Ca2+ current and Ca(2+)-activated chloride current in isolated smooth muscle cells of the sheep urethra.

1. Isolated sheep urethral cells were studied using the perforated patch clamp technique (T = 37 degrees C). Depolarizing steps ranging from -40 to -10 mV evoked an inward current that peaked within 10 ms and a slower inward current. Stepping back to the holding potential of -80 mV evoked large inward tail currents. All three currents were abolished by nifedipine (1 microM). Substitution of external Ca2+ with Ba2+ resulted in potentiation of the fast inward current and blockade of the slow current and tails. 2. Changing the chloride equilibrium potential (ECl) from 0 to +27 mV shifted the reversal potential of the tail currents from 1 +/- 1 to 27 +/- 1 mV (number of cells, n = 5). Chloride channel blockers, niflumic acid (10 microM) and anthracene-9-carboxylic acid (9AC, 1 mM), reduced the slow current and tails suggesting that these were Ca(2+)-activated Cl- currents, ICl(Ca). 4. Caffeine (10 mM) induced currents that reversed at ECl and were blocked by niflumic acid (10 microM). 5. In current clamp mode, some cells developed spontaneous transient depolarizations (STDs) and action potentials. Short exposure to nifedipine blocked the action potentials and unmasked STDs. In contrast, 9AC and niflumic acid reduced the amplitude of the STDs and blocked the action potentials. 6. In conclusion, these cells have both L-type ICa and ICl(Ca). The former appears to be responsible for the upstroke of the action potential, while the latter may act as a pacemaker current.

Tetrodotoxin-sensitive sodium current in sheep lymphatic smooth muscle.

1. Fast inward currents were elicited in freshly isolated sheep lymphatic smooth muscle cells by depolarization from a holding potential of -80 mV using the whole-cell patch-clamp technique. The currents activated at voltages positive to -40 mV and peaked at 0 mV. 2. When sodium chloride in the bathing solution was replaced isosmotically with choline chloride inward currents were abolished at all potentials. 3. These currents were very sensitive to tetrodotoxin (TTX). Peak current was almost abolished at 1 microM with half-maximal inhibition at 17 nM. 4. Examination of the voltage dependence of steady state inactivation showed that more than 90% of the current was available at the normal resting potential of these cells (-60 mV). 5. The time course of recovery from inactivation was studied using a double-pulse protocol and showed that recovery was complete within 100 ms with a time constant of recovery of 20 ms. 6. Under current clamp, action potentials were elicited by depolarizing current pulses. These had a rapid upstroke and a short duration and could be blocked with 1 microM TTX. 7. Spontaneous contractions of isolated rings of sheep mesenteric lymphatic vessels were abolished or significantly depressed by 1 microM TTX.

Effect of purinergic blockers on outward current in isolated smooth muscle cells of the sheep bladder

Freshly dispersed cells from sheep urinary bladder were voltage clamped using the whole cell and inside-out patch-clamp technique. Cibacron and Basilen blue increased outward current in a dose-dependent manner with a half-maximal response at 10(-5) M. Suramin, in concentrations to 10(-3) M, had no such effect. The Cibacron blue response was abolished in Ca2+-free physiological salt solution, suggesting that it was acting on a Ca2+-dependent current. Similarly, the Cibacron blue-sensitive current was significantly attenuated by charybdotoxin. Cibacron blue did not modulate inward current nor were its effects modified by caffeine or heparin, suggesting that its effect on outward current was not secondary to an increase in intracellular Ca2+. Application of 10(-4) M Cibacron blue to the inside membrane of excised patches caused a rapid increase in open probability of a large-conductance (300 pS) K+ channel. These results suggest that Cibacron blue is a potent activator of a Ca2+-dependent outward current in bladder smooth muscle cells in addition to its action as a purinergic blocker.

Kv1.5 is a major component underlying the A-type potassium current in retinal arteriolar smooth muscle.

Little is known about the molecular characteristics of the voltage-activated K(+) (K(v)) channels that underlie the A-type K(+) current in vascular smooth muscle cells of the systemic circulation. We investigated the molecular identity of the A-type K(+) current in retinal arteriolar myocytes using patch-clamp techniques, RT-PCR, immunohistochemistry, and neutralizing antibody studies. The A-type K(+) current was resistant to the actions of specific inhibitors for K(v)3 and K(v)4 channels but was blocked by the K(v)1 antagonist correolide. No effects were observed with pharmacological agents against K(v)1.1/2/3/6 and 7 channels, but the current was partially blocked by riluzole, a K(v)1.4 and K(v)1.5 inhibitor. The current was not altered by the removal of extracellular K(+) but was abolished by flecainide, indicative of K(v)1.5 rather than K(v)1.4 channels. Transcripts encoding K(v)1.5 and not K(v)1.4 were identified in freshly isolated retinal arterioles. Immunofluorescence labeling confirmed a lack of K(v)1.4 expression and revealed K(v)1.5 to be localized to the plasma membrane of the arteriolar smooth muscle cells. Anti-K(v)1.5 antibody applied intracellularly inhibited the A-type K(+) current, whereas anti-K(v)1.4 antibody had no effect. Co-expression of K(v)1.5 with K(v)beta1 or K(v)beta3 accessory subunits is known to transform K(v)1.5 currents from delayed rectifers into A-type currents. K(v)beta1 mRNA expression was detected in retinal arterioles, but K(v)beta3 was not observed. K(v)beta1 immunofluorescence was detected on the plasma membrane of retinal arteriolar myocytes. The findings of this study suggest that K(v)1.5, most likely co-assembled with K(v)beta1 subunits, comprises a major component underlying the A-type K(+) current in retinal arteriolar smooth muscle cells

Current-induced transition in atomic-sized contacts of metallic alloys

We have measured conductance histograms of atomic point contacts made from the noble-transition-metal alloys CuNi, AgPd, and AuPt for a concentration ratio of 1:1. For all alloys these histograms at low-bias voltage (below 300 mV) resemble those of the noble metals, whereas at high bias (above 300 mV) they resemble those of the transition metals. We interpret this effect as a change in the composition of the point contact with bias voltage. We discuss possible explanations in terms of electromigration and differential diffusion induced by current heating.

On the reasons for stepwise changes in the tunnelling current across nanogaps

The role of the crystalline orientation of the STM tip in the generation of metal clusters is studied by atom dynamics simulations. When a (111) facet is facing the surface, the process is accompanied by a perturbation of the surface stronger than that observed for more open tip structures. This implies a technological application: the possibility of orienting a nanocrystallite deposited on a tip according to the changes observed in the force on the tip.

Current-driven magnetic rearrangements in spin-polarized point contacts

A method for investigating the dynamics of atomic magnetic moments in current-carrying magnetic point contacts under bias is presented. This combines the nonequilibrium Green's function (NEGF) method for evaluating the current and the charge density with a description of the dynamics of the magnetization in terms of quasistatic thermally activated transitions between stationary configurations. This method is then implemented in a tight-binding (TB) model with parameters chosen to simulate the main features of the electronic structures of magnetic transition metals. We investigate the domain wall (DW) migration in magnetic monoatomic chains sandwiched between magnetic leads, and for realistic parameters find that collinear arrangement of the magnetic moments of the chain is always favorable. Several stationary magnetic configurations are identified, corresponding to a different number of Bloch walls in the chain and to a different current. The relative stability of these configurations depends on the geometrical details of the junction and on the bias; however, we predict transitions between different configurations with activation barriers of the order of a few tens of meV. Since different magnetic configurations are associated with different resistances, this suggests an intrinsic random telegraph noise at microwave frequencies in the I-V curves of magnetic atomic point contacts at room temperature. Finally, we investigate whether or not current-induced torques are conservative.

A Maxwell relation for current-induced forces

A Maxwell relation is presented involving current-induced forces. It provides a new physical picture of the origin of current-induced forces and in the small-voltage limit it enables the identification of a simple thermodynamic potential which drives electromigration. The question of whether current-induced forces are conservative or non-conservative is discussed briefly in the light of these insights.

Are current-induced forces conservative?

The expression for the force on an ion in the presence of current can be derived from first principles without any assumption about its conservative character. However, energy functionals have been constructed that indicate that this force can be written as the derivative of a potential. On the other hand, there exist specific arguments that strongly suggest the contrary. We propose physical mechanisms that invalidate such arguments and demonstrate their existence with first-principles calculations. While our results do not constitute a formal resolution to the fundamental question of whether current-induced forces are conservative, they represent a substantial step forward in this direction.