Different action of ethosuximide on low- and high-threshold calcium currents in rat sensory neurons.

The effects of ethosuximide on calcium channels were studied on dorsal root ganglion neurons from one-day-old rats using the patch-clamp technique. Bath application of ethosuximide induced dose-dependent and reversible suppression of calcium currents without affecting their time-course. Substantial differences between the effects of ethosuximide on the low-threshold and high-threshold (T- and L-) currents were observed. Ethosuximide reduced the T-current with greater potency than the L-current (Kd for T-current is 7 microM vs 15 microM for L-current). This relative specificity of its action still remained if applied at concentrations up to 1 mM. These data support the hypothesis according to which the anti-epileptic action of ethosuximide is related to reduction of the low-threshold calcium currents in sensory neurons.

Patch-clamp recordings on rat cardiac muscle slices.

Thin slices (100-200 microns) from cardiac ventricles of newborn (1-14 days old) rats were used for patch-clamp recordings. High resistance seals (greater than 10 GOhms) could be obtained on this preparation without enzymatic treatment of tissue. Resting potential for cardiac fibres, measured in whole-cell configuration, ranged between -30 and -65 mV. Averaged sodium currents and inward rectifying potassium single-channel currents, recorded in cell-attached mode, displayed similar features to those previously reported for isolated rat ventricular cells. Application of patch-clamp techniques to cardiac slices allows single-channel recordings without complicated procedures of cell isolation. Moreover, possible alteration of channel properties caused by proteolytic enzymes can be avoided. This technique provides a new tool for studying electrophysiological properties of heart cells in situ.

Cultured glial precursor cells from mouse cortex express two types of calcium currents.

Glial precursor cells of the oligodendrocyte lineage express two types of Ca2+ channels which can be distinguished by their peak activation. Ca2+ currents were recorded using the patch-clamp technique in the whole cell configuration and were found to be similar to the low and the high voltage activated Ca2+ channel described in neurons. While the majority of precursor cells expressed only one of the two current types respectively, in 30% of cells both types of current could be identified. These Ca2+ channels might play an important role in signal transduction during target recognition of the developing glial cell.

Omega-conotoxin blockade of calcium currents in cultured neonatal rat cardiomyocytes: different action on EGTA-modified calcium channels.

Calcium currents from neonatal rat ventricular heart muscle cells grown in primary culture were examined using the "whole-cell" voltage clamp technique. An inward current characterized by large amplitude and slow inactivation decay was induced when the extracellular Ca2+ concentration was reduced by EGTA. This current was suppressed by extracellular Na+ removal, or by calcium antagonists, and increased by epinephrine and BAY K 8644. These findings suggest that this current is carried by sodium ions through Ca channels. Both Ca and Na currents through calcium channels were irreversibly blocked by omega-conotoxin. Complete blockade developed 10-15 minutes after the toxin introduction in the extracellular solution. Blockade of Na currents through calcium channels was characterized by a transient increase of current amplitude without any changes in its kinetics and voltage-dependent properties. Structural differences between calcium channels in rat and guinea-pig and frog cardiomyocytes were suggested.

Possible existence of transient TTX-sensitive chloride current in the membrane of isolated cardiomyocytes.

Cardiomyocytes enzymatically isolated from rat and guinea pig ventricular tissue were investigated under conditions of intracellular perfusion and voltage clamp at 18-20 degrees C. Perfusion with 135 mmol/l Tris(HF), pH 7.2 was used to eliminate outward potassium currents. The dependence of inward current (elicited by depolarizing pulses from a holding potential level of--120 mV) on low external TTX concentrations (from 10(-13) to 10(-10) mol/l) was studied. Similar TTX concentrations increased the amplitude of the inward current and changed its kinetics in a large number of cells tested. The effect was fully reversible. The effect could be evaluated in a net form by digital subtraction of the current obtained after the application of a low external TTX concentration from the initial current in a TTX-free solution. The TTX concentration dependence of the difference current could be fitted by one-to-one binding curve with Kd = (1.0 +/= 0.4) x 10(-12) mol/l. TTX-induced current changes were absent in low sodium or chloride-free external solutions. The outward current (a block of which by TTX produced the inward current changes observed) showed a reversal potential consistent with the chloride nature of such a current. The existence of a transient TTX-sensitive Na-dependent potential gated chloride current in the membrane of isolated cardiomyocytes is postulated.

Sodium currents in the membrane of isolated cardiomyocytes.

In the experiments on ventricular myocytes which were enzymatically isolated from hearts of 4 weeks old rats and were internally perfused in voltage clamp conditions the existence of two sodium currents was demonstrated. The data obtained suggest that the so called fast sodium current in the membrane of ventricular myocytes is in fact a sum of two currents with different kinetics and potential dependent properties, sensitivity to tetrodotoxin and external pH which are supposed to be passed through separate membrane channel populations. Due to about four fold difference in their time courses of inactivation the currents were called fast sodium current (IfNa) and slow sodium current (IsNa).