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Hypertension complicates approximately 10% of all pregnancies and is a leading cause of maternal and foetal mortality and morbidity world-wide. Pre-eclampsia is a major subgroup of these hypertensive disorders. It is defined as a rise in blood pressure to 140/90mm Hg or greater accompanied by proteinuria and usually presents after 20 weeks of pregnancy. Much of the early research into this disorder has concentrated on the determination of vasoactive compounds such as the renin-angiotensin system and prostacyclin. In the 1980's it was discovered that an inorganic free radical molecule, nitric oxide (NO), was released from the endothelium cell lining of the vasculature and was involved in regulating vasodilation of the vasculature walls via smooth muscle.It was also shown to have cytotoxic effects on bacteria, to inhibit platelet aggregation and to act as a neurotransmitter. The aim of this research was to assess the role of nitric oxide in preeclampsia. This was accomplished by the analysis of its oxidation products nitrite and nitrate in plasma from women with pre-eclampsia compared with those from normotensive pregnancies. A simple and robust assay for nitrite and nitrate was developed using ion chromatography. Initial experiments using isocratic elution with conductivity detection on a Dionex QIC system with an AS4A-SC column showed promise but were unsatisfactory due to the interference from chloride ions. Successive improvements to the technique involved changing the elution system to a gradient, initially to one with carbonate and subsequently to chloride, changing the detector system to direct UV detection at 214nm and changing the column to a high capacity, strong exchanger type. The resulting method shows good resolution, does not suffer from chloride overload and was simple to use. Control results for 200 serum samples showed that the male mean nitrite and nitrate levels were 3.34 ± 5.17 μmol L" and 42.1 ± 33.1 μmol L-1 respectively while female levels were 4.74 ± 11.7 μmol L-' and 37.5 ± 27.9 μmol L" respectively. Addition information on the free-radical status of the pregnant study groups was assessed by determination of lipid peroxides and the peroxynitrite product, 3-nitrotyrosine. An improved GC-MS method was developed to quantify total fatty acids and lipid peroxides. A new reversed phase HPLC technique for the analysis of free 3-nitrotyrosine in human plasma/serum was also developed although sample numbers were not as great as expected. Statistical analysis using F-tests, t-tests and the Mann-Whitney analysis did not show any difference in nitric oxide metabolites, lipids, lipid peroxides or peroxynitrite between gestation matched normotensive pregnant women and those with pre-eclampsia or pregnancy-induced hypertension. Published research has shown a vital role for nitric oxide in the maintenance of blood flow in normal pregnancy. This research does not support evidence for diminished or enhanced nitric oxide production in pre-eclampsia compared with normal pregnancy.

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An interaction of the renin-angiotensin system and the autonomic nervous system was demonstrated in vivo during activation of the former by sodium depletion in the dog and the latter during application of lower body negative pressure (LBNP) in the cat. In the dog, inhibitors of angiotensin converting enzyme (ACE), teprotide and captopril, together with an angiotensin II (AII) antagonist, saralasin, and a peptide inhibitor of renin, H77, given intravenously lowered blood pressure (BP) by reducing peripheral resistance in relation to the prevailing level of plasma renin activity (PRA). They did so without changing cardiac output or heart rate as PRA rose above the resting level. The lack of tachycardia was due to a resetting of the baroreflex without a change in sensitivity as teprotide unmasked an action of AII at a peripheral site since when administered into a lateral cerebral ventricle it was ineffective. In the cat, teprotide and saralasin enhanced the fall in BP induced by LBNP and impaired its recovery. When these inhibitors were given during LBNP, a greater and more sustained fall in BP was seen than with either inhibitor alone. This occurred before activation of plasma renin and was not associated with a reduction in sympathetic efferent nerve activity. Further studies revealed that teprotide, captopril and enalapril interfered with neurogenic vasoconstriction involving AII in pithed rats and moreover, captopril was active in lowering BP in two strains of rat shown to be particularly sensitive to the adrenergic potentiating effect of AII. These findings have provided physiological evidence in vivo supporting a peripheral interaction between the autonomic nervous system and AII even at low levels of activation which potentiate adrenergic mechanisms and maintain homeostatic reflexes. They suggest that a significant part of the hypotensive activity of ACE inhibitors is due to interference with facilitatory actions of AII on the autonomic nervous system.

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