Biomedical subjects

C Baud

Publications and source records attributed to C Baud.

48 records · Page 3Linked to original sources

PubMed1984

A voltage-gated hydrogen ion current in the oocyte membrane of the axolotl, Ambystoma.

Membrane currents in the immature oocyte of the urodele amphibian Ambystoma were studied using the two-micro-electrode voltage-clamp technique. A current carried by H ions (IH) constituted the major portion of outward current activated by depolarizations from the resting voltage (about -60 mV). Net inward current was not observed at this developmental stage. The reversal potential for IH measured from tail currents obtained in two step voltage-clamp experiments shifted by 54 mV per unit change in external pH between pH 6.9 and 8.4. The reversal potential at pH 7.4 was not affected by changes in external K or Cl concentrations. A small change in reversal potential was observed with removal of external Na. The amplitude of IH was not affected by removal of external Ca (Mg or Sr substitution). Ca ionophore A23187 shifted the current-voltage relation towards negative voltages. Activation of IH did not appear to depend on Ca influx. The instantaneous current-voltage relation for IH, measured from tail currents at approximately equal internal and external H ion concentrations, was linear between -40 and +30 mV. The steady-state conductance-voltage relationship was sigmoidal with membrane voltage, and, at pH 7.4, was one-half maximum at about +15 mV (V1/2). The time courses of activation and deactivation were proportional to 1-exp (-t/tau). A plot of time constant (tau) against voltage was bell-shaped, with a maximum near V1/2. These results suggested that the activation of IH is voltage dependent. Increases in the external H ion concentration shifted the conductance-voltage and time constant-voltage relations in parallel towards positive voltages. The magnitude of these shifts showed a lower saturation near pH 9. Low concentrations of external Cd (10-300 microM) reduced current amplitude by shifting the current-voltage relation in the positive direction. Cd also reduced the limiting slope conductance. These effects were partially reversible.

Action Potentials↗

PubMed1984

Induction and disappearance of excitability in the oocyte of Xenopus laevis: a voltage-clamp study.

Electrically excitable, sodium-selective channels are induced in the membrane of the oocytes of Xenopus laevis when it is submitted to prolonged positive potentials (Kado, Marcher & Ozon, 1979; Baud, Kado & Marcher, 1982). Under a long positive voltage-clamp step, the membrane current, initially outward, becomes inward with a sigmoidal time course. The mean time to half-maximal inward current (t 1/2) is about 18 s at 16 degrees C when stepping the membrane potential to +55 mV. The rate of channel induction was very temperature dependent (Q10 about 5). In an Arrhenius plot, the t 1/2 for induction at temperatures between 5 and 22 degrees C showed a single slope. The rate of induction was dependent on the membrane potential, increasing exponentially with positive membrane potential (e-fold for a 20 mV change). When the membrane was maintained at resting potential after induction, the ability to produce inward currents with short depolarizing steps slowly disappeared with a t 1/2 of 4 min at 16 degrees C. The temperature dependence for disappearance was larger than that found for induction (Q10 about 7). The rate of disappearance was not dependent on holding the membrane potential in the range -30 to -100 mV. Induction proceeded in calcium-free medium. Cycloheximide, a potent protein synthesis inhibitor had no effect (100 micrograms/ml) on the induction rate. Isobutylmethylxanthine (IBMX) or theophyllin (phosphodiesterase inhibitors) applied externally (10(-4) M) did not affect the induction or disappearance rates. From the present results, mechanisms such as protein synthesis or a second messenger (such as calcium or cyclic AMP) do not appear to be involved. During the depolarization necessary to obtain induction, another conductance was also activated. It was more slowly established, appeared to be non-saturable and had a reversal potential between zero and -10 mV. It was found to be very much reduced at temperatures below about 16 degrees C.

Animals↗

PubMed1983

Developmental change of a depolarization-induced sodium permeability in the oocyte of Xenopus laevis.

In the full grown oocyte of Xenopus laevis, a sodium permeability can be induced by depolarization to positive potential with current injection. Voltage-clamp analysis has shown that depolarization causes a long-lasting modification of the membrane during which voltage-gated sodium channels become functional. In the present study, I have looked for the existence of these channels during cell growth. The channels appear during a very restricted period of cell growth, corresponding to Dumont stage V. A possible biological function is discussed.

Animals↗

PubMed1982

Sodium channels induced by depolarization of the Xenopus laevis oocyte.

An electrically gated Na+ channel can be made to appear in the membrane of the Xenopus laevis oocyte by simple depolarization. This membrane normally responds passively to imposed transmembrane currents with resting potentials around -60 mV, but when it is held depolarized to more than about +30 mV it becomes possible to obtain long-lasting regenerative depolarizations up to +80 mV; these depolarizations can last as long as 20 min. This potential is due to an "induction" of a Na+-dependent channel that is electrically gated open and closed. Its threshold for opening is about -20 mV and it is selective for Na+ over Cs+ and choline+ but is blocked by relatively small quantities of Li+. When a long voltage clamp step to a positive potential under ENa (+70 to +90 mV) is applied, an inward current is observed for many minutes, implying that this channel does not have an inactivation mechanism. The inward Na+ current is blocked by 0.50 mM tetrodotoxin. When the membrane is held at or near resting potential, the excitability will disappear with time, but it can be made to reappear by again depolarizing the membrane.

Action Potentials↗

PubMed1981

The rise and fall of electrical excitability in the oocyte of Xenopus laevis.

1. An electrically excited (gated) sodium selective channel has been found in the Xenopus laevis oocyte, a cell membrane previously considered non-excitable. 2. The channel is produced by prolonged depolarization of the membrane and is removed by prolonged repolarization. Both processes are very dependent on temperature and potential. 3. Once produced, the channel can be opened and closed electrically, but does not show inactivation as is found in other sodium selective channels. 4. The sodium selectivity and the electrical gating properties of this channel make it a potentially useful candidate for the study of these general channel characteristics. The fact that this membrane can be made to show these properties reversibly offers the possibility of the studying the origins of this channels.

Animals↗

PubMed1973