STEADY-STATE GRADIENT IN CALCIUM-ION ACTIVITY ACROSS THE INTERCELLULAR BRIDGES CONNECTING OOCYTES AND NURSE CELLS IN HYALOPHORA-CECROPIA

Intracellular activities of K+, H+, Mg2+, Ca2+, and Cl-, measured with ion selective microelectrodes in the oocyte and the nurse cells in ov arian follicles of Hyalophora cecropia, indicated that a Ca2+ current is a key component of the electrical potential that is maintained acro ss the intercellular bridges connecting these two cells. In vitellogen ic follicles, Ca2+ activity averaged 650 nM in the oocyte and 190 nM i n the nurse cells, whereas activities of the other ions studied differ ed between these cells by no more than 6%. Incubation in 200 mu M ammo nium vanadate caused a reversal of electrical potential from 8.3 mV, n urse cell negative, to 3.0 mV, oocyte negative, and at the same time t he Ca2+ gradient was reversed: activities rose to an average 3.0 mu M in the nurse cells and 1.6 mu M in the oocyte, whereas transbridge rat ios of the other cations remained at 0-3%. In immature follicles that had not yet initiated their transbridge potentials, Ca2+ activities av eraged similar to 2 mu M in both oocyte and nurse cells. The results s uggest that vitellogenic follicles possess a vanadate-sensitive Ca2+ e xtrusion mechanism that is more powerful in the nurse cells than in th e oocyte. (C) 1994 Wiley-Liss, Inc.