (laparoscopic biliary surgery) have evolved. The present paper evaluates the risks of ERCP procedures in a modern setting. METHODS: A prospective audit of all ERCP carried out by a single unit across two campuses over 12 months was undertaken. All procedures were included and predetermined morbidity criteria were recorded and evaluated by independent observers who did not perform the procedures. RESULTS: During this period 372 procedures were performed. A total of 9.4% of procedures failed to achieve the preprocedure-stated goal. There were five deaths (30-day mortality of 1.3%) and in 16 patients complications were recorded (morbidity of 4.3%). Two clinical and two technical factors were shown to be associated with the morbidity and mortality by multiple logistic regression analysis: diagnosis of sphincter of Oddi dysfunction; presence of jaundice; need to perform percutaneous transhepatic drainage of an obstructed biliary system after a failed endoscopic approach; and multiple ERCP. CONCLUSIONS: These results compare favourably with results from other reported series and serve to illustrate the relative safety of diagnostic and therapeutic ERCP.

Adolescent↗
PubMed2000

Alternative technique for complex spectra analysis

The choice of a suitable random matrix model of a complex system is very sensitive to the nature of its complexity. The statistical spectral analysis of various complex systems requires, therefore, a thorough probing of a wide range of random matrix ensembles which is not an easy task. It is highly desirable, if possible, to identify a common mathematical structure among all the ensembles and analyze it to gain information about the ensemble properties. Our successful search in this direction leads to the Calogero Hamiltonian, a one-dimensional quantum Hamiltonian with inverse-square interaction, as the common base. This is because both the eigenvalues of the ensembles and a general state of the Calogero Hamiltonian evolve in an analogous way for arbitrary initial conditions. The varying nature of the complexity is reflected in different forms of the evolution parameter in each case. A complete investigation of the Calogero Hamiltonian can then help us in the spectral analysis of complex systems.

Journal Article↗
PubMed1999

Time-resolved rhodopsin activation currents in a unicellular expression system.

The early receptor current (ERC) is the charge redistribution occurring in plasma membrane rhodopsin during light activation of photoreceptors. Both the molecular mechanism of the ERC and its relationship to rhodopsin conformational activation are unknown. To investigate whether the ERC could be a time-resolved assay of rhodopsin structure-function relationships, the distinct sensitivity of modern electrophysiological tools was employed to test for flash-activated ERC signals in cells stably expressing normal human rod opsin after regeneration with 11-cis-retinal. ERCs are similar in waveform and kinetics to those found in photoreceptors. The action spectrum of the major R(2) charge motion is consistent with a rhodopsin photopigment. The R(1) phase is not kinetically resolvable and the R(2) phase, which overlaps metarhodopsin-II formation, has a rapid risetime and complex multiexponential decay. These experiments demonstrate, for the first time, kinetically resolved electrical state transitions during activation of expressed visual pigment in a unicellular environment (single or fused giant cells) containing only 6 x 10(6)-8 x 10(7) molecules of rhodopsin. This method improves measurement sensitivity 7 to 8 orders of magnitude compared to other time-resolved techniques applied to rhodopsin to study the role particular amino acids play in conformational activation and the forces that govern those transitions.

Cell Fusion↗
PubMed1999

Normal and mutant rhodopsin activation measured with the early receptor current in a unicellular expression system.

The early receptor current (ERC) represents molecular charge movement during rhodopsin conformational dynamics. To determine whether this time-resolved assay can probe various aspects of structure-function relationships in rhodopsin, we first measured properties of expressed normal human rhodopsin with ERC recordings. These studies were conducted in single fused giant cells containing on the order of a picogram of regenerated pigment. The action spectrum of the ERC of normal human opsin regenerated with 11-cis-retinal was fit by the human rhodopsin absorbance spectrum. Successive flashes extinguished ERC signals consistent with bleaching of a rhodopsin photopigment with a normal range of photosensitivity. ERC signals followed the univariance principle since millisecond-order relaxation kinetics were independent of the wavelength of the flash stimulus. After signal extinction, dark adaptation without added 11-cis-retinal resulted in spontaneous pigment regeneration from an intracellular store of chromophore remaining from earlier loading. After the ERC was extinguished, 350-nm flashes overlapping metarhodopsin-II absorption promoted immediate recovery of ERC charge motions identified by subsequent 500-nm flashes. Small inverted R(2) signals were seen in response to some 350-nm flashes. These results indicate that the ERC can be photoregenerated from the metarhodopsin-II state. Regeneration with 9-cis-retinal permits recording of ERC signals consistent with flash activation of isorhodopsin. We initiated structure-function studies by measuring ERC signals in cells expressing the D83N and E134Q mutant human rhodopsin pigments. D83N ERCs were simplified in comparison with normal rhodopsin, while E134Q ERCs had only the early phase of charge motion. This study demonstrates that properties of normal rhodopsin can be accurately measured with the ERC assay and that a structure-function investigation of rapid activation processes in analogue and mutant visual pigments is feasible in a live unicellular environment.

Cell Line↗
PubMed1999

Universal level dynamics of complex systems.

We study the evolution of the distribution of eigenvalues of a N x N matrix subject to a random perturbation drawn from (i) a generalized Gaussian ensemble and (ii) a non-Gaussian ensemble with a measure variable under the change of basis. It turns out that, in case (i), a redefiniton of the parameter governing the evolution leads to a Fokker-Planck equation similar to the one obtained when the perturbation is taken from a standard Gaussian ensemble (with invaraiant measure). This equivalence can therefore help us to obtain the correlations for various physically significant cases modeled by generalized Gaussian ensembles by using the already known correlations for standard Gaussian ensembles. For large N values, our results for both cases (i) and (ii) are similar to those obtained for the Wigner-Dyson gas as well as for the perturbation taken from a standard Gaussian ensemble. This seems to suggest the independence of evolution, in the thermodynamic limit, from the nature of perturbation involved as well as the initial conditions and therefore the universality of dynamics of the eigenvalues of complex systems.

Journal Article↗
PubMed1997

Use of amphotropic retroviral vectors for gene transfer in human colon carcinoma cells.

Previous studies in rodent models have demonstrated the feasibility of gene transfer to the stem cells of the intestinal epithelium using ecotropic retroviral vectors delivered luminally. This report represents a next step toward targeting the human intestine as a site for somatic gene therapy. The first experiment assessed the viability of amphotropic retroviral vectors in the luminal environment. It was found that after 4 hr at 37 degrees C in luminal effluent, the loss of titer was no greater than when incubated in control media. Likewise, neither the vector nor the target cells were adversely affected by N-acetylcysteine, which is likely to be used as a preparatory agent for mucus removal. To determine whether human intestinal cells are transducible by these vectors, three colon carcinoma cell lines were studied: HT-29, T84, and Caco-2. All were transduced; however, the expression of the reporter gene was highest in the HT-29 cells. Subsequent studies using these cells showed that with regular stocks of vector, gene transfer peaked at a stock dilution of 1/10 and declined at full strength. This problem could be partially overcome by centrifugal concentration of the retroviral stocks. With this approach, gene transfer increased with increasing particles up to 10x regular stock titers but was inefficient at 100x. Overall, these findings provide encouraging evidence that amphotropic retroviral vectors may eventually be used for in vivo gene transfer into human intestinal epithelium. However, they also point to the need for improved methods of concentrating retroviral vectors.

3T3 Cells↗
PubMed1996

Gene transfer into fetal rat intestine.

To assess the fetal intestine as a site for gene therapy, we have explored a xenograft model in which fetal rat intestine is grafted subcutaneously into nu/nu mice. Prior to grafting, the tissue was exposed to a replication-deficient retroviral vector bearing the neo gene. Transduction efficiency was assessed by quantitative polymerase chain reaction (PCR) of neo in DNA recovered from the grafts. Three methods of infection were employed: (i) simple flushing of the fetal intestine with the vector; (ii) incubation with the vector for 2 hr; and (iii) a combination of both. The first method gave the highest transduction efficiencies in terms of both the proportion of samples that were neo-positive and the number of neo-positive cells per sample. Using this approach, the time course of persistence of neo-positive cells was analyzed by collecting grafts at 1 versus 3 weeks post-infection. The results showed approximately five-fold more positive cells at the earlier time point than at the later, suggesting loss of transduced cells due to cell turnover. Nevertheless, the persistence of a portion of the positive cells for at least 3 weeks is encouraging for future studies with fetal intestine.

Animals↗
PubMed1994

Optimization of gene transfer into intestinal epithelial cells using a retroviral vector.

Somatic gene therapy has been proposed as a method of treating various metabolic diseases and conditions associated with a deficiency in secretory proteins. The intestine, because of its accessibility, large size, rapid rate of cell turnover, and known location of stem cells, is an attractive site for retroviral gene therapy. Stem cells in the intestine are known to reside in the crypts of Lieberkühn. In this study the IEC-6 cell line, derived from rat intestinal crypt cells, was used as an in vitro model to study the feasibility and dynamics of retroviral gene transfer in intestinal cells. Using a replication-deficient retrovirus, we delivered a reporter gene, bacterial beta-galactosidase (beta-gal) into NIH-3T3 fibroblasts and IEC-6 cells. Successful transduction was measured by X-gal histochemistry. Viral titers on IEC-6 cells were lower than on NIH-3T3 cells but were within the same order of magnitude. Gene transfer increased linearly with retroviral concentration up to a 1:5 dilution of retroviral supernatant. With undiluted viral medium, gene transfer was inhibited and this effect was more pronounced with the IEC-6 cells. The negative effect of the undiluted retroviral supernatant was minimized by decreasing the harvest time from the packaging cell line. An optimal plating density 12 h prior to infection was found to be approximately 10(5) cells/6-cm dish in both cell lines. The infection rate was proportionally enhanced by the use of multiple infections. In conclusion, under ideal conditions, the IEC-6 cells were infected at similar levels of efficacy as NIH-3T3 cells.(ABSTRACT TRUNCATED AT 250 WORDS)

3T3 Cells↗