N omega-nitro-L-arginine, an inhibitor of nitric oxide synthesis, increases blood pressure in rats and reverses the pregnancy-induced refractoriness to vasopressor agents

Objective: With N omega-nitro-L-arginine, a potent inhibitor of nitric oxide synthesis, we tested the hypothesis that nitric oxide plays a functional role in the blunted pressor responsiveness seen during pregnancy.

Study design: A group of six pregnant rats were instrumented on the fourteenth day of gestation and studied on days 19 and 20, as well as 7 days post partum. Another group of six virgin rats were similarly prepared and used 5 days after surgery. Blood pressure and heart rate were monitored in conscious freely moving Animals before and during the administration of drugs or placebo. Results were analyzed, by one-way repeated-measures analysis of variance, with Dunnett's t test, or by paired t test where applicable.

Results: Basal mean arterial pressure and heart rate were 90.8 +/- 3.0 mm Hg and 330 +/- 6 beats/min in pregnant Animals and 107.1 +/- 3.2 mm Hg and 315 +/- 7 beats/min in nonpregnant Animals. Pressor responses to angiotensin II, vasopressin, and norepinephrine were attenuated in gravid Animals. Infusion of N omega-nitro-L-arginine significantly and in a dose-dependent manner increased mean arterial pressure and reduced heart rate. These effects could be completely reversed by L-arginine administration. Changes in mean arterial pressure were higher during pregnancy as compared with postpartum values. N omega-nitro-L-arginine infusion potentiated pressor responses to all three vasopressors, resulting in dose-response curves that were significantly shifted to the left, making them virtually identical in pregnant and postpartum rats.

Conclusion: Our data support the emerging view that nitric oxide plays a key role in the regulation of blood pressure during pregnancy.