In Xenopus embryos, the successive and rapid cell divisions that follow fertilization are accompanied by periodic oscillations of intracellular pH (pHi). Cycling of pHi occurs in phase with several other oscillatory activities, namely nuclear divisions, M phase-promoting factor (MPF) activity, and surface contraction waves (SCWs). We report that treatments that abolish cycling of MPF activity and the SCWs also suppress the pHi oscillations, whereas those that block cell division without affecting neither MPF activity nor the SCWs do not suppress the pHi oscillations. Experiments on enucleated oocytes, matured in vitro and activated, demonstrated that the activity governing the rhythmicity of the pHi oscillations resided in the cytoplasm of the oocyte. In this respect, the activity responsible for the pHi oscillations was different from that which drives the SCWs, which necessitated the presence of the oocyte germinal vesicle (Ohsumi et al., 1986), but more closely resembled MPF activity that did not require the presence of the oocyte germinal vesicle (Dabauvalle et al., 1988). In mature eggs enucleated at the time of egg activation, the pHi oscillations were similar to those in control nucleated eggs, whereas the period between two peaks of SCWs was 35-60 min vs. 20-35 min in nucleated control eggs. Previous studies had shown that the periodicity of SCWs was larger in anucleate egg fragments than in their nucleate counterparts (Sakai and Kubota, 1981), the difference being on the order of 6-15 min (Shinagawa, 1983). However, in these previous studies, enucleation was performed 30-50 min after fertilization. Our results clearly demonstrate that the periodicity of the SCWs is lengthened when the interval between egg activation and enucleation is shortened, thereby providing an easier way to assess the nuclear dependency of the SCWs. Finally, the various possibilities concerning the role of pHi cycling during cell division are discussed.

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