Electrical currents through full-grown and maturing Xenopus oocytes.

Abstract

An extracellular vibrating electrode was used to map the current pattern around Xenopus laevis oocytes. Current was found to enter the animal hemisphere and leave the vegetal hemisphere; in fully grown oocytes from which the follicle cells had been removed, the maximal current density was about 1 uA/cm2. This current decreased to nearly zero in response to progesterone and several other maturation-producing agents. In the case of progesterone, the decline began within a few minutes of the addition of the hormone and proceeded with a half-time of about 20 min.An analysis of the effects on the current of the removal or addition of various ions and drugs led to the inference that the major current-carrying ion was chloride and that the chloride permeability was controlled by calcium. An ubiquitous feature of nonmammalian animal eggs is the presence of an animal-vegetal axis. The nucleus of the egg is generally displaced toward the animal pole and this pole is the site of polar body formation. In many amphibian eggs, the axis is readily distinguished by the pigment accumulation at the animal pole and the yolk mass at the vegetal pole. Superimposed on this visible polarity is a developmental polarity: the animal pole gives rise to the ectodermal parts of the embryo and the vegetal pole, to the endodermal. (For a recent review, see refs. 1 and 2.) When the full-grown anuran oocyte is exposed to progesterone, a sequence of events is triggered, leading to the reinitiation of meiosis and formation of a fertilizable egg. This process is also a polar one. The basal portion of the germinal vesicle is the first to break down as the entire germinal vesicle moves toward the animal pole (3).Our knowledge of the physical forces that might produce such asymmetries and direct polar movements remains limited. The oocytes are randomly oriented in the ovary; thus gravity is not the causal agent. Strong evidence has now accumulated in the case of fucoid algae eggs that electrical currents and ion gradients are involved in transducing environmental asymmetries into cellular polarity (4-6). Electrical currents associated with growth and localization processes have also been shown to exist in insect oocytes (7), pollen tubes (8), and regenerating Acetabularia (9). There are also a few older surface potential measurements from which the existence of polar currents in animal eggs can be inferred (10). It is against this background that this investigation of animal-vegetal currents in Xenopus oocytes and eggs was begun.METHODS AND MATERIALS Sexually mature Xenopus laevls females were obtained from the South African Snake Farm, Fish Hoek, South Africa. To obtain oocytes, females were anesthetized by chilling in an ice bath, portions of their ovaries were removed, and the oocytes were then manually dissected from their follicles. In some cases, the remaining follicle cells were removed by a 5-min treatment with Pronase (50 ;tg per ml in Steinberg's solution).A vibrating platinum electrode was used to measure extracellular currents. T...

Cholinergic and catecholaminergic receptors in the Xenopus oocyte membrane

“…It is interesting that an increase in Cl permeability has also been implicated in the generation of the activation potential in Bufo eggs (Maeno, 1959) and in ACh-induced depolarizations in mouse oocytes (Eusebi et al 1979). Furthermore, a steady polar current flow, from animal to vegetal hemispheres, occures in fully grown Xenopus oocytes and here again Cl ions act as the major current carrier (Robinson, 1979). It would be interesting to know if the channels involved in all these cases are different, or are the same but are gated by different processes.…”

Ion currents modulating oocyte maturation in animals

“…In Xenopus laevis, the ability to respond to neurotransmitters is related to chloride channels which generate a steady current in the early immature oocyte. Earlier studies by Robinson [1979] suggested that the electric field generated by chloride channels may serve to localize components in the growing oocyte and, more generally, these electrical currents might play a role in the establishment of cell polarity during oogenesis. In Xenopus, voltage-sensitive ion channels, selective for sodium and capable of generating action potentials, were shown to first appear during oocyte growth [Baud 1983].…”

A transient calcium‐dependent chloride current in the immature Xenopus oocyte.

“…The Cl-dependent egg-activation potential in Rana has been elicited with Ca ionophore (Schlichter & Elinson, 1981) and by intracellular Ca injection (Cross, 1981). And the standing current in Xenopu8 oocytes has been blocked by treatments expected to block Ca entry (Robinson, 1979).…”

“…In the amphibian oocyte recent experiments have suggested that variations in Cl permeability may be involved in a number of processes associated with the cell membrane. These include responses to acetylcholine (Xenopu8: Kusano, Miledi & Stinnakre, 1977, to adenosine (Xenopwu: Lotan, Dascal, Cohen & Lass, 1982), and to fertilization and activation (Bufo: Maeno, 1959;Ito, 1972;Rana: Cross & Elinson, 1980; Xenopws: Grey, Bastiani, Webb & Schertel, 1982), and a standing electrical current found in immature oocytes (Xenopuw: Robinson, 1979).…”