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

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Abstract
The zona-free hamster egg allows multiple entries of heterologous as well as homologous sperm. The hamster egg inseminated with mouse sperm (M .times. H egg) showed recurring, transient hyperpolarizing responses (h r) 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. Unlike the hamster sperm, the cessation of flagellar motion of the 1st mouse sperm (1-stop) failed to induce the 1st h r but produced only a small hyperpolarizing step of 3-7 mV. Similar steps occurred for each of additional sperm with a 1-to-1 correspondence, 4-50 s ahead of the cessation of sperm motion. In M .times. H eggs, the h r first appeared .apprx. 15 min after the 1-stop. The intervals of the h r thereafter were in the range between 2 and 10 min, in contrast to 30-45 s in hamster eggs inseminated with hamster sperm (H .times. H eggs). The h r in M .times. H eggs were abolished by intracellular injection of EGTA [ethyenoglycol bis-(.beta.-amino-ethyl-ether)-N,N,N'',N''-tetraacetic acid], suggesting that they were caused by a periodic increase in the intracellular Ca2+ concentration ([Ca2+]i) as in H .times. H eggs. The gradual h s in M .times. 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. The resumption of the 2nd meiosis, the indication of egg activation, is delayed in M .times. H eggs by .apprx. 15 min, compared with that in H .times. H eggs. There was a good correlation between the delay of activation and that of the occurrence of the 1st h r. In M .times. H eggs, the probability of egg activation within 70 min was dependent on the number of sperm penetrations: 90% for more than 10 sperm while 20-30% for less than 5 sperm. Eggs in which sperm penetration was not followed by activation showed no h r. The mouse egg inseminated with mouse sperm showed small h r (3-4 mV) superimposed on the h s from -35 to -55 mV in 50 min after insemination. Both h r and h s were associated with an increase in the membrane conductance. The h s was considered to be due mainly to a Ca2+ into the unfertilized mouse egg could not increase the K conductance with injection currents up to 4 nA. The h r were suggested to be resulted from a periodic increase in [Ca2+]i, since they were abolished by injection of EGTA.