. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 334 T. TOGO ET AL. 6 r. 100 200 300 400 Na* Concentration (niM) 500 Figure 5. Induction of polyspermy in low-sodium ASW. Oocytes were incubated in low-sodium ASWs for 10 min, and inseminated at R,/0 of 5 x 102 - 1 x 103. Values are the mean ± SEM of five experiments. depolarized to + ± mV (n = 5) and then repo- larized to a steady resting potential of ± mV (n = 5) at ± s (n = 5) after the onset of depolar- ization (Fig. 6A, Table I). After measurement, all the oo- cytes became normal 2-cell-stage e
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 334 T. TOGO ET AL. 6 r. 100 200 300 400 Na* Concentration (niM) 500 Figure 5. Induction of polyspermy in low-sodium ASW. Oocytes were incubated in low-sodium ASWs for 10 min, and inseminated at R,/0 of 5 x 102 - 1 x 103. Values are the mean ± SEM of five experiments. depolarized to + ± mV (n = 5) and then repo- larized to a steady resting potential of ± mV (n = 5) at ± s (n = 5) after the onset of depolar- ization (Fig. 6A, Table I). After measurement, all the oo- cytes became normal 2-cell-stage embryos (Table I). A series of oscillating spikes of depolarization (arrows in Fig. 6A) was observed after the membrane potential re- turned to a resting potential in all oocytes measured in NSW. These spikes may be due to sperm contacts with the oocyte surface, since the frequency of the spikes in- creased as the concentration of sperm was increased (data not shown). When the concentration of sodium in ASW was re- duced to 50 mAf, the resting potential of unfertilized oo- cytes was — ± mV (« = 3). The membrane of the oocytes depolarized upon insemination, but the peak value in low-sodium ASW was remarkably lower ( ± mV, n = 3) than that in NSW (Fig. 6B, Table I). All the oocytes transferred into normal ASW after the measurement in low-sodium ASW exhibited abnormal cleavage (Table I). Although we attempted to examine fertilization under current-clamp or voltage-clamp con- ditions, the experiments were unsuccessful because of the difficulty of electrode penetration. The late polyspermy block When the fertilized oocytes were heavily re-inseminated (Rs/0 = 8 X 103 - 1 X 104) at 1 min after an initial light insemination (Rs/0 = 5 X 102 - 1 X 103), the number of bound sperm was smaller () than that in the oocytes initially inseminated at the same Rs/0 (Rs/0 = 8 X 103 - 1 X 104) ( at time zero in Fig. 7). The number of bound sperm was reduced to ,
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