Potassium and chloride dependence of the membrane potential of vitellogenic follicles of Sarcophaga bullata (Diptera)

Abstract

The membrane potential of the oöcyte and the nurse cells of the vitellogenic follicles of the fleshfly Sarcophaga bullata has been measured with microelectrodes. During the early (4A) vitellogenic stages, the membrane potential of the oöcyte is more positive than that of the nurse cells, both being interconnected by ring canals or cytoplasmic bridges. A possible role for this potential gradient in the physiology of the developing oöcyte is discussed. The gradient is not sustained during the further (4B, 4C) vitellogenic stages. The dependence of the membrane potential on K + and Cl − has been examined in several physiological salines. A slope of 39–48 mV for a 10-fold change in [K +] 0, depending on the exact experimental conditions and on the cell type, is demonstrated. In salines without Cl −, the potential gradient between oöcyte and nurse cells in the 4A stage disappears. This suggests that Cl − is of prime importance in the generation of the potential gradient. A model is proposed which shows that a transfollicular ionic current can be generated by an asymmetric distribution of ion channels.