Increased uptake of thymidine in the activation of sea urchin eggs: Specificity of uptake and dependence on internal pH, the cortical reaction, and external sodium

Abstract

The uptake of thymidine in sea urchin eggs is considered in terms of its specificity, the cortical reaction, and the increase of intracellular pH following fertilization. The rate of uptake increases greater than 50-fold after fertilization. All deoxyribonucleosides and ribonucleosides tested compete with thymidine for transport sites. Free pyrimidine and purine bases, deoxyribonucleotides, and amino acids do not compete, showing that the specificity of this uptake lies at the nucleoside level. Uptake may be turned on in unfertilized eggs by treatment with ammonia, a treatment known to by-pass the cortical reaction and raise intracellular pH. However, when compared with uptake in fertilized eggs, it proceeds later and at a lower rate. Both of these deficiencies are overcome by fertilizing the ammonia-treated eggs or treating them with butyric acid or ionophore A23187. These treatments induce the cortical reaction and stimulate an immediate and complete turn-on of thymidine uptake. Superseding these apparent involvements of the cortical reaction and mtracellular pH in thymidine uptake is an extremely strict requirement for extracellular Na +.