Vaso-active factors in pregnancy

Dietary calcium, magnesium, and phosphorus intakes and risk of stroke in Chinese adults

Controversial results have been reported about the association of calcium, magnesium, and phosphorus and stroke risk, but none in China. To investigate the association between dietary calcium, magnesium, phosphorus, and stroke incidence in Chinese adults, we collected data from the China Health and Nutrition Survey (CHNS) from 2004 to 2011, including 6411 participants aged 45-79 years and free of stroke at baseline. Diet was assessed by interviews combining 3-d 24-h food recalls and household food inventory weighing at each survey round. The stroke incident was identified based on the validated self-report. Multivariate Cox regression models were used to estimate hazard ratios (HRs) and 95% confidence intervals (CIs). For 32,024 person-years of follow-up, 179 stroke cases were documented. After adjustment for major lifestyle and dietary risk factors, calcium intake was positively associated with reduced stroke risk, and the HR of stroke comparing extreme quartiles was 0.53 (95% CI 0.29-0.96, P_trend = 0.03). In further stratified analyses, significant heterogeneity across sex strata was found (P_interaction = 0.03). Dietary calcium intake among men was more inversely related to stroke, with HRs being 0.33 (95% CI 0.15-0.76, P _trend = 0.02), compared to 1.24 (95% CI 0.46-3.35, P_trend = 0.89) among women. However, no significant association between stroke and magnesium or phosphorus was revealed. Our findings suggest that higher dietary calcium intake was associated with a lower risk of stroke in Chinese adults, particularly in men.

Possible mechanisms for the suppressing action of 17beta-estradiol on beta-adrenoceptor-mediated vasorelaxation in rat aorta

The mode of action of estrogen on beta-adrenoceptor-mediated relaxation was investigated by using isolated ring preparations of thoracic aorta from ovariectomized rats. Administration of 17beta-estradiol to ovariectomized rats significantly suppressed isoprenaline-induced relaxation of aortic rings. There was no alteration in the beta-adrenoceptor binding characteristics. The suppressing action of 17beta-estradiol on the N(G)-nitro-L-arginine and indomethacin-resistant relaxation induced by isoprenaline disappeared after pretreatment with N,N-diethylaminoethyl-2,2-diphenylvalerate hydrochloride (SKF 525A), an inhibitor of cytochrome P450 (CYP). The levels of CYP2C11 expression were the highest of the CYP mRNAs examined in rat aorta. 17beta-Estradiol replacement increased the expression of CYP2C11 mRNA in the aorta, compared with that in ovariectomized rats. These results suggest that estrogen suppresses beta-adrenoceptor-mediated vasorelaxation, and that the mechanisms may be associated with alterations in CYP2C11 metabolites.

Role of peroxynitrite in altered fetal-placental vascular reactivity in diabetes or preeclampsia

Oxidative stress may increase production of superoxide and nitric oxide, leading to formation of prooxidant peroxynitrite to cause vascular dysfunction. Having found nitrotyrosine residues, a marker of peroxynitrite action, in placental vessels of preeclamptic and diabetic pregnancies, we determined whether vasoreactivity is altered in these placentas and treatment with peroxynitrite produces vascular dysfunction. The responses of diabetic, preeclamptic, and normal placentas to increasing concentrations of the vasoconstrictors U-46619 (10(-9)-10(-7) M) and ANG II (10(-9)-10(-7) M) and the vasodilators glyceryl trinitrate (10(-9)-10(-7) M) and prostacyclin (PGI(2); 10(-8)-10(-6) M) were compared as were responses to these agents in normal placentas before and after treatment with 3.16 x 10(-4) M peroxynitrite for 30 min. Responses to both vasoconstrictors and vasodilators were significantly attenuated in diabetic and preeclamptic placentas compared with controls. Similarly, responses to U-46619, nitroglycerin, and PGI(2), but not ANG II, were significantly attenuated following peroxynitrite treatment. The presence of nitrotyrosine residues confirmed peroxynitrite interaction with placental vessels. Overall, our data suggest that peroxynitrite formation is capable of attenuating vascular responses in the human placenta.

Calcium metabolism, calcium supplementation and hypertensive disorders of pregnancy

In recent years growing attention has been directed towards the possible role of calcium in the development of pregnancy-induced hypertension and preeclampsia. Several studies describe calcium metabolism in normal and hypertensive pregnancy, but so far, they have shown discrepant and inconsistent results. Intracellular free calcium, which plays an important role in vascular smooth muscle contraction, has been claimed as a pathogenic factor in hypertensive disorders of pregnancy. Although there is discordance in the data, a possible role of intracellular calcium in the development of hypertensive disorders of pregnancy cannot be excluded. Observational studies in pregnant women suggest an inverse association between calcium intake and the incidence of hypertensive disorders of pregnancy. Despite large methodological differences, the results from the calcium supplementation trials support this finding. Although it is rather difficult to isolate the effect of calcium intake from the intake of other mineral elements, results from calcium supplementation trials are supportive for calcium being the most important. Proposed mechanisms by which calcium supplementation may lower blood pressure involve changes in parathyroid hormone (PTH) level, the renin-angiotensin system and calcium as a modifier of vascular agent regulation, but none of these have yet been elucidated. At present, circumstantial evidence suggest a positive role for calcium in the prevention of hypertensive disorders of pregnancy, but definite evidence is lacking and further research is warranted.

Endothelium-dependent relaxation of human resistance arteries in pregnancy

OBJECTIVE

Reduced peripheral vascular resistance is an integral component of vascular adaptation in pregnancy. The precise mediators of the reduced resistance are unknown, however, and are often assessed by indirect means. In this study we investigated endothelium-dependent relaxation directly by examining acetylcholine-mediated relaxation in small arteries dissected from the subcutaneous fat layer undergoing biopsy at the time of gynecologic surgery and cesarean section.

STUDY DESIGN

By means of a small vessel myograph we measured tension in resistance arteries of normal pregnant (n = 22) and nonpregnant (n = 10) women and assessed the contributions of vasodilatory prostanoids and endothelium-derived relaxing factor on endothelium-dependent relaxation, as elicited by acetylcholine (1 nmol/L to 10 mumol/L) after precontraction with 3 mumol/L norepinephrine.

RESULTS

Endothelium-dependent relaxation was similar in arteries of pregnant and nonpregnant women. Sodium nitroprusside-induced relaxation was similar in arteries of pregnant and nonpregnant women.

CONCLUSION

This study fails to demonstrate any increase in acetylcholine-induced endothelium-dependent relaxation in small subcutaneous arteries in pregnancy.

The role of extracellular calcium in pregnancy-induced attenuation of phenylephrine contraction in rat aorta with functional endothelium

The effect of pregnancy on the supply of calcium ions for the contractile responses of rat aortic rings to phenylephrine was investigated. The contractility of intact aortic rings from pregnant rats, compared with that of similar rings from non-pregnant rats, to phenylephrine and potassium chloride was significantly decreased. Contractions of rings from non-pregnant rats, pretreated with phenylephrine or potassium chloride, in response to calcium chloride were greater than those of similarly treated rings from pregnant rats. When the concentration of calcium chloride in the medium bathing the rings was reduced to 0.8 mmol.l-1, the contractile response to phenylephrine was significantly (P < 0.005) inhibited in rings from both pregnant and non-pregnant rats but to a greater extent in rings from non-pregnant rats. Contractions of aortic rings from pregnant rats in response to phenylephrine in calcium-free medium were similar to those of rings from non-pregnant rats, suggesting equal dependence on calcium from intracellular stores. The results suggest that pregnancy decreased the response to calcium influx into the aortic smooth muscle cells through both receptor- and voltage-operated calcium entry pathways. Since de-endothelialization reversed the pregnancy-induced diminished contraction to phenylephrine, it is likely that pregnancy interfered with contractions induced by activation of receptors with phenylephrine through enhanced production of endothelium-derived relaxing factor(s).

The role of endothelium in phenylephrine- and potassium-induced contractions of the rat aorta during pregnancy

The involvement of endothelium in the contractile responses of rat aortic rings to phenylephrine and potassium chloride in pregnancy was examined. Contractions in response to both agents were significantly greater in rings from non-pregnant rats than in rings from pregnant rats, and they were unaltered by treatment of the rings with indomethacin. De-endothelialization potentiated the contractions of rings from pregnant rats in response to phenylephrine, but had no significant effect on similar rings contracted with potassium chloride. Whereas de-endothelialization had no significant effect on the contractions to phenylephrine in rings from non-pregnant rats, it decreased those of rings from the same type of rats, contracted with potassium chloride. Pregnancy significantly inhibited contractions in response to calcium chloride of rings treated with phenylephrine or potassium chloride. The effect of endothelium removal on contractions to calcium chloride in rings from pregnant and non-pregnant rats treated with phenylephrine or potassium chloride was similar to that observed for phenylephrine-induced and potassium chloride-induced contractions, respectively. Contractions of intact aortic rings from pregnant and non-pregnant rats to phenylephrine in calcium-free medium were similar. Results of the study suggest that the effect of pregnancy on the contractions of the rat aorta in response to phenylephrine and potassium chloride is at least partly mediated by the endothelium and is independent of prostaglandin synthesis. The endothelial factor involved in this effect appears to modulate contractions by interfering with calcium influx through the receptor-operated calcium channels and the voltage-operated calcium channels.

Plasma from normal pregnant women alters the reactivity of rabbit aortic smooth muscle with functional endothelium

1. The effects of normal human pregnant plasma on the reactivity of rabbit aortic rings with documented endothelium have been studied. The rings were incubated in plasma from either normal pregnant or non-pregnant women and the effects of incubation on their responses to noradrenaline (NA), calcium chloride stimulation and potassium chloride-induced relaxation were examined. 2. There was no significant difference in the responses to NA in rings incubated in plasma from either pregnant or non-pregnant women. 3. The responses of the rings, incubated in both types of plasma, to calcium chloride stimulation through receptor-operated channels were similar, but rings incubated in plasma from pregnant women demonstrated decreased sensitivity and contractility to calcium chloride stimulation through voltage operated channels. 4. Potassium chloride-induced relaxation was significantly enhanced in rings incubated in plasma from normal pregnant women. The pattern of the responses was unaltered by either de-endothelialization or pretreatment of the rings with indomethacin. 5. The effects of plasma from pregnant women are similar to what has been reported as characteristic of vascular smooth muscle in normal pregnancy.