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Voltage-clamp experiments in full-grown frog oocytes, in a range of membrane potentials from 90 mV negative to 30 mV positive, have revealed the presence of voltage-dependent channels selective for K+, blocked by extracellular TEA. The percentage of open K+-channels increases with membrane depolarizations over a range from -40 mV to +10 mV, thus supporting the outward rectification in the I/V relationship. The current transport through the K+-channels open at different potential levels and in various [K+]o takes place in accordance with the constant-field assumptions. The leakage current of the oocyte membrane was found to be considerable large.

Variations of [K+]o have been found to influence the membrane potential in frog full-grown oocytes at a greater extent than other modifications in extracellular medium composition; however, the relationship between membrane potential and [K+]o was far from that expected for an ideal K electrode. The I/V relationship, as determined in current clamp conditions, displays an evident "inward-going" rectification at potentials more negative than -80, -90 mV as well as an evident "outward-going" rectification at potentials more positive than -20, -10 mV, thus suggesting the presence of voltage-dependent channels in this membrane. In the linearity range of the I/V relationship, Rm was found to be about 3 + 15 K omega cm2 and Cm about 20 + 30 microF/cm2.

The I/V relationship of frog oocytes is markedly modified following maturation, lacking the mature unfertilized egg cell membrane of K-selective channels and the leakage being strongly reduced. The only kind of voltage-dependent channels still present in this membrane are activated by depolarizing steps to potentials more positive than 30-40 mV; the current flowing through these channels is carried at least partly by Na+. At difference with the immature oocyte, where several ionic mechanisms are oriented to maintain the membrane potential at a constant level, the mature egg-cell admits considerable voltage deflections. The observed variations in membrane properties could be of importance in allowing the maturation process and/or in preparing the mature egg-cell to successful fertilization.

In addition to K-selective channels, two kinds of voltage-dependent channels are present in the membrane of full-grown frog oocytes: 1) non-selective channels, activated by hyperpolarizing steps to potentials more negative than -80, -90 mV; 2) cationic channels, activated by depolarizing steps to potentials more positive than +30, +40 mV. The former are related to the inward rectification; the latter could contribute to the outward rectification of the I/V relationship. Therefore the membrane potential in full-grown oocytes tends to be maintained at a constant value due to the changes of permeability that follow possible potential variations occurring in both directions.