title: A hyperpolarization-activated ion current of amphibian oocytes.
abstract: A comparative analysis of a hyperpolarization-activated ion current present in amphibian oocytes was performed using the two-electrode voltage-clamp technique in Xenopus laevis, Xenopus tropicalis, and Ambystoma mexicanum. This current appears to be driven mainly by Cl(-) ions, is independent of Ca(2+), and is made evident by applying extremely negative voltage pulses; it shows a slow activating phase and little or no desensitization. The pharmacological profile of the current is complex. The different channel blocker used for Cl(-), K(+), Na(+) and Ca(2+) conductances, exhibited various degrees of inhibition depending of the species. The profiles illustrate the intricacy of the components that give rise to this current. During X. laevis oogenesis, the hyperpolarization-activated current is present at all stages of oocytes tested (II-VI), and the amplitude of the current increases from about 50 nA in stage I to more than 1 μA in stage VI; nevertheless, there was no apparent modification of the kinetics. Our results suggest that the hyperpolarization-activated current is present both in order Anura and Urodela oocytes. However, the electrophysiological and pharmacological characteristics are quite perplexing and seem to suggest a mixture of ionic conductances that includes the activation of both anionic and cationic channels, most probably transiently opened due to the extreme hyperpolarizion of the plasma membrane. As a possible mechanism for the generation of the current, a kinetic model which fits the data suggests the opening of pores in the plasma membrane whose ion selectivity is dependent on the extracellular Cl(-) concentration. The extreme voltage conditions could induce the opening of otherwise latent pores in plasma membrane proteins (i.e., carriers), resembling the ´slippage´ events already described for some carriers. These observations should be valuable for other groups trying to express cloned, voltage-dependent ion channels in oocytes of amphibian in which hyperpolarizing voltage pulses are applied to activate the channels.
authors: Departamento de Neurobiología Celular y Molecular, Laboratorio de Neurobiología Molecular y Celular, Instituto de Neurobiología, Campus UNAM Juriquilla, Querétaro, Qro, CP 76230, Mexico. ochoadelapaz@yahoo.com.mx
authors: L D
authors: LD
authors: Ochoa-de la Paz
authors: D B
authors: DB
authors: Salazar-Soto
authors: J P
authors: JP
authors: Reyes
authors: R
authors: R
authors: Miledi
authors: A
authors: A
authors: Martinez-Torres
mesh_terms: D000558:Ambystoma mexicanum / Q000378:metabolism / Q000502:physiology; D000663:Amphibians / Q000378:metabolism* / Q000502:physiology*; D000818:Animals; D027321:Anion Transport Proteins / Q000378:metabolism; D027682:Cation Transport Proteins / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism / Q000502:physiology; D055724:Electrophysiological Phenomena / Q000502:physiology; D006863:Hydrogen-Ion Concentration; D007473:Ion Channels / Q000378:metabolism*; D008564:Membrane Potentials / Q000502:physiology*; D009865:Oocytes / Q000378:metabolism*; D014982:Xenopus laevis / Q000378:metabolism / Q000502:physiology
chemical_list: D027321:Anion Transport Proteins; D027682:Cation Transport Proteins; D007473:Ion Channels