Inflammatory processes enhance cAMP-mediated uterus relaxation in the pregnant rat: the role of TNF-alpha.

The objective of this study was to assess the in vitro uterus relaxing potency of beta(2)-adrenergic receptor (beta(2)-AR) agonists in pregnant rats after in utero administration of the bacterial lipopolysaccharide, Escherichia coli endotoxin (LPS). The LPS (100 microg/kg) was injected into the uterine lumen on day 16 of pregnancy. The effects of beta(2)-AR agonist terbutaline was tested in vitro, in isolated uterine rings precontracted by electric field stimulation. Uterine beta(2)-AR densities were detected by radioligand binding assay, the activated G-protein levels were investigated by a radiolabelled GTP binding assay. Uterine cAMP accumulation and the serum tumor necrosis factor-alpha (TNF-alpha) levels were measured by enzyme immunoassay. The endotoxin-evoked preterm delivery occurred on day 21. Higher pD(2) values of terbutaline (p < 0.001) were detected in endotoxin-treated rats: 9.14 +/- 0.36 vs. 7.71 +/- 0.12 compared with sham-operated rats. The densities or the equilibrium dissociation constants of beta(2)-ARs were not different (p > 0.05) in LPS-treated vs. control animals. Serum TNF-alpha level rose threefold after LPS treatment, but this rise was abolished by thalidomide. In LPS + thalidomide-treated rats, the effect of terbutaline became similar to that in sham-operated controls. By the measurement of myometrial cAMP levels, we documented that the concentration-response curve of terbutaline on cAMP accumulation was shifted to the left in the LPS-treated rats, with a significant rise in the pD(2). We concluded that in the case of uterine inflammation, the in vitro uterus-relaxing potency of beta(2)-agonists enhances, which is possibly mediated by TNF-alpha and uterine cAMP levels and that may serve as a rationale for the use of beta(2)-AR agonists in the attenuation of preterm uterine contractions on an inflammatory basis.