Ca2+ and K+ current in cultured vascular smooth muscle cells from rat aorta

Ca²⁺ (I_Ca) and K⁺ (I_K) currents were recorded in single cultured cells from rat aorta using the whole cell clamp technique with patch electrodes. I_Ca was detected at–30 mV, and at 20 mV it reached a peak in about 10 ms and decayed with a t_1/2=50 ms. The mean maximum slope conductance (G_Ca) was 30 S/cm². I_K was detected at–10 mV and at 20 mV reached its maximum with a t_1/2=12 ms. For I_K, G_K=200 S/cm². These channels can be activated during action potentials and play a role in the excitation and contraction of vascular smooth muscle cells.Doctoral training program UAM-1     

Expression of voltage-dependent sodium and transient potassium currents in an identified sub-population of dorsal root ganglion cells acutely isolated from 12-day-old mouse embryos

The electrophysiological properties of a subset of dorsal root ganglion (DRG) neurons microdissected from 12-day-old (E12) mouse embryos and acutely isolated were analyzed as soon as 3 after their isolation. Two classes of neurons were defined according to their mean diameter. The larger diameter class was examined in this study. They display uniform cytoskeletal properties with co-expression of vimentin and neurofilament triplet proteins. Patch-clamp methods also revealed a homogeneous and limited repertoire of ionic channels that included (1) a TTX-sensitive Na⁺ current whose properties are similar to that reported in mature mammalian neurons, and (2) two types of K⁺ currents that can be compared with the delayed rectifier (I _k ) and the transient (I _a) potassium currents found in other mammalian preparations. It may be possible to use this in vitro model to examine the development of new types of currents, such as Ca²⁺ currents during neuronal growth and differentiation.     

Effects of acetylcholine on time-dependent currents in sheep cardiac Purkinje fibers

Voltage clamp experiments were carried out on sheep Purkinje fibers to determine the effect of Ach on the time-dependent currents.On the pacemaker current (i _{K 2}) Ach 10^–6 mol·l^–1 had the following effects: shift of the activation curve by a few mV in the depolarizing direction, without change in the rectifier ratio. The potential dependence of the time constants for activation and deactivation was influenced in a similar way as the activation curve.Ach had no effect on the positive dynamic current (i _qr ) or the late plateau outward current (i _x ).The slow inward current (i _si ) as well as the transient inward current (T.I.) were reduced in amplitude and slowed in time course by Ach.The changes in pacemaker current are important in explaining the increased rate of diastolic depolarization in the presence of Ach. The decrease of slow inward current by Ach cannot be made responsible for the plateau shift or the prolongation of the action potential.Supported by F.G.W.O. Belgium 3.0087.74     

The hyperpolarisation-activated current,I f,is not required for pacemaking in single cells from the rabbit atrioventricular node

Using electrophysiological and radiotracer studies in parallel, we have investigated the characteristics of the endogenous Na⁺-dependent amino acid transporter (system B^0,+) in Xenopus oocytes with regard to ion dependence, voltage dependence and transport stoichiometry. In voltage-clamped oocytes (–60 mV) superfusion with saturating concentrations of amino acids (1 mM) in 100 mM NaCl resulted in reversible, inward currents (mean±SEM): alanine, 1.83±0.09 nA (n=21); arginine, 2.54±0.18 nA (n=17); glutamine, 1.73±0.10 nA (n=19). Only arginine evoked a current in choline medium (0.50±0.13 nA, n=10), whereas Cl^– replacement had no effect on evoked currents. The glutamine-evoked current was saturable (I _max=1.73 nA, glutamine K _m=0.12 mM) and linearly dependent upon voltage between –90 and –30 mV. Using direct and indirect (activation) methods, we found that transport can proceed with Na⁺/amino acid coupling stoichiometry of either 11 or 21, but coupling was the same for each amino acid tested (alanine, arginine and glutamine) within a batch of oocytes (i.e. from a single toad). Despite the net single positive charge on arginine, the magnitude of the net transmembrane charge movement during Na⁺-coupled arginine transport was identical to that for the zwitterionic neutral amino acids glutamine and alanine; this may be explained by a concomitant stimulation of K⁺ efflux during arginine transport with a putative coupling of 1 K⁺1 arginine.     

Atypical miniature end-plate currents in the normal rat neuromuscular synapse and after acetylcholinesterase inhibition and cooling

Leningrad State University, A. A. Ukhtomskii Leningrad Institute of Physiology. (Presented by Academician of the Academy of Medical Sciences of the USSR S. N. Golikov.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 109, No. 6, pp. 523–525, June, 1990.     

A model of electrical activity and cytosolic calcium dynamics in vascular endothelial cells in response to fluid shear stress

A mathematical model is proposed to describe the intracellularCa ²⁺ (Ca _i) transient and electrical activity of vascular endothelial cells (VEC) elicited by fluid shear stress (τ). The intracellularCa ²⁺ store of the model VEC is comprised of aCa _i-sensitive (sc) and an inositol (1,4,5)-trisphosphate (IP ₃)-sensitive compartment (dc). The dc [Ca ²⁺] is refilled by the sc whose [Ca ²⁺] is the same as extracellular [Ca ²⁺].IP ₃ produced by the τ-deformed mechanoreceptors discharges the dcCa ²⁺ into the cytosol. The increase of cytosolic[Ca ²⁺] inducesCa ²⁺ release (CICR) from the sc. The raisedCa _i activates aCa _i-activatedK ⁺ current (I _{K, Ca}) and inhibitsIP ₃ production. The cell membrane potential is determined byI _{K, Ca}, voltage-dependentNa ⁺ andK ⁺ currents. Steady τ>0.1 dyne/cm² elicits aCa _i varies sigmoidally withLog ₁₀(τ) with a maximal peakCa _i of 150 nM at τ=4 dynes/cm². Step increases of τ fail to elicit aCa ²⁺ response in cells previously stimulated by a lower shear. TheCa ²⁺ response gradually decreases with repetitive τ stimuli. Pulsatile shear elicits two to three times higherCa _i and hyperpolarizes the cell more than steady shear of the same magnitude. The simulatedCa ²⁺ responses to τ are quantitatively and qualitatively similar to those observed in cultured VEC. The model provides a possible explanation of why the vasodilating stimulus is greater for pulsatile flow than for nonpulsatile flow.     

Modification of rat brain Kv1.4 channel gating by association with accessory Kvβ1.1 and β2.1 subunits

 We have examined the effects of co-expression of Kvβ1.1 and Kvβ2.1 subunits on the gating of rat brain Kv1.4 channels, expressed in Xenopus oocytes. Expression of Kv1.4 subunits alone produced a rapidly inactivating ”A” type current, which activated at potentials beyond –60 mV in a solution containing high levels of rubidium. Current activation curves obtained from tail current measurements were fitted with a Boltzmann function, with V _1/2 = –47 mV and k = 10 mV. Neither the Kvβ1.1 nor Kvβ2.1 subunits altered the voltage dependence of activation. Both subunits accelerated the activation time constant of Kv1.4, without affecting its voltage dependence. Surprisingly, the Kvβ2.1 subunit, which lacks an N-terminal inactivation domain, was almost as effective as the Kvβ1.1 subunit in speeding up Kv1.4. Steady-state inactivation of Kv1.4 was unchanged upon co-expression with either Kvβ1.1 or Kvβ2.1 subunits. Kv1.4 recovered from inactivation with two time constants; apart from an ≈ 50% lengthening of the slow time constant with a high Kvβ2.1 injection ratio, neither time constant was altered by either the Kvβ1.1 or Kvβ2.1 subunits, suggesting little interaction with recovery from C-type inactivation. Clearly, β subunits have the potential to modify the gating of Kv1.4 channels in the brain more subtly than has been suggested previously. Received: 17 March 1997 / Accepted: 30 June 1997     

Axonal and glial currents activated during the post-tetanic hyperpolarization in non-myelinated nerve

 Changes in membrane potential and potassium concentration in the extracellular space ([K⁺]_e) of rabbit vagus nerve were measured simultaneously during electrical activity and during the period of recovery using a modified sucrose-gap method and potassium-sensitive microelectrodes. After stimulation for 15 s at 15 Hz the main activity-induced increase in [K⁺]_e reached 16.9 mM. This increase in [K⁺]_e was paralleled by a depolarization of the preparation. The period of activity was followed by a post-tetanic hyperpolarization (PTH) lasting tens of seconds, generated by the axonal electrogenic Na⁺-K⁺ pump and to a lesser extent by the pump of the surrounding Schwann cells. The amplitude of the PTH dramatically increased in experiments in which inward currents were blocked by removal of Cl^– or after application of Cs⁺ or Ba²⁺, indicating that under normal conditions the current generated by the Na⁺-K⁺ pump is strongly short-circuited. A pharmacological and kinetic study showed that these currents are: (1) the hyperpolarization-activated current I _h, and (2) the inwardly rectifying I _KIR current. The results show that the latter originates from Schwann cells. Our data indicate that in non-myelinated nerves there is a subtle association of inward ionic channels which (1) helps the cell to maintain an optimal membrane potential after a period of activity, and (2) contributes to the removal of excess K⁺ from the extracellular space. Received: 7 August 1997 / Received after revision 6 April 1998 / Accepted: 15 April 1998     

Activating effect of <Emphasis Type="Italic">Tanacetum vulgare</Emphasis> L. pectin polysaccharide on ionic channels of neuronal membrane

The membranotropic effects of TVF tanacetan pectin polysaccharide derived from Tanacetum vulgare L. was studied by the voltage-clamp method on isolated neurons of Lymnaea stagnalis mollusk. TVF in concentrations of 0.1-10.0 µg/ml nonselectively activated the outward potassium and total inward (sodium and calcium) ionic currents (slightly dose-dependently and reversibly increased their amplitude by 5–10%) and decreased nonspecific leakage current.Translated from Byulleten Eksperimentalnoi Biologii i Meditsiny, Vol. 138, No. 10, pp. 439–441, October, 2004     

Suppression of charge movement by calcium antagonists is not related to calcium channel block

The calcium channel-inhibiting drugs nitrendipine and diltiazem represent two important classes of organic calcium antagonists. In the present study, the effect of these drugs on calcium currents and charge displacement currents in bullfrog semitendinosus muscle fibers was examined using a vaseline gap voltage clamp. Nitrendipine (10 M) reduced the quantity of charge that moved both during the ON phase (Q_ON) and the OFF phase (Q_OFF) of charge movement. This action appeared to be most selective for Q_ON. However, at this same concentration, nitrendipine had no blocking action on inward calcium currents. In contrast to these findings, diltiazem blocked calcium currents in a concentration-dependent manner, while slightly increasing the quantity of charge moved during Q_ON and Q_OFF. The enhancement of charge movement by diltiazem resulted from two actions. First, diltiazem shifted the voltage-dependence of charge movement to more negative potentials. Second, diltiazem increased the maximum amount of charge moved. (Supported by NIH NS 03178 and HL 07382.)