Calcium Current in the Unfertilized Egg of the Hamster

Calcium Current in the Unfertilized Egg of the Hamster

The presence of a calcium current (i_Ca²⁺) passed via a specific channel was examined in the unfertilized hamster egg using the whole-cell voltage clamp technique. Pure inward current was isolated using a Ca²⁺-rich pipette solution containing 10 mM TEA. This current was independent of external Na⁺ and was highly sensitive to the Ca²⁺ concentration in the bathing solution, indicating that the inward current is carried by Ca²⁺. The maximal amplitude was -4.12±0.58nA (n=12) with 10mM Ca²⁺ at -3OmV from a holding potential of -8OmV. This current reached its maximum within 20ms beyond -3OmV and decayed rapidly with an inactivation time constant τ of 15ms. Activation and inactivation of this i_Ca²⁺ was steeply dependent on the membrane potential. The i_Ca²⁺ began to activate at the lower voltage of -55 mV and reached its peak at -35 mV, being completely inactivated at potentials more positive than -40 mV. These result suggest that i_Ca²⁺ in hamster eggs passes through channels with electrical properties similar to low voltage-activated T-type channels. Other results from the present study support this suggestion; First, the inhibitory effect of Ni²⁺ (IC₅₀=13.7 μM) was more potent than Cd²⁺ (IC₅₀=123 μM). Second, Ba²⁺ conductance was equal to or below that of Ca²⁺. Third, i_Ca²⁺ in hamster eggs was relatively insensitive to nifedipine (IC₅₀=96.6 μM), known to be a specific t-type blocker. The physiological role of i_Ca²⁺ in the unfertilized hamster eggs remains unclear. Analysis from steady-state inactivation activation curves reveals that only a small amount of this current will pass in the voltage range (-70 ~ -30 mV) which partially overlaps with the resting membrane potential. This current has the property that it can be easily activated by a weak depolarization, thus it may trigger a certain kind of a intracellular event following fertilization which may cause oscillations in the membrane potential.