Nitric oxide synthase activity in pregnant rabbit uterus decreases on the last day of pregnancy.

OBJECTIVE: Our purpose was to test a potential role for the endogenous smooth muscle relaxant nitric oxide in the control of gestational uterine activity by quantifying and characterizing its synthetic enzyme, nitric oxide synthase, in uterine tissue at the end of pregnancy. STUDY
DESIGN: We measured nitric oxide synthase activity through the conversion of tritiated L-arginine to tritiated L-citrulline in subcellular preparations of decidua and myometrium from pregnant rabbits at 27, 30, and 31 days' (term)gestation. Nitric oxide synthase was characterized by measuring its relative inhibition by arginine analogs and its calcium-calmodulin requirement. Nitric oxide synthase activities were compared by one-way analysis of variance with Fisher's post hoc test.
RESULTS: Nitric oxide synthase activity in decidua was high at 27 days' gestation (6.32 +/- 1.10 pmol/mg protein per minute, n = 6), less with the approach of labor (30 days = 3.16 +/- 1.25 pmol/mg per minute, n = 4), and lowest at 31 days (1.07 +/- 0.29 pmol/mg per minute, n = 4, p < 0.05). Decidual nitric oxide synthase was calcium insensitive, and arginine analogs reduced activity with potencies consistent with their effect on the induced form of nitric oxide synthase.
CONCLUSION: Decidual nitric oxide synthase activity, which has the characteristics of the inducible isoform of the enzyme, is significantly lower on the last day of gestation. This suggests a role for nitric oxide in the control of uterine contractility during pregnancy.