BKCa channel activation by membrane-associated cGMP kinase may contribute to uterine quiescence in pregnancy

We investigated the influence of pregnancy on large-conductance calcium-act ivated potassium channel (BKCa) activity (NPo) and on channel expression in membranes of isolated human myometrial smooth muscle cells. NPo in inside- out patches was higher in pregnant myometria (PM) compared with nonpregnant myometria (NPM), and the half-maximal activation potential was shifted by 39 mV to more negative potentials. This effect was not due to an enhanced B KCa channel expression. In the presence of cAMP kinase (PKA) or cGMP kinase (PKG), NPo increased in patches from PM but decreased in those from NPM. W estern blot analysis and use of a specific PKG inhibitor (1 muM KT-5823) ve rified the existence of a partially active membrane-associated PKG. Inhibit ion of PKA by 100 nM PKI, the inhibitory peptide of PKA, had no effect on N Po. 8-p-Chlorophenylthio-cGMP (8-pCPT-cGMP) hyperpolarized cells from PM. T his effect was abolished by iberiotoxin, a specific blocker of BKCa channel s. It is concluded that an endogenous, membrane-bound PKG in myometrial cel ls specifically enhances BKCa channel activity during pregnancy and thus ma y contribute to uterine quiescence during pregnancy.