Features of superexchange nonresonant tunneling conductance in anchored molecular wires

NAS Ukraine via Project No. 0116U002067A modified superexchange model is used to clarify the physical mechanisms for the formation of nonresonant tunneling conductance in terminated molecular wires. Due to the specific relationship between its key parameters, this model has wider areas of applicability compared to the flat-barrier model and the standard superexchange model, which are widely involved for the physical interpretation of experimental results. Moreover, the results obtained in the two latest models appear in the modified model as characteristic limiting cases. Our estimates show that the exponential decay of conductance, characterized by an attenuation factor β (per repeating unit), is limited by the conditions β ≤ 1.2 and β ≥ 3.7 for the flat-barrier and standard models, respectively. At the same time, the modified superexchange model yields β > 0, which, thus, allows us to analyze the tunneling conductance in molecular wires containing both saturated and conjugated bonds. We also show that for a small number of N repeating wire units (about 3-6 depending on the value of β), the exponential dependence of conductance on N is violated and, accordingly, contact conductance is not identical to conductance at N = 0. Formulas are found which, on the basis of experimental data, make it possible to establish the values of superexchange parameters as well as indicate the conditions of possible hybridization between the orbitals of the anchor groups and the adjacent end units belonging to the interior wire region. One example is the establishment of features in the tunneling conductance of terminated alkane chains caused by the nature of their anchor groups.publishersversionpublishe

The Coxsackievirus and Adenovirus Receptor Has a Short Half-Life in Epithelial Cells

The coxsackievirus and adenovirus receptor (CAR) is an essential cellular protein that is involved in cell adhesion, cell signaling, and viral infection. The 8-exon encoded isoform (CAREx8) resides at the apical surface of polarized epithelia, where it is accessible as a receptor for adenovirus entering the airway lumen. Given its pivotal role in viral infection, it is a target for antiviral strategies. To understand the regulation of CAREx8 and determine the feasibility of receptor down regulation, the half-life of total and apical localized CAREx8 was determined and correlated with adenovirus transduction. Total and apical CAREx8 has a relatively short half-life of approximately 2 h. The half-life of apical CAREx8 correlates well with adenovirus transduction. These results suggest that antiviral strategies that aim to degrade the primary receptor for apical adenovirus infection will be effective within a relatively short time frame after application

Overshooting Cytosolic Ca2+ Signals Evoked By Capacitative Ca2+ Entry Result from Delayed Stimulation of a Plasma Membrane Ca2+ Pump

The effect of capacitative Ca2+ entry on cytosolic free Ca2+ concentration ([Ca2+](c)) was examined in calf pulmonary artery endothelial cells treated with thapsigargin. Restoration of extracellular Ca2+ evoked an overshoot in [Ca2+](c): the intial rate of Ca2+ influx was 12.4 ± 0.5 nM/s as [Ca2+](c) rose monoexponentially (time constant, τ = 36 ± 2 s) to a peak (322 ± 16 nM) before declining to 109 ± 14 nM after 2000 s. Rates of Ca2+ removal from the cytosol were measured throughout the overshoot by recording the monoexponential decrease in [Ca2+](c) after rapid removal of extracellular Ca2+. The time constant for recovery (τ(rec) decreased from 54 ± 4 s when Ca2+ was removed after 10 s to its limiting value of 8.8 ± 1.0 s when it was removed after 2000 s. The time dependence of the changes in τ(rec) indicate that an increase in [Ca2+](c) is followed by a delayed (τ = 408 s) stimulation of Ca2+ removal, which fully reverses (τ ~ 185 s) after Ca2+ entry ceases. Numerical simulation indicated that the changes in Ca2+ removal were largely responsible for the overshooting pattern of [Ca2+](c). Because prolonged (30 min) Ca2+ entry did not increase the total 45Ca2+ content of the cells, an increased rate of Ca2+ extrusion across the plasma membrane most likely mediates the Ca2+ removal, and since it persists in the absence of extracellular Na+, it probably results from stimulation of a plasma membrane Ca2+ pump. We conclude that delayed stimulation of a plasma membrane Ca2+ pump by capacitative Ca2+ entry may protect cells from excessive increases in [Ca2+](c) and contribute to oscillatory changes in [Ca2+](c)

Parameter Optimization for Excitable Cells

The electrophysiology of nodose ganglia neurons is of great interest in the analysis of cell membrane currents and action potential behavior. This behavior was initially outlined in the Hodgkin-Huxley conductance model using a system of nonlinear differential equations. Later, Schild et al. developed an extension of the Hodgkin-Huxley model to provide a more exhaustive description of ion channels involved in nodose neuronal action potential activity. We consider a variety of methods to fit the parameters of both the Hodgkin-Huxley and Schild et al. models to an empirical stimulus response dataset. Our methods were validated using synthetic datasets, as well as voltage-clamp data for nodose neurons. The fitting procedure that we implemented demonstrates the predictive efficacy of the Schild et al. model as well as its ability to sufficiently characterize electrical signatures of nodose neurons

Endogenous Heavy Metal Ions Perturb Fura-2 Measurements of Basal and Hormone-Evoked Ca2+ Signals

Using the membrane-permeant chelator of heavy metal ions, N,N,N\u27,N\u27- tetrakis(2-pyridylmethyl)ethylene diamine (TPEN), we demonstrate that in pancreatic acinar cells, hepatocytes, and a variety of mammalian cell lines, endogenous heavy metal ions bind to cytosolic fura-2 causing basal cytosolic free [Ca2+] ([Ca2+](i)) to be overestimated. TPEN had most effect in cells lightly loaded with fura-2, suggesting the presence of a limited pool of heavy metal ions (≤12 μM in pancreatic acinar cells) that does not rapidly exchange across the plasma membrane. In fura-2-loaded hepatocytes, vasopressin failed to evoke a detectable change in fluorescence, but after preincubation of cells with TPEN, it caused fluorescence changes characteristic of an increase in [Ca2+](i). We conclude that in many mammalian cells, a slowly exchanging mixture of cytosolic heavy metal ions binds to fura-2 both to quench its fluorescence and to mimic the effects of Ca2+ binding, thereby causing basal [Ca2+](i) to be overestimated. By chelating endogenous heavy metal ions, TPEN allows basal [Ca2+](i) to be accurately measured and, by preventing competition between heavy metal ions and Ca2+ for binding to fura-2, unmasks the full effect of agonists in increasing [Ca2+](i)

Inhibition of Carbonic Anhydrase Activity by Green Tea antioxidant EGCG

Previous studies have shown that carbonic anhydrase (CA) and its isozymes contribute to development and progression of cancer. In several studies, an antioxidant from green tea (-)- Epigallocatechin 3-O-Gallate (EGCG) has been shown to slow down proliferation of cancer cells. We hypothesized that this effect of EGCG may be due to inhibition of CA. In addition, we tested if the products of EGCG hydrolysis, Gallic acid, and (-)-Epigallocatechin inhibit Carbonic Anhydrase. Using the electrometric Wilbur-Anderson assay, we successfully measured the CA activity and showed its inhibition by acetazolamide, a known CA inhibitor. The assay involves determination of the time T in seconds required for 4 mL of water saturated with carbon dioxide to lower pH from 8.3 to 6.3 when added to 6 mL of 0.02M Tris buffer at 0°C without (T0) and with (TE) the enzyme. The activity is defined as A = 2*(T0/TE-1) assay units (AU). In 8 blank trials, T0 was 56 ± 3 s (Mean ± SD), and with 0.001mg/10mL (~3.3nM) CA, TE = 23 ± 1 s (3084 ± 297 Wilbur-Anderson units). The same concentration of carbonic anhydrase in the presence of 10µM EGCG resulted in T = 37 ± 1 sec (1095 ± 146 Wilbur-Anderson units), p \u3c 0.001. In the presence of 10µM Gallic Acid, T = 18.4 ± 0.4 sec (3970 ± 204 Wilbur-Anderson units), p = 0.391. In the presence of 10µM (-)-Epigallocatechin, T = 20 ± 2 sec (3606 ± 594 Wilbur- Anderson units), p=0.320. These results show that EGCG partially inhibited CA, reducing its activity by 35%. The products of EGCG hydrolysis, Gallic acid, and (-)-Epigallocatechin did not affect carbonic anhydrase activity. This supports our hypothesis that cancer protective properties of EGCG may be in part due to its inhibition of CA. Thus, EGCG and its derivatives could potentially be used for cancer therapy. This research was funded by Wehner Research Fund, MSU College of Science and Mathematics Summer Student Research Program, and the Garden State Louis Stokes Alliance for Minority Participation Program

Neuronal Prostacyclin Is an Autocrine Regulator of Arterial Baroreceptor Activity

We tested the hypothesis that neuronal prostacyclin is an autocrine regulator of arterial baroreceptor neuronal activity. In isolated rat aortic nodose baroreceptor neurons, mechanical stimulation depolarized 12 neurons by 13.1±3.4 mV and triggered action potentials in 5 of them, averaging 18.2±9.5 spikes. Current injections depolarized 21 neurons by 29.9±8.0 mV and triggered action potentials averaging 17.0±2.4 spikes. After a period of prolonged neuronal activation with pulses of 1 nA at 20 Hz for 1 minute, the action potential responses to mechanical stimulation and to current injections were first markedly suppressed (0.2±0.2 and 2.1±0.7 spikes, respectively) and then enhanced, reaching levels above control (29.0±8.0 and 21.7±2.6 spikes, respectively) over the subsequent 15 minutes. In contrast, there was no inhibition of the depolarizations caused by mechanical stimulation or current injections. The recovery and enhancement of action potentials, which reached 150±5.4% of control values at 15 minutes (n=13), were abrogated by 10 μmol/L of indomethacin and replaced by sustained inhibition for over 15 minutes. Carbacyclin (10 μmol/L) reversed the inhibition and restored action potential responses. Prostacyclin production by cultured nodose neurons was enhanced by arachidonic acid and electrical field stimulation and inhibited by indomethacin. We conclude that prostacyclin provides an autocrine feedback that restores and enhances the responsiveness of arterial baroreceptor neurons after their inhibition from excessive neuronal activation. We speculate that reduced synthesis of neuronal prostacyclin contributes to the resetting phenomenon and the suppressed activity of arterial baroreceptors in hypertension

Mechanosensory Transduction of Vagal and Baroreceptor Afferents Revealed By Study of Isolated Nodose Neurons in Culture

Changes in arterial pressure and blood volume are sensed by baroreceptor and vagal afferent nerves innervating aorta and heart with soma in nodose ganglia. The inability to measure membrane potential at the nerve terminals has limited our understanding of mechanosensory transduction. Goals of the present study were to: (1) Characterize membrane potential and action potential responses to mechanical stimulation of isolated nodose sensory neurons in culture; and (2) Determine whether the degenerin/epithelial sodium channel (DEG/ENaC) blocker amiloride selectively blocks mechanically induced depolarization without suppressing membrane excitability. Membrane potential of isolated rat nodose neurons was measured with sharp microelectrodes. Mechanical stimulation with buffer ejected from a micropipette (5, 10, 20 psi) depolarized 6 of 10 nodose neurons (60%) in an intensity-dependent manner. The depolarization evoked action potentials in 4 of the 6 neurons. Amiloride (1 μM) essentially abolished mechanically induced depolarization (15±4 mV during control vs. 1±2 mV during amiloride with 20-psi stimulation, n=6) and action potential discharge. In contrast, amiloride did not inhibit the frequency of action potential discharge in response to depolarizing current injection (n=6). In summary, mechanical stimulation depolarizes and triggers action potentials in a subpopulation of nodose sensory neurons in culture. The DEG/ENaC blocker amiloride at a concentration of 1 μM inhibits responses to mechanical stimulation without suppressing membrane excitability. The results support the hypothesis that DEG/ENaC subunits are components of mechanosensitive ion channels on vagal afferent and baroreceptor neurons