We reported previously that plasma levels, urinary excretion, and metabolic production of cyclic guanosine 3',5'-monophosphate (cGMP) are increased in gravid rats, and postulated that endogenous nitric oxide (NO), a potent vasodilator and immune modulator, may mediate this change. Four lines of evidence are now presented demonstrating increased biosynthesis of NO during pregnancy in rats: 1) Urinary excretion and plasma levels of the stable NO metabolite, nitrate, are elevated in pregnant rats; urinary excretion of nitrate is increased in pseudopregnant rats. 2) The urinary excretion of cGMP also increases during pregnancy and pseudopregnancy, paralleling the rise in urinary nitrate excretion. 3) Chronic treatment with the NO synthase inhibitor, NG-nitroarginine methyl ester (NAME), inhibits the increase in urinary nitrate excretion. 4) Nitric oxide hemoglobin is detected by electron paramagnetic resonance spectroscopy in the blood of pregnant, but not in nonpregnant, rats. The results show endogenous NO production is increased in gravid rats. This finding raises the possibility that NO may contribute to maternal vasodilation and uterine immune suppression of normal pregnancy.

We tested the hypothesis that nitric oxide (NO) biosynthesis increases during normal human pregnancy and decreases in preeclampsia. The major metabolites of NO, nitrate and nitrite (NOx), were measured in both the plasma and 24-h urine of women subjected to a reduced NOx diet. In this way, the plasma and urinary levels mainly reflected endogenous production rather than dietary intake. Moreover, we assessed cGMP, a second messenger of NO, in the same samples. Both NOx and cGMP assays were validated in our laboratory. We first conducted a cross-sectional study of nonpregnant women ( n = 15), normal pregnant women in the first ( n = 9), second ( n = 17) and third ( n = 22) trimesters, as well as women with preeclampsia ( n = 15) and transient hypertension of pregnancy ( n= 7). We also performed a serial study in the same women ( n = 9) before, during, and after pregnancy. Taken together, the results of the two investigations suggested marked increases in cGMP production especially during the first trimester when the maternal circulation is rapidly vasodilating. In contrast, whole body NO production as estimated by the plasma level and urinary excretion of NOx was not elevated during the first trimester. Finally, unequivocal demonstration of reduced NO biosynthesis in preeclampsia was not forthcoming.

Knowledge about possible alterations in cardiac output (CO), total peripheral vascular resistance (TPVR), and their time course and magnitude of change is conspicuously lacking for the conscious gravid rat. Therefore, we assessed CO using Fick methodology in unrestrained, chronically instrumented, conscious rats. The rats were studied during early (day 7), mid (day 13), or late gestation (day 18) along with nonpregnant control rats matched with respect to age and days postsurgery. Significant differences between pregnant and nonpregnant rats were observed during midgestation, when CO was increased by 26 +/- 12% and TPVR was decreased by 23 +/- 9% in the pregnant animals. These changes were accompanied by a narrowed arterial-mixed venous oxygen content difference (AVD; P < 0.05 vs. nonpregnant). In late gravid rats, CO was higher than nonpregnant values by 49 +/- 8%, and TPVR was lower by 34 +/- 7% (both P < 0.05). Oxygen consumption and carbon dioxide production were significantly increased, and AVD further narrowed when compared with the nonpregnant control group. With the exception of absent chronic respiratory alkalosis in pregnant rats, we conclude that cardiovascular and respiratory changes in conscious, gravid rats and in pregnant women are comparable. We speculate that the ultimate purpose of many of these adaptations is to increase CO so that oxygen delivery and the supply of nutrients to the uteroplacental units are sufficient or more than sufficient to meet oxygen and nutrient demands. At midgestation, the rise in CO seems to anticipate the oxygen needs of the nascent uteroplacental units.

No abstract

Attenuation of pressor responsiveness to several administered vasoconstrictors is a constant feature of normal gestation in humans and other species, such as the rat. However, the mechanism of this physiological adaptation remains uncertain. Because plasma levels of 17 beta-estradiol (E2) and progesterone (P) increase markedly during pregnancy, we tested the hypothesis that these hormones may mediate the reduced pressor responses. Seven days after bilateral ovariectomy and chronic instrumentation of rats, the pressor responses of arginine vasopressin, angiotensin II, and norepinephrine were tested on two occasions > or = 48 h apart. Then E2, P, or a combination of E2 and P was administered by subcutaneous implantation of 21-day-release steroid pellets. Pressor responses were again tested at various times throughout the period of steroid treatment. The plasma concentrations of the steroids were assessed by radio-immunoassay, and doses of the hormones were given that both approximated and exceeded circulating levels found in our laboratory for gravid rats. Despite chronic elevation of plasma E2 and/or P, we did not observe consistent attenuation of pressor responsiveness in any of the steroid-treatment regimens, nor was a decline in mean arterial pressure observed, which is typically found in rats during late gestation. In conclusion, we are unable to support the hypothesis that E2 and/or P contributes to the diminished pressor responsiveness of rat pregnancy.