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.

[The characteristics of the sodium currents across EGTA-modified calcium channels in the membrane of cultured rat cardiomyocytes]. / Kharakteristika natrievykh tokov cherez EGTA-modifitsirovannye kal'tsievye kanaly membrany kul'tiviruemykh kardiomiotsitov krys.

In isolated, cultured neonatal rat ventricular myocytes sodium currents through calcium channels induced by lowering of extracellular calcium concentration 100 nmol/l have been investigated by whole-cell patch clamp technique. Such Na(+)-carried currents are modulated by classic Ca2+ agonists and antagonists. The potential-dependent characteristics of Na+ current are shifted at 20 mV in hyperpolarizing direction as compared to initial Ca(2+)-carried current. The inactivation decay of Na+ current through Ca2+ channels has the monoexponential behaviour. The possible action of extracellular Ca2+ lowering on Ca2+ channel selective filter and gating mechanisms is suggested.

[Lack of low-threshold calcium channels in the membrane of cultured cells of the heart ventricle of newborn rats]. / Otsutstvie nizkoporogovykh kal'tsievykh kanalov b membrane kul'tiviruemykh kletok zheludochka serdtsa novorozhdennykh krys.

In isolated, cultured neonatal rat ventricular myocytes calcium current was investigated by whole-cell patch clamp technique. The shift of holding potential values from -100-80 mV to -40 mV caused no significant changes in calcium current potential-dependent and kinetic properties. No calcium current component with preferential steady-state inactivation at membrane potentials ranged between -100 and -60 mV was observed. Calcium current component resistant to classic Ca-antagonists action was not found either. All data listed above permit a conclusion on lack of the low-threshold calcium channels in the membrane of single cultured neonatal rat ventricular myocytes. Possible reasons of the absence of this type of calcium are discussed.

[Use of a microfluorometric method for measuring free calcium in the cytoplasm of isolated cultured rat cardiomyocytes]. / Primenenie metoda mikrofliuorometrii dlia izmereniia svobodnogo ka'ltsiia v tsitoplazme odinochnykh ku'ltiviruemykh kardiomiotsitov krysy.

Dual wavelength microfluorometry was used to measure the cytoplasmic free calcium concentration [( Ca2+]in) in single cultured cells from ventricular myocytes of neonatal rats loaded with the indicator fura-2. At 2.5 nmol/l extracellular Ca2+ in the resting cells [Ca2+]in was between 80 and 110 nmol/l. Sometimes, spontaneous low-frequency (approximately 0.1 Hz) [Ca2+]in oscillations were observed. High-potassium depolarization led to a Ca2+-antagonists-sensitive rise of [Ca2+]in. Both caffeine++ (5-10 mmol/l) and thymol (lmmol/l) initialized transient increase of [Ca2+]in. Mechanisms of [Ca2+]in homeostasis in heart muscle cells were discussed.

[Effect of reduced extracellular level of sodium ions on the intracellular level of calcium ions in the cytoplasm of cultured rat cardiomyocytes]. / Vliianie snizheniia vnekletochnoi kontsentratsii ionov natriia na vnutrikletochnuiu kontsentratsiiu ionov ka'ltsiia v tsitoplazme ku'ltiviruemykh kardiomiotsitov krys.

Cytosolic free calcium [( Ca2+]in) was measured using fura-2 in isolated cultured ventricular myocytes of neonatal rat. Exposure of the cardiomyocyte to a solution in which all Na+ have been replaced by impermeable cations results in a 400-600 nmol/l increase of [Ca2+]in. This increase is followed by a slow decrease to the initial level. A decrease of the extracellular calcium concentration from 2.5 to 0.5 mmol./l or increase to 10 mmol/l produced, respectively, decrease and increase of the amplitude of [Ca2+]in rise in response to low-Na+ superfusion. Exposure of cardiomyocytes to low-Na+ solutions also led to a 2-3 fold increase of caffeine++-dependent Ca2+ release from intracellular stores. Changes in [Ca2+]in can be attributed to the operation of a sodium-calcium exchanger in heart cells.