Periodic hyperpolarizing responses in hamster and mouse eggs fertilized with mouse sperm

Abstract

1. The zona-free hamster egg allows multiple entries of heterologous as well as homologous sperm. The hamster egg inseminated with mouse sperm (M × H egg) showed recurring, transient hyperpolarizing responses (h.r.s) with the peak of -70 to -80 mV. They were superimposed on a hyperpolarizing shift of the resting potential (h.s.) which gradually reached -60 mV in 50 min after insemination.

2. Unlike the hamster sperm, the cessation of flagellar motion of the first mouse sperm (`1-stop') failed to induce the first h.r. but produced only a small hyperpolarizing `step' of 3-7 mV. Similar steps occurred for each of additional sperm with a one-to-one correspondence, 4-50 sec ahead of the cessation of sperm motion.

3. In M × H eggs, the h.r. first appeared about 15 min after the `1-stop'. The intervals of the h.r.s thereafter were in the range between 2-10 min, in contrast to 30-45 sec in hamster eggs inseminated with hamster sperm (H × H eggs).

4. The h.r.s in M × H eggs were abolished by intracellular injection of EGTA, suggesting that they were caused by periodic increase in the intracellular Ca²⁺ concentration ([Ca²⁺]_i) as in H × H eggs.

5. The gradual h.s. in M × H eggs was considered to be due mainly to an increase in Ca-independent K permeability, since the resting potential beyond -60 mV at 50-70 min after insemination was changed by only 3-5 mV on the removal of Cl ions and on EGTA injection.

6. Histological observations revealed that the resumption of the second meiosis, the indication of egg activation, is delayed in M × H eggs by about 15 min, compared with that in H × H eggs. There was a good correlation between the delay of activation and that of the occurrence of the first h.r.

7. In M × H eggs, the probability of egg activation within 70 min was dependent on the number of sperm penetrations: 90% for more than ten sperm while 20-30% for less than five sperm. Eggs in which sperm penetration was not followed by activation showed no h.r.s.

8. The mouse egg inseminated with mouse sperm showed small h.r.s (3-4 mV) superimposed on the h.s. from -35 to -55 mV in 50 min after insemination. Both h.r.s and h.s. were associated with an increase in the membrane conductance. The h.s. was considered to be due mainly to a Ca-independent increase in K permeability.

9. Iontophoretic injection of Ca²⁺ into the unfertilized mouse egg could not increase the K conductance with injection currents up to 4 nA. However, the h.r.s were suggested to be resulted from a periodic increase in [Ca²⁺]_i, since they were abolished by injection of EGTA.