Fractional contribution of major ions to the membrane potential of Drosophila melanogaster oocytes

Abstract

In ovarian follicles of Drosophila melanogaster, ion substitution experiments revealed that K(+) is the greatest contributor (68%) in setting oocyte steady-state potential (E(m)), while Mg(2+) and a metabolic component account for the rest. Because of the intense use made of Drosophila ovarian follicles in many lines of research, it is important to know how changes in the surrounding medium, particularly in major diffusible ions, may affect the physiology of the cells. The contributions made to the Drosophila oocyte membrane potential (E(m)) by [Na(+)](o), [K(+)](o), [Mg(2+)](o), [Ca(2+)](o), [Cl(-)](o), and pH (protons) were determined by substitutions made to the composition of the incubation medium. Only K(+) and Mg(2+) were found to participate in setting the level of E(m). In follicles subjected to changes in external pH from the normal 7.3 to either pH 6 or pH 8, E(m) changed rapidly by about 6 mV, but within 8 min had returned to the original E(m). Approximately half of all follicles exposed to reduced [Cl(-)](o) showed no change in E(m), and these all had input resistances of 330 kOmega or greater. The remaining follicles had smaller input resistances, and these first depolarized by about 5 mV. Over several minutes, their input resistances increased and they repolarized to a value more electronegative than their value prior to reduction in [Cl(-)](o). Together, K(+) and Mg(2+) accounted for up to 87% of measured steady-state potential. Treatment with sodium azide, ammonium vanadate, or chilling revealed a metabolically driven component that could account for the remaining 13%.