Intracellular ionic distribution, cell membrane permeability and membrane potential of the Xenopus egg during first cleavage

Abstract

SUMMARYPreviously we have presented evidence for a direct relationship between post-mitotic new membrane formation and changes in the electrical membrane characteristics during cytokinesis of Xenopus eggs [l, 21. In the present study the phenomena underlying the hyperpolarization of the electrical membrane potential during cytokinesis were investigated. Total Na+ and K+ contents at the onset of the first and second cleavage were measured independently by flame spectophotometry and by means of ion-selective electrodes. Total Cl-content was measured by the latter method only. The water content was determined from the difference between wet weight and dry weight. 3H,0-influx experiments yielded an independent estimate of the water content (0.737 ,ul/egg), a rate constant for the influx of 1.412 x 10e3 set,-r and a low water permeability of 1.87 x 1O-5 cm set-I. They furthermore revealed the absence of intracellular water compartmentation. The Naf, K+ and Cl-concentrations remained constant during first cleavage at 58.6, 87.3 and 62.6 mM/l cell water, respectively. The membrane potential (E,), the membrane resistance (R,), and the intracellular ion activities of Na+, K+ and Cl-(a;,, ak and a$ were measured simultaneously and continuously during cleavage, using conventional glass microelectrodes and ion-selective microelectrodes. a' na showed an increase from 19.4 to 22.4 mM concomitant with the hyperpolarization of Em and the decline of R,. a; and a& remained constant at 51.4 and 53.1 mM, respectively. From the calculated activity coefficients it was concluded that all Cl-ions were free, whereas 30 % of the Kf ions and 60 % of the Naf ions were bound. The influence of changes in the ionic composition of the medium on E,,, was analysed in the uncleaved egg, in normally cleaving eggs, and in eggs cleaving outside the vitelline membrane. The latter conditions leads to exposure of the whole area of newly formed membrane to the medium. The cell membrane of the uncleaved egg exhibits no permselectivity. In normally cleaving eggs the relative permeability P&Px was 0.73, while in eggs cleaving outside the vitelline membrane it was 0.19. It was concluded that the changes of E,,, during cytokinesis are due to the insertion of a part of the newly formed membrane into the cell surface. The K+ permeability of this new membrane is at least five times greater that that of the pre-existing membrane. The possible role of the hyperpolarization of E,,, in the regulation of the cell cycle is briefly discussed.In previous reports [2, 31 the characteristic changes of the electrical membrane potential (E,) and the electrical membrane resistance (R,) during the first cytokinetic cycle of Xenopus laevis eggs were described. A temporal and causal relationship was found between these changes and new membrane formation. Ihe electrical membrane characteristics start to change at 6-8 min from the onset of the cleavage. From that time on morphological signs of the incorporation of new membrane material in the bottom of the cleavage furrow are ...

Cholinergic and catecholaminergic receptors in the Xenopus oocyte membrane

“…Table 1 illustrates a representative set of results. These results are similar to those obtained from Rana pipiens oocytes (Morrill, 1965) and Xenopus oocytes (De Laat, Buwalda & Habets, 1974).…”

The distribution of sodium, potassium and chloride in the nucleus and cytoplasm of Bufo bufo oocytes measured by electron microprobe analysis.

“…The present studies thus confirm previous studies which indicate the presence of an internal fraction of Na in the oocyte which is inexchangeable or very slowly exchangeable with external Li (Dick & Lea, 1964;Dick & McLaughlin, 1969;Dick et al 1970;De Laat et al 1974;Dick & Fry, 1973. This fraction lies in the cytoplasm but is readily dialysable against either Na or Li (Dick & Fry, 1973).…”

“…It may also be noted that a K-sensitive glass micro-electrode in most cells detects essentially all the cell K and thus behaves similarly to the electron microprobe (Lev & Armstrong, 1976), although insome frog musclesthere is evidence of a sequestered fraction of K (Lee & Armstrong, 1974). In amphibian oocytes, reported undetected K has varied from zero (Dick & McLaughlin, 1969) to 30 % (De Laat, Buwalda & Habets, 1974).…”

Observations on intracellular pH during cleavage of eggs of Xenopus laevis.

“…Arguing against this are those experiments in which the cytoplasm was held at artificially low pH values without disrupting cell division . It seems more likely that any pH change is a secondary effect related to the cycle of intracellular sodium activity (4,29) or the insertion of highly K-permeable new membrane during furrowing (28,40).…”