Low-Voltage Activated Ca²⁺ Current Carried via T-Type Channels in the Mouse Egg

Low-Voltage Activated Ca²⁺ Current Carried via T-Type Channels in the Mouse Egg

Most of voltage operated Ca²⁺ channels can be divided into three types (T-, N-, and L-type), according to the electrical and pharmacological properties. Their distribution is closely related to cell specific functions. Properties of the voltage activated Ca²⁺ current in mouse eggs were examined to classify channel types and to deduce the function by using whole cell voltage clamp technique. Ca²⁺ currents appeared below -40 mV and reached a maximum at -15 mV (half maximum was -31 mV), then decayed rapidly (inactivation time constant τ=28.2±9.59 ms at -10 mV within 50 ms after the onset of step depolarization. Activation and inactivation of the Ca²⁺ channel was steeply dependent on voltage, in a relatively low range of -70 mV ~ -10 mV, half maximum of activation was -31 mV and that of inactivation was -39 mV, respectively. This current was not decreased significantly by nifedipine, a specific dihydropyridine Ca²⁺ channel blocker in the range of 1 μM to 100μM. The inhibitory effect of Ni²⁺ on Ca²⁺ current was greater than that of Cd²⁺. The conductance of Ba²⁺ through the channel was equal to or lower than that of Ca²⁺ These results implied that Ca²⁺ current activated at a lower voltage in the mouse egg is carried via a Ca²⁺ channel with similar properties that of the T-type channel.