Developmental changes in membrane transport and permeability in the early mouse embryo

Electrophysiological and isotope tracer flux experiments were performed to measure the membrane ion permeability and transport properties of the two-cell mouse embryo. The results show that the internal exchangeable Na and K are 151 and 130 mM, respectively, and their membrane permeabilities are (P_Na) 16 × 10^-8 cm sec^-1 and (P_K) 21 × 10^-8 cm sec^-1. These values predict a membrane potential of -24 mV (inside negative) which agrees well with -19 mV measured with microelectrodes. Ouabain-sensitive isotope fluxes demonstrate a Na/K pump mechanism with a stoichiometry of 1.7:1 (Na:K). An external-Na-dependent Na efflux is demonstrated by the reduction of unidirectional Na efflux in Na-free medium, but there is no evidence for a similar mechanism of K efflux at this stage of development. These results are compared with the values reported for the mouse oocyte [Powers, R. D., and Tupper, J. T. (1974). Develop. Biol. 38, 320-331; (1975). Exp. Cell Res. 91, 413-421]. The hyperpolarization of the membrane potential as compared with the oocyte (-13 mV) results primarily from the increased P_K P_Na ratio. A similar phenomenon has been noted in other developing embryos. The increase in pump-mediated K influx at the two-cell stage is accompanied by a decrease of similar magnitude in the external-K-dependent K efflux which is found in the oocyte. This suggests that the KK exchange mechanism may be converted to an active pump. Because of the changes in ion concentrations and movements and the unusual metabolic requirements of the mouse embryo, the effect of external Na on the uptake of glucose and pyruvate in the oocyte and two- and eight-cell stage was examined. No Na-dependent carbohydrate transport could be found at these stages.