The alpha(1B)-adrenergic receptor subtype activates the phospholipase C signaling pathway in rat myometrium at parturition

Abstract

This study demonstrates that α1-adrenergic receptors previously identified in the pregnant rat myometrium are heterogeneous. They can be subtyped α(1A)- and α(1B)-adrenergic receptors on the basis of their affinity for the antagonists WB4101 (α(1A) > α(1B)) and chloroethylclonidine (α(1B) selective). Between Day 21 of pregnancy and term, the proportion of [3H]prazosin binding sites with low affinity for WB4101 and sensitive to inactivation by 10-5 M chloroethylclonidine under hypotonic conditions (α(1B) subtype) remained constant. In contrast, the number of [3H]prazosin binding sites with a high affinity for WB4101 and insensitive to chloroethylclonidine (α(1A) subtype) increased by 88% at term. The effect of 5'-guanylylimidodiphosphate (Gpp[NH]p) on competition of the agonist phenylephrine for [3H]prazosin binding in the presence of WB4101 or after chloroethylclonidine pretreatment indicates that the α(1A)- adrenergic receptor underwent uncoupling whereas the α(1B)-adrenergic receptor-G protein coupled state was increased (+ 63%). Phenylephrine consistently stimulated phospholipase C activity on membrane fractions prepared from term myometria. This stimulation was completely inhibited after 10-5 M chloroethylclonidine but was not consistently decreased with 5- methylurapidyl, a selective α(1A)-antagonist. Furthermore QL antibody (anti- Gαq/Gα11) also specifically blocked the phenylephrine-stimulated phospholipase C activity. Altogether these results strongly suggest that activation of the α(1B)-adrenergic receptor subtype in the pregnant myometrium at term may contribute to the stimulation of the Gαq/Gα11/phospholipase C signaling pathway.