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,psi 13-14]Bn were inhibitory for both Bn-mediated tension and generation of inositol phosphates. [125I-Tyr4]Bn bound to myometrial membranes with high affinity (Kd = 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 (Ki = 0.4 to 0.6 nM) >> neuromedin B (Ki = 10.3 nM). The high affinity displayed by [D-Phe6]Bn-(6-13) methyl ester (Ki = 2.8 nM) and [Leu14,psi 13-14]Bn (Ki = 35 nM) for competing for [Tyr4]Bn binding supported the involvement of a GRP-preferring Bn receptor. Guanine nucleotides decreased the binding of [125I-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, pertussis toxin-insensitive event that contributes to Bn-mediated uterine contractions.

Evidence that adhesion of electrically permeabilized platelets to collagen is mediated by guanine nucleotide regulatory proteins

Adhesion of electrically permeabilized platelets to collagen was found to be essentially independent of free Ca2+ concentration in the medium. Addition of stable GTP analogues increased the proportion of adhering cells about 5-fold. This effect was inhibited by guanosine 5′-[beta-thio]diphosphate, cytochalasin D or monoclonal antibodies to glycoprotein Ia. In contrast, the protein kinase C inhibitor staurosporine had only a small effect on the GTP-analogue-enhanced adhesion of the permeabilized cells to collagen. These results suggest that a guanine nucleotide regulatory (G)-protein is directly linked to the collagen receptor and is involved in the actin-dependent recruitment of additional collagen receptors.

Prostanoid biosynthesis and mechanisms of action

Prostanoids are local hormones formed from arachidonic acid that coordinate responses to circulating hormones which elicit prostanoid synthesis. For example, in the kidney, prostaglandin (PG) E2 synthesized by collecting tubule epithelia in response to arginine vasopressin (AVP) acts on the parent collecting tubule as well as the neighboring thick limb to modulate NaCl and water reabsorption occurring in response to AVP. Studies performed over the last 15 years have defined the major cellular and subcellular sites of PG synthesis in the kidney. In addition, it is now recognized that the multiple cellular actions of prostanoids in the kidney are mediated through receptors coupled to guanine nucleotide regulatory proteins. The goal of this review is to summarize recent biochemical and molecular biological studies on prostanoid biosynthetic enzymes and on prostanoid receptors. The major topics to be addressed are 1) phospholipid precursors of arachidonate, 2) membrane-associated and cytosolic phospholipase A2s, 3) PG endoperoxide (PGH) synthase isozymes, 4) thromboxane A (TxA) synthase, and 5) TxA/PGH and PGE receptors.

Modulation of NO and endothelin by chronic increases in blood flow in canine femoral arteries

Experiments were designed to determine whether chronic increases in arterial blood flow alter production of or response to nitric oxide and endothelin. Canine femoral arteries proximal to an arteriovenous fistula- and from the contralateral sham-operated blood vessels were removed, cut into rings, and suspended for measurement of isometric force in organ chambers. The remainder of the artery was homogenized for measurement of endothelin content by radioimmunoassay. NG-monomethyl-L-arginine (10(-4) M) caused concentration-dependent increases in tension only in fistula-operated arteries. Endothelium-dependent relaxations to acetylcholine and BHT-920 were greater in fistula- compared with sham-operated arteries. These differences were reduced by the arginine analogue. Pertussis toxin (100 ng/ml) inhibited relaxations to acetylcholine only in fistula-operated arteries and to BHT-920 only in sham-operated arteries. Contractions to endothelin-1 were greater in fistula- compared with sham-operated arteries. These results suggest that chronic increases in blood flow enhance the tonic and receptor-stimulated production of nitric oxide and its release by receptors coupled to pertussis toxin-sensitive guanine nucleotide regulatory proteins. Furthermore, chronic increases in blood flow may either inhibit the production of endothelin or promote its depletion from endothelial cells while simultaneously increasing the sensitivity of the smooth muscle to its contractile effects.