The Ca2+-activated, large conductance K+-channel (BKCa) is a player in the LH/hCG signaling cascade in testicular Leydig cells

► The Ca2+-activated K+-channel (BKCa) is expressed by Leydig cells in adult hamster and human testis. ► BKCa is activated upon LH-receptor activation. ► It is responsible for the hCG-induced hyperpolarization of the Leydig cell membrane. ► Inhibition of BKCa abolishes hyperpolarization but increases testosterone and StAR levels. ► Thus BKCa is involved in limiting the production of testosterone evoked by LH-receptor activation. In Leydig cells, hormonal stimulation by LH/hCG entails increased intracellular Ca2+ levels and steroid production, as well as hyperpolarization of the cell membrane. The large-conductance Ca2+-activated K+-channel (BKCa) is activated by raised intracellular Ca2+ and voltage and typically hyperpolarizes the cell membrane. Whether BKCa is functionally involved in steroid production of Leydig cells is not known. In order to explore this point we first investigated the localization of BKCa in human and hamster testes and then used a highly specific toxin, the BKCa blocker iberiotoxin (IbTx), to experimentally dissect a role of BKCa. Immunohistochemistry and RT-PCR revealed that adult Leydig cells of both species are endowed with these channels. Ontogeny studies in hamsters indicated that BKCa becomes strongly detectable in Leydig cells only after they acquire the ability to produce androgens. Using purified Leydig cells from adult hamsters, membrane potential changes in response to hCG were monitored. HCG hyperpolarized the cell membrane, which was prevented by the selective BKCa blocker IbTx. Steroidogenic acute regulatory (StAR) mRNA expression and testosterone production were not affected by IbTx under basal conditions but markedly increased when hCG, in submaximal and maximal concentration or when db-cAMP was added to the incubation media. A blocker of KV4-channels, expressed by Leydig cells, namely phrixotoxin-2 (PhTx-2) was not effective. In summary, the data reveal BKCa as a crucial part of the signaling cascade of LH/hCG in Leydig cells. The hyperpolarizing effect of BKCa in the Leydig cell membrane appears to set in motion events limiting the production of testosterone evoked by stimulatory endocrine mechanisms.