Paradoxical Role of Large-Conductance Calcium-Activated K+ (BK) Channels in Controlling Action Potential-Driven Ca2+ Entry in Anterior Pituitary Cells

Activation of high-conductance Ca2+-activated K+ (BK) channels normally limits action potential duration and the associated voltage-gated Ca2+ entry by facilitating membrane repolarization. Here we report that BK channel activation in rat pituitary somatotrophs prolongs membrane depolarization, leading to the generation of plateau-bursting activity and facilitated Ca2+ entry. Such a paradoxical role of BK channels is determined by their rapid activation by domain Ca2+, which truncates the action potential amplitude and thereby limits the participation of delayed rectifying K+ channels during membrane repolarization. Conversely, pituitary gonadotrophs express relatively few BK channels and fire single spikes with a low capacity to promote Ca2+ entry, whereas an elevation in BK current expression in a gonadotroph model system leads to the generation of plateau-bursting activity and high-amplitude Ca2+transients.