Recently, we have described the Ca²⁺-dependent K⁺ channel of big conductance (BK_Ca) in primary granulosa cells (GCs) of the human ovary. The channel had the characteristic single channel conductance of 250 pS and its opening was shown to be necessary for human chorionic gonadotropin (hCG)-induced progesterone production. The question arose, whether the trophic hormone hCG has an impact on BK_Ca channel expression and characteristics in GCs. Treatment with hCG for more than 3 days reduced the fraction of GCs exhibiting functional BK_Ca channels as tested by opening with niflumic acid, whereas the current normalised to cell size (I_specific ) was not affected. I_specific was significantly lower at day 2-3 of culture compared to day 4–6. This might result from in vivo hCG-treatment of the in vitrofertilisation patients from whom GCs originate. The Ca²⁺ concentration causing half maximal activation varied over two orders of magnitude for single BK_Ca channels (mean 15 mM, at +60 mV) and was not significantly affected by treatment with hCG. The variability might be due to heterogeneity of the channel composition with respect to regulatory b subunits. In accordance, we detected mRNA encoding b2, b3 and b4, but not b1. This variability and potential further Ca²⁺-dependent K⁺ channel types could enable differentiated responses of GCs upon stimuli

(e.g. acetylcholine, oxytocin) raising intracellular Ca²⁺ levels. (Supported by DFG Graduate School '333')