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
