摘要 :
The voltage-clamp technique is applicable only to spherical cells. In nonspherical cells, such as neurons, the membrane potential is not clamped distal to the voltage-clamp electrode. This means that the current recorded by the vo...
展开
The voltage-clamp technique is applicable only to spherical cells. In nonspherical cells, such as neurons, the membrane potential is not clamped distal to the voltage-clamp electrode. This means that the current recorded by the voltage-clamp electrode is the sum of the local current and of axial currents from locations experiencing different membrane potentials. Furthermore, voltage-gated currents recorded from a nonspherical cell are, by definition, severely distorted due to the lack of space clamp. Justifications for voltage clamping in nonspherical cells are, first, that the lack of space clamp is not severe in neurons with short dendrites. Second, passive cable theory may be invoked to justify application of voltage clamp to branching neurons, suggesting that the potential decay is sufficiently shallow to allow spatial clamping of the neuron. Here, using numerical simulations, we show that the distortions of voltage-gated K(+) and Ca(2+) currents are substantial even in neurons with short dendrites. The simulations also demonstrate that passive cable theory cannot be used to justify voltage clamping of neurons due to significant shunting to the reversal potential of the voltage-gated conductance during channel activation. Some of the predictions made by the simulations were verified using somatic and dendritic voltage-clamp experiments in rat somatosensory cortex. Our results demonstrate that voltage-gated K(+) and Ca(2+) currents recorded from branching neurons are almost always severely distorted.
收起
摘要 :
Fifteen National Metrology Institutes have participated in DC voltage ratio comparison CCEM-K8. The method followed to normalize the participants' results, the calculation of the key comparison reference values and the comparison ...
展开
Fifteen National Metrology Institutes have participated in DC voltage ratio comparison CCEM-K8. The method followed to normalize the participants' results, the calculation of the key comparison reference values and the comparison results are reported for the two mandatory ratios of the comparison, 1000 V/10 V and 100 V/10 V.
收起
摘要 :
The voltage-sensing domain of voltage-gated ion channels is characterized by specific, conserved, charged residues. Positively charged residues on segment S4 are the main contributors to voltage-sensing and negatively charged resi...
展开
The voltage-sensing domain of voltage-gated ion channels is characterized by specific, conserved, charged residues. Positively charged residues on segment S4 are the main contributors to voltage-sensing and negatively charged residues on the S2 and S3 segments are believed to participate to the process. However, their function in the voltage sensor is not well understood. To probe the role of three acidic residues in NaChBac (D-58 and E-68 in S2, and D-91 in S3), we employed site-directed mutagenesis to substitute native acidic residues with cysteine (neutral), lysine (positive charge), or either aspartate or glutamate (negative charge). We used a combination of the patch-clamp technique to record Na+ currents and molecular modeling to visualize interacting amino acid residues. We suggest that the acidic residues on the S2 and S3 segments form specific interactions with adjacent amino acids in the voltage-sensor domain. The main interactions in NaChBac are D-58 (S2) with A-97-G-98 (S3) and R-120 (S4), E-68 (S2) with R-129 (L4-5), and D-91 (S3) with R-72 (S2). Changing these acidic residues modified the interactions, which in turn altered the sensitivity of the voltage sensor.
收起
摘要 :
The use of ionization chambers in linac radiotherapy dosimetry requires various corrections to the measured charges, one of these being the recombination correction. The recombination correction factor (k(s)) is generally estimate...
展开
The use of ionization chambers in linac radiotherapy dosimetry requires various corrections to the measured charges, one of these being the recombination correction. The recombination correction factor (k(s)) is generally estimated from the two-voltage analysis (TVA) for each beam quality. However, it is possible that the ionization chamber above some threshold polarizing voltage does not follow the accepted Boag theory very well. Secondly the TVA is time-consuming as the chamber needs to stabilize after each polarizing voltage change and since it must be performed for each beam quality. Another approach consists in using the fact that k(s) is predicted to depend linearly on dose per pulse by Boag theory: determining this relationship once and for all using a multi-voltage analysis (MVA), one also checks the range validity of the Boag theory for the chamber considered. This work presents a thorough analysis of k(s) dependence on dose per pulse of FC65-G (cylindrical) and Roos (plane-parallel) ionization chambers in pulsed photon and electron beams, respectively. Within the uncertainties, the recombination factors are found to be independent of beam quality, and no deviation from the Boag theory is observed within the tested range of polarizing voltages. Before adapting the equations given using the MVA other users should check that their ionization chambers show the same dose per pulse dependence using the TVA for a few beam qualities.
收起
摘要 :
The mechanism of blockade of P/Q Ca(2+) channels by antimigraine, dotarizine, was studied in voltage-clamped bovine adrenal chromaffin cells. Inward currents through P/Q channels were pharmacologically isolated by superfusing the ...
展开
The mechanism of blockade of P/Q Ca(2+) channels by antimigraine, dotarizine, was studied in voltage-clamped bovine adrenal chromaffin cells. Inward currents through P/Q channels were pharmacologically isolated by superfusing the cells with omega-conotoxin GVIA (1 microM) plus nifedipine (3 microM). Dotarizine (10-30 microM) blocked the P/Q fraction of I(Ba) and promoted current inactivation. Thus, dotarizine caused a greater blockade of the late I(Ba), compared with blockade of the early peak I(Ba). This effect was more prominent, the longer was the duration of the depolarising pulse. The blockade of I(Ba) was also greater at more depolarising holding potentials (i.e. -60 mV), than was the blockade produced at more hyperpolarising holding potentials (i.e. -80 or -110 mV). Catecholamine secretory responses to brief pulses (2 s) of a Krebs-HEPES solution containing 100 mM K(+) and 2 mM Ca(2+) was blocked by 3 microM dotarizine. Blockade was faster and greater when dotarizine was applied on cells that were previously depolarised with Krebs-HEPES deprived of Ca(2+) and containing increasing concentrations of K(+). This voltage-dependent blockade of P/Q channels and exocytosis might be the underlying mechanism explaining the dotarizine prophylaxis of migraine attacks.
收起
摘要 :
Many studies have used the channel blocker ZD 7288 to assess possible physiological and pathophysiological roles of hyperpolarization-activated cation currents (I(h)). In view of the known interplay between I(h) and other membrane...
展开
Many studies have used the channel blocker ZD 7288 to assess possible physiological and pathophysiological roles of hyperpolarization-activated cation currents (I(h)). In view of the known interplay between I(h) and other membrane conductances, the effects in Wistar rats of ZD 7288 on low-voltage-activated (LVA (- or T-type)) Ca(2+) channels were examined in whole-cell patch-clamp recordings from CA1 pyramidal cells in the presence of TTX, TEA, 4-AP, CsCl, BaCl(2) and nifedipine. ZD 7288 reduced T-type calcium channel currents and this effect was concentration dependant. ZD 7288 blocked T-type currents when applied extracellularly, but not when included in the recording pipette. Furthermore, ZD 7288 altered the steady-state voltage-dependent inactivation of T-currents. These results indicate that the blocker ZD 7288 has effects on voltage sensitive channels additional to those reported for the I(h) current.
收起
摘要 :
In this paper, an input receiver with a hysteresis characteristic that can work at voltage levels between 0.9 V and 5 V is proposed. The input receiver can be used as a wide voltage range Schmitt trigger also. At the same time, re...
展开
In this paper, an input receiver with a hysteresis characteristic that can work at voltage levels between 0.9 V and 5 V is proposed. The input receiver can be used as a wide voltage range Schmitt trigger also. At the same time, reliable circuit operation is ensured. According to the research findings, this is the first time a wide voltage range Schmitt trigger is being reported. The proposed circuit is compared with previously reported input receivers, and it is shown that the circuit has better noise immunity. The proposed input receiver ends the need for a separate Schmitt trigger and input buffer. The frequency of operation is also higher than that of the previously reported receiver. The circuit is simulated using HSPICE at 035-um standard thin oxide technology. Monte Carlo analysis is conducted at different process conditions, showing that the proposed circuit works well for different process conditions at different voltage levels of operation. A noise impulse of (V_cc/2) magnitude is added to the input voltage to show that the receiver receives the correct logic level even in the presence of noise. Here, V_cc is the fixed voltage supply of 33 V.
收起
摘要 :
Modulation of the L-type Ca(2+) channel (LTCC) by sorcin was investigated by measuring the L-type Ca(2+) current (I (Ca,L)) in isolated rabbit ventricular myocytes using ruptured patch, single electrode voltage clamp in the absenc...
展开
Modulation of the L-type Ca(2+) channel (LTCC) by sorcin was investigated by measuring the L-type Ca(2+) current (I (Ca,L)) in isolated rabbit ventricular myocytes using ruptured patch, single electrode voltage clamp in the absence of extracellular Na(+). Fifty millimolars EGTA (170 nM Ca(2+)) in the pipette solution buffered bulk cytoplasmic [Ca(2+)], but retained rapid Ca(2+)-dependant inactivation of I (Ca,L,). Recombinant sorcin (3 muM) in the pipette significantly slowed time-dependant inactivation (tau (fast): 8.8 +/- 0.9 vs. 15.1 +/- 1.7 ms). Sorcin had no significant effect on I (Ca,L,) after inhibition of the sarcoplasmic reticulum (SR). Using 10 mM 1,2-bis(o-N,N,N',N'-tetraacetic acid (170 nM Ca(2+)), I (Ca,L) inactivation was then determined by a Ca(2+) -independent, voltage-dependant process. Under these conditions, 3 muM sorcin speeded up inactivation. A similar effect was observed by substitution of Ca(2+) with Ba(2+). Down-regulation of endogenous sorcin to 27 +/- 7% using an RNAi adenoviral vector slowed inactivation of I (Ca,L) by approximately 42%. The effects of sorcin on voltage-dependant inactivation were mimicked by a truncated form of the protein containing only the Ca(2+)-binding domain. This data is consistent with two independent actions of sorcin on the LTCC: (1) slowing Ca(2+)-dependant inactivation and (2) stimulating voltage-dependant inactivation. The net effect of sorcin on the time-dependent inactivation of I (Ca,L) was a balance between these two effects. Under normal conditions, sorcin slows I (Ca,L) inactivation because the effects of Ca(2+)-dependant inactivation out-weigh the effects on voltage-dependant inactivation.
收起
摘要 :
This review addresses the localized regulation of voltage-gated ion channels by phosphorylation. Comprehensive data on channel regulation by associated protein kinases, phosphatases, and related regulatory proteins are mainly avai...
展开
This review addresses the localized regulation of voltage-gated ion channels by phosphorylation. Comprehensive data on channel regulation by associated protein kinases, phosphatases, and related regulatory proteins are mainly available for voltage-gated Ca(2+) channels, which form the main focus of this review. Other voltage-gated ion channels and especially K(v)7.1-3 (KCNQ1-3), the large- and small-conductance Ca(2+)-activated K(+) channels BK and SK2, and the inward-rectifying K(+) channels K(ir)3 have also been studied to quite some extent and will be included. Regulation of the L-type Ca(2+) channel Ca(v)1.2 by PKA has been studied most thoroughly as it underlies the cardiac fight-or-flight response. A prototypical Ca(v)1.2 signaling complex containing the beta(2) adrenergic receptor, the heterotrimeric G protein G(s), adenylyl cyclase, and PKA has been identified that supports highly localized via cAMP. The type 2 ryanodine receptor as well as AMPA- and NMDA-type glutamate receptors are in close proximity to Ca(v)1.2 in cardiomyocytes and neurons, respectively, yet independently anchor PKA, CaMKII, and the serine/threonine phosphatases PP1, PP2A, and PP2B, as is discussed in detail. Descriptions of the structural and functional aspects of the interactions of PKA, PKC, CaMKII, Src, and various phosphatases with Ca(v)1.2 will include comparisons with analogous interactions with other channels such as the ryanodine receptor or ionotropic glutamate receptors. Regulation of Na(+) and K(+) channel phosphorylation complexes will be discussed in separate papers. This review is thus intended for readers interested in ion channel regulation or in localization of kinases, phosphatases, and their upstream regulators.
收起
摘要 :
Metaflumizone is the latest addition to the armamentarium of the Na + channel inhibitor insecticide family. We used the Xenopus oocyte expression system and a Markovian model to assess the effect of metaflumizone on Apis ? mellife...
展开
Metaflumizone is the latest addition to the armamentarium of the Na + channel inhibitor insecticide family. We used the Xenopus oocyte expression system and a Markovian model to assess the effect of metaflumizone on Apis ? mellifera Na + channels (AmNa V 1). Our results reveal that metaflumizone inhibits AmNa V 1 channels by targeting the kinetics of recovery from slow inactivation. Multistate modeling of fast and slow inactivation of the AmNa V 1 channel made it possible to study the effects of metaflumizone on a set of rate constants underlying the transition between the open and inactivated conformations and provided insights into their specificity. We conclude that the methods we used could be extended to assessing the toxicity of other Na + channel inhibitor insecticides.
收起
原文获取