Potassium channel blockers unmask electrical excitability of insect follicles

Abstract Standard techniques for intracellular recording and current injection were used to determine if the developing ovarian follicles of six species of insect were electrically excitable. Follicles were dissected from the panoistic ovarioles of Locusta migratoria and Periplaneta americana , the polytrophic meroistic ovarioles of the fruit fly Drosophila rnelanogaster and the silkworm moths Hyalophora cecropia and H. Columbia , and the telotrophic meroistic ovarioles of the mealworm beetle Tenebrio mlitor. Follicles of all species except Tenebrio were inexcitable in control saline; Tenebrio follicles produced short action potentials which did not outlast the duration of the current pulse. Follicles of all species except Drosophila produced long duration overshooting action potentials when depolarized in salines containing the potassium channel blockers tetraethylammonium (TEA; 10–80 mM), Bazf (10–20 mM) and 4‐aminopyridine (4‐AP; 10 mM). Hyalophora follicles were also excitable in saline containing TEA and Ba 2+ but not 4‐AP, and Tenebrio follicles were excitable in saline containing Ba 2+ but not TEA and 4‐AP. Excitability in follicles of all species was unaffected by sodium replacement but was reversibly inhibited by the calcium channel blockers La 3+ (1 mM), Co 2+ (10 mM), or verapamil (0.1‐1.0 mM). It is suggested that the action potentials are caused by the movement of calcium or barium through voltage‐dependent channels that are masked in control saline by a large counteracting potassium conductance. Excitability appears to be a property of the oolemma and not the follicle cell membranes. Possible functions of voltage‐dependent ion channels in insect oocytes are discussed.