Changes in ion permeability and membrane potential during early echinoderm development: Electrophysiological and tracer‐flux determinations

After fertilization the membrane potential of the Asterias embryo undergoes an approximately fourfold increase in internal negativity, as measured by intracellular microelectrodes. Our previous experiments have demonstrated that the ionic basis of the membrane potential in this embryo may be described by a simplified form of the constant field equation. Using this relationship, the present experiments illustrate that the relative permeability of Na to K (P_Na/P_K) may be determined at any time during the early cleavage of this embryo. The results of these experiments indicate a relative increase of P_K as related to P_Na as the mechanism responsible for the large change in membrane potential. This has been confirmed by independent determination of absolute values for P_Na and P_K by means of ²⁴Na and ⁴²K tracer‐flux analysis. Experiments carried out during early cleavage demonstrate a greater than two fold increase in K permeability (3.8 × 10⁻⁷ cm/sec to 9.5 × 10⁻⁷ cm/sec) with no concurrent alteration in Na permeability (0.1 × 10⁻⁷ cm/sec). P_Na/P_K in 15 minute and two hour post‐fertilized embryos is 0.03 and 0.01 respectively. Its magnitude is the same whether determined by the constant field assumption or from tracer‐flux analysis. These changes in ionic permeability occur on a time scale which is similar to that of increased metabolic and synthetic activities in other echinoderm embryos. A possible regulatory role in the early differentiation of this embryo is discussed.