Changes in ATP utilization and plasma membrane surface area during the meiotic divisions
The decrease in kf shown in Figure 4 indicates a drop in cytosolic ADP concentration and an increase in the cytosolic phosphoryl potential of the oocyte prior to nuclear membrane breakdown (4 - 6 h). The decreased availability of ADP, normally generated in large part by hydrolysis of ATP during ion-transport by Na/K-ATPase, could arise from the observed internalization of the plasma membrane Na/K-ATPase (Na-pump) over the same time period [2]. This seems probable, based on our data showing a simultaneous 50 - 60% decrease in membrane capacitance (cell surface area), increased endocytosis [18], and the disappearance of more than 95% of the high affinity [3H]ouabain binding sites in progesterone-treated denuded Rana pipiens oocytes over the same time period [2].
[3H]ouabain specifically binds to the α-subunit of the Na/K-ATPase (reviewed in [2,3]). By 6 h the [3H]ouabain is recovered bound to intracellular vesicles of progesterone-treated oocytes; it does not diffuse out of the progesterone-treated denuded oocytes [2]. Both scanning [18] and transmission (Figure 1) electron micrographs indicate that the prophase oocyte is covered with numerous microvilli. Estimates of the surface area in control oocytes reveal that the microvilli increase the oocyte surface area 10-12 times compared with that of a sphere of the same diameter [18]. Scanning electron micrographs reveal that the microvilli of progesterone-treated oocytes disappear coincident with the decrease in membrane capacitance, leaving only stumps on the oocyte surface [18].