Potent activation of large-conductance Ca²⁺-activated K ⁺ channels by the diphenylurea 1,3-bis-[2-hydroxy-5-(trifluoromethyl) phenyl]urea (NS1643) in pituitary tumor (GH3) cells

1,3-Bis-[2-hydroxy-5-(trifluoromethyl)phenyl]urea (NS1643) is reported to be an activator of human ether-à-go-go-related gene current. However, it remains unknown whether it has any effects on other types of ion channels. The effects of NS1643 on ion currents and membrane potential were investigated in this study. NS1643 stimulated Ca²⁺-activated K⁺ current [I_K(Ca)] in a concentration-dependent manner with an EC₅₀ value of 1.8 μM in pituitary tumor (GH₃) cells. In inside-out recordings, this compound applied to the intracellular side of the detached channels stimulated large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels with no change in single-channel conductance. It shifted the activation curve of BK_Ca channels to less depolarized voltages without altering the gating charge of the channels. NS1643-stimulated channel activity depended on intracellular Ca²⁺, and mean closed time during exposure to NS1643 was reduced. NS1643 (3 μM) had little or no effect on peak amplitude of ether-à-go-go-related gene-mediated K⁺ current evoked by membrane hyperpolarization, although it increased the amplitude of late-sustained components of K⁺ inward current, which was suppressed by paxilline but not by azimilide. NS1643 (3 μM) had no effect on L-type Ca²⁺ current. This compound reduced repetitive firing of action potentials, and further application of paxilline attenuated its decrease in firing rate. In addition, NS1643 enhanced BK_Ca-channel activity in human embryonic kidney 293T cells expressing α-hSlo. In summary, we clearly show that NS1643 interacts directly with the BK_Ca channel to increase the amplitude of I_K(Ca) in pituitary tumor (GH₃) cells. The α-subunit of the channel may be a target for the action of this small compound.