BKCa channel regulates calcium oscillations induced by alpha-2-macroglobulin in human myometrial smooth muscle cells

The large-conductance, voltage-gated, calcium (Ca2+)-activated potassium channel (BKCa) plays an important role in regulating Ca2+ signaling and is implicated in the maintenance of uterine quiescence during pregnancy. We used immunopurification and mass spectrometry to identify proteins that interact with BKCa in myometrium samples from term pregnant (>= 37 wk gestation) women. From this screen, we identified alpha-2-macroglobulin (alpha 2M). We then used immunoprecipitation followed by immunoblot and the proximity ligation assay to confirm the interaction between BKCa and both alpha 2M and its receptor, low-density lipoprotein receptor-related protein 1 (LRP1), in cultured primary human myometrial smooth muscle cells (hMSMCs). Single-channel electrophysiological recordings in the cell-attached configuration demonstrated that activated alpha 2M (alpha 2M*) increased the open probability of BKCa in an oscillatory pattern in hMSMCs. Furthermore, alpha 2M* caused intracellular levels of Ca2+ to oscillate in oxytocin-primed hMSMCs. The initiation of oscillations required an interaction between alpha 2M* and LRP1. By using Ca2+-free medium and inhibitors of various Ca2+ signaling pathways, we demonstrated that the oscillations required entry of extracellular Ca2+ through store-operated Ca2+ channels. Finally, we found that the specific BKCa blocker paxilline inhibited the oscillations, whereas the channel opener NS11021 increased the rate of these oscillations. These data demonstrate that alpha 2M* and LRP1 modulate the BKCa channel in human myometrium and that BKCa and its immunomodulatory interacting partners regulate Ca2+ dynamics in hMSMCs during pregnancy.