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 decline 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 suggest 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.