PubMed Health⌕ Search

Biomedical subjects

A Petris

Publications and source records attributed to A Petris.

At least 19 recordsLinked to original sources
PubMed2003

(2+1)-dimensional soliton formation in photorefractive Bi12SiO20 crystals.

(2+1)-dimensional spatial solitons in Bi12SiO20 (BSO) photorefractive crystals with large optical activity are experimentally demonstrated. The soliton formation when a Gaussian beam is injected at the input has been previously analyzed numerically and then experimentally investigated. We demonstrate analytically, numerically, and experimentally that by applying static electric biases of high values, the polarization rotation accelerates: this acceleration prevents the beam from broadening if the polarization rotation period becomes shorter than the diffraction length. Contemporary to this nonlinear optical activity, an induced birefringence affects the beam polarization state. Analysis of the polarization dynamics shows that the polarization changes nonuniformly across the beam (with a field dependent speed) until about 30-35 kV/cm; above this limit, the whole beam has just one polarization state. Representation on the Poincaré sphere of the polarization dynamics reveals the existence of a stable polarization trajectory closed around a polarization attractor that depends on the linear optical activity and on the photorefractive nonlinearity. The experimental soliton is well described by the analytical solutions already obtained [Fazio et al., Phys. Rev. E 66, 016605 (2002)].

Journal Article↗
PubMed2001

Verapamil block of large-conductance Ca-activated K channels in rat aortic myocytes.

The effects of verapamil on the large conductance Ca-activated K (BK) channel from rat aortic smooth muscle cells were examined at the single channel level. Micromolar concentrations of verapamil produced a reversible flickering block of the BK channel activity. Kinetic analysis showed that verapamil decreased markedly the time constants of the open states, without any significant change in the time constants of the closed states. The appearance of an additional closed state-specifically, a nonconducting, open-blocked state--was also observed, whose time constant would reflect the mean residence time of verapamil on the channel. These observations are indicative of a state-dependent, open-channel block mechanism. Dedicated kinetic (group) analysis confirmed the state-dependent block exerted by verapamil. D600 (gallopamil), the methoxy derivative of verapamil, was also tested and found to exert a similar type of block, but with a higher affinity than verapamil. The permanently charged and membrane impermeant verapamil analogue D890 was used to address other important features of verapamil block, such as the sidedness of action and the location of the binding site on the channel protein. D890 induced a flickering block of BK channels similar to that observed with verapamil only when applied to the internal side of the membrane, indicating that D890 binds to a site accessible from the cytoplasmic side. Finally, the voltage dependence of D890 block was assessed. The experimental data fitted with a Langmuir equation incorporating the Woodhull model for charged blockers confirms that the D890-binding site is accessed from the internal mouth of the BK channel, and locates it approximately 40% of the membrane voltage drop along the permeation pathway.

Animals↗
PubMed2001

Large-conductance calcium-activated potassium channels in neonatal rat intracardiac ganglion neurons.

The properties of single Ca2+-activated K+ (BK) channels in neonatal rat intracardiac neurons were investigated using the patch-clamp recording technique. In symmetrical 140 mM K+, the single-channel slope conductance was linear in the voltage range -60/+60 mV, and was 207+/-19 pS. Na+ ions were not measurably permeant through the open channel. Channel activity increased with the cytoplasmic free Ca2+ concentration ([Ca2+]i) with a Hill plot giving a half-saturating [Ca2+] (K0.5) of 1.35 microM and slope of approximately equals 3. The BK channel was inhibited reversibly by external tetraethylammonium (TEA) ions, charybdotoxin, and quinine and was resistant to block by 4-aminopyridine and apamin. Ionomycin (1-10 microM) increased BK channel activity in the cell-attached recording configuration. The resting activity was consistent with a [Ca2+]i <100 nM and the increased channel activity evoked by ionomycin was consistent with a rise in [Ca2+]i to > or =0.3 microM. TEA (0.2-1 mM) increased the action potential duration approximately equals 1.5-fold and reduced the amplitude and duration of the afterhyperpolarization (AHP) by 26%. Charybdotoxin (100 nM) did not significantly alter the action potential duration or AHP amplitude but reduced the AHP duration by approximately equals 40%. Taken together, these data indicate that BK channel activation contributes to the action potential and AHP duration in rat intracardiac neurons.

Action Potentials↗
PubMed2000

Characterization of the large-conductance Ca-activated K channel in myocytes of rat saphenous artery.

We used the patch-clamp method to characterize the BK channel in freshly isolated myocytes from the saphenous branch of the rat femoral artery. Single-channel recordings revealed that the BK channel had a conductance of 187 pS in symmetrical 150 mM KCl, was blocked by external tetraethylammonium (TEA) with a KD(TEA) of approx. 300 microM at +40 mV, and by submicromolar charybdotoxin (CTX). The sensitivity of the BK channel to Ca was especially high (KD(ca) approx. 0.1 microM at +60 mV) compared to skeletal muscle and neuronal tissues. We also investigated the macroscopic K current, which under certain conditions is essentially sustained by BK channels. This conclusion is based on the findings that the macroscopic current activated upon depolarization follows a single exponential time course and is virtually fully blocked by 100 nM CTX and 5 mM external TEA. We made use of this occurrence to assess the voltage and Ca dependence of the macroscopic BK current. In intact myocytes, the BK channel showed a strong and voltage-dependent reduction of the outward current (62% at +40 mV), most likely due to block by intracellular Ba and polyamines. The results obtained from macroscopic and unitary current indicate that approx. 2.5% of the BK channels are active under physiological conditions, sustaining approx. 20 pA of outward current. Given the high input resistance of these cells, few BK channels are required to open in order to cause a significant membrane hyperpolarization, and thus function to limit the contraction resulting from acute increases in intravascular pressure, or in response to hypertensive pathologies.

Animals↗
PubMed1999

Bimodal kinetics of a chloride channel from human fibroblasts.

Excised patches were used to study the kinetics of a Cl channel newly identified in cultured human fibroblasts (L132). The conductance of ca. 70 pS in 150 mm symmetrical Cl, and the marked outward rectification ascribe this channel to the ICOR family. Long single-channel recordings (>30 min) revealed that the channel spontaneously switches from a kinetic mode characterized by high voltage dependence (with activity increasing with depolarization; mode 1), into a second mode (mode 2) insensitive to voltage, and characterized by a high activity in the voltage range +/-120 mV. On patch excision the channel always appeared in mode 1, which was maintained for a variable time (5-20 min). In most instances the channels then switched into mode 2, and never were seen to switch back, in spite of the eight patches that cumulatively dwelled in this mode 2.33-fold as compared to mode 1. Stability plots of long recordings showed that the channel was kinetically stable in both modes, allowing standard analysis of steady-state kinetics to be performed. Open and closed time distributions of mode 1 and mode 2 revealed that the apparent number of kinetic states of the channel was the same in the two modes. The transition from mode 1 into mode 2 was not instantaneous, but required a variable time in the range 5-60 sec. During the transition the channel mean open time was intermediate between mode 1 and mode 2. The intermediate duration in the stability plot however is not to be interpreted as if the channel, during the transition, rapidly switches between mode 1 and mode 2, but represents a distinct kinetic feature of the transitional channel.

Cell Line↗
PubMed1999

Mechanism of verapamil block of a neuronal delayed rectifier K channel: active form of the blocker and location of its binding domain.

1. The mechanism of verapamil block of the delayed rectifier K currents (I K(DR)) in chick dorsal root ganglion (DRG) neurons was investigated using the whole-cell patch clamp configuration. In particular we focused on the location of the blocking site, and the active form (neutral or charged) of verapamil using the permanently charged verapamil analogue D890. 2. Block by D890 displayed similar characteristics to that of verapamil, indicating the same state-dependent nature of block. In contrast with verapamil, D890 was effective only when applied internally, and its block was voltage dependent (136 mV/e-fold change of the on rate). Given that verapamil block is insensitive to voltage (Trequattrini et al., 1998), these observations indicate that verapamil reaches its binding site in the uncharged form, and accesses the binding domain from the cytoplasm. 3. In external K and saturating verapamil we recorded tail currents that did not decay monotonically but showed an initial increase (hook). As these currents can only be observed if verapamil unblock is significantly voltage dependent, it has been suggested (DeCoursey, 1995) that neutral drug is protonated upon binding. We tested this hypothesis by assessing the voltage dependence of the unblock rate from the hooked tail currents for verapamil and D890. 4. The voltage dependence of the off rate of D890, but not of verapamil, was well described by adopting the classical Woodhull (1973) model for ionic blockage of Na channels. The voltage dependence of verapamil off rate was consistent with a kinetic scheme where the bound drug can be protonated with rapid equilibrium, and both charged and neutral verapamil can unbind from the site, but with distinct kinetics and voltage dependencies.

Animals↗
PubMed1999

Mechanisms of verapamil inhibition of action potential firing in rat intracardiac ganglion neurons.

The effects of verapamil and related phenylalkylamines on neuronal excitability were investigated in isolated neurons of rat intracardiac ganglia using whole-cell perforated patch-clamp recording. Verapamil (>/=10 microM) inhibits tonic firing observed in response to depolarizing current pulses at 22 degrees C. The inhibition of discharge activity is not due to block of voltage-dependent Ca2+ channels because firing is not affected by 100 microM Cd2+. The K+ channel inhibitors charybdotoxin (100 nM), 4-aminopyridine (0.5 mM), apamin (30-100 nM), and tetraethylammonium ions (1 mM) also have no effect on firing behavior at 22 degrees C. Verapamil does not antagonize the acetylcholine-induced inhibition of the muscarine-sensitive K+ current (M-current) in rat intracardiac neurons. Verapamil inhibits the delayed outwardly rectifying K+ current with an IC50 value of 11 microM, which is approximately 7-fold more potent than its inhibition of high voltage-activated Ca2+ channel currents. These data suggest that verapamil inhibits tonic firing in rat intracardiac neurons primarily via inhibition of delayed outwardly rectifying K+ current. Verapamil inhibition of action potential firing in intracardiac neurons may contribute, in part, to verapamil-induced tachycardia.

4-Aminopyridine↗
PubMed1998

Verapamil block of the delayed rectifier K current in chick embryo dorsal root ganglion neurons.

We have used the patch-clamp method in the whole-cell configuration to investigate the mechanism of block of the delayed rectifier K current (IDRK) by verapamil in embryonic chick dorsal root ganglion (DRG) neurons. Verapamil induced a dose-dependent decay of the current, without altering its activation kinetics. This observation, together with the good description of IDRK time course at various blocker concentrations with the computer simulation of a three-state chain model (closed left and right arrow open left and right arrow open-blocked), indicates that verapamil acts as a state-dependent, open-channel blocker. To account for the double-exponential time course of recovery from block, this minimal kinetics scheme was expanded to include a closed-blocked state resulting from channel closure (at hyperpolarized voltages) with verapamil still bound to it. The apparent block and unblock rate constants assessed from verapamil-induced current decay in the presence of external Na were 0.95 +/- 0.05 ms-1mM-1 and 0.0037 +/- 0.0016 ms-1, respectively. When external Na was replaced by K, only the unblock rate constant changed, to 0.02 +/- 0.009 ms-1. Under these ionic conditions it was also observed that the recovery from block was modified from the double-exponential time course in the presence of external Na (tau1 = 160 ms; tau2 = 1600 ms), to a faster single-exponential recovery (tau = 100 ms). We tested the voltage dependence of block by applying stimulation protocols aimed at eliminating bias easily introduced by the shift of the gating equilibrium and by the coupling of channel activation and block. Under these experimental conditions the resulting block rate constant was not measurably voltage dependent.

Animals↗
PubMed1996

Characterization of a neuronal delayed rectifier K current permeant to Cs and blocked by verapamil.

We have used the patch-clamp method in the whole-cell configuration to characterize the delayed rectifier K current (IDRK) in embryonic chick dorsal root ganglion (DRG) neurons. The IDRK is activated by depolarizing pulses positive to -40 mV, and its V1/2 is near -20 mV. The slope factor of 10.4 mV for an e-fold change in conductance indicates an equivalent gating charge of 2.4e. Inactivation during sustained depolarizing pulses displays two distinct time constants of 200-300 msec and 6-9 sec, respectively. Outward current through the delayed rectifier K (DRK) channels could also be carried by internal Cs, which however exerts mild block when in mixtures with K, as evidenced by the anomalous mole fraction effect. The relative permeability of Cs vs. K, PCs/PK, as calculated from reversal potential measurements, is 0.25. Rb likewise permeates the DRK channel (PRb/PK = 0.67). The IDRK was effectively suppressed by external application of the Ca channel blocker Verapamil, with apparent dissociation constant of ca. 4 microM. The time course of Verapamil block, its good description by equations derived from open-channel block kinetic scheme, and the frequency-dependent effect of the blocker indicate that Verapamil can bind to the channel only when it is in the open state.

Animals↗
PubMed1995

Tityus bahiensis toxin IV-5b selectively affects Na channel inactivation in chick dorsal root ganglion neurons.

A novel toxin was isolated from the venom of the Brazilian scorpion Tityus (T.) bahiensis. The N-terminal amino acid sequence of this toxin was shown to be 80% identical to the corresponding segment of T. serrulatus toxin IV-5. The new toxin was thus named toxin IV-5b. Toxin IV-5b was found to markedly slow inactivation of Na channel in dorsal root ganglion neurons from chick embryo. By contrast, Na channel activation was only negligibly delayed, and deactivation completely unaffected. Similarly unaffected by the toxin were K and Ca currents. The slowing effect of the toxin starts to appear at concentrations of c. 80 nM, and shows a KD of 143 nM. With a toxin concentration of 2.4 microM, the Na channel inactivation time constant was increased c. 3-fold with respect to the control. The slowing of inactivation was voltage dependent, and increased with depolarization.

Animals↗
PubMed1993

Ion channels of biological membranes. A structuralist view.

Ion channels are integral membrane proteins, crossed by a central pore which serves to ions as a pathway between intra- and extracellular environment. Their main characteristics, gating and selectivity, are discussed as a contribution to the problem of the dynamic structuring of the organism.

Animals↗
PubMed1993

Effects of high salt concentration on the open probability of the background chloride channel.

We report here single channel data showing that high concentration of NaCl or CsCl at the cytoplasmic side of the membrane increases the open probability of the background Cl channel from hippocampal neurons. The open probability vs. voltage curve was shifted by 36 mV towards more negative voltages, when salt concentration was raised from 300 to 1000 mM. The steepness of the curve remained unchanged. Possible explanations for this effect are discussed.

Animals↗
PubMed1992

Newly identified steady-state potassium channels in rat hippocampal neurons.

We report two new types of potassium channels in cultured hippocampal neurons of rat. Both channels occurred in the soma membrane of these cells at very low density. They were active in steady-state conditions, within a wide voltage range that included the resting membrane potential. Their open probability was enhanced by membrane depolarization, but not influenced by Ca ions. In symmetrical 150 mM KCl the channels showed a slope conductance of ca. 40 and 80 pS, respectively. Current-voltage relations of both K channels show a negative slope at high positive voltages.

Animals↗
PubMed1989

Evolution of ionic channels of biological membranes.

This paper presents a view of the evolution and phylogenetic distribution of ionic channels of biological membranes. The view is based on the assumptions that ionic channels (1) appeared very early in the history of life, (2) have evolved from a common ancestor, and (3) have been subjected to evolutionary pressure to reach precision and high speed of signaling. We propose that Ca2+ was the intracellular messenger and modulator of the most primitive biological systems, which implies that the first channel to appear may have been a calcium channel. Then, very soon the entire group of potassium channels evolved from the calcium channel to improve the shape of signals and to restore initial conditions. Sodium channels probably appeared relatively late, diversifying from calcium channels in the early metazoan groups. Mainly because Na+ ions do not interfere with cellular metabolism (thus allowing the inward current--and, consequently, the speed of conduction--to be greatly increased), sodium channels probably proved advantageous in the generation of the action potential, and selection replaced calcium channels with sodium channels in this function. Finally, with the acquisition of multicellularity, channels responsible for synaptic transmission appeared. The case of the acetylcholine receptor channel is briefly discussed.

Animals↗
PubMed1988

Single channel recording and gating function of ionic channels.

This review discusses several aspects of the kinetic analysis of the gating function of membrane channels, as carried out on single channel data obtained by the patch clamp method. In particular, the following three aspects of channel behavior are reviewed in some detail: 1) estimate of the number of states or conformations the channel can enter; 2) the lifetime of each state as a function of transmembrane voltage (voltage-gated channels), or of ligand concentration (ligand-gated channels); 3) estimates of the rates at which the transitions between the various states occur, and assessment of reaction mechanisms. An introductory description of conformational transitions of channel proteins is also provided.

Animals↗
PubMed1988

Single chloride channels in cultured rat neurones.

Single-channel currents through chloride channels were recorded in cultured hippocampal neurones from rats using the patch-clamp method. The channel is active at voltages between -80 and +80 mV, and the time spent in the open state increases with depolarization (almost fourfold for 120 mV). The channel conductance is 62 pS in symmetrical 150 mM NaCl saline. In salt gradient conditions the channel was measurably permeable to Na+. Substitution of NO3- and Br- for Cl- gave higher single-channel currents, meaning a higher permeability of the channel toward the two anions than to Cl-. SO4(2-) ions were poorer charge carriers, yet contributed measurable inward current at negative voltages. Channel activity appeared independent of intracellular Ca2+ concentration. Taken together, these features would suggest for this channel a role in stabilizing resting membrane potential and in maintaining normal cell excitability.

Animals↗