Cardiovascular responses attenuate with repeated NO synthesis inhibition in conscious fetal sheep

The cardiovascular effects of repeated administration of the nitric oxide (NO) synthesis inhibitor N omega-nitro-L-arginine methyl ester (L-NAME) were assessed daily for 3 days in fetal sheep near term (124-126 days gestation) beginning 4 days after surgery (n = 7). In the first hour on day 1, fetal infusion of L-NAME (30 mg bolus, 6 mg/min infusion iv for 3 h) significantly increased fetal arterial pressure from 41 +/- 2 to 58 +/- 3 mmHg, decreased heart rate from 173 +/- 5 to 134 +/- 3 beats/min, increased umbilicoplacental resistance from 0.16 +/- 0.02 to 0.28 +/- 0.07 mmHg.ml-1.min, and inhibited the hypotensive response to acetylcholine (ACh; 2 micrograms iv bolus). All changes were sustained except for arterial pressure, which decreased significantly to 50 +/- 3 mmHg in the third hour. Within 17 h, all cardiovascular variables returned to control. L-NAME readministered on days 2 and 3 had no effect on cardiovascular variables. L-NAME did not potentiate the pressor response to angiotensin II on day 2 and caused a surprising attenuation of the pressor response to endothelin-1 on day 3. We conclude that, whereas NO normally contributes to low arterial pressure, high heart rate, and low umbilicoplacental vascular resistance in fetal sheep near term, the role of NO in these functions is replaced by an alternate mechanism within 17 h after NO synthesis inhibition with L-NAME.