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PubMed1995

A computer-based system for the measurement of membrane capacitance to monitor exocytosis in secretory cells.

A software-based system for real-time measurement of membrane capacitance was developed for monitoring exocytosis in a single cell. The IBM PC compatible software, linked to an interface box, provides capacitance measurement capabilities to a patch-clamp setup utilizing a Labmaster data acquisition board. In order to measure the time-dependent changes in membrane capacitance, the system uses a software-based phase detector which isolates capacitive membrane current from a cell subject to sinusoidal voltage stimulation. The sampling program can monitor changes in membrane capacitance in response to depolarizing voltage steps, intracellular calcium stimulation, or drug application. A separate analysis program was constructed to implement the quantitative analysis, plotting and exporting of the capacitance, conductance and current data acquired by the sampling program.

Algorithms↗
PubMed1990

Capacitance hydrophones for pressure determination in lithotripsy.

Capacitance hydrophones have been designed and applied for pressure measurements of shock waves in lithotripsy. Their working principle, properties, and design are discussed. Formulae are given, permitting the determination of pressures and calibration of hydrophones from electric measurements. Examples of comparative measurements with electrodynamic and PVDF hydrophones are presented, confirming the correctness of obtained peak pressure values. The hydrophones are characterized by high durability, small controlled nonlinearity, and a wide frequency band. However, they cause distortions in the shape of the overshoot of the measured shock wave pulses and their application is limited to measurements in the focal region.

Calibration↗
PubMed2006

Charge accumulation and polarization in titanium dioxide electrodes.

Nanocrystalline TiO(2) electrodes were studied spectroelectrochemically by observing the simultaneous relaxation of the current and absorbance after applying a voltage step. The absorbance behaved differently in two time regimes: (1) ionic polarization in the oxide electrode, in which charged ions, such as Ti(3+) sites and/or interstitial Ti(4+) sites, move in response to the applied electric field, and (2) the diffusion of Li(+) ions into the TiO(2). These two behaviors were analyzed with equivalent circuit models. Li(+) ions reduce the resistance of the TiO(2) by approximately 90%, increase the capacitance by approximately 350%, and decrease the inductance by approximately 30%. Voltage cycling produces a buildup of intercalated Li(+) ions, lessening the electrode's response to the potential step, and causing it to become a more efficient inductor. The potential distribution in the nanoparticles is described by using a dielectric model in which roughly half the applied potential is dropped across the interface with a Li(+)-ion-containing electrolyte.

Journal Article↗
PubMed1990

G protein control of potassium channel activity in a mast cell line.

Using the patch-clamp technique, we studied regulation of potassium channels by G protein activators in the histamine-secreting rat basophilic leukemia (RBL-2H3) cell line. These cells normally express inward rectifier K+ channels, with a macroscopic whole-cell conductance in normal Ringer ranging from 1 to 16 nS/cell. This conductance is stabilized by including ATP or GTP in the pipette solution. Intracellular dialysis with any of three different activators of G proteins (GTP gamma S, GppNHp, or AlF-4) completely inhibited the inward rectifier K+ conductance with a half-time for decline averaging approximately 300 s after "break-in" to achieve whole-cell recording. In addition, with a half-time averaging approximately 200 s, G protein activators induced the appearance of a novel time-independent outwardly rectifying K+ conductance, which reached a maximum of 1-14 nS. The induced K+ channels are distinct from inward rectifier channels, having a smaller single-channel conductance of approximately 8 pS in symmetrical 160 mM K+, and being more sensitive to block by quinidine, but less sensitive to block by Ba2+. The induced K+ channels were also highly permeable to Rb+ but not to Na+ or Cs+. The current was not activated by the second messengers Ca2+, inositol 1,4,5-trisphosphate, inositol 1,3,4,5-tetrakisphosphate, or by cyclic AMP-dependent phosphorylation. Pretreatment of cells with pertussis toxin (0.1 microgram/ml for 12-13 h) prevented this current's induction both by guanine nucleotides and aluminum fluoride, but had no effect on the decrease in inward rectifier conductance. Since GTP gamma S is known to stimulate secretion from patch-clamped rat peritoneal mast cells, it is conceivable that K+ channels become inserted into the plasma membrane from secretory granules. However, total membrane capacitance remained nearly constant during appearance of the K+ channels, suggesting that secretion induced by GTP gamma S was minimal. Furthermore, pertussis toxin had no effect on secretion triggered by antigen, and triggering of secretion before electrical recording failed to induce the outward K+ current. Finally, GTP gamma S activated the K+ channel in excised inside-out patches of membrane. We conclude that two different GTP-binding proteins differentially regulate two subsets of K+ channels, causing the inward rectifier to close and a novel K+ channel to open when activated.

Aluminum↗
PubMed1995

Effects of dihydroergotamine on cerebral circulation during experimental intracranial hypertension.

Different cerebral vasoconstrictors have recently been suggested for the treatment of raised intracranial pressure (ICP), in patients with severe traumatic brain lesions. Such treatment may be associated with severe side effects. A porcine model simulating an intracranial mass lesion was utilized to examine the haemodynamic cerebral effects of dihydroergotamine (DHE), a recently introduced pharmacological treatment for raised intracranial pressure. Intracranial hypertension was induced by inflation of two tonometric gastric balloons placed extradurally covering the parieto-occipital region bilaterally. The animals were randomized into one group with six animals receiving 1.0 mg of DHE i.v. followed by a continuous infusion of 0.2 mg/h (high dose) and another group of six animals receiving 0.15 mg i.v. followed by 0.03 mg/h (low dose). Measurements of cerebral blood flow (CBF) and arterio-venous difference in oxygen content (CaVO2) were performed by 5, 20 and 60 min after the DHE infusion. Intracranial pressure (ICP), mean arterial blood pressure (MAP) and cerebral electrical activity (EEG) were recorded continuously. In both groups infusion of DHE caused a lasting decrease in ICP probably achieved mainly by a decrease in cerebral blood volume due to constriction of both arterial and venous capacitance vessels. In the group treated with high-dose DHE, but not in that given low-dose DHE, a progressive increase in CaVO2, a fall in jugular venous pH and an increase in EEG delta activity were observed indicating cerebral hypoxia. The study supports the view that DHE may be a valuable tool in the pharmacological treatment of increased ICP in traumatic brain lesions but underscores the importance of a proper dosage.

Animals↗
PubMed1984

Trifluoperazine reduces inward ionic currents and secretion by separate mechanisms in bovine chromaffin cells.

Using patch-clamp techniques, excitation and secretion in chromaffin cells were studied by measurement of unitary inward currents and of stimulus-evoked increments in membrane capacitance. The effect of the calmodulin inhibitor trifluoperazine (TFP) on Na, Ca and acetylcholine-induced (ACh) currents as well as on capacitance increments was investigated. TFP in concentrations up to 10 microM had no effect on Na channel currents. TFP was a potent anticholinergic agent. TFP in concentrations of 100 nM-1 microM decreased net ACh-induced currents by a slow block or allosteric modification of the channel. The effect was only partially reversible. Recovery from desensitization was retarded in direct relation to [TFP]. At the single channel level, TFP was found to slightly shorten open times in 0.5 and 20 microM-ACh. As reported previously, desensitization can be modelled by at least two desensitized states, as reflected by the bursting and clustering behaviour of single channels. TFP shortened clusters mainly by reducing the number of bursts per cluster. Whole-cell Ca currents (ICa) were reduced in 10 microM-TFP from an average of 29 microA cm-2-13 microA cm-2. Changes in capacitance of 1-200 fF were elicited in controls by maximal activation of the Ca current. We interpreted these steps to be the summed result of many exocytotic vesicular fusion events. Capacitance steps depended on ICa and were absent when extracellular Ca was removed. Application of 10 microM-TFP inhibited capacitance steps. The block of capacitance steps by TFP was shown to be independent of the reduction of ACh and Ca inward ionic currents. We conclude that the prevention of exocytosis by TFP is not completely described by its inhibition of electrical excitability but also results from intracellular actions.

Acetylcholine↗
PubMed1984

An investigation of threshold properties among cat spinal alpha-motoneurones.

In anaesthetized cats, thresholds for long (rheobase) and brief duration current pulses have been obtained from spinal motoneurones and compared with other cell parameters and membrane properties. Rheobase showed only weak over-all relationships with conduction velocity and with cell size, estimated as the total capacitance of individual motoneuronal equivalent cylinders. Rheobase showed a clear tendency to vary inversely with after-hyperpolarization (a.h.p.) duration and was strongly correlated with the input conductance and with the inverse of the membrane time constant. However, the range of rheobase current exceeded that of input conductance by almost a factor of 2. Part of this range discrepancy arose because threshold depolarization tended to increase with rheobase current. Thus, among motoneurones grouped according to rheobase magnitude (three groups), those within the lowest rheobase group had threshold depolarizations about 6 mV on average lower than those within the highest rheobase group. Even though this difference was not directly related to resting potential differences between the groups, further analysis suggested that it may have arisen secondarily to impalement-induced depolarization. The finding that experimentally estimated threshold depolarizations in individual motoneurones were generally larger than those predicted by the product of input resistance and rheobase indicated that a subthreshold rectification process also contributed to the range of rheobase. The difference was largest in the low-rheobase group and smallest in the high-rheobase group. Because these differences were proportional to the differences in input resistance between the separate motoneurone groups, it is suggested that the magnitude of the current underlying the rectification process does not differ systematically among motoneurones. Within groups of motoneurones classified on the basis of rheobase or a.h.p. duration, significant correlations existed between rheobase current and input conductance. An analysis of variance indicated that even within such functional subgroups of motoneurones, rheobase was appreciably better correlated with membrane time constant than with estimated cell size. Although showing a range approximately half that of rheobase, the brief current threshold was similar to rheobase in its relations with total cell capacitance, a.h.p. duration and the inverse of membrane time constant.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1991

Effect of adenosine and glucagon on hepatic blood volume responses to sympathetic nerves.

Hepatic blood volume responses were studied in cats using in vivo plethysmography. The maximal response (Rmax) to sympathetic nerve stimulation and to infusions of norepinephrine into the hepatic artery or portal vein was similar (12-14 mL expelled per liver in 2.9-kg cats; average liver weight, 76.8 +/- 6.8 g). The ED50 for norepinephrine intraportal (0.44 +/- 0.13) and intrahepatic arterial infusions (0.33 +/- 0.08 micrograms.kg-1.min-1) were similar indicating equal access of both blood supplies to the capacitance vessels. Adenosine (2.0 mg.kg-1.min-1) did not cause significant volume changes but produced a mild (27%) suppression of Rmax due to nerve stimulation with no change in the frequency (3.4 Hz) needed to produce 50% of Rmax. Rmax tended (not statistically significant) to decrease during glucagon (1.0 micrograms.kg-1.min-1) infusion but the nerve frequency needed to produce 50% of Rmax rose to 5.6 Hz. Thus both adenosine and glucagon produced modulation of sympathetic nerve-induced capacitance responses without having significant effects on basal blood volume. Adenosine, by virtue of its marked effects on arterial resistance vessels (at substantially lower doses than those used here) and the relative lack of effect on venous capacitance vessels, may be useful for producing clinical afterload reduction without venous pooling.

Adenosine↗
PubMed2006

Electromotility in outer hair cells: a supporting role for fast potassium conductance.

Motility of outer hair cells underlies the cochlear amplifier, which is critical for the ear's sensitivity and fine tuning. Of the two motile mechanisms present in these cells, electromotility at the lateral wall depends on the receptor potential and thus depends on currents through the cell body. We found that, in the guinea pig cochlea, basal turn outer hair cells have a fast-activating ion current (tau < 0.3 ms at 23 degrees C), which is absent in apical turn cells. This finding is consistent with our previous theoretical analysis that a fast-activating potassium current is required only in the basal turn to counteract the capacitive current and thereby to enhance the effectiveness of electromotility. Thus, our finding is consistent with the functional significance of electromotility. We conjecture therefore that the current reduces the capacitance of the outer hair cell in order to increase hearing bandwidth.

Animals↗
PubMed1993

Voltage-clamp frequency domain analysis of NMDA-activated neurons.

1. Voltage and current-clamp steps were added to a sum of sine waves to measure the tetrodotoxin-insensitive membrane properties of neurons in the intact lamprey spinal cord. A systems analysis in the frequency domain was carried out on two types of cells that have very different morphologies in order to investigate the structural dependence of their electrophysiological properties. The method explicitly takes into account the geometrical shapes of (i) nearly spherical dorsal cells with one or two processes and (ii) motoneurons and interneurons that have branched dendritic structures. Impedance functions were analysed to obtain the cable properties of these in situ neurons. These measurements show that branched neurons are not isopotential and, therefore, a conventional voltage-clamp analysis is not valid. 2. The electrophysiological data from branched neurons were curve-fitted with a lumped soma-equivalent cylinder model consisting of eight equal compartments coupled to an isopotential cell body to obtain membrane parameters for both passive and active properties. The analysis provides a quantitative description of both the passive electrical properties imposed by the geometrical structure of neurons and the voltage-dependent ionic conductances determined by ion channel kinetics. The model fitting of dorsal cells was dominated by a one-compartment resistance and capacitance in parallel (RC) corresponding to the spherical, non-branched shape of these cells. Branched neurons required a model that contained both an RC compartment and a cable that reflected the structure of the cells. At rest, the electrotonic length of the cable was about two. Uniformly distributed voltage-dependent ionic conductance sites were adequate to describe the data at different membrane potentials. 3. The frequency domain admittance method in conjunction with a step voltage clamp was used to control and measure the oscillatory behavior induced by N-methyl-D-aspartate (NMDA) on lamprey spinal cord neurons. Voltage-clamp currents and impedance functions were measured at different membrane potentials. The impedance functions had a voltage-dependent resonance and phase shift characteristic of a negative conductance. These measurements provide a quantitative analysis of the conductances induced by NMDA in central neurons of the lamprey spinal cord and directly establish the basis of the non-linear oscillatory behavior previously observed in the presence of NMDA. NMDA was shown specifically to activate a negative and a positive conductance, both of which were markedly affected by the membrane potential. It is shown that the net current in the presence of NMDA must be considered as the algebraic sum of currents in opposite directions.(ABSTRACT TRUNCATED AT 400 WORDS)

Amino Acids↗
PubMed1991

Reversible inhibition of voltage-dependent outer hair cell motility and capacitance.

Outer hair cells (OHC) from the organ of Corti are capable of fast voltage-induced length changes (Santos-Sacchi and Dilger, 1988), suggesting that an associated voltage sensor should reside in the OHC plasma membrane. Voltage-dependent mechanical responses and nonlinear charge movement of isolated OHCs from the guinea pig were analyzed using the whole-cell voltage-clamp technique. Ionic currents in the cells were blocked. Nonlinear voltage-dependent charge movement or, correspondingly, voltage-dependent capacitance was measured with step or AC analysis. OHC movements were measured either from video or using a differential photodiode technique. Maximum charge movements up to 2.5 pC were measured in OHCs from the low-frequency region of the cochlea. Both AC and step analyses indicated a peak nonlinear capacitance of 16-17 pF. The voltage dependence was fit to a Boltzmann relation with the step analysis indicating a maximum nonlinear capacitance at -23 mV step potential from a holding potential of about -120 mV, and AC analysis indicating a maximum at a holding potential near -40 mV. AC analysis probably provides a more accurate evaluation of voltage dependence. Measures of OHC motility magnitude versus voltage follow the nonlinear capacitance-voltage function obtained from AC measures. Treatment of the cells with gadolinium ions (0.5-1 mM) blocked OHC motility. This treatment also produced a shift of the nonlinear capacitance function along the voltage axis in the depolarizing direction, which can be explained by membrane surface charge screening. However, maximum capacitance was reduced as well and may correspond to the reduction or abolition of OHC motility in response to gadolinium treatment. Gadolinium effects were reversible. Nonlinear capacitance is not a function of membrane deformation due to length changes, since removal of OHC cytosol via the patch pipette abolished longitudinal movement but did not reduce nonlinear charge movement. It is interesting to note that the nonlinear capacitance will dynamically influence the time constant of the OHC during acoustically evoked receptor potential generation.

Animals↗
PubMed1989

Ca2+ and Na+ currents in developing skeletal myoblasts are expressed in a sequential program: reversible suppression by transforming growth factor beta-1, an inhibitor of the myogenic pathway.

We have analyzed the biophysical and developmental properties of Ca2+ and Na+ currents in C2 muscle cells, whose morphological and biochemical phenotype closely resembles differentiated skeletal muscle. Both fused and unfused C2 myocytes possessed: (1) membrane capacitance consistent with the presence of complex sarcotubular invaginations, (2) tetrodotoxin-sensitive Na+ channels, and (3) "fast" and "slow" Ca2+ channels that inactivated at holding potentials of -40 and -20 mV, respectively. Thus, the passive electrical properties, Na+ currents, and Ca2+ currents expressed in C2 cells each differed from those found in the nonfusing muscle cell line, BC3H1, and corresponded more precisely to characteristic findings observed in skeletal muscle fibers. In further contrast to BC3H1 cells, C2 muscle also expressed "transient" Ca2+ channels similar to those reported in embryonic or neonatal skeletal muscle, which were detected within 12-24 hr of mitogen withdrawal, up to 60 hr before appearance of "fast" and "slow" currents. Na+ channels also were induced 12-24 hr after mitogen withdrawal. Unlike the "fast" and "slow" Ca2+ currents, which were maximally expressed at 8-14 d of serum withdrawal, "transient" Ca2+ channels became down-regulated upon prolonged differentiation (as found in postnatal skeletal muscle in vivo) and were no longer expressed at 14 d. Despite their divergent kinetic and developmental properties, all components of Ca2+ and Na+ current in C2 myocytes were suppressed reversibly in the presence of transforming growth factor beta-1, a purified growth factor that inhibits the myogenic phenotype. The results indicate that fusion is not essential for skeletal myoblasts to produce developmentally regulated voltage-gated channels that resemble those of intact muscle and demonstrate that the formation of diverse Ca2+ and Na+ channels can be mediated by a single peptide that affects the myogenic pathway.

Calcium↗
PubMed1977

[Capacitive effects during transurethral resections with high-frequency currents (author's transl)].

The cutting probe of transurethral resection devices has to be fed through a line inside of the shaft. The capacitive high-frequency currents between this feeder and the shaft flow through the urethra and may cause destructive heat within the urethra if a metallic shaft is used. This paper shows that no danger is caused from these capacitive currents as long as the capacity between feeder and shaft is less than 120 pF.

Electric Injuries↗
PubMed2000

Second-order model of membrane electric field induced by alternating external electric fields.

With biological cells exposed to ac electric fields below 100 kHz, external field is amplified in the cell membrane by a factor of several thousands (low-frequency plateau), while above 100 kHz, this amplification gradually decreases with frequency. Below 10 MHz, this situation is well described by the established first-order theory which treats the cytoplasm and the external medium as pure conductors. At higher frequencies, capacitive properties of the cytoplasm and the external medium become increasingly important and thus must be accounted for. This leads to a broader, second-order model, which is treated in detail in this paper. Unlike the first-order model, this model shows that above 10 MHz, the membrane field amplification stops decreasing and levels off again in the range of tens (high-frequency plateau). Existence of the high-frequency plateau could have an important impact on present theories of high-frequency electric fields effects on cells and their membranes.

Biomedical Engineering↗
PubMed1993

Electrical impedance in the lower limbs of patients with Duchenne muscular dystrophy: a preliminary study.

Electrical impedances were measured, at a range of frequencies from 2 to 200 kHz, in the lower limbs of nine patients with Duchenne muscular dystrophy and 12 normal subjects. The specific admittance was calculated from the impedance, based on the assumption that the shape of the part of the limb measured is a truncated cone. Curve-fitting to the admittance locus on the complex plane gave the intracellular and extracellular resistances and the membrane capacitance. The intracellular resistance was higher and the membrane capacitance was lower in the patients than in the normal subjects. The membrane capacitance in the normal subjects increased with age. In the patients, the membrane capacitance decreased, and the intracellular resistance increased with the stage of disability, which was assessed by a physician. These experimental results agree well with the theoretical explanation, which uses simplified spherical models to represent living tissue.

Adolescent↗
PubMed2000

Voltage-dependent K(+) currents in spiral prominence epithelial cells of rat cochlea.

It has been suggested that spiral prominence is associated with ion transport, but the characterization of ion channels has not been explored so far. We studied the electrical properties and ion conductances of the spiral prominence epithelial cells (SPECs), which are epithelial cells covering the luminal side of spiral prominence, in the upper turn of neonatal rat cochlea using a whole-cell variant patch clamp technique. The cell capacitance was 16.3+/-2.1 pF (n=33) and the resting membrane potential was -68. 9+/-2.5 mV (n=14) in perilymph-like bath solution. It was found that those SPECs possess a large voltage-activated, outwardly rectifying K(+) current and a small inwardly rectifying K(+) current. The outward K(+) current was activated by depolarizing pulses more positive than -30 mV, and sensitive to tetraethylammonium chloride (20 mM), 4-aminopyridine (10 mM), but not to Ba(2+) (0.5 mM). Tail current analysis revealed that it was primarily K(+)-selective. The time course of activation was well fitted by an exponential function raised to second power. The small inwardly rectifying K(+) current was sensitive to Ba(2+) (0.5 mM), and the Ba(2+)-sensitive current was K(+)-selective. In cell-attached or inside-out patch recordings, no discernible K(+) channel currents were found in the apical membrane of SPECs. Based on these results, we conclude that SPECs have two types of voltage-dependent K(+) currents, which are most likely located in the basolateral membrane.

Animals↗