Voltage-Dependent Ionic Currents and Their Regulation by GTP and Phorbol Ester in the Unfertilized Eggs of Mouse and Hamster

Voltage-Dependent Ionic Currents and Their Regulation by GTP and Phorbol Ester in the Unfertilized Eggs of Mouse and Hamster

The present study was performed to investigate the properties of ionic currents elicited by voltage pulses in the unfertilized eggs of mouse and hamster by using the whole cell voltage clamp techniques and to find out if there are any differences in properties between eggs of the two rodents. In addition, the modulatory effect of G proteins and protein kinase C (PKC) on the ionic channels were observed. The inward current in hamster eggs was shown to be due to Ca²⁺ current (i_ca)). The current voltage relations of these currents in hamster egg were analogous to those in mouse eggs. The amplitude of i_ca in the hamster egg was larger than that in the mouse egg (-3.12±1.07 nA vs. -1.71±0.71 nA, mean± SD). These results suggest that the Ca²⁺ channels in both kinds of eggs have similar channel properties but their density, and/or conduct ance per unit area is higher in hamster eggs than in mouse eggs. Outward currents in eggs of both mouse and hamster were carried by K⁺. In hamster eggs, they appeared to comprise at least two components; a transient outward component (i_to) and a steady state component (i_∞ The i_to was found to be dependent on intracellular Ca²⁺ concentration; whereas on the other hand i_∞ was Ca²⁺-independent. Ca²⁺ currents were increased in eggs treated with GTP (or GTPγS) or fluoroaluminate (AIF_4^-). In the hamster egg these increments were antagonized by GDP (or GDPβS) application. In contrast to the enhancement of i_ca, i_k was reduced following GTP (or GTPγS) perfusion in mouse eggs. The transient component (i_to) in hamster eggs was increased by adding GTP but decreased by phorbol ester, TPA or dioctanoyl glycerol (DOG). Simultaneous application of GTPγS and DOG suppressed i_to more effectively than a single application or DOG or TPA. From the above results, we have shown that ionic currents elicited by voltage pulses existed in the unfertilized eggs of mouse and hamster. There are at least two types of currents, i_ca and i_k in mouse eggs, while three types, i_ca, Ca²⁺-dependent i_k and Ca²⁺-independent i_k exist in hamster eggs. ionic channels in these eggs may be regulated either directly by GTP and PKC or indirectly by the substances linked with GTP and PKC.