Platelet intracellular free calcium concentration in normotensive and hypertensive pregnancies in the human

Platelet function in preeclampsia - a systematic review and meta-analysis

Preeclampsia is a serious pregnancy-related complication. Platelets are potentially important in the pathogenesis of preeclampsia, and platelet function analyses may prove as sensitive preeclampsia biomarkers. This study aimed to systematically review and summarise the literature on platelet function markers in preeclampsia. This systematic review was conducted according to PRISMA and registered in PROSPERO. Relevant studies were identified through PubMed and Embase on 15/08/17. As platelet function markers platelet activation, platelet aggregation and platelet adhesion markers were included. If possible, relevant data were extracted for each marker to perform a meta-analysis of the mean difference between women with and without preeclampsia. All 69 included articles underwent quality rating. Some platelet activation markers, especially p-selectin and mean platelet volume (MPV), were significantly increased comparing the two groups of women, while others were not. The meta-analysis demonstrated that, overall, women with preeclampsia had significantly higher MPV than in women without preeclampsia. No significant difference was found regarding platelet aggreg`ation comparing the two groups. Platelet adhesion was investigated in noneof the included studies. In conclusion, further studies are warranted to investigate platelet activation markers future role as predictive markers in preeclampsia. MPV is suggested as the most promising biomarker for evaluating platelet function in preeclampsia.

Physiologic and pathologic changes of platelets in pregnancy

Platelets are key players in haemostasis and thrombus formation. Defects affecting platelets during pregnancy can lead to heterogeneous complications, such as thrombosis, first trimester miscarriage and postpartum haemorrhage. The incidence of complications is increased in women who have heritable platelet function disorders. Modifications of platelet count or platelet functions during normal pregnancy and preeclampsia will be summarized and the management of pregnant women with heritable platelet function disorders will be discussed.

Vascular and cellular calcium in normal and hypertensive pregnancy

Normal pregnancy is associated with significant hemodynamic changes in the cardiovascular system in order to meet the metabolic demands of mother and fetus. These changes include increased cardiac output, decreased vascular resistance, and vascular remodeling in the uterine and systemic circulation. Preeclampsia (PE) is a major complication of pregnancy characterized by proteinuria and hypertension. Several risk factors have been implicated in the pathogenesis of PE including genetic and dietary factors. Ca2+ is an essential dietary element and an important regulator of many cellular processes including vascular function. The importance of adequate dietary Ca2+ intake during pregnancy is supported by many studies. Pregnancy-associated changes in Ca2+ metabolism and plasma Ca2+ have been observed. During pregnancy, changes in intracellular free Ca2+ concentration ([Ca2+](i)) have been described in red blood cells, platelets and immune cells. Also, during pregnancy, an increase in [Ca2+](i) in endothelial cells (EC) stimulates the production of vasodilator substances such as nitric oxide and prostacyclin. Normal pregnancy is also associated with decreased vascular smooth muscle (VSM) [Ca2+](i) and possibly the Ca2+-sensitization pathways of VSM contraction including protein kinase C, Rho-kinase, and mitogen-activated protein kinase. Ca2+-dependent matrix metalloproteinases could also promote extracellular matrix degradation and vascular remodeling during pregnancy. Disruption in the balance between dietary, plasma and vascular cell Ca2+ may be responsible for some of the manifestation of PE including procoagulation, decreased vasodilation, and increased vasoconstriction and vascular resistance. The potential benefits of Ca2+ supplements during pregnancy, and the use of modulators of vascular Ca2+ to reduce the manifestations of PE in susceptible women remain an important area for experimental and clinical research.

Vaso-active factors in pregnancy

Pregnancy results in profound physiological changes in the cardiovascular system, yet these changes are completely reversible. It is apparent that vaso-active factors, some as yet probably unidentified, which act as humoral or local autocrine or paracrine regulators of vasular resistance, play a major role in these cardio-vascular changes. This role may be heightened in pregnancy when there has to be a large increase in blood flow to the uterus and placenta while maintaining adequate flow to other vascular beds. Our knowledge of the mechanisms of action of these vaso-active factors and their interactions with each other still remains incomplete. Alterations in synthesis and action of these vaso-active factors may occur in pregnancies associated with pregnancy-induced hypertension, pre-eclampsia or intra-uterine growth retardation. Investigation of such alterations may help to elucidate the roles of vaso-active factors in both normal and pathological situations. The gestational hormones oestrogen and progesterone, are obviously prime candidates as overall regulators of the cardiovascular changes of pregnancy and as agents which alter the synthesis or action of other vaso-active factors. Currently, much attention is being focused on the role of local autocrine or paracrine vaso-active factors which may be produced by the endothelium or by the underlying vascular smooth muscle cells and alterations in their production or action in the hyptertensive disorders of pregnancy. The endothelium forms the largest endocrine organ within the body and so its importance in the mediation of vascular events should not be under-estimated. The principal objective of this review is to examine the roles of these many autocrine and paracrine vaso-active factors during pregnancy and their relation with the overall regulation of the vascular system. Changes which may occur and be involved in the aetiology of pre-eclampsia and growth retardation will also be examined.

Dietary calcium and pregnancy-induced hypertension: is there a relation?

The evidence that calcium plays a role in the etiology, prevention, and treatment of pregnancy-induced hypertension (PIH) is reviewed. The precise factors involved in the pathogenesis of PIH are unclear, but several alterations in calcium metabolism have been identified. Epidemiologic data suggest an inverse correlation between dietary calcium intake and incidence of PIH. Although evidence suggests a possible beneficial effect of supplemental calcium, contradictions persist in clinical trials of pregnant women. Presently, there is insufficient evidence to support routine calcium supplementation of all pregnant women. However, high-risk groups, such as pregnant teens, populations with inadequate calcium intake, and women at risk of developing PIH, may benefit from consuming additional dietary calcium.

Association of circulating endothelium and noradrenaline with increased calcium-channel binding sites in the placental bed in pre-eclampsia

OBJECTIVE

To evaluate factors contributing to both placental hypoperfusion and maternal vasoconstriction in pre-eclampsia.

DESIGN

Single centre, comparative study of calcium-channel density and affinity in the placental bed of pregnant women with normotension and pre-eclampsia.

SETTING

Teaching hospital.

PARTICIPANTS

Twenty-two primigravidae in the third trimester of pregnancy: 10 with pre-eclampsia and 12 normotensive.

METHODS

Plasma levels of endothelin-1 (by RIA) and noradrenaline (by HPLC-ED) were measured. Both pharmacological characterisation and anatomical localisation of dihydropyridine-sensitive binding sites (using radioligand-binding studies and autorradiographic techniques) were determined with 3H-isradipine in placental bed tissues to determine both the density (Bmax) and the affinity (Kd) of receptor sites.

RESULTS

Higher plasma levels of endothelin-1 and noradrenalin were found in women with pre-eclampsia compared with normotensive women. Placental bed tissues bound 3H-isradipine in a saturable, reversible time and temperature-dependent manner with very low Kd values. Study of the 3H-isradipine specificity binding included the use of several dihydropyridine displacers. In the group with pre-eclampsia the Scatchard analysis of the results showed a significant increase (P < 0.001) both in the affinity [Kd = 0.23 nmol (0.04) vs 0.45 nmol (0.03), pre-eclampsia vs normotensive] and in the density of calcium-channel binding sites [Bmax = 77.70 fmol/mg (1.30) vs 64.30 fmol/mg (1 80) tissue, pre-eclampsia vs normotensive]. Autoradiography confirmed that in the placental bed tissue of those with pre-eclampsia there was a much higher silver grain density in the arteries walls, compared with normotensive women.

CONCLUSIONS

In pre-eclampsia there is an increase in the maternal circulation of two strong vasoconstrictor factors (endothelin-1 and noradrenalin) and a sharp increase both in the density and the affinity of calcium-channel binding sites in placental bed central area. The latter may strongly contribute to the perpetuation of the uteroplacental hypoperfusion either by itself or by amplifying the local actions of circulating factors, such as endothelin-1 and noradrenalin.

The role of serotonin in preeclamptic hypertension. A review and case report

Generalized arteriolar vasospasm and subsequent damage to vascular endothelium are fundamental to the development of preeclampsia. A possible mechanism underlying this involves increased deportation of trophoblast cells into maternal venous circulation with 5-hydroxytryptamine (serotonin, 5-HT) release as a consequence of platelet aggregation onto these fragments. Ketanserin, a 5-HT(2) receptor blocker, has been used successfully to treat preeclampsia. An examination of the therapeutic role of ketanserin indicates that there should now be a much more detailed assessment of serotonin antagonists in the treatment and prophylaxis of preeclampsia and eclampsia.

Intraplatelet free calcium and calcium-regulating hormones in plasma are not related to the antihypertensive effect of nifedipine in hypertensive pregnancy

Intracellular free calcium regulates contraction-relaxation processes in vascular smooth muscle. We compared intraplatelet free calcium ([Ca2+]i) and pH ([pH]i) in hypertensive pregnant women to those in normotensive pregnant and non-pregnant women. Plasma parathormone and vitamin D metabolite were simultaneously assessed. In hypertensive pregnancy, [Ca2+]i tended to be lower than in normotensive pregnant (P = 0.08) and non-pregnant subjects (P = 0.06). In hypertensive pregnancy, 1,25, (OH)2 vitamin D in plasma was in the same range as in non-pregnant women and significantly lower than in normotensive pregnancy (p < 0.01). The other two vitamin D metabolites, parathormone and [pH]i were equal in the three groups. A five-day nifedipine treatment (10 mg t.i.d.) increased [Ca2+]i in hypertensive pregnant (P < 0.05) and normotensive non-pregnant subjects (P = 0.06), whereas [pH]i (P < 0.05) and 25 (OH) vitamin D (P < 0.05) decreased in the former and 24,25 (OH)2 vitamin D increased in the latter group (P < 0.05). Initial [Ca2+]i did not correlate with blood pressure in any group. The antihypertensive effect of nifedipine did not correlate with any variable measured. In conclusion, [Ca2+]i and calcium-regulating hormones seem not to be related to the antihypertensive effect of nifedipine in hypertensive pregnancy. In this type of hypertension, intraplatelet calcium may not reflect calcium balance in smooth muscle cells regulating vascular tone and blood pressure.

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.

Platelet intracellular free calcium response to arginine vasopressin is similar in preeclampsia and normal pregnancy

OBJECTIVE

Our purpose was to compare the effect of arginine vasopressin stimulation on platelet free intracellular calcium levels in normal pregnancy, incipient preeclampsia, and established preeclampsia.

STUDY DESIGN

Cross-sectional and prospective observational study designs were used. Platelet free intracellular calcium was measured by flow cytometry in 10 nonpregnant female volunteers, 10 women with established preeclampsia and their normal pregnant matched controls, and 64 normal pregnant women at 28 weeks' gestation. All pregnant women were nulliparous.

RESULTS

Increased response to arginine vasopressin stimulation was observed in nonpregnant compared with pregnant women (p < 0.001). No differences were observed between women with normal pregnancy, incipient preeclampsia, or established preeclampsia.

CONCLUSION

The platelet intracellular free calcium response to arginine vasopressin is not a feature of incipient or established preeclampsia. Therefore, in contrast to a previous report, it does not appear to have value as a predictive test for preeclampsia.

Preeclampsia is not associated with altered platelet vasopressin binding and cytosolic Ca++ concentration

OBJECTIVES

Preeclampsia is an important cause of fetal and maternal morbidity and mortality. Recently it was described that platelet cytosolic Ca++ levels could be used to screen for preeclampsia. The current study investigated platelet arginine vasopressin receptor characteristics, platelet cytosolic Ca++ levels, plasma- and platelet-bound arginine vasopressin in white pregnant women.

STUDY DESIGN

In a cross-sectional study nine third-trimester nulliparous pregnant women with gestational hypertension (seven with proteinuria, two with excessive weight gain without proteinuria) were compared with nine healthy nulliparous pregnant women matched for gestation length and age and 10 healthy age-matched nonpregnant women. Determined were (1) platelet arginine vasopressin receptor number and affinity, (2) platelet cytosolic Ca++ levels, both basal and on arginine vasopressin or thrombin stimulation, and (3) plasma- and platelet-bound arginine vasopressin levels.

RESULTS

None of the measured parameters differed significantly among the three groups studied. Mean arginine vasopressin receptor number and affinity ranged from 108 to 143 receptors per platelet and 0.35 to 0.40 nmol/L, respectively. A single population of binding sites was found (Hill number 0.96). Basal Ca++ levels ranged from 113.4 to 133.3 nmol/L, on arginine vasopressin stimulation from 199 to 250 nmol/L. Median arginine vasopressin levels in platelet-poor plasma were between 1.2 and 2.4 pg/ml, with circulating platelets being estimated to possess two to five molecules of arginine vasopressin per platelet. A significant correlation was found between platelet cytosolic Ca++ levels before and after arginine vasopressin stimulation (r = 0.69, p < 0.001) and a weak correlation between platelet receptor density and arginine vasopressin-stimulated platelet cytosolic Ca++ levels (r = 0.38, p < 0.05).

CONCLUSIONS

The studied parameters, platelet cytosolic Ca++ levels, whether basal or after stimulation with arginine vasopressin and vasopressin platelet receptor density and affinity, do not discriminate already hypertensive or preeclamptic white women from nondiseased subjects. A valuable test to screen for preeclampsia awaits further prospective studies.

Platelet cytosolic free calcium before and after antihypertensive treatment in perinephritis hypertension of the rabbit

Cytosolic free calcium concentration ([Ca2+]i) in platelets has been reported to be elevated in human essential hypertension, to be positively correlated with blood pressure and to decrease with blood pressure reduction. However, some groups have been unable to confirm these findings in either humans or hypertensive rats. We have examined the relationship between platelet [Ca2+]i and blood pressure in the perinephritis model of hypertension in the rabbit. In addition, the effects of both acute and chronic treatment with verapamil or prazosin were studied. Mean arterial pressure, heart rate and platelet [Ca2+]i were measured before and after treatment. Platelet [Ca2+]i was measured by the Quin 2 fluorescence technique. Platelet [Ca2+]i was similar for the normotensive and hypertensive rabbits, and no correlation between platelet [Ca2+]i and blood pressure was observed. None of the antihypertensive treatments produced a lowering of platelet [Ca2+]i. Therefore we conclude that platelet [Ca2+]i is unlikely to be a universally useful index of ([Ca2+]i in vascular smooth muscle of resistance vessels.