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

摘要

class="enumerated" style="list-style-type:decimal">We used large conductance Ca²⁺-activated K⁺ (BKCa) channel activity as a probe to characterize the inhibitory/stimulatory G protein (Gi/Gs) signalling pathways in intact cells from pregnant (PM) and non-pregnant (NPM) myometrium.Isoprenaline (10 μM) enhanced the outward current (Iout) in PM cells and inhibited Iout in NPM cells. Additional application of the α2-adrenoceptor (α2-AR) agonist clonidine (10 μM) further enhanced the isoprenaline-modulated Iout in PM cells but partially antagonized Iout in NPM cells. Clonidine alone did not affect Iout. The specific cAMP kinase (PKA) inhibitor H-89 (1 μM) abolished the effects of isoprenaline and clonidine. The specific BKCa channel blocker iberiotoxin (0·1 μM) inhibited Iout by ≈80 %; the residual current was insensitive to isoprenaline.Inhibition of Gi activity by either pertussis toxin or the GTPase activating protein RGS16 abolished inhibitory as well as stimulatory effects of clonidine on Iout.Transducin-α, a scavenger of Gi βγ dimers, converted the stimulatory action of clonidine on Iout into an inhibitory effect. Free transducin-βγ enhanced both the stimulatory and the inhibitory effects of isoprenaline on I_out.The results demonstrate that BK_Ca channel activity is a sensitive probe to follow adenylyl cyclase–cAMP–PKA signalling in myometrial smooth muscle cells. Both G_iα-mediated inhibition and G_iβγ-mediated stimulation can occur in the same cell, irrespective of pregnancy. It is speculated that the coupling between α₂-AR and G_i proteins is more efficient during pregnancy and that G_iβγ at high levels simply override the inhibitory action of G_i α.