Calcium-activated and voltage-dependent potassium conductances in clonal pituitary cells

Voltage clamp recordings of GH3/B6 pituitary cells reveal the presence of non linear steady state membrane properties at the level of the resting potential (about ?41 mV). Clamping the cells to potentials more depolarized than ?60 mV is associated with a potential dependent increase in membrane conductance and membrane current variance. Tetra-ethylammonium (TEA), Cobalt (Co²⁺) and methoxy-verapamil (D-600) each attenuate these potential-dependent changes. Spectral analysis of membrane current fluctuations shows that power spectral densities calculated for fluctuations occuring over the ? 70 to ? 40 mV range declin? monotonically as a function of frequency, while spectra derived from fluctuations obtained over the ? 20 mV to 0 mV range decline as the square of frequency and are usually well fitted by a single Lorentzian equation. The half-power frequency of these spectra varies from 45 to 65 Hz. If we assume that the activities of two-state (open-closed) ion channels underlie the electrical behaviour of the membrane at the resting potential and at more depolarized levels, then the results suggests the presence of K⁺ ion channels whose activation depends both on potential and Ca²⁺ ions. These K⁺ ion channels have estimated electrical properties (conductance : 15 ps ; duration : 3 msec) similar to those present in other excitable membranes.