Pregnancy-induced hypertension: current concepts

The Role of Macula Densa Nitric Oxide Synthase 1 Beta Splice Variant in Modulating Tubuloglomerular Feedback

Abnormalities in renal electrolyte and water excretion may result in inappropriate salt and water retention, which facilitates the development and maintenance of hypertension, as well as acid-base and electrolyte disorders. A key mechanism by which the kidney regulates renal hemodynamics and electrolyte excretion is via tubuloglomerular feedback (TGF), an intrarenal negative feedback between tubules and arterioles. TGF is initiated by an increase of NaCl delivery at the macula densa cells. The increased NaCl activates luminal Na-K-2Cl cotransporter (NKCC2) of the macula densa cells, which leads to activation of several intracellular processes followed by the production of paracrine signals that ultimately result in a constriction of the afferent arteriole and a tonic inhibition of single nephron glomerular filtration rate. Neuronal nitric oxide (NOS1) is highly expressed in the macula densa. NOS1β is the major splice variant and accounts for most of NO generation by the macula densa, which inhibits TGF response. Macula densa NOS1β-mediated modulation of TGF responses plays an essential role in control of sodium excretion, volume and electrolyte hemostasis, and blood pressure. In this article, we describe the mechanisms that regulate macula densa-derived NO and their effect on TGF response in physiologic and pathologic conditions. © 2023 American Physiological Society. Compr Physiol 13:4215-4229, 2023.

Macula Densa NOS1β Modulates Renal Hemodynamics and Blood Pressure during Pregnancy: Role in Gestational Hypertension

BACKGROUND

Regulation of renal hemodynamics and BP via tubuloglomerular feedback (TGF) may be an important adaptive mechanism during pregnancy. Because the β-splice variant of nitric oxide synthase 1 (NOS1β) in the macula densa is a primary modulator of TGF, we evaluated its role in normal pregnancy and gestational hypertension in a mouse model. We hypothesized that pregnancy upregulates NOS1β in the macula densa, thus blunting TGF, allowing the GFR to increase and BP to decrease.

METHODS

We used sophisticated techniques, including microperfusion of juxtaglomerular apparatus in vitro, micropuncture of renal tubules in vivo, clearance kinetics of plasma FITC-sinistrin, and radiotelemetry BP monitoring, to determine the effects of normal pregnancy or reduced uterine perfusion pressure (RUPP) on macula densa NOS1β/NO levels, TGF responsiveness, GFR, and BP in wild-type and macula densa-specific NOS1 knockout (MD-NOS1KO) mice.

RESULTS

Macula densa NOS1β was upregulated during pregnancy, resulting in blunted TGF, increased GFR, and decreased BP. These pregnancy-induced changes in TGF and GFR were largely diminished, with a significant rise in BP, in MD-NOS1KO mice. In addition, RUPP resulted in a downregulation in macula densa NOS1β, enhanced TGF, decreased GFR, and hypertension. The superimposition of RUPP into MD-NOS1KO mice only caused a modest further alteration in TGF and its associated changes in GFR and BP. Finally, in African green monkeys, renal cortical NOS1β expression increased in normotensive pregnancies, but decreased in spontaneous gestational hypertensive pregnancies.

CONCLUSIONS

Macula densa NOS1β plays a critical role in the control of renal hemodynamics and BP during pregnancy.

Pregnancy-induced Cardiovascular Pathologies: Importance of Structural Components and Lipids

Pregnancy leads to adaptations for maternal and fetal energy needs. The cardiovascular system bears the brunt of the adaptations as the heart and vessels enable nutrient supply to maternal organs facilitated by the placenta to the fetus. The components of the cardiovascular system are critical in the balance between maternal homeostatic and fetus driven homeorhetic regulation. Since lipids intersect maternal cardiovascular function and fetal needs with growth and in stress, factors affecting lipid deposition and mobilization impact risk outcomes. Here, the cardiovascular components and functional derangements associated with cardiovascular pathology in pregnancy, vis-à-vis lipid deposition, mobilization and maternal and/or cardiac and fetal energy needs are detailed. Most reports on the components and associated pathology in pregnancy, are on derangements affecting the extracellular matrix and epicardial fat, followed by the endothelium, vascular smooth muscle, pericytes and myocytes. Targeted studies on all cardiovascular components and pathological outcomes in pregnancy will enhance targeted interventions.

Long-chain polyunsaturated fatty acids, pregnancy, and pregnancy outcome

During pregnancy, essential long-chain polyunsaturated fatty acids (LCPUFAs) play important roles as precursors of prostaglandins and as structural elements of cell membranes. Throughout gestation, accretion of maternal, placental, and fetal tissue occurs and consequently the LCPUFA requirements of pregnant women and their developing fetuses are high. This is particularly true for docosahexaenoic acid (DHA; 22:6n-3). The ratio of DHA to its status marker, docosapentaenoic acid (22:5n-6), in maternal plasma phospholipids decreases significantly during pregnancy. This suggests that pregnancy is associated with maternal difficulty in coping with the high demand for DHA. The DHA status of newborn multiplets is significantly lower than that of singletons; the same is true for infants of multigravidas as compared with those of primigravidas and for preterm compared with term neonates. Because the LCPUFA status at birth seems to have a long-term effect, the fetus should receive an adequate supply of LCPUFAs. Data from an international comparative study indicated that, especially for n-3 LCPUFAs, the fetus is dependent on maternal fatty acid intake; maternal supplementation with LCPUFAs, their precursors, or both increased LCPUFA concentrations in maternal and umbilical plasma phospholipids. However, significant competition between the 2 LCPUFA families was observed, which implies that effective supplementation requires a mixture of n-6 and n-3 fatty acids. Further research is needed to determine whether higher LCPUFA concentrations in plasma phospholipid will have functional benefits for mothers and children.

Anaesthesia for caesarean section in a patient with recent subarachnoid haemorrhage and severe pre-eclampsia

Subarachnoid haemorrhage is a leading 'indirect' cause of maternal death in the UK. We describe the case of a 43-year-old woman who presented with headache, photophobia and neck stiffness of sudden onset at 32 weeks' gestation. Cerebral computed tomography demonstrated subarachnoid blood in the cisterns around the midbrain, and oral nimodipine was started to prevent vasospasm. Preparations were made for endovascular coil embolisation in the event of identification of a posterior circulation aneurysm. However, angiography under general anaesthesia failed to reveal any vascular abnormality. On emergence from anaesthesia, headache persisted, and over the next 24 h severe pre-eclampsia developed. Magnesium sulphate was started, and urgent Caesarean section performed under general anaesthesia without incident. The rationale for the neuroradiological, obstetric and anaesthetic management is discussed.

A prospective study of the impact of psychosocial and lifestyle variables on pregnancy complications

This prospective study examines the influence of maternal stress, social support and lifestyle variables reported over the course of pregnancy on subsequent gestational and intrapartum complications. Demographic and biomedical factors were also studied. One hundred and two women were followed on a monthly basis beginning in the third month of pregnancy. Measures of daily stress (hassles), state-anxiety (STAI-state) and pregnancy-specific stress were taken monthly. Pregnancy progress and lifestyle behaviors such as smoking, caffeine and alcohol intake were assessed by trimester. One month following delivery, a telephone interview was conducted to inquire about the labor/delivery and infant status. Sixty-three per cent of the women experienced a pregnancy complication. Three groups consisting of women who experienced gestational complications, intrapartum complications only, and no complications were formed. Women who subsequently experienced gestational complications reported over the course of their pregnancy higher levels of state-anxiety, daily hassles and pregnancy-specific stress beginning in the third month of pregnancy. Women who experienced complications during the intrapartum period only reported higher daily hassles during pregnancy and consumed more caffeinated beverages compared to the other groups. Primiparous women were more likely to experience gestational and/or intrapartum complications than multiparous women. These findings support a role for psychosocial variables in pregnancy complications. The results indicate that certain psychosocial and lifestyle variables may be differentially associated with complications occurring at various phases of pregnancy.

Thromboprophylaxis, coagulation disorders, and regional anaesthesia

The surgery and trauma-induced modulation of the coagulation system includes a considerable risk of perioperative thromboembolic complications unless effective thromboprophylactic treatment is given. In the present survey the patient at risk of deep vein thrombosis (DVT) and pulmonary embolism (PE) is characterized and the documented efficacy of different currently used thromboprophylactic regimens is summarized. Systemic thromboprophylactic treatment may include a risk of an increased bleeding tendency which may lead to haemorrhagic complications. In patients with a coagulation abnormality or in patients receiving anticoagulants for perioperative thromboprophylaxis there is a fear among anaesthesiologists that the use of regional anaesthesia (spinal or epidural) may be associated with spinal haemorrhagic complications, i.e. with spinal haematoma formation leading to compression of the spinal cord and severe neurologic sequelae. Present aspects on the risk of spinal haematoma formation at the combined use of pharmacological thromboprophylactic regimens and spinal or epidural anaesthesia/ analgesia are therefore summarized. Pregnancy is associated with changes in the haemostatic system, which in the preeclamptic or eclamptic patient may be rather pronounced and constitute a clinical problem since regional anaesthetic techniques are often preferred for obstetric anaesthesia/analgesia. The specific problems to be considered prior to the choice of regional anaesthesia/analgesia for a parturient with a suspected coagulation disorder are therefore commented on in more detail. Finally, recommendations are given for safe spinal and epidural analgesic and anaesthetic routines in patients with potential haemostatic disturbances due to thromboprophylactic treatment with anticoagulants or bleeding disorders.

First antenatal visit haematocrit and pregnancy induced hypertension

First antenatal visit haematocrit values during the first and second trimesters were retrospectively collected from 546 nulliparas with singleton pregnancies. The results were analysed for correlation with development of pregnancy induced hypertension (PIH) later in pregnancy. It was found that women with higher haematocrit values, especially over 0.40, had an increased risk of developing PIH. However, there appeared to be no absolute level of haematocrit which had sufficient discriminative value to be useful in clinical practice.

Sera from preeclamptic women increase the content of triglycerides and reduce the release of prostacyclin in cultured endothelial cells

The causes of the "endothelial dysfunction" accompanying preeclampsia are unknown. Women with preeclampsia have a marked hyperlipidemia which reflects altered lipid metabolism. We asked if the hyperlipidemic sera of preeclamptic women could cause altered endothelial cell properties. Cultured endothelial cells were incubated with sera from women with preeclampsia (PE) or normal pregnancies as controls. Fifty PE-sera were tested and in 45 cases the endothelial cells acquired a large number of sudanophilic granules which by electron microscopy had lipid appearance. In 31 incubations with 31 individual control sera cellular lipid granules were observed in 4 cases. The cellular triglyceride content was increased to 153 +/- 30 compared to 48 +/- 10 micrograms/mg cell protein in the control cells. Furthermore, the endothelial release of prostacyclin, measured as 6-keto PGF1 alpha, was 8.8 +/- 0.6 ng/mg cell protein in cells incubated with PE-sera as compared to 40.3 +/- 6.4 ng/mg in the control cells.

CONCLUSION

The hyperlipidemic sera from preeclamptic women induced triglyceride accumulation in cultured endothelial cells. This was accompanied by altered endothelial function as demonstrated by reduced prostacyclin release.

Epidurals, spinals and bleeding disorders in pregnancy: a review

Paraplegia caused by spinal haemorrhage is a very rare but disastrous complication of spinal or epidural insertion. The risk in uncomplicated surgical and obstetric patients is outlined. Bleeding disorders in pregnant patients may prevent the use of major regional anaesthesia. Factors which influence the choice of anaesthetic technique for patients with pregnancy-induced hypertension, von Willebrand's disease, and anticoagulation therapy, are discussed.