Increased potency of α1-adrenergic receptors to induce inositol phosphates production correlates with the up-regulation of α1d/Ghα/phospholipase Cδ1 signaling pathway in term rat myometrium

Abstract

In the present study, we studied the potential regulation by rat myometrial alpha1-adrenergic receptors (alpha1-AR) of the newly identified Gh alpha protein/phospholipase C delta 1 (PLC delta 1) signaling pathway and compared myometrial inositol phosphates (InsP) production and activity of the uterine circular muscle in response to alpha1-AR activation between mid-pregnancy and term. For this, we quantified the level of rat myometrial alpha1-AR coupling to Gh alpha protein by photoaffinity-labeling, the cytosolic amount of PLC delta 1 enzyme by immunoblotting, and the expression level of alpha1-AR subtypes by RT-PCR. The results showed an increased level of alpha1-AR/Gh alpha protein coupling and the amount of PLC delta 1 at term (+147 and +65% respectively, versus mid-pregnancy). This was correlated with an up-regulation of alpha 1d-AR subtype (+70% versus mid-pregnancy). Incubation of myometrial strips with phenylephrine (Phe), a global alpha1-agonist, increased InsP production in a dose-dependent manner at both mid-pregnancy and term, but with an enhanced potency (tenfold decrease in EC(50) value) at term. Phe also dose-dependently induced contraction of the circular muscle at both mid-pregnancy and term. However, unlike InsP response, no amelioration of potency was observed at term. Similar results were obtained with the endogenous agonist norepinephrine. Our results show, for the first time, that rat myometrial alpha 1d-AR/Gh alpha/PLC delta 1 signaling pathway is up-regulated at term. This is associated with an increased potency of alpha1-AR to elicit InsP production but not uterine contraction at this period. It is thus hypothesized that alpha1-AR, through activation of Gh alpha/PLC delta 1 system, are not primarily involved in the initiation of labor but may rather regulate responses such as myometrial cell proliferation or hypertrophy.