VOLTAGE-CLAMP OF THE NUCLEAR-ENVELOPE

By using conventional intracellular microelectrodes, we show that the starfish germinal vesicle in situ has a resting potential of +4 mV to +30 mV with respect to the cytoplasm. Patch-clamp electrodes reveal that the surface of the isolated nucleus contains ion channels of 120 pS single-channel conductance. Under whole-cell voltage-clamp conditions, the starfish germinal vesicle generates asymmetric time-variant currents that depend on the ionic environment. Fluorescent dyes in the whole-cell clamp electrode fill the nucleus, showing that the time-variant and the steady-state conductances are properties of the outer and inner nuclear membranes or something that spans them. Because scanning electron microscopy shows a density of nuclear pores in the range 60-110 per square micrometre, and the steady-state resistance in the range is 30-100 M Omega, it seems probable that the pores are not freely permeable to ions. Furthermore, fluorescent dyes of molecular weight up to 500 Da micro-injected either into the cytoplasm or the nucleus of intact oocytes accumulate in the nucleus. Our results show that the nucleus acts as an electrically isolated compartment that separates charged low molecular mass molecules including inorganic ions.