Effects of pregnancy on femoral microvascular responses in the rat

Uteroplacental insufficiency programmes vascular dysfunction in non-pregnant rats: compensatory adaptations in pregnancy

Intrauterine growth restriction is a risk factor for cardiovascular disease in adulthood. We have previously shown that intrauterine growth restriction caused by uteroplacental insufficiency programmes uterine vascular dysfunction and increased arterial stiffness in adult female rat offspring. The aim of this study was to investigate vascular adaptations in growth restricted female offspring when they in turn become pregnant. Uteroplacental insufficiency was induced in WKY rats by bilateral uterine vessel ligation (Restricted) or sham surgery (Control) on day 18 of pregnancy. F0 pregnant females delivered naturally at term. F1 Control and Restricted offspring were mated at 4 months of age and studied on day 20 of pregnancy. Age-matched non-pregnant F1 Control and Restricted females were also studied. Wire and pressure myography were used to test endothelial and smooth muscle function, and passive mechanical wall properties, respectively, in uterine, mesenteric, renal and femoral arteries of all four groups. Collagen and elastin fibres were quantified using polarized light microscopy and qRT-PCR. F1 Restricted females were born 10–15% lighter than Controls (P <0.05). Non-pregnant Restricted females had increased uterine and renal artery stiffness compared with Controls (P <0.05), but this difference was abolished at day 20 of pregnancy. Vascular smooth muscle and endothelial function were preserved in all arteries of non-pregnant and pregnant Restricted rats. Collagen and elastin content were unaltered in uterine arteries of Restricted females. Growth restricted females develop compensatory vascular changes during late pregnancy, such that region-specific vascular deficits observed in the non-pregnant state did not persist in late pregnancy.

Does nitric oxide contribute to the basal vasodilation of pregnancy in conscious rabbits?

Pregnancy produces marked systemic vasodilation, but the mechanism is unknown. Experiments were performed in conscious rabbits to test the hypotheses that increased nitric oxide (NO) production contributes to the increased vascular conductance, but that the contribution varies among vascular beds. Rabbits were instrumented with aortic and vena caval catheters and ultrasonic flow probes implanted around the ascending aorta, superior mesenteric artery, terminal aorta, and/or a femoral artery. Hemodynamic responses to intravenous injection of N(omega)-nitro-L-arginine (L-NA; 20 mg/kg or increasing doses of 2, 5, 10, 15, and 20 mg/kg) were determined in rabbits first before pregnancy (NP) and then at the end of gestation (P). L-NA produced similar increases in arterial pressure between groups, but the following responses were larger (P < 0.05) when the rabbits were pregnant: 1) decreases in total peripheral conductance [-3.7 +/- 0.3 (NP), -5.0 +/- 0.5 (P) ml x min(-1) x mmHg(-1)], 2) decreases in mesenteric conductance [-0.47 +/- 0.05 (NP), -0.63 +/- 0.07 (P) ml x min(-1) x mmHg(-1)], 3) decreases in terminal aortic conductance [-0.43 +/- 0.05 (NP), -0.95 +/- 0.19 ml x min(-1) x mmHg(-1) (P)], and 4) decreases in heart rate [-41 +/- 4 (NP), -62 +/- 5 beats/min (P)]. Nevertheless, total peripheral and terminal aortic conductances remained elevated in the pregnant rabbits (P < 0.05) after L-NA. Furthermore, decreases in cardiac output and femoral conductance were not different between the reproductive states. We conclude that the contribution of NO to vascular tone increases during pregnancy, but only in some vascular beds. Moreover, the data support a role for NO in the pregnancy-induced increase in basal heart rate. Finally, unknown factors in addition to NO must also underlie the basal vasodilation observed during pregnancy.

Regional conductance changes during hemorrhage in pregnant and nonpregnant conscious rabbits

Late pregnant (P) conscious rabbits are less able to maintain arterial pressure during hemorrhage than nonpregnant (NP) animals. This study tested the hypothesis that the difference is due in part to less reflex vasoconstriction when the rabbits are P. Rabbits (n = 14) were instrumented with arterial and venous catheters as well as ultrasonic flow probes around the superior mesenteric, renal, and/or terminal aortic arteries. Pregnancy increased (P < 0.05) blood volume [235 +/- 5 (P) vs. 171 +/- 3 (NP) ml], terminal aortic conductance [1.88 +/- 0.11 (P) vs. 0.98 +/- 0.06 (NP) ml. min(-1). mmHg(-1)], mesenteric conductance [1.20 +/- 0.19 (P) vs. 0.80 +/- 0. 05 (NP) ml. min(-1). mmHg(-1)], and heart rate [191 +/- 4 (P) vs. 162 +/- 3 (NP) beats/min] and decreased arterial pressure [59 +/- 1 (P) vs. 67 +/- 2 (NP) mmHg; P < 0.05]. Renal conductance was unaltered. The rabbits were bled in both the NP and P states at 2% of the initial blood volume per minute until arterial pressure fell below 45 mmHg. Arterial pressure fell with less blood loss in P rabbits [28 +/- 2% (P) vs. 39 +/- 2% (NP) of initial blood volume; P < 0.001]. Terminal aortic conductance decreased (P < 0.001) before the pressure fall in both groups, but the response was reduced in P rabbits. Mesenteric and renal conductances did not change in either group before the blood pressure fall. During the pressure fall, terminal aortic conductance increased (P < 0.05) only in NP rabbits. Mesenteric conductance increased in both groups. In summary, rabbits in late gestation are less able to maintain arterial pressure during hemorrhage, at least in part because of reduced vasoconstriction in tissues perfused by the terminal aorta.

Preeclampsia selectively impairs endothelium-dependent relaxation and leads to oscillatory activity in small omental arteries

The vascular pathophysiology of preeclampsia, a hypertensive disorder unique to human pregnancy, has been postulated to be due to endothelial dysfunction, primarily manifest as deficient nitric oxide (NO) synthesis. We evaluated contraction (KCl and arginine vasopressin [AVP]) and dilation (acetylcholine and bradykinin) in small resistance-size omental arteries obtained during surgery from women with preeclampsia, postulating that these vessels would exhibit augmented contraction and diminished endothelium-dependent relaxation, most likely due to decreased NO synthesis. For comparison, vessels were also obtained from normotensive gravidas, pregnant women with chronic hypertension, or with chronic hypertension and superimposed preeclampsia, as well as from premenopausal nonpregnant controls. Vessels of approximately 200 micron in internal diameter were studied in vitro using a Mulvany-Halpern myograph. Maximal contraction due to either KCl or AVP was significantly augmented in vessels from women with preeclampsia; these vessels all exhibited endothelium- and cyclooxygenase-dependent phasic oscillations while vessels from all other groups exhibited only tonic contractions. Acetylcholine and bradykinin both led to dose- and endothelium-dependent relaxation which was unaffected by inhibitors of NO synthesis. Responses to bradykinin were similar in vessels from normal pregnant and preeclamptic women while those to acetylcholine were absent in vessels from women with preeclampsia. These data suggest specific defects in resistance-artery endothelium from women with preeclampsia.

Nitric oxide and pregnancy

We have hypothesized that an alteration in the production of endothelium-dependent factors by sex hormones is a potential unifying mechanism for both the decreased arterial contractility and the redistribution of cardiac output characteristic of normal pregnancy. Thus, the effect of pregnancy/ estradiol on any one vascular bed will reflect the number and distribution of estrogen receptors. In this article, we review what is known about the effects of pregnancy and estrogen on nitric oxide synthase. Pregnancy increases Ca(2+)-dependent NOS activity early in gestation. The timing of the increase parallels the increase in plasma estradiol concentration. The increase in maternal brain NOS during pregnancy is blocked by tamoxifen. cGMP content increases along a similar time course in most but not all tissues. The changes in cGMP more closely approximate the changes in blood flow during pregnancy. This suggests that multiple elements of the NO:cGMP pathway are altered by pregnancy. It also shows that cGMP content cannot always be used as a surrogate for NOS activity. Estradiol, but not progesterone or testosterone, increases CA(2+)-dependent NOS activity. NO accounts for some, but not all of the pregnancy-associated changes in maternal arterial contractile response. It is not involved in uterine quiescence. Nitric oxide synthase is developmentally regulated in the fetus and is likely important in regulating the distribution of fetal blood flow.