Ca²⁺-activated K⁺ currents were recorded from unfertilised and fertilised golden hamster
eggs, using conventional intracellular recording techniques. Activation of the currents was
accomplished by various treatments producing a rise in the intracellular free calcium
concentration ([Ca²⁺]ᵢ;). The resulting hyperpolarisations of the membrane were investigated
using Ca²⁺ extrusion blockers such as La³⁺, quercetin, 2,4-Dinitrophenol, high pH₀ solution and
Na⁺-free solution, to alter the time-course of the hyperpolarisations.
Iontophoretic injection of Ca²⁺ and Sr²⁺ produced a transient hyperpolarisation of the
hamster egg membrane accompanied by an increase in membrane conductance, mediated by a
Ca²⁺-activated K⁺ current. The effects of La³⁺, quercetin, high pH₀ solution and Na⁺-free solution
in prolonging the hyperpolarisation suggest that both a Na⁺-Ca²⁺ exchange system and an active
Ca²⁺ pump are responsible for the recovery phase of the Ca²⁺-evoked hyperpolarisations.
A combination of high pH₀ and high [Ca²⁺]₀ solution was found to evoke a sustained
hyperpolarisation of the membrane, with an estimated value of the reversal potential of about 85 mV. The membrane potential changed linearly with log [K⁺]₀ with a slope of 43 ± 2 mV
(mean ± S.D., n=4) for a 10-fold change in [K⁺]₀, while it was unaltered by the removal of CI
from the solution. The amplitude of the pH₀-induced hyperpolarisation decreased substantially
when [Ca²⁺]₀ was lowered from 20 to 1 mM and with addition of TEA. Injection of EGTA into
the egg prevented the pH₀-induced hyperpolarisation, suggesting that a rise in [Ca²⁺]ᵢ is required.
The high pH₀ does not produce a large increase in pHᵢ and the onset of the response is rapid,
compatible with an external site of action.
A transient hyperpolarisation and increase in conductance could be evoked after a long
latency (ca. 9 s) by a single Ca²⁺ action potential in unfertilised hamster eggs. The estimated
reversal potential was close to Ek. A second action potential elicited soon after the first did not
induce a similar response. A number of treatments (insertion of a Ca²⁺ micro-pipette, aplication
of Na⁺-free solution, La³⁺or high pH₀) likely to raise [Ca²⁺]ᵢ also induced similar large
hyperpolarisations, after which a single Ca² action potential failed to evoke a large delayed
hyperpolarisation. This suggests that a small rise in [Ca²⁺]ᵢ activates a slow process leading to a
further large increase in [Ca²⁺]ᵢ.
The fertilisation potential of hamster eggs, consisting of transient, repetitive
hyperpolarisations was recorded using intracellular recording techniques. The duration of
hyperpolarisations were prolonged by La³⁺, high pH₀ and Na⁺-free solution. Low pH₀ decreased
the duration. The frequency of the hyperpolarisations was also reduced by Co²⁺ and La³⁺.
The first single channel recordings from the hamster egg were made, using the excised
inside-out patch conformation. Evidence of a channel activated by intracellular Ca²⁺ was
obtained. This channel had a reversal potential at 0 mV in symmetrical KCl, and displayed
outward rectification over the potential range - 60 - + 60 mV. The conductance was 65.3 ± 12.9
pS at depolarised potentials and 23.4 ± 11.8 pS (n=4) at hyperpolarised potentials.
