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

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.