Calcium‐evoked Opening of Potassium Channels in Hamster Eggs

Abstract

SUMMARYMeasurements of membrane potential and resistance have been made in zona-free hamster eggs. The resting potential lay in the range -9 to -100 mV and the input resistance fell in the range 14 to 440 MQ; high resting potentials were associated with large input resistances. Calcium injected ionophoretically into an egg from an intracellular micro-electrode caused a reduction of the membrane resistance. The estimated reversal potential for the calcium-evoked response was about -80 mV and its amplitude depended on the extracellular concentration ofpotassium but not on the chloride concentration. We conclude that membrane potassium channels open in response to a rise in the cytosolic concentration of calcium ions. Evidence is presented to suggest that micro-electrode recordings of the membrane potential and resistance of eggs suffer from an impalement leak artifact. The presence of the artifact lowers the resting potential and resistance of the cell so that intracellular calcium injection causes a hyperpolarization. We conclude that a hyperpolarizing response to calcium would be unlikely in the absence of an impalement artifact.

Calcium Action Potentials in Unfertilized Eggs of Mice and Hamsters

“…This suggests that the relative sodium permeability of the mouse egg is not Previous electrophysiological studies (Powers & Tupper, 1974;Okamoto et al 1977;Miyazaki & Igusa, 1982;Georgiou et al 1983) have reported a slight influence of potassium ions on the resting potential ofmouse and hamster eggs. However, these experiments were carried out on eggs depolarized by impalement-leak artifacts and therefore it was necessary to re-examine the effect of potassium on potential and resistance.…”

“…Moreover, when a second micro-electrode is inserted into a hamster or mouse egg after the high potential and resistance state is attained there is always a transient fall both in the potential and the resistance of the egg (Fig. 4, Georgiou et al 1983; Fig. 1, present study) as would be expected for a transient leak pathway at the micro-electrode tip.…”

On the mechanism of a pH‐induced rise in membrane potassium conductance in hamster eggs.

“…Although the presence of Ca2+-activated K+ channels has been demonstrated in mammalian eggs (e.g. Miyazaki & Igusa, 1983;Georgiou et al 1983) Cl-channels also activated by calcium exist in Xenopus oocytes (Barish, 1982;Miledi & Parker, 1984) and so it was necessary to test for the latter type in hamster eggs. Methane sulphonate is a useful replacement anion for Cl-as it is impermeant and does not alter intracellular pH (Sharp & Thomas, 1981).…”

“…Voltage-clamp studies (Okamoto, Takahashi & Yamashita, 1977) and intracellular recordings (Miyazaki & Igusa, 1982;Georgiou et al 1983) The mean maximum rate of rise at pH. = 7-5 was 1P7+0-2 V/s (n = 12).…”

“…Each hyperpolarizing response results from an increase in the membrane K+ conductance activated by a rise in the intracellular free Ca2+ concentration ([Ca2+]i) (Miyazaki & Igusa, 1982). lonophoretic injection of Ca2+ into eggs will activate this K+ conductance, producing a transient hyperpolarization of the membrane (Miyazaki & Igusa, 1982;Georgiou, Bountra, Bland & House, 1983). The recovery phase of this transient hyperpolarization is prolonged by the application of La3+, high [Ca2+]o or quercetin (Georgiou, Bountra, McNiven & House, 1987a) which are known to inhibit the Ca2+ pump in red blood cells, mouse fibroblasts and toad retinal cells (Sarkadi, Szasz, Gerlocozy & Gardos, 1977;Okada, Tsuchiya & Yada, 1982;Cervetto & MeNaughton, 1986).…”