Progesterone activation of β1-containing BK channels involves two binding sites

Progesterone (>= 1 mu M) is used in recovery of cerebral ischemia, an effect likely contributed to by cerebrovascular dilation. The targets of this progesterone action are unknown. We report that micromolar (mu M) progesterone activates mouse cerebrovascular myocyte BK channels; this action is lost in beta(-/-)(1) mice myocytes and in lipid bilayers containing BK alpha subunit homomeric channels but sustained on beta(1)/beta(4)-containing heteromers. Progesterone binds to both regulatory subunits, involving two steroid binding sites conserved in beta(1)/beta(4): high-affinity (sub-mu M), which involves Trp87 in beta(1) loop, and low-affinity (mu M) defined by TM1 Tyr32 and TM2 Trp163. Thus progesterone, but not its oxime, bridges TM1-TM2. Mutation of the high-affinity site blunts channel activation by progesterone underscoring a permissive role of the high-affinity site: progesterone binding to this site enables steroid binding at the low-affinity site, which activates the channel. In support of our model, cerebrovascular dilation evoked by mu M progesterone is lost by mutating Tyr32 or Trp163 in beta(1) whereas these mutations do not affect alcohol-induced cerebrovascular constriction. Furthermore, this alcohol action is effectively counteracted both in vitro and in vivo by progesterone but not by its oxime.

Automated summary

Progesterone activates BK channels in mouse cerebrovascular myocytes by binding to the regulatory subunits, with a high-affinity site enabling steroid binding at a low-affinity site to activate the channel. Mutation of the high-affinity site weakens channel activation by progesterone, indicating its permissive role.