Changes in permeability of sea urchin egg membrane to urea after fertilization or activation.

1. Fertilization or artificial activation of the sea urchin egg, are known to increase the plasma membrane permeability to ions and small metabolites. We have examined the permeability properties of the egg membrane to urea. 2. Urea influx is not saturable and is slightly dependent on temperature. It is inhibited by phloretin. 3. Fertilization leads to a biphasic increase in urea flux kinetics. The first acceleration lasts about 10 min. The second acceleration is maintained at least up to the first cleavage. 4. The ionophore A 23187 mimics exactly fertilization. 5. Weak bases such as procaine lead to a gradual acceleration in urea flux kinetics. The response depends on the dose of procaine added. 6. The inhibitor of oxydative phosphorylation 2,4-DNP fails to block the accelerations triggered by fertilization or activation by A 23187. 7. We discuss the results in light of the early and late events of egg activation.