GRP-PREFERRING BOMBESIN RECEPTORS INCREASE GENERATION OF INOSITOL PHOSPHATES AND TENSION IN RAT MYOMETRIUM

In the estrogen-treated rat myometrium, bombesin (Bn) and related agonists triggered contraction and the increased generation of inositol phosphates. The relative order of potencies was identical for both responses: Bn = gastrin releasing peptide (GRP) = litorin = neuromedin C >> neuromedin B. Two specific GRP-preferring receptor antagonists, namely [D-Phe6]Bn-(6-13) methyl ester and [Leu14,phi13-14]Bn were inhibitory for both Bn-mediated tension and generation of inositol phosphates. [I-125-Tyr4]Bn bound to myometrial membranes with high affinity (K(d) = 104 pM) to a single class of sites in a saturable and reversible manner. The relative potencies for inhibiting binding were GRP = litorin = [Tyr4]Bn (K(i) = 0.4 to 0.6 nM) >> neuromedin B (K(i) = 10.3 nM). The high affinity displayed by [D-Phe6]Bn-(6-13) methyl ester (K(i) = 2.8 nM) and (Leu14,phi13-14]Bn (K(i) = 35 nM) for competing for [Tyr4]Bn binding supported the involvement of a GRP-preferring Bn receptor. Guanine nucleotides decreased the binding of [I-125-Tyr4]Bn and accelerated the rate of ligand dissociation, reflecting the coupling of receptors to guanine nucleotide regulatory proteins (G proteins). The results demonstrate that rat myometrium expresses functional GRP-preferring Bn receptors whose activation stimulates the phospholipase C pathway, a pertussis toxin-insensitive event that contributes to Bn-mediated uterine contractions.