Vinpocetine-induced stimulation of calcium-activated potassium currents in rat pituitary GH₃ cells

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 (I_K(Ca)) with an EC₅₀ value of 4 μM. When the recording pipettes were filled with 10 mM EGTA, vinpocetine also stimulated I_K(Ca). In the cell-attached configuration, application of vinpocetine to the bath increased the activity of large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels. In excised membrane patches, application of vinpocetine (10 μM) to the bath did not change the single-channel conductance of BK_Ca channels; however, it did increase channel activity. In the inside-out configuration, neither 8-bromo cyclic GMP nor YC-1 applied intracellularly affected BK_Ca channel activity. The vinpocetine-induced change in the kinetic behavior of BK_Ca 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 BK_Ca channels without altering single-channel conductance. Thus, the present study suggests that vinpocetine-mediated stimulation of I_K(Ca) may result from the direct activation of BK_Ca channels and indirectly from elevated cytosolic Ca²⁺.