Search

Cell experiments with large, short electric field pulses of opposite polarity reveal a remarkable phenomenon: Bipolar cancellation (BPC). Typical defining experiments involve quantitative observation of tracer molecule influx at times of order 100 s post pulsing. Gowrishankar et al. BBRC 2018 503:1194-1199 shows that long-lived pores and altered partitioning or hindrance due to inserted occluding molecules can account for BPC. In stark contrast, the Assisted Discharge (AD) hypothesis, Pakhomov et al. CellMol- LifeSci 2014 71(22):4431-4441; Fig. 6, only involves early times of a microsecond down to nanoseconds. Further, well established terminology for cell membrane discharge relates to membrane potential decays shortly after pulsing. Discharge is silent on molecular or ionic transport, and does not address the fact that tracer molecule uptake vs time is measure at about 100s after pulsing ceases. Our critique of AD notes that there can be an association of AD with BPC, but associations are only necessary, not sufficient. A BPC mechanism hypothesis must be shown to be causal, able to describe time-dependent molecular influx. The two hypotheses involve very different time-scales (less than a microsecond vs 100 s) and very different quantities (volts/s vs molecules/s). Unlike pore-based hypotheses the AD hypothesis lacks explicit molecular transport mechanisms, and does not address the greatly delayed measured molecular uptake. We conclude that AD is an implausible candidate for explaining BPC., Comment: Version 01

Extraordinarily large but short electric field pulses are reported by many experiments to cause bipolar cancellation (BPC). This unusual cell response occurs if a first pulse is followed by a second pulse with opposite polarity. Possibly universal, BPC presently lacks a mechanistic explanation. Multiple versions of the "standard model" of cell electroporation (EP) fail to account for BPC. Here we show, for the first time, how an extension of the standard model can account for a key experimental observation that essentially defines BPC: the amount of a tracer that enters a cell, and how tracer influx can be decreased by the second part of a bipolar pulse. The extended model can also account for the recovery of BPC wherein the extent of BPC is diminished if the spacing between the first and second pulses is increased. Our approach is reverse engineering, meaning that we identify and introduce an additional biophysical mechanism that allows pore transport to change. We hypothesize that occluding molecules from outside the membrane enter or relocate within a pore. Significantly, the additional mechanism is fundamental and general, involving a combination of partitioning and hindrance. Molecules near the membrane can enter pores to block transport of tracer molecules while still passing small ions (+/- 1) that govern electrical behavior. Accounting for such behavior requires an extension of the standard model., Comment: 12 pages, 4 figures

Standard model of electropration (EP) has long emphasized a single pore lifetime to explain post-pulse transport across cell membranes. However, pore lifetimes estimated from molecular dynamics (MD) models and and those measured from experimental data differ by several orders of magnitude. We hypothesize that a broad distribution of lifetimes may describe the post-pulse behavior. Here, we show that pore distribution, number and size of pores, show interesting behavior in different ranges of pore lifetimes. Interestingly, for large electric fields (greater than 1 kV/cm) and short pore lifetimes (approximately 100 ns), an significant loss in pore number occurs during the pulse. Given the large number of EP applications that apply such fields, this phenomenon may be crucial to post-pulse response of cell membrane to such fields., Comment: Version 1, Oct 10, 2017

We present results of studying the post-pulse transport properties of single pores. Single pores exhibit active transport (drift), and not just passive transport (diffusion) at early post-pulse times. In addition, we suggest experimental design methods obtained from model parameters that would enable experimentalists to find localized regions of a few pores., Comment: Version 1, Oct 10, 2017

We describe a concept that has the potential to change how we think about the underlying mechanisms of cell membrane electroporation (EP). Prior experimental, theoretical and modeling have emphasized a single pore lifetime as adequate for particular conditions. Here we introduce a much more complex response: The rapid creation of many types of pore structures, of which some are traditional transient lipidic pores (TPs), but the great majority are complex pores (CPs) based on both lipids and other molecules or molecular segments. At the inner leaflet of the cell plasma membrane (PM) non-lipidic molecules come from the over-crowded cytoplasm. At the outer leaflet they originate from the extracellular medium and extracellular matrix. Some partially or fully insert into TPs during or shortly after TP formation, or bind to the membrane nearby. This process is complex, leading to mostly short-lived structures, with relatively few lasting for long times. We speculate that the characteristic pore lifetimes range from $\sim$100 ns to 1,000 s, based on implications from experiments. The frequency-of-occurrence probably falls off extremely rapidly with increasing lifetime, $\tau_{CP}$, which implies that most are inaccessible to traditional experimental methods. It also suggests that unexpected behavior can occur early in pulsing, vanishing before post-pulse observations begin., Comment: Version01 August 24, 2017

We performed a 3D radiative MHD simulation of the solar atmosphere. This simulation shows a jet-like feature that shows similarities to the type II spicules observed for the first time with Hinode. Rapid Blueshifted Events (RBEs) on the solar disk are associated with these spicules. Observational results suggest they may contribute significantly in supplying the corona with hot plasma. We perform a detailed comparison of the properties of the simulated jet with those of type II spicules (observed with Hinode) and RBEs (with ground-based instruments). We analyze variety of synthetic emission and absorption lines from the simulations including chromospheric Ca II and Ha to TR and coronal temperatures (10E4 to several 10E6K). We compare their synthetic intensities, line profiles, Doppler shifts, line widths and asymmetries with observations from Hinode/SOT and EIS, SOHO/SUMER, SST and SDO/AIA. Many properties of the synthetic observables resemble the observations, and we describe in detail the physical processes that lead to these observables. Detailed analysis of the synthetic observables provides insight into how observations should be analyzed to derive information about physical variables in such a dynamic event. For example, we find that LOS superposition in the optically thin atmosphere requires the combination of Doppler shifts and spectral line asymmetry to determine the velocity in the jet. Other properties differ from the observations, especially in the chromospheric lines. The mass density of the part of the spicule with a chromospheric temperature is too low to produce significant opacity in chromospheric lines. These and other discrepancies are described in detail, and we discuss which mechanisms and physical processes may need to be included in the MHD simulations to mimic the thermodynamic processes of the chromosphere and corona, in particular to reproduce type II spicules., Comment: 57 pages, 23 Figures, Accepted in ApJ

We present results of a 3D model of optical to gamma-ray emission from the slot gap accelerator of a rotation-powered pulsar. Primary electrons accelerating to high-altitudes in the unscreened electric field of the slot gap reach radiation-reaction limited Lorentz factors of 2 x 10^7, while electron-positron pairs from lower-altitude cascades flow along field lines interior to the slot gap. The curvature, synchrotron and inverse Compton radiation of both primary electrons and pairs produce a broad spectrum of emission from infra-red to GeV energies. Both primaries and pairs undergo cyclotron resonant absorption of radio photons, allowing them to maintain significant pitch angles. Synchrotron radiation from pairs with a power-law energy spectrum with Lorentz factors 10^2 - 10^5, dominate the spectrum up to 10 MeV. Synchrotron and curvature radiation of primaries dominates from 10 MeV up to a few GeV. We examine the energy-dependent pulse profiles and phase-resolved spectra for parameters of the Crab pulsar as a function of magnetic inclination and viewing angle, comparing to broad-band data. In most cases, the pulse profiles are dominated by caustics on trailing field lines. We also explore the relation of the high-energy and the radio profiles, as well as the possibility of caustic formation in the radio cone emission. We find that the Crab pulsar profiles and spectrum can be reasonably well reproduced by a model with viewing angle 45 degrees and inclination angle 100 or 80 degrees. This model predicts that the slot gap emission below 200 MeV will exhibit correlations in time and phase with the radio emission., Comment: 35 pages, 7 figures, accepted for publication in Astrophysical Journal

We present results of a 3D model of optical to gamma-ray emission from the slot gap accelerator of a rotation-powered pulsar. Primary electrons accelerating to high altitude in the pulsar magnetosphere at the outer edge of the open field volume, as well as electron-positron pairs on field line interior to the slot gap, radiate curvature, inverse Compton and synchrotron radiation. Both primaries and pairs undergo cyclotron resonant absorption of radio photons, allowing them to maintain significant pitch angles and to produce a broad spectrum of emission from infra-red to GeV energies. Synchrotron radiation from pairs with a power-law energy spectrum dominate the spectrum up to 10 MeV. Synchrotron and curvature radiation of primaries dominates from 10 MeV up to a few GeV. The high-energy pulse profiles are dominated by caustics on trailing field lines. In the case of the Crab pulsar, the radio conal emission may also form caustics in phase with the high-energy peaks. If resonant absorption of radio emission produces high-energy synchroti-on radiation, emission below 200 Mev is expected to exhibit correlations in time and phase with the radio emission.

We present results of a 3D model of optical to gamma-ray emission from the slot gap accelerator of a rotation-powered pulsar. Primary electrons accelerating to high-altitudes in the unscreened electric field of the slot gap reach radiation-reaction limited Lorentz factors of approx. 2 x 10(exp 7), while electron-positron pairs from lower-altitude cascades flow along field lines interior to the slot gap. The curvature, synchrotron and inverse Compton radiation of both primary electrons and pairs produce a broad spectrum of emission from infra-red to GeV energies. Both primaries and pairs undergo cyclotron resonant absorption of radio photons, allowing them to maintain significant pitch angles. Synchrotron radiation from pairs with a power-law energy spectrum from gamma = 10(exp 2) - 10(exp 5), dominate the spectrum up to approx. 10 MeV. Synchrotron and curvature radiation of primaries dominates from 10 MeV up to a few GeV. We examine the energy-dependent pulse profiles and phase-resolved spectra for parameters of the Crab pulsar as a function of magnetic inclination alpha and viewing angle zeta, comparing to broad-band data. In most cases, the pulse profiles are dominated by caustics on trailing field lines. We also explore the relation of the high-energy and the radio profiles, as well as the possibility of caustic formation in the radio cone emission. We find that the Crab pulsar profiles and spectrum can be reasonably well reproduced by a model with alpha = 45deg and zeta approx. 100deg or 80deg. This model predicts that the slot gap emission below 200 MeV will exhibit correlations in time and phase with the radio emission.