Dual ionic controls for the activation of protein synthesis at fertilization

The general metabolic activation of the sea urchin egg at fertilization is dependent on a release of intracellular stores of calcium and the subsequent transient elevation of intracellular Ca2+ (refs 1--3). However, this elevation does not by itself lead to increased macromolecular synthesis and development but initiates steps which result in a long-term elevation of intracellular pH (refs 4--6). Among the developmental processes dependent on the elevation of intracellular pH is the large acceleration in the rate of protein synthesis at fertilization. Weak penetrating bases such as ammonia can be used to mimic the processes resulting in an increase in intracellular pH and so show the corresponding increases in protein synthesis rate. Conversely, it is possible to demonstrate a gradual but complete shut down of protein synthesis if the intracellular pH is reduced to the unfertilized level with penetrating weak acids. However, the rate of protein synthesis in ammonia-activated eggs lags behind that of fertilized controls even though ammonia activation can result in an intracellular pH increase greater than occurs in the fertilized egg. This result has led to the suggestion that factors other than intracellular pH may be regulating protein synthesis following fertilization. To investigate the possibility that the Ca2+ transient may have such a role, we measured the rate of amino acid incorporation in eggs that were activated in various ionic conditions which enabled the effects of Ca2+ and pH changes to be studied separately. Our results, reported here, show that if intracellular pH is elevated, increases in intracellular Ca2+ play an additional part in the activation of protein synthesis at fertilization.