FRACTIONAL CONTRIBUTION OF MAJOR IONS TO THE MEMBRANE POTENTIAL OF Drosophila melanogaster OOCYTES

In ovarian follicles of Drosophila melanogaster, ion susbtitution experiments revealed that K+ is th greates contributor (68%) in setting oocyte steady-state potential (E-m), while Mg2+ 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), [Mg2+](o), [Ca2+](o), [Cl-](o), and pH (protons) were determined by substitutions made to the composition of the incubation medium. Only K+ and Mg2+ 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 pH8, E-m changed rapidly by about 6mV, 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 resistance of 330k Omega or greater. The remaining follicles had smaller input resistance, and these first depolarized by about 5mV. Over several minutes, their input resistance increased and they repolarized to a value more electronegative than their valve prior to reduction in [Cl-](o). Together, K+ and Mg2+ accounted for up to 87% of measured, steady-state potential. Treatment with sodium azide, ammonium vandate, or chilling revealed a metabolically driven component that could account for the remaining 13%. (c) 2009 Wiley Periodicals, Inc.