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This study demonstrates that alpha1-adrenergic receptors previously identified in the pregnant rat myometrium are heterogeneous. They can be subtyped alpha1A- and alpha1B-adrenergic receptors on the basis of their affinity for the antagonists WB4101 (alpha1A &gt; alpha1B) and chloroethylclonidine (alpha1B 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 (alpha1B subtype) remained constant. In contrast, the number of [3H]prazosin binding sites with a high affinity for WB4101 and insensitive to chloroethylclonidine (alpha1A 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 alpha1A-adrenergic receptor underwent uncoupling whereas the alpha1B-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 alpha1A-antagonist. Furthermore QL antibody (anti-G alpha(q)/G alpha11) also specifically blocked the phenylephrine-stimulated phospholipase C activity. Altogether these results strongly suggest that activation of the alpha1B-adrenergic receptor subtype in the pregnant myometrium at term may contribute to the stimulation of the G alpha(q)/G alpha11/phospholipase C signaling pathway.

The Alpha1B-Adrenergic Receptor Subtype Activates the Phospholipase C Signaling Pathway in Rat Myometrium at Parturition1