The effects of vinpocetine, an inhibitor of cyclic GMP phosphodiesterase, on ionic currents were examined in rat pituitary GH3 lactotrophs with the aid of the patch-clamp technique. In GH3 cells bathed in normal Tyrode's solution, vinpocetine (10 μM) reversibly increased the amplitude of Ca2+-activated K+ current (IK(Ca)) with an EC50 value of 4 μM. When the recording pipettes were filled with 10 mM EGTA, vinpocetine also stimulated IK(Ca). In the cell-attached configuration, application of vinpocetine to the bath increased the activity of large-conductance Ca2+-activated K+ (BKCa) channels. In excised membrane patches, application of vinpocetine (10 μM) to the bath did not change the single-channel conductance of BKCa channels; however, it did increase channel activity. In the inside-out configuration, neither 8-bromo cyclic GMP nor YC-1 applied intracellularly affected BKCa channel activity. The vinpocetine-induced change in the kinetic behavior of BKCa channels was due to an increase in mean open time and a decrease in mean closed time. Vinpocetine (10 μM) caused a leftward shift in the midpoint for the voltage-dependent opening. Under the current-clamp mode, vinpocetine (10 μM) decreased the firing rate of spontaneous action potentials induced by thyrotropin-releasing hormone (10 μM) in GH3 cells. In pheochromocytoma PC12 cells, vinpocetine (10 μM) applied intracellularly also enhanced the activity of BKCa channels without altering single-channel conductance. Thus, the present study suggests that vinpocetine-mediated stimulation of IK(Ca) may result from the direct activation of BKCa channels and indirectly from elevated cytosolic Ca2+.
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