Steady‐state gradient in calcium ion activity across the intercellular bridges connecting oocytes and nurse cells in Hyalophora cecropia

Intracellular activities of K + , H + , Mg 2+ , Ca 2+ , and Cl − , measured with ion selective microelectrodes in the oocyte and the nurse cells in ovarian follicles of Hyalophora cecropia , indicated that a Ca 2+ current is a key component of the electrical potential that is maintained across the intercellular bridges connecting these two cells. In vitellogenic follicles, Ca 2+ activity averaged 650 nM in the oocyte and 190 nM in the nurse cells, whereas activities of the other ions studied differed between these cells by no more than 6%. Incubation in 200 μM ammonium vanadate caused a reversal of electrical potential from 8.3 mV, nurse cell negative, to 3.0 mV, oocyte negative, and at the same time the Ca 2+ gradient was reversed: activities rose to an average 3.0 μM in the nurse cells and 1.6 μM in the oocyte, whereas transbridge ratios of the other cations remained at 0–3%. In immature follicles that had not yet initiated their transbridge potentials, Ca 2+ activities averaged ∼ 2 μM in both oocyte and nurse cells. The results suggest that vitellogenic follicles possess a vanadatesensitive Ca 2+ extrusion mechanism that is more powerful in the nurse cells than in the oocyte. © 1994 Wiley‐Liss, Inc.