Novel properties of the depolarization-induced endogenous sodium conductance in the Xenopus laevis oocyte.
It has been shown by means of the two-microelectrode voltage-clamp technique that in membranes of Xenopus laevis oocytes a Na+-selective permeability can be activated by long-lasting or repetitive depolarization (R.T. Kado and C. Baud, Journal of Physiology, Paris, 77:1113-1117, 1981). In this study the permeability in inside-out giant membrane patches with diameters of 20-30 microm was analysed. Once induced, the Na+ permeability has a voltage-dependent open probability that increases with positive potentials and half-maximally activates at about 0 mV. Sudden changes of membrane potential elicit transient currents with strongly voltage-dependent time constants of from less than 1 ms at -150 mV to several hundreds of milliseconds at positive potentials. In contrast to the on-cell configuration, the permeability ceases completely within a few minutes in the cell-free inside-out configuration. This rundown can be prevented by including MgATP, but not Mg2+ or ATP alone, in the intracellular solution. Intracellular Mg2+ ions, in addition to being a co-factor for ATP in the activation process, decrease the permeability in a dose-dependent manner. Steady-state fluctuation analysis gave no evidence that an increased noise level is caused by open-close kinetics of an ion channel, suggesting that the single-channel conductance is below 1 pS if a channel-like structure is the origin of the endogenous Na+ permeability.
Rettinger
J
J
Max-Planck-Institut für Biophysik/JWG-University Frankfurt, Kennedyallee 70, D-60596 Frankfurt/Main, Germany. jrett@okay.net
D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D002462:Cell Membrane / Q000502:physiology*; D002463:Cell Membrane Permeability / Q000187:drug effects; D004553:Electric Conductivity; D005260:Female; D008274:Magnesium / Q000494:pharmacology; D008564:Membrane Potentials; D009865:Oocytes / Q000648:ultrastructure*; D012964:Sodium / Q000378:metabolism*; D015222:Sodium Channels / Q000502:physiology*; D014982:Xenopus laevis
D015222:Sodium Channels; D000255:Adenosine Triphosphate; D012964:Sodium; D008274:Magnesium