Transient photocurrent in molecular junctions: singlet switching on and triplet blocking.

The kinetic approach adapted to describe charge transmission in molecular junctions, is used for the analysis of the photocurrent under conditions of moderate light intensity of the photochromic molecule. In the framework of the HOMO-LUMO model for the single electron molecular states, the analytic expressions describing the temporary behavior of the transient and steady state sequential (hopping) as well as direct (tunnel) current components have been derived. The conditions at which the current components achieve their maximal values are indicated. It is shown that if the rates of charge transmission in the unbiased molecular diode are much lower than the intramolecular singlet-singlet excitation/de-excitation rate, and the threefold degenerated triplet excited state of the molecule behaves like a trap blocking the charge transmission, a possibility of a large peak-like transient switch-on photocurrent arises.

Imaging synaptic activity in intact brain and slices with FM1-43 in C. elegans, lamprey, and rat.

The fluorescent probe FM1-43 has been used extensively for imaging vesicle recycling; however, high nonspecific adsorption resulting in elevated background levels has precluded its use in certain tissues, notably brain slices. We have found that a sulfobutylated derivative of beta-cyclodextrin (ADVASEP-7) has a higher affinity for FM1-43 than the plasma membrane. ADVASEP-7 was used as a carrier to remove FM1-43 nonspecifically bound to the outer leaflet of the plasma membrane or extracellular molecules, significantly reducing background staining. This has enabled us to visualize synaptic vesicle recycling in the nematode C. elegans, intact lamprey spinal cord, and rat brain slices.

The effect of acetylcholine and serotonin on calcium transients and calcium currents in identified Helix pomatia L. neurons.

The results presented demonstrate that in D neurons of the snail Helix pomatia L., acetylcholine (ACh) (10 divided by 100 microM) and serotonin (5-HT) (0.1 divided by 1000 microM) applications reduce both the basal intracellular concentration level ([Ca2+]in) and the amplitudes of calcium transients induced by membrane depolarization. It is likely that the mechanism of [Ca2+]in changes in the suppression of calcium inward currents (ICa). Influences of Ach and 5-HT on ICa were studied. Both effects were dose-dependent (ACh--0.01 divided by 100 microM and 5-HT--0.1 divided by 1000 microM). The half-maximal effects (IC50) were evoked by ACh concentration of 0.15 microM and 5-HT--15 microM. Furthermore we have also shown that in some cells 5-HT could evoke a transient increase in ICa (IC50 = 2 microM). The effects of Ach and 5-HT were nonadditive--the subsequent application of ACh after 5-HT, and vice versa, produced no inhibitory effects. This may indicate that both substances act through a common intermediate (possibly, G-protein).

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 initial rate of Ca2+ influx was 12.4 +/- 0.5 nM/s as [Ca2+]c rose monoexponentially (time constant, tau = 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 (tau 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 tau rec indicate that an increase in [Ca2+]c is followed by a delayed (tau = 408 s) stimulation of Ca2+ removal, which fully reverses (tau approximately 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.

Calcium clamp in single nerve cells.

Free calcium concentration in isolated single neurons was clamped using a new technical approach based on a feed-back connection between the Fura-2 fluorescence signal measuring the intracellular Ca2+ concentration ([Ca2+]i) and iontophoretic current injecting Ca2+ into the cell. Beginning of [Ca2+]i clamping at a level above the basal one triggered fast (few seconds) current transients equal to injection of 36 +/- 20 microM Ca2+ (for a 0.1 microM change of [Ca2+]i), representing the filling of a fast cytosolic buffer. Continuation of clamping required very small clamping currents (corresponding to injection of 0.39 +/- 0.20 microM.s-1 Ca2+). This value increased proportionally to the magnitude of the change of [Ca2+]i above basal level, indicating the activation of calcium-dependent mechanisms for Ca2+ removal from the cytosol. The described approach allowed measurement, under physiological conditions, of the capacitative and kinetic properties of different Ca-regulating systems functioning in a single nerve cell as well as other types of cells.

Dual mechanisms of angiotensin-induced activation of mouse sympathetic neurones.

Ang II directly activates neurones in sympathetic ganglia. Our goal was to define the electrophysiological basis of this activation. Neurones from mouse aortic-renal and coeliac ganglia were identified as either 'tonic' or 'phasic'. With injections of depolarizing currents, action potentials (APs) were abundant and sustained in tonic neurones (TNs) and scarce or absent in phasic neurones (PNs). Resting membrane potentials were equivalent in TNs (-48 +/- 2 mV, n = 18) and PNs (-48 +/- 1 mV, n = 23) while membrane resistance was significantly higher in TNs. Ang II depolarized and increased membrane resistance equally in both TNs (n = 8) and PNs (n = 8) but it induced APs only in TNs, and enhanced current-evoked APs much more markedly in TNs (P < 0.05). The AT1 receptor antagonist losartan (2 microm, n = 6) abolished all responses to Ang II, whereas the AT2 receptor blocker PD123,319 had no effect. The transient K+ current (IA), which was more than twice as large in TNs as in PNs, was significantly inhibited by Ang II in TNs only whereas the delayed sustained K+ current (IK), which was comparable in both TNs and PNs, was not inhibited. M currents were more prominent in PNs and were inhibited by Ang II. The IA channel blocker 4-aminopyridine triggered AP generation in TNs and prevented the Ang II-induced APs but not the depolarization. Blockade of M currents by oxotremorine M or linopirdine prevented the depolarizing action of Ang II. The protein kinase C (PKC) inhibitor H7 (10 microm, n = 9) also prevented the Ang II-induced inhibition of IA and the generation APs but not the depolarization nor the inhibition of M currents. Conversely, the PKC agonist phorbol 12-myristate 13-acetate mimicked the Ang II effects by triggering APs. The results indicate that Ang II may increase AP generation in sympathetic neurones by inducing a PKC-dependent inhibition of IA currents, and a PKC-independent depolarization through inhibition of M currents. The differential expression of various K+ channels and their sensitivity to phosphorylation by PKC may determine the degree of activation of sympathetic neurones and hence may influence the severity of the hypertensive response.

Calcium clamp in isolated neurones of the snail Helix pomatia.

1. Intracellular free calcium concentration ([Ca2+]i) in isolated non-identified Helix pomatia neurones has been clamped at different physiologically significant levels by a feedback system between the fluorescent signal of fura-2 probe loaded into the cell and ionophoretic injection of Ca2+ ions through a CaCl2-loaded microelectrode. The membrane potential of the neurone has also been clamped using a conventional two-microelectrode method. 2. Special measurements have shown that the transport indices of injecting microelectrodes filled with 50 mM CaCl2 are quite variable (0.11 +/- 0.06, mean +/- S.D.). However, for each electrode the transport indices remained stable during several injection trials into a solution drop having the size of a neurone. The spread of calcium ions from the tip of the microelectrode across the cytosol of the neurone terminated within 2-4 s. The spatial difference in [Ca2+]i at this time did not exceed 10%. 3. Clamping of [Ca2+]i at a new increased level was accompanied by a transient of the Ca(2+)-injecting current. To increase [Ca2+]i by 0.1 microM, the amount of calcium ions injected during this stage had to be 36 +/- 20 microM Ca2+ per cell volume. Obviously, this transient represents the filling of a fast cytosolic buffer which has to be saturated to reach a new increased level of [Ca2+]i. It was followed by a steady component of Ca(2+)-injecting current, which was quite low (corresponding to injection of 0.39 +/- 0.20 microM s-1 for a 0.1 microM change of [Ca2+]i). This may represent the functioning of Ca(2+)-eliminating systems and corresponds to a similar amount of Ca2+ extruded from the cytoplasm. 4. Changes in the injection current also developed when Ca2+ influx through the membrane was triggered by the activation of voltage-gated calcium channels. The amount of Ca2+ entering the cell during the first seconds of depolarization to--15 mV was equal to 0.59 +/- 0.31 microM s-1 per cell volume. 5. No activation of Ca(2+)-dependent potassium current was observed during the changes in [Ca2+]i to levels exceeding the basal one by several times. Obviously, to activate this current, a much stronger increase in [Ca2+]i is needed in the immediate vicinity of the corresponding channels.

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',N'-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 (> or = 12 microM 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.

The droplet technique: measurement of calcium extrusion from single isolated mammalian cells.

This paper contains a description of the modified droplet technique that is designed to monitor Ca2+ extrusion from single isolated pancreatic acinar cells. A cell loaded with calcium indicator is maintained in a small droplet of solution containing another calcium indicator. Differences in the optical properties of the intracellular and extracellular indicators allows one to monitor simultaneously intracellular and extracellular calcium concentrations. The paper contains a description of the calibration procedure that is used to calculate intracellular and extracellular calcium concentrations. The advantages and disadvantages of different pairs of extracellular and intracellular indicators are discussed.

Extrusion of calcium from a single isolated neuron of the snail Helix pomatia.

Simultaneous optical measurements of extra- and intracellular Ca2+ concentrations were carried out on isolated snail neurons injected iontophoretically with Ca2+. The fluorescent indicator Fura-2 was used to measure intracellular concentration of free Ca, and the absorbant indicator Antipyrylazo III to measure changes in extracellular calcium concentration in the micro-chamber containing the cell. The velocity of Ca2+ extrusion from a single cell has been shown to be in accordance with the level of free Ca in the neuronal cytoplasm. After an increase in intracellular free Ca by iontophoretic injection from a microeletrode to 0.2-0.5 microM, the velocity of Ca2+ extrusion from the neuron was approximately 0.3-4.6 microM/sec per cell volume. During caffeine-induced calcium-dependent calcium release of Ca2+ from intracellular stores a stimulation of calcium extrusion took place, reaching the velocity of 5.0 microM/sec per cell volume.

Fluorescent detection of Zn(2+)-rich vesicles with Zinquin: mechanism of action in lipid environments.

High concentrations of free Zn2+ ions are found in certain glutamatergic synaptic vesicles in the mammalian brain. These terminals can be visualized histochemically with quinoline sulfonamide compounds that form fluorescent complexes with Zn2+. The present study was undertaken to examine the interaction of the water-soluble quinoline sulfonamide probe, Zinquin (2-methyl-8-(toluene-p-sulfonamido)-6-quinolyloxyacetic acid) with the complex heterogeneous cellular environment. Experiments on rat hippocampal and neocortical slices gave indications that Zinquin in its free acid form was able to diffuse across the plasma and synaptic vesicle membranes. Further experiments were undertaken on unilamellar liposomes to study the interaction of Zinquin and its metal complexes in membranes. These experiments confirmed that Zinquin is able to diffuse across lipid bilayers. Steady-state and time-resolved fluorimetric studies showed that Zinquin in aqueous solution mainly forms a 1:2 (metal:ligand) complex with small amounts of a 1:1 complex. Formation of the 1:1 complex was favored by the presence of lipid, suggesting that it partitions into membranes. Evidence is presented that Zinquin can act as a Zn(2+)-ionophore, exchanging Zn2+ for two protons. The presence of a pH gradient across vesicles traps the Zn(2+)-probe complex within the vesicles. Zinquin is useful as a qualitative probe for detecting the presence of vesicular Zn2+; however, its tendency to partition into membranes and to serve as an ionophore should be borne in mind.