Pregnancy switches adrenergic signal transduction in rat and human uterinemyocytes as probed by BKCa channel activity

1. We used large conductance Ca2+-activated K+ (BKCa) channel activity as a probe to characterize the inhibitory/stimulatory G protein (G(1)/G(s)) sig nalling pathways in intact cells from pregnant (PM) and non-pregnant (NPM) myometrium. 2. Isoprenaline (10 mu M) enhanced the outward current (I-out) in PM cells and inhibited I-out in NPN cells. Additional application of the alpha(2)-ad renoceptor (alpha(2)-AR) agonist clonidine (10 mu M) further enhanced the i soprenaline-modulated I-out in PM cells but partially antagonized I-out in NPM cells. Clonidine alone did not affect I-out. The specific cAMP kinase ( PKA) inhibitor H-89 (1 mu M) abolished the effects of isoprenaline and clon idine. The specific BKCa channel blocker iberiotoxin (0.1 mu M) inhibited I -out by similar to 80%; the residual current was insensitive to isoprenalin e. 3. Inhibition of G(i) activity by either pertussis toxin or the GTPase acti vating protein RGS16 abolished inhibitory as well as stimulatory effects of clonidine on I-out. 4. Transducin-alpha, a scavenger of G(i)beta gamma dimers, converted the st imulatory action of clonidine on I-out into an inhibitory effect. Free tran sducin-beta gamma enhanced both the stimulatory and the inhibitory effects of isoprenaline on I-out. 5. The results demonstrate that BKCa channel activity is a sensitive probe to follow adenylyl cyclase-cAMP-PKA signalling in myometrial smooth muscle cells. Both GP-mediated inhibition and G(i)beta gamma-mediated stimulation can occur in the same cell, irrespective of pregnancy It is speculated that the coupling between alpha(2)-AR and G(i) proteins is more efficient durin g pregnancy and that G(i)beta gamma at high levels simply override the inhi bitory action of G(i)alpha.