Context: beta(3)-Adrenoreceptor modulation in human myometrium during pregnancy is linked functionally to myometrial inhibition. Maxi-K+ channels (BKCa) play a significant role in modulating cell membrane potential and excitability.Objective: This study was designed to investigate the potential involvement of BKCa channel function in the response of human myometrium to beta(3)-adrenoceptor activation.Design: Single and whole-cell electrophysiological BKCa channel recordings from freshly dispersed myocytes were obtained in the presence and absence of BRL37344, a specific beta(3)-adrenoreceptor agonist. The in vitro effects of BRL37344 on isolated myometrial contractions, in the presence and absence of the specific BKCa channel blocker, iberiotoxin ( IbTX), were investigated.Setting: The study was carried out at the Clinical Science Institute. Patients orOther Participants: Myometrial biopsies were obtained at elective cesarean delivery.Intervention: No intervention was applied.Main Outcome Measures: Open state probability of single channel recordings, whole cell currents, and myometrial contractile activity were measured.Results: Single-channel recordings identified the BKCa channel as a target of BRL37344. BRL37344 significantly increased the open state probability of this channel in a concentration-dependent manner ( control 0.031 +/- 0.004; 50 mu M BRL37344 0.073 +/- 0.005 (P < 0.001); and 100 mu M BRL37344 0.101 +/- 0.005 ( P < 0.001). This effect was completely blocked after preincubation of the cells with 1 mu M bupranolol, a nonspecific - adrenoreceptor blocker, or 100 nM SR59230a, a specific beta(3)- adrenoreceptor antagonist. In addition, BRL37344 increased whole- cell currents over a range of membrane potentials, and this effect was reversed by 100 nM IbTX. In vitro isometric tension studies demonstrated that BRL37344 exerted a significant concentrationdependent relaxant effect on human myometrial tissue (P < 0.05), and preincubation of these strips with IbTX attenuated this effect on both spontaneous and oxytocin- induced contractions (44.44 and 57.84% at 10(-5) M, respectively).Conclusions: These findings outline that activation of the BKCa channel may explain the potent uterorelaxant effect of beta(3)-adrenoreceptor agonists.