Calcium-activated potassium-channels in mammalian eggs

Abstract

Intracellular microelectrode recordings from zonafree hamster eggs revealed a mean value for the membrane potential of -34+9mV (mean ± SD, n=36) and a mean value for the input resistance of 290±170MΩ.(mean ± SD) .Ionophoretic injection of calcium ions into a hamster egg caused a marked membrane hyperpolarization (mean ± SD = 27±9mV, n=63) which was associated with a large reduction in the egg input resistance (viz. from 140±94MΩ prior to the injection to 40±24MΩ at the peak of the response, n=63). The estimated reversal potential of the calcium-evoked hyperpolarization was found to be -69±11mV (mean ± SD,n=63). The mean slope of the relation between the reversal potential and log₁₀[ᴷ⁺]ₒ was 50mV in good agreement with the value predicted by the Nernst equation for a rise in conductance primarily selective for potassium ions. In addition the reversal potential was unaffected by about a tenfold reduction in the concentration of external chloride. It is concluded that the calcium-evoked hyperpolarization was caused by the opening of membrane potassium channels that are activated by an increase in the intracellular concentration of ionized free calcium.The duration of the calcium-evoked response was markedly extended in the presence of ImM external lanthanum or 20mM calcium or by external application of 0.2-2mM 2,4,dinitrophenol.Double microelectrode experiments identified the presence of an impalement leak artifact which might cause an underestimate of the true membrane potential and input resistance. Evidence is also presented for a second type of artifact, namely the generation of a leak conductance pathway during the passage of large depolarizing current pulses (>10nA, 1 sec) used for the ionophoretic injection of calcium into cells.