Intracellular pH and the sodium requirement at fertilisation

Abstract

Several lines of evidence suggest that ionic messengers are primary agents in the metabolic derepression which occurs at fertilisation. The derepression at fertilisation or partheno-genetic activation of the sea urchin egg occurs in two main phases. The first phase, which triggers the early events of fertilisation, is mediated by transitory increase of intracellular free calcium^1–3. The second, which triggers the late events of fertilisation, is mediated by a rise in the intracellular _pH (refs 4–6). The transition from the early events of fertilisation of sea urchin eggs to the late events requires a minimal concentration of sodium in the external medium⁷. External Na⁺ is required for the acid efflux which follows fertilisation⁸. Na⁺ requirement and the acid efflux have been correlated in a hypothesis which proposes that internal protons are exchanged for external Na⁺ (refs 8, 9). By using _pH-sensitive microelectrodes, we have examined the relationship between external Na⁺ and internal _pH more closely. We demonstrate here that the increase of the intracellular pH following egg activation does require external Na⁺. However, the relative insensitivity of the alkalisation of the egg cytoplasm to large reductions of external Na⁺ is evidence against the Na–H exchange hypothesis.