Single-channel Recording Experiments Research Articles

Live Cell Interactions with Biocompatible Ultra-Short Carbon Nanotube Porins

Various techniques, such as peptide chemistry, DNA origami, or nanotubes cutting, have been tried to develop synthetic analogues of biological membrane channels for transmembrane transport of ions and molecules, but challenges still remain to achieve the desired affinity, transport properties and biocompatibility for delivery applications. Here we report the development of CNT porins, an ultra-short biocompatible carbon nanotube with the dimension of 5-15 nm in length and 1.5 nm in diameters and with structure and function resembling membrane channel. We explored concentration-dependent interaction of the CNT porins with CHO cells using microscopy and OmniLog phenotype microarray system. Interestingly, the CNT porins didn’t inhibit the cell viability at low concentration over a 72 hr co-incubation period. Single channel recording experiments also showed well-defined channels formation on the cell membrane. Those results indicated those artificial transmembrane channels could be established as a promising biomimetic platform for developing cell interfaces, studying transport in biological channels, and creating stochastic sensors. Their inherent robustness towards biological and chemical challenges and exceptional biocompatibility should prove valuable for bionanofluidic and cellular interface applications.

Barium, Tea and Sodium Sensitive Potassium Channels are Present in the Human Placental Syncytiotrophoblast Apical Membrane

The human placental syncytiotrophoblast (hSTB) is a polarized epithelial structure, without paracellular routes, forming the main barrier for materno–fetal exchange. There is ample evidence suggesting the presence of potassium (K +) channels in the placental apical membrane; which could contribute to membrane potential and volume regulation. We have therefore examined the K + currents of isolated apical membranes from human term placenta using electrophysiological methods: reconstitution of ion channels from apical membranes into giant liposomes (single channel recordings, patch clamp method) or their functional transplantation into Xenopus laevis oocytes (total currents recording, voltage clamp method). Single channel recording experiments show the presence of K + channels in the hSTB microvillous membrane sensitive to Tetraethylammonium (TEA) and Barium (Ba +2). Patch current activity was diminished 50% and 70% by 20 mmol/L TEA and 5 mmol/L Ba +2 respectively. The more frequent conductance was approximately 73 pS, however several levels of current were detected suggesting the presence of more than one type of K + channel. In addition, sodium (Na +) sensitivity was detected in the patch current thus, over 10 mmol/L Na + reduced the seal current to 38%. These results were corroborated by the total current experiments where the K + current elicited in injected oocytes with apical purified membrane was blocked by Ba +2 and TEA. The total current was also affected by Na +, becoming larger when a Na +-free solution was used. Our results show the existence of at least two types of Ba +2-sensitive K + channels including a TEA sensitive sub-population, and some of them Na + sensitive K + channels. These channels could be the conductive pathways proposed previously for this cation in placental hSTB. Our novel contribution has been to successfully obtain K + channel recordings in systems suitable for electrophysiological studies of isolated apical membranes.

Calsequestrin is essential for the Ca2+ release induced by myotoxin alpha in skeletal muscle sarcoplasmic reticulum.

Myotoxin alpha (MYTX), a polypeptide toxin purified from the venom of prairie rattlesnakes (Crotalus viridis viridis), induced Ca2+ release from the heavy fraction of skeletal sarcoplasmic reticulum (HSR), using a Ca2+ electrode. The effect of MYTX was nearly abolished by pretreatment with ryanodine, an alkaloid-based Ca2+ channel blocker. In the stopped-flow experiments, MYTX increased the choline+ permeability of HSR in the presence of calsequestrin (CS). Single channel recording experiments showed that in the presence of CS, the channel currents were markedly enhanced by MYTX applied to the cis side, but not to the trans side. However, in the absence of CS, MYTX failed to cause the excitatory effect in both the experiments. These results suggest that CS is essential for MYTX-induced Ca2+ release through the Ca2+ release channels in skeletal HSR.

Open-channel block of N-methyl-D-aspartate (NMDA) responses by memantine: therapeutic advantage against NMDA receptor-mediated neurotoxicity

Excessive activation of NMDA receptors is thought to mediate the calcium-dependent neurotoxicity associated with hypoxic-ischemic brain injury, trauma, epilepsy, and several neurodegenerative diseases. For this reason, various NMDA antagonists have been investigated for their therapeutic potential in these diseases, but heretofore none have proven to be both effective and safe. In the present study, memantine, an adamantane derivative similar to the antiviral drug amantadine, is shown to block the channels activated by NMDA receptor stimulation. From whole-cell and single-channel recording experiments, the mechanism of action of memantine is deduced to be open-channel block, similar to MK-801; however, unlike MK-801, memantine is well tolerated clinically. Compared to MK-801, memantine's safety may be related to its faster kinetics of action with rapid blocking and unblocking rates at low micromolar concentrations. Furthermore, at these levels memantine is an uncompetitive antagonist and should theoretically allow near-normal physiological NMDA activity throughout the brain even in the face of pathologically high focal concentrations of glutamate. These pharmacological properties confer upon memantine a therapeutic advantage against NMDA receptor-mediated neurotoxicity with few side effects compared with other organic NMDA open-channel blockers. Moreover, memantine is increasingly effective against escalating levels of glutamate, such as those observed during a stroke. Low micromolar concentrations of memantine, levels known to be tolerated by patients receiving the drug for the treatment of Parkinson's disease, prevent NMDA receptor-mediated neurotoxicity in cultures of rat cortical and retinal ganglion cell neurons; memantine also appears to be both safe and effective in a rat stroke model. These results suggest that memantine has considerable therapeutic potential for the myriad of clinical entities associated with NMDA receptor-mediated neurotoxicity.

Estimating the number of channels in patch recordings

The estimation of the number of channels in a patch was assumed to be equivalent to the estimation of the binomial parameter n. Seven estimators were evaluated, using data sets simulated for a range of parameters appropriate for single channel recording experiments. No single estimator was best for all parameters; a combination of estimators is a possible option to avoid the biases of individual estimators. All estimators were highly accurate in estimating n in the case that n = 1. For n </= 4 the simplest estimator, the maximum number of simultaneously open channels, was the best, For larger values of n the best estimators were Bayesian.

Effects of diltiazem and verapamil on responses to acetylcholine.

1. The calcium channel antagonists diltiazem and verapamil were found to alter the average lifetime of ion channels activated by acetylcholine (ACh). 2. Average channel lifetime was determined from the decay phase of miniature endplate currents at the neuromuscular junction of mouse hemidiaphragms and from direct recording of single channel currents activated by ACh from BC3H1 mouse tumour cells in culture. 3. Both diltiazem and verapamil reduced average channel lifetime in a dose-dependent manner. For each drug, concentrations as high as 20 microM-100 microM were required to decrease channel lifetime by 50%. 4. Single channel recording experiments also showed that both diltiazem and verapamil greatly decreased the frequency of opening events at concentrations as low as 2 microM to 5 microM. This finding is consistent with an enhancement of receptor desensitization.

Toxins That Modulate the Sodium Channel Gating Mechanisma

A variety of toxins and chemicals has been shown to modulate the gating kinetics of the sodium channel. Studies of batrachotoxin, grayanotoxins and pyrethroids are summarized here as examples. Batrachotoxin and grayanotoxins eliminate the sodium channel inactivation thereby causing a prolonged, steady-state sodium current to flow during a depolarizing step. The sodium channel activation kinetics are not affected markedly. Batrachotoxin appears to bind to a site in the sodium channel to which the inactivation gate normally binds, thus causing an inhibition of sodium inactivation. Single channel recording experiments have shown that the mean open time of individual sodium channels is greatly prolonged by batrachotoxin. It appears that individual sodium channels are modified by batrachotoxin in an all-or-none manner. Pyrethroids which are synthetic derivatives of pyrethrins also modify the kinetics of sodium channels in a very drastic manner. In the presence of type I pyrethroids which lack a cyano group at the alpha position (e.g., allethrin and tetramethrin), a large steady-state sodium current appears during a step depolarization and a large slowly decaying sodium tail current appears upon repolarization. Thus both the activation and inactivation kinetics are slowed. Type II pyrethroids which contain an alpha-cyano group (e.g., deltamethrin, cyphenothrin, and fenvalerate) exert effects on sodium channels qualitatively similar to those of type I pyrethroids. However, the amplitudes of the steady-state sodium current and sodium tail current are smaller and the time constant of tail current decay is much longer. The mean open time of single sodium channels is greatly prolonged by the pyrethroids, and the effect is much more pronounced in type II than in type I pyrethroids. A high degree of stereospecificity has been found among four isomers of tetramethrin, (+)-trans and (+)-cis isomers being highly active and (-)-trans and (-)-cis isomers almost totally inactive. The inactive isomers bind to the sodium channel sites, thus preventing the action of the active isomers. Because of the unique action of pyrethroids in modulating the sodium channels, they are becoming useful tools for channel physiology and pharmacology.