Recording and analysis of uterine activity in pregnant guinea-pigs

Challenge with ovalbumin antigen increases uterine and cervical contractile activity in sensitized guinea pigs

OBJECTIVE

The aims of this study were to investigate the effects of ovalbumin challenge on uterine and cervical contractility, intrauterine pressure, and uterine electromyography activity in sensitized guinea pigs.

STUDY DESIGN

Guinea pigs were sensitized by injection of ovalbumin-aluminum hydroxide suspension. Control animals were injected with the aluminum hydroxide suspension only. On days 55-57 of pregnancy, longitudinal uterine and cervical strips from guinea pigs were prepared for isometric tension recording. Nonpregnant guinea pigs were outfitted with telemetric transducers to record intrauterine pressure and uterine electromyography.

RESULTS

Ovalbumin significantly increased contractility of uterine and cervical strips from sensitized versus nonsensitized animals. These effects were abolished by histamine H(1) receptor antagonist in uterine strips and by histamine H(1) receptor antagonist and a mast cell stabilizer in cervical strips from sensitized animals. Cyclooxygenase and 5-lipoxygenase inhibitors had no significant effect on the response to ovalbumin. Treatment with ovalbumin in vivo significantly increased intrauterine pressure and uterine electromyography activity in sensitized but not in nonsensitized, animals.

CONCLUSION

Our findings indicate that type I hypersensitivity reactions may be important in mediating uterine contractility in pregnant and nonpregnant states.

Uterine contractile response to the graded infusion of oxytocin in guinea pigs near term

OBJECTIVE

Myometrial oxytocin binding characteristics do not change near term in guinea pigs. We tested the hypothesis that the uterine contractile response to oxytocin does not change near term.

STUDY DESIGN

Chronically instrumented guinea pigs were given graded infusions of oxytocin (n = 19 animals) or saline solution (n = 4 animals) on days 58, 60, and 62 and then daily until delivery (term, 68 days). Uterine contractile response was assessed by the area of the quasi-integrated electromyogram. Concentrations of 13,14-dihydro-15-keto-prostaglandin F(2)(alpha) were measured in maternal plasma.

RESULTS

The uterine contractile response to oxytocin increased with advancing gestation (P =.007, random coefficients model) and labor (P =.005). Plasma concentrations of 13,14-dihydro-15-keto-prostaglandin F(2)(alpha) increased during oxytocin infusion (P <.022 vs saline solution infusion).

CONCLUSION

The uterine contractile response to oxytocin increases during the 1 to 2 weeks before term in guinea pigs, despite stable myometrial oxytocin receptor density. Oxytocin stimulates the production of prostaglandin F(2)(alpha).

Oxytocin receptors in guinea pig myometrium near term and during labor

Oxytocin receptors in myometrium of women, rats, and rabbits rise markedly before the onset of labor, suggesting a role in the initiation of labor. In guinea pigs, a previous study reported no such rise by one-point determination of oxytocin binding. The purpose of this study was to use a more rigorous method to determine whether the binding characteristics of myometrial oxytocin receptors change in relation to labor in guinea pigs. Competitive binding studies were carried out in microsomes from inner and outer myometrium between 42 days of gestation and labor. Binding to analogs was also tested. Data were analyzed with affinity spectra and LIGAND. Oxytocin bound to one site with a dissociation constant of 6.3 +/- 0.65 x 10(-9) M. Binding capacity was 1.0 +/- 0.1 x 10(-12) mol/mg protein. The Hill coefficient was near unity. No significant changes occurred with gestation or labor in dissociation constant, binding capacity, or Hill coefficient (all P >/= 0.2, nested ANOVA). Binding capacity was higher in the outer than in the inner layer (1.2 +/- 0.2 vs. 0.8 +/- 0.1 x 10(-12) mol/mg protein, P = 0.02), but the dissociation constants were similar. Differences existed in the dissociation constants of the analogs tested. The main conclusion is that oxytocin receptors are unlikely to have a regulatory role in the initiation of labor in guinea pigs.

Production of prostaglandin F2 alpha and E2 in explants of intrauterine tissues of guinea pigs during late pregnancy and labor

Prostaglandin production in amnion and decidua is considered important for human parturition. We investigated in pregnant guinea pigs, a species similar to women in regard to the endocrinology of pregnancy, whether the production rates of PGE2 and PGF2 alpha in various intrauterine tissues are compatible with a role in parturition. Net production rates were measured at 45, 55 and 65 days of gestation and during labor in amnion, chorion, myo-endometrium, the outer layer of the myometrium, the site of placental implantation, and placenta. Net production rates in amnion increased between 45 days and labor (30-fold for PGE2 and 8-fold for PGF2 alpha, P < 0.0001). During labor, the production rates in amnion of PGE2 (P = 0.006) and PGF2 alpha (P = 0.019) were higher than at 45, 55, and 65 days of gestation. In myo-endometrium, the production rates of PGF2 alpha were higher at 65 days of gestation than at 55 days and during labor (P = 0.046). Addition of arachidonic acid (10(-5) M) increased production of PGE2 and/or PGF2 alpha in all tissues (P < 0.05) except placenta. In amnion, the response to arachidonic acid increased with advancing gestation. This suggests that 1) PGE2 and PGF2 alpha produced by amnion have a potential role in the initiation and maintenance of labor, 2) PGF2 alpha produced by myo-endometrium has a potential role in the initiation of labor, 3) cyclooxygenase(s) are not rate-limiting except in placenta, and 4) the expression of cyclooxygenase in amnion increases with advancing gestation.