Oöplasmic pH and electrical properties of developing locust (Locusta migratoria) eggs

Oöplasmic pH and the electrical potential difference across the egg coverings (PDegg) were measured in developing locust (Locusta migratoria) eggs with double-barrelled pH microelectrodes. Successful microelectrode impalements were obtained up to the ninth day of development, the day before hatching at 37°C. PDegg for eggs bathed in control saline changed substantially during development, from a minimum of −23 mV in day-1 eggs to a maximum of −78 mV in day-3 eggs, whereas oöplasmic pH remained near 7.2–7.3 before fertilization and up to day 5. Dechorionation reduced PDegg in day-3 eggs by only 5 mV. Input resistance decreased from 1.2 MΩ in chorionated eggs before fertilization to less than 0.2 MΩ in day-1 to day-5 eggs. PDegg changed by up to 19 mV for a 10-fold change in bathing saline Cl− concentration, but less than 5 mV for a 10-fold change in the concentration of Na+ or K+. Experiments with internally perfused eggs indicated that the Cl−-dependent component of PDegg is developed primarily across the serosal cuticle, and that up to 28% of PDegg consists of a second potential developed across the serosal epithelium. Contribution of a metabolic pump to the latter component was suggested by the effects of chilling or anoxia on PDegg in intact eggs. Acidification of egg oöplasm by 0.2 pH units in hypercapnic bathing saline (7% carbon dioxide) was associated with a 66% hyperpolarization of PDegg. The results suggest that oöplasmic pH increases as ambient pCO2 declines during oviposition, and that an electrogenic proton pump contributes to both PDegg and oöplasmic pH regulation.