Identification and characterization of Ca2+-activated K+ channels in granulosa cells of the human ovary

Abstract

Abstract Background Granulosa cells (GCs) represent a major endocrine compartment of the ovary producing sex steroid hormones. Recently, we identified in human GCs a Ca2+-activated K+ channel (KCa) of big conductance (BKCa), which is involved in steroidogenesis. This channel is activated by intraovarian signalling molecules (e.g. acetylcholine) via raised intracellular Ca2+ levels. In this study, we aimed at characterizing 1. expression and functions of KCa channels (including BKCa beta-subunits), and 2. biophysical properties of BKCa channels. Methods GCs were obtained from in vitro-fertilization patients and cultured. Expression of mRNA was determined by standard RT-PCR and protein expression in human ovarian slices was detected by immunohistochemistry. Progesterone production was measured in cell culture supernatants using ELISAs. Single channels were recorded in the inside-out configuration of the patch-clamp technique. Results We identified two KCa types in human GCs, the intermediate- (IK) and the small-conductance KCa (SK). Their functionality was concluded from attenuation of human chorionic gonadotropin-stimulated progesterone production by KCa blockers (TRAM-34, apamin). Functional IK channels were also demonstrated by electrophysiological recording of single KCa channels with distinctive features. Both, IK and BKCa channels were found to be simultaneously active in individual GCs. In agreement with functional data, we identified mRNAs encoding IK, SK1, SK2 and SK3 in human GCs and proteins of IK and SK2 in corresponding human ovarian cells. Molecular characterization of the BKCa channel revealed the presence of mRNAs encoding several BKCa beta-subunits (beta2, beta3, beta4) in human GCs. The multitude of beta-subunits detected might contribute to variations in Ca2+ dependence of individual BKCa channels which we observed in electrophysiological recordings. Conclusion Functional and molecular studies indicate the presence of active IK and SK channels in human GCs. Considering the already described BKCa, they express all three KCa types known. We suggest that the plurality and co-expression of different KCa channels and BKCa beta-subunits might allow differentiated responses to Ca2+ signals over a wide range caused by various intraovarian signalling molecules (e.g. acetylcholine, ATP, dopamine). The knowledge of ovarian KCa channel properties and functions should help to understand the link between endocrine and paracrine/autocrine control in the human ovary.