Stimulatory effects of chlorzoxazone, a centrally acting muscle relaxant, on large conductance calcium-activated potassium channels in pituitary GH₃ cells

Chlorzoxazone, a centrally acting muscle relaxant, has been used as a marker for hepatic CYP2E1 activity. However, little is known about the mechanism of chlorzoxazone actions on ion currents in neurons or neuroendocrine cells. We thus investigated its effects on ion currents in GH₃ lactotrophs. Chlorzoxazone reversibly increased Ca²⁺-activated K⁺ current (I_K(Ca)) in a concentration-dependent manner with an EC₅₀ value of 30 μM. The chlorzoxazone-stimulated I_K(Ca) was inhibited by iberitoxin (200 nM) or clotrimazole (10 μM), but not by glibenclamide (10 μM) or apamin (200 nM). Chlorzoxazone (30 μM) suppressed voltage-dependent L-type Ca²⁺ current. In the inside-out configuration, chlorzoxazone applied to the intracellular side of the patch did not modify single-channel conductance of large conductance Ca²⁺-activated K⁺ (BK_Ca) channels, but did increase channel activity by increasing mean open time and decreasing mean closed time. Chlorzoxazone also caused a left shift in the activation curve of BK_Ca channels. However, Ca²⁺-sensitivity of these channels was unaffected by chlorzoxazone. 1-Ethyl-2-benzimidazolinone (30 μM), 2-amino-5-chlorobenzoxazole (30 μM) or chlormezanone (30 μM) enhanced BK_Ca channel activity, while 6-hydroxychlorzoxazone (30 μM) slightly increased it; however, chlorphenesin carbamate (30 μM) had no effect on it. Under the current-clamp condition, chlorzoxazone (10 μM) reduced the firing rate of action potentials. In neuroblastoma IMR-32 cells, chlorzoxazone (30 μM) also stimulated BK_Ca channel activity. The stimulatory effects of chlorzoxazone on these channels may be responsible for the underlying mechanism of chlorzoxazone actions on neurons and neuroendocrine cells.