Intracellular pH in early Xenopus embryos: its effect on current flow between blastomeres.

Abstract

Electrophysiological techniques were used to monitor the flow of electric current from one cell to the next in X. laevis embryos between the 4-cell and early blastula stages of development. Intracellular pH and blastocoel pH were determined using pH-sensitive micro-electrodes. The resting intracellular pH was 7.74 .+-. 0.02 (standard error of mean [SEM], n [number] = 29); there were no systematic differences between developmental stages. Blastocoel cavity pH was 8.4 .+-. 0.06 (SEM, n = 10). The intracellular value was 18 meq H+/pH unit per l. In embryos treated with bicarbonate buffered Holtfreter solution equilibrated with 100% CO2 the intracellular pH fell to 6.3 .+-. 0.17 (SD, n = 8). The membrane potential fell and the input resistance increased. The size of the effect on membrane potential and input resistance varied. From the 32-cell stage onwards current flow from one cell to the next was abolished when the intracellular pH fell below 6.5; the effect was rapid in onset and completely reversible. At cleavage stages of development lowering intracellular pH with CO2 had no effect on current flow from cell to cell. The relationship between intracellular pH and current flow from cell to cell was sigmoid and covered between 0.2 and 0.4 pH units. The pH at which current flow was completely abolished ranged from 6.85-6.4. Alterations in extraembryonic pH over the range 5.8-7.5 had no effect on any parameter measured. Lowering the intracellular pH increases the resistance of non-junctional and junctional membranes. The pH-junctional conductance relationship cannot be extracted from these data. Variations in intracellular pH may provide a useful tool for the study of the functional role of direct cell to cell communication in adult organs and early embryos.