Comparison of the effects of unfractionated heparin and the low-molecular-weight heparins dalteparin and enoxaparin on spontaneous platelet aggregation and adenosine diphosphate activity in platelets during the third trimester of pregnancy.

Low-molecular-weight heparins (LMWHs) have demonstrable pharmacokinetic, pharmacodynamic and safety advantages over unfractionated heparin (UH) in routine clinical use and are now the preferred agents in routine anticoagulant therapy. However, the utility and impact of the LMWH compared with that of UH has not been studied extensively in human pregnancy, wherein the prophylaxis against venous thromboembolism is imperative. Human pregnancy is a hypercoagulable state with an increase in spontaneous platelet aggregation (SPA) in vivo. We evaluated and compared the effects of UH and the LMWHs dalteparin and enoxaparin (10 U/ml) on SPA in citrated whole blood with an ultraflow platelet counter in pregnancy and also investigated the role of adenosine diphosphate (ADP) in heparin-induced platelet aggregation in the third trimester of pregnant women (aged 28 +/- 3 years, gestational age 34 +/- 5 weeks) and in healthy, age-matched nonpregnant women. Pregnant women showed a significantly increased SPA of 37% 6 5% compared with 16% 6 3% in nonpregnant women (p < 0.01). UH exerted a significantly greater proaggregatory effect on SPA compared with that of LMWHs or saline (p < 0.0002; ANOVA). The maximum values of SPA were as follows: UH, 69% +/- 5%; dalteparin, 46% +/- 5%; and enoxaparin, 54% +/- 3%. There was no difference between SPA induced by LMWHs and saline or between enoxaparin and dalteparin. At 480 s, there was no difference in SPA induced by LMWH between pregnant and nonpregnant women, but UH substantially and specifically increased SPA in pregnant women compared with that in nonpregnant women (p < 0.01). This heparin-induced platelet activation and thrombocytopenic response was reversed by apyrase grade II (ADP scavenger) that also inhibited SPA in pregnancy to a level similar to that of nonpregnant women (p < 0.0002; ANOVA). These results indicate that the LMWHs dalteparin and enoxaparin cause significantly less platelet aggregation in whole blood in pregnancy and in the nonpregnant state when compared with UH. The proaggregatory platelet effects of UH is substantially enhanced in pregnancy, a property not shared by LMWHs. The reversal of the heparin-induced platelet activation by apyrase grade II suggests that the mechanism is, at least in part, mediated by copious ADP release from platelets or red cells by heparin but not LMWHs.

Endogenous opioids and baroreflex control in humans.

We describe two studies designed to elucidate the role of endogenous opioids in blood pressure control in humans. In the first study, nine normal subjects received infusions of DAMME (a metenkephalin analog), naloxone, or saline, and blood pressure, heart rate, and plasma norepinephrine concentration were determined supine and following 5 minutes of 70 degrees head-up tilt at intervals for 6 hours. Blood pressure following tilt was significantly decreased by DAMME but not influenced by naloxone, the effect being most marked at 3 hours (placebo = 110 +/- 6/78 +/- 7 mm Hg; naloxone = 106 +/- 10/79 +/- 5 mm Hg; DAMME = 96 +/- 16/67 +/- 8 mm Hg (p less than 0.01). However, heart rate and plasma norepinephrine did not rise in response to this hypotension. Heart rates at 3 hours were: placebo = 87 +/- 16 bpm; naloxone = 88 +/- 19 bpm; DAMME = 89 +/- 23 bpm. Plasma norepinephrine levels (nmol/liter) at 3 hours were: placebo = 6.0 +/- 2.2; naloxone = 5.8 +/- 1.9; DAMME = 6.0 +/- 1.9. In the second study, seven normal subjects had blood pressure reduced by incremental infusions of sodium nitroprusside, and the effects of placebo, naloxone, and DAMME on the slope of the heart period/blood pressure relationship investigated. Naloxone significantly increased the slope by 90% and DAMME significantly reduced the slope by 30%. It is concluded that endogenous opioids modulate the baroreflex control of blood pressure in normal humans.

Effect of naloxone on the actions of captopril.

Captopril inhibits the metabolism of endogenous opioids in vitro and potentiates their effects in vivo. We examined the hypothesis that endogenous opioids contribute to the actions of captopril in man. The acute cardiovascular and autonomic effects of oral captopril, intravenous naloxone, and their combination were examined in eight healthy men with normotension in a double-blind, placebo-controlled study of a Latin squares design. Naloxone altered neither blood pressure nor heart rate. There were significant falls in systolic blood pressure during captopril dosing alone, but there was no fall in blood pressure during combination therapy. Heart rates were higher during the combination than during captopril alone. The combination caused sedation, but neither captopril nor naloxone alone had any behavioral effects. Modification of the acute circulatory effects of captopril by naloxone suggests a role for endogenous opioids in the responses to converting enzyme inhibition. The sedation caused by the combination raises the possibility that captopril may exert central nervous actions in man.

The pharmacological management of hypertension in pregnancy.

PURPOSE: The management of pregnancy-induced hypertension (PIH) and preeclampsia using antihypertensive drug therapy remains contentious. Conflicts arise due to differences in diagnostic criteria and varying attitudes regarding the value of treating hypertension, which is only one aspect of this systemic disorder. The following review assesses the role of individual agents and their effects upon both maternal and foetal/neonatal wellbeing. STUDY SELECTION: Human clinical trials of each of the main antihypertensive drugs used in the management of PIH/preeclampsia are reviewed. The value of randomized, placebo-controlled trials and long-term paediatric follow up is stressed. RESULTS OF DATA ANALYSIS: A number of agents have a favourable benefit-risk profile for use in women with PIH/preeclampsia; these include alpha-methyldopa, beta-blockers, hydralazine, prazosin, calcium channel antagonists and ketanserin. Diazoxide and sodium nitroprusside may also be used for acute severe hypertension. Angiotensin converting enzyme inhibitors are contra-indicated. Low-dose aspirin is presently being investigated in multicentre trials and may play a major role in the prevention of preeclampsia. CONCLUSION: Decisions regarding the need for antihypertensive treatment during pregnancy and the selection of a specific antihypertensive agent should be based upon an assessment of the relative benefits and risks for the individual patient. In future studies, the effects of antihypertensive agents upon the underlying pathophysiological processes involved in PIH/preeclampsia may guide therapeutic decision making.

Platelet reactivity in vitro in relation to thromboxane in healthy pregnancy.

There is substantial evidence of increased platelet reactivity in vivo and in vitro during pregnancy. Previous in vitro studies suggest that platelets from pregnant women show increased sensitivity to agonists, the response to which has a thromboxane dependent component. The aim of this study was to determine whether this is due to increased activity of the thromboxane biosynthetic pathway or to increased platelet sensitivity to the effects of thromboxane. During pregnancy, platelets were more sensitive to the pro-aggregatory effects in vitro of the thromboxane mimetic U46619, in whole blood and in platelet rich plasma, compared to those from non-pregnant controls. The difference in extent of U46619-induced platelet aggregation between groups was abolished in the presence of a high concentration of the specific thromboxane antagonist ICI 192605, but not by prior incubation of blood with aspirin. Platelets from pregnant women were significantly less sensitive to inhibition of arachidonic acid induced activation by the thromboxane synthetase inhibitor dazmegrel, but there was no change in platelet cyclic AMP accumulation under these conditions. Arachidonic acid induced platelet thromboxane B2 production was similar in pregnant and non-pregnant subjects. In conclusion, platelets are more sensitive to the activating effects of thromboxane during pregnancy, but there is no change in the intrinsic reactivity of the thromboxane biosynthetic pathway.

Magnesium modifies the responses of platelets to inhibitory agents which act via cAMP.

This study has investigated the interaction of raised extracellular magnesium and agents which act via cAMP with respect to inhibition of platelet aggregation and effects on platelet cAMP accumulation. Iloprost (3 ng/ml) and PGD2 (0.2 microgram/ml) each caused time dependent increases in platelet cAMP which were significantly greater in the presence of 3 mM added MgSO4 (p < 0.01). Addition of ADP (5 microM) resulted in a fall in cAMP which remained higher in the presence of MgSO4 (p < 0.01). Forskolin (5 micrograms/ml) and DN9693 (100 microM) also caused increments in platelet cAMP but these responses were not influenced by added MgSO4. Addition of ADP resulted in a further increase in cAMP which was augmented by MgSO4 (p < 0.03). This increase was abolished by adenosine deaminase (1.2 U/ml). These experiments show that MgSO4 can modify the cAMP responses produced by iloprost and PGD2 and by forskolin and DN9693 when ADP is present. It appears that as well as inhibiting, ADP can also stimulate cAMP production under certain experimental conditions. This appears to be due to breakdown of ADP to adenosine.

Clinical pharmacokinetics of prazosin--1985.

Prazosin is a selective alpha 1-adrenoceptor antagonist which is useful alone or in combination for the treatment of hypertension and heart failure. Unlike many other antihypertensive drugs, the action of prazosin appears to be closely related to its concentration in plasma or whole blood. Prazosin is variably absorbed, is subject to first-pass metabolism, and is eliminated almost entirely as metabolites of much lower hypotensive activity than the parent drug. Prazosin is highly bound to plasma and tissue proteins. The influences of renal, hepatic and cardiac disease on the disposition of prazosin are reviewed, as are the effects of pregnancy and ageing. The optimum use of prazosin in clinical practice depends on an understanding of the pharmacokinetic properties of the drug.

Effects of NG-nitro-L-arginine methyl ester on regional haemodynamic responses to MgSO4 in conscious rats.

1. We assessed regional haemodynamic responses to the vasodilator, MgSO4, in the absence and presence of the nitric oxide synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME), in conscious chronically instrumented Long Evans rats (n = 9). 2. MgSO4 (loading dose 220 mumol kg-1 min-1 for 7 min, maintenance dose 56 mumol kg-1 min-1 for 7 min), alone, caused slight bradycardia and hypotension accompanied by reductions in renal and mesenteric flows, but a marked hyperaemic vasodilatation in the hindquarters (flow, delta 54 +/- 6%, vascular conductance, delta 77 +/- 5%). 3. L-NAME (183 nmol kg-1 min-1) caused hypertension (29 +/- 2 mmHg) accompanied by bradycardia (-51 +/- 6 beats min-1) and reductions in flow and vascular conductance in the renal (-18 +/- 4% and -35 +/- 3%, respectively), mesenteric (-35 +/- 3% and -49 +/- 3%, respectively), and hindquarters (-26 +/- 3% and -42 +/- 3%, respectively) vascular beds. In the presence of L-NAME, the hypotensive and bradycardic effects of MgSO4 were still apparent, but its hindquarters hyperaemic vasodilator effect was significantly attenuated. 4. In order to determine if the inhibitory action of L-NAME on the hindquarters hyperaemic vasodilator action of MgSO4 was a non-specific effect, due to the change in baseline conditions caused by L-NAME, we also examined responses to MgSO4 in the presence of endothelin-1 (12.5 pmol kg-1 min-1) or angiotensin II (50 pmol kg-1 min-1). In the presence of either peptide, the overall effects of MgSO4 on hindquarters flow and vascular conductance were unchanged. 5. In a separate experiment (n = 8) we determined that the inhibitory effect of L-NAME on the hyperaemic vasodilator response to MgSO4 was prevented by L-arginine, and also demonstrated that the Beta2-adrenoceptor antagonist, ICI 118551, caused significant inhibition of the hindquarters haemodynamic effects of MgSO4.6. We conclude that the hindquarters haemodynamic effects of MgSO4 in conscious rats involve a substantial L-NAME-sensitive component which depends on activation of Beta2-adrenoceptors, probably asa consequence of adrenal medullary adrenaline release.

Assessment of the effects of endothelin-1 and magnesium sulphate on regional blood flows in conscious rats, by the coloured microsphere reference technique.

There is evidence to suggest that magnesium (Mg2+) is beneficial in the treatment of a number of conditions, including pre-eclampsia and acute myocardial infarction. The mode of action of Mg2+ in these conditions is not clear, although the vasodilator properties of Mg2+ are well documented both in vitro and in vivo. Previously, we demonstrated that i.v. infusion of magnesium sulphate (MgSO4) alone, or in the presence of vasoconstrictors, caused increases in flow and conductance in the common carotid, internal carotid and hindquarters vascular beds, in conscious rats. Therefore, the objective of the present study was to investigate the regional and subregional changes in haemodynamics in response to the vasoconstrictor peptide endothelin-1 (ET-1) and MgSO4 in more detail, using the coloured microsphere reference technique. Infusion of ET-1 and MgSO4 had similar effects on heart rate and mean arterial pressure as in our previous study. Infusion of ET-1 caused a rise in mean arterial pressure and a fall in heart rate, and infusion of MgSO4 returned mean arterial pressure to control levels with no effect on heart rate. The responses to MgSO4 in the presence of ET-1 showed considerable regional heterogeneity with blood flow increasing (e.g. skeletal muscle), decreasing (e.g. stomach) or not changing (e.g. kidney). Of particular interest was the finding that MgSO4 caused increases in flow in the cerebral and coronary vascular beds. This, and our previous studies, have shown that MgSO4 can reverse vasoconstriction in a number of vascular beds, and indicate that this compound may have therapeutic benefit in conditions associated with vasospasm.

Distribution of receptors mediating phosphoinositide hydrolysis in cultured human umbilical artery smooth muscle and endothelial cells.

Cultures of human umbilical artery smooth muscle and endothelial cells have been established and the effect of a range of calcium-mobilizing receptor agonists on inositol phospholipid hydrolysis has been compared in the two cell types. In human umbilical artery endothelial cells, histamine (EC50 20 microM), ATP (EC50 6.7 microM), sodium fluoride (20 mM) and thrombin (1 U/mL) produced marked increases in [3H]inositol phosphate accumulation. In contrast, bradykinin (1 microM), 5-hydroxytryptamine (5-HT) (0.1 mM) and carbachol (1 mM) produced only a small (< 1% of the response to 1 mM histamine) effect on [3H]inositol phosphate accumulation in these cells. In human umbilical artery smooth muscle cells, histamine (EC50 16 microM), bradykinin (EC50 4.5 nM), 5-HT (EC50 0.7 microM) and carbachol (EC50 21 microM) produced substantial effects (> 20% of the response to 1 mM histamine) on inositol phospholipid hydrolysis while ATP (1 mM) and thrombin (1 U/mL) were much less effective. The response to histamine in both smooth muscle and endothelial cells was antagonized by 50 nM mepyramine (apparent Kd = 5.6 and 2.9 nM in the two cell types, respectively). The response to 5-HT in smooth muscle cells was antagonized by 50 nM ketanserin (apparent Kd = 4.5 nM). In human umbilical artery smooth muscle cells the inositol phosphate response to carbachol was antagonized by 4-diphenylacetoxy-N-methylpiperidine (4-DAMP; pKd = 9.3), atropine (pKd = 9.7), pirenzepine (pKd = 6.7) and methoctramine (pKd = 6.9). These data are consistent with the involvement of an M3-muscarinic receptor in this response. These studies suggest that receptors mediating inositol phospholipid hydrolysis are differentially distributed between human umbilical artery endothelial and smooth muscle cells.

Receptor-stimulated phospholipase C activity in human umbilical artery cultured endothelial cells grown in a low oxygen environment.

Endothelial cells of the human umbilical blood vessels are widely cultured in an oxygen tension (21%) far above that in which they exist in vivo (3%). This study investigates the effect of the long term culture (ca. 1 month) of human umbilical artery endothelial cells in a reduced oxygen environment (3%: HUAEC3) in comparison to cells grown in a 'normoxic' environment (21%: HUAEC21). Despite reports of altered metabolic pathways and reduced membrane integrity in other cell types, the characteristics of HUAEC3 were found to be similar to those of HUAEC21 with respect to morphology, immunocytochemical profile and in vitro growth rates. Cellular glutathione was maintained in these cells although ATP levels in HUAEC3 were found to be significantly lower than those observed in HUAEC21. The phosphoinositide responses of the HUAEC3 to a variety of agonists were also found to be of similar magnitude to those observed in HUAEC21. In addition, the pharmacological characteristics of the phospholipase C-linked histamine H1 and P2y2 (P2U) receptors were not changed by culture of cells in a low oxygen environment.

Nifedipine and platelets in preeclampsia.

In addition to its antihypertensive properties, nifedipine inhibits platelet aggregation in vitro. Because increased platelet aggregation is a feature of preeclampsia, we have investigated nifedipine in this condition. Ten women at 31 +/- 2.8 weeks gestation, with blood pressure 162 +/- 18/102 +/- 10 mmHg (despite atenolol 200 mg/day) and proteinuria 2.0 +/- 1.1 g/24 hr, were treated with nifedipine. Pregnancies were prolonged by 17 +/- 15 days (range 5 to 56). Blood pressure was controlled in eight of the ten patients, final values before delivery being 142 +/- 16/89 +/- 12 mmHg (P less than 0.02). Platelet count rose in all women from 190 +/- 80 to 261 +/- 78 X 10(9)/1 (P less than 0.001). Nifedipine appears to reverse the thrombocytopenia of pre-eclampsia, in addition to controlling the blood pressure.

The use of Doppler ultrasound in the study of fetal cardiovascular physiology.

Doppler velocimetry of the uteroplacental and umbilical arteries provides an opportunity for the safe, reproducible and repeatable study of these circulations and already has led to an increased knowledge of the pathophysiology of pregnancy induced hypertension and intra-uterine growth retardation. This technique, which has only recently been introduced widely into obstetrics, also permits the study of the cardiovascular effects on the fetus of maternally administered drugs. The use of Doppler for these purposes is illustrated with particular reference to intra-uterine growth retardation and the use of the antihypertensive drugs, atenolol and nifedipine.

Risk factor modification in hypertension--success and failure.

A group of 175 patients have attended for four years a microcomputer based, nurse practitioner managed clinic for the long term care of hypertension. Improvement in blood pressure (BP) control was seen initially and has been maintained. Drug treatment has been with a beta blocker followed if necessary by a diuretic and/or a vasodilator. Despite individual dietary and 'stop smoking' advice, there was no marked change in body weight while only males reduced smoking. A significant increase in non fasting serum cholesterol (6.4 +/- 1.3 to 6.6 +/- 1.3 mmol/l) and glucose (5.5 +/- to 6.1 +/- 2.5 mmol/l) was seen overall, but was restricted to those patients taking a thiazide diuretic (with or without other drugs). These findings indicate the need for a more systematic collection of information on effects of long term treatment and for the development of better strategies for improving lifestyle.

A comparison of the management of hypertensive patients by nurse practitioners compared with conventional hospital care.

Patients with hypertension (n = 198) managed by a nurse practitioner hypertension clinic (NPC) were compared over a four-year period with an age-sex matched group of patients attending conventional hospital hypertension outpatient clinics (CHC) for follow-up in two other hospitals in the same city. Mortality is reported for an additional two years from initial presentation. There was no difference between the groups for initial supine systolic or diastolic blood pressure, weight, obesity index, alcohol use, cholesterol or glucose. The reported causes of hypertension were similar. There were more smokers initially in the CHC (46% vs. 37%). Initial drug treatment was less varied in the NPC. At the end of one year there was no significant weight loss in either group, and drug treatment was broadly similar between the two groups. Those attending the NPC, however, had a greater decrease in both supine systolic blood pressure (P less than 0.05) and diastolic blood pressure (P less than 0.01). At the end of four years there were 136 patients (69%) attending the NPC and 70 (35%) remaining in the CHC. For those remaining systolic blood pressure control was similar. Diastolic blood pressures were lower in the NPC (P less than 0.001). Mortality was not significantly different between the groups over a six-year period. This study shows that a nurse practitioner clinic can control blood pressure in a greater proportion of patients, maintain contact with more efficient follow-up of a greater percentage of patients and collect the information needed to assess the effectiveness of care over the long term.

Umbilical artery blood velocity waveform analysis in predicting the small-for-dates fetus.

Doppler umbilical artery blood velocity waveforms were recorded longitudinally in 78 randomly selected pregnancies. The waveform maximal (A) to minimal (B) Doppler shift frequency ratios were determined to provide a semi-quantitative measure of impedance to blood flow in the fetoplacental circulation. Fifty-eight of these pregnancies and their outcomes were normal. The A/B ratios from this normal group were used to derive a normal range of this index throughout the second half of pregnancy. A further 45 pregnancies, also randomly selected, were studied once within 2 weeks of delivery. Using those measurements, plus those from the longitudinal groups sampled within 2 weeks of delivery, the predictive values of the A/B ratio in detecting the small for gestational age fetus have been derived. The sensitivity of the umbilical artery A/B ratio in predicting a small for gestational age infant within 2 weeks of delivery was 58% with a specificity of 94%. The positive test predictive value was 69% with a negative test predictive value of 90%. These predictive values are lower than those previously published, probably because high-risk pregnancies were over-represented in earlier studies. Waveform analysis within 2 weeks of delivery did, however, appear to identify fetuses destined to develop a range of perinatal problems. Large prospective studies are needed before the predictive value of Doppler waveform analysis is clarified.

Atenolol in essential hypertension during pregnancy.

OBJECTIVE: To determine the effect of atenolol on the outcome of pregnancy in women with essential hypertension. DESIGN: Prospective, randomised, double blind, placebo controlled study. SETTING: Hospital clinic. PATIENTS: 33 Women with mild essential hypertension (systolic blood pressure 140-170 mm Hg or diastolic pressure 90-110 mm Hg on two occasions at least 24 hours apart) consecutively referred to two obstetric medical clinics. Four patients in the placebo group were withdrawn from the study: control of blood pressure was inadequate in two, one developed breathlessness, and one changed her mind about participating. The mean gestation in the 29 remaining women on entry to the study was 15.9 weeks. MAIN OUTCOME MEASURES: Blood pressure and birth weight. INTERVENTION: 14 Women received placebo. 15 Women received atenolol 50 mg daily initially, increasing until either the blood pressure was less than 140/90 mm Hg or a dose of 200 micrograms daily was reached. RESULTS: The mean blood pressure on entry was 148/86 mm Hg in the group given atenolol and 144/86 mm Hg in the group given placebo. During treatment the mean diastolic pressure was significantly reduced by atenolol compared with placebo (to 74 v 81 mm Hg; difference in means (95% confidence interval) 7.0 (2.9 to 10.0) mm Hg) but the effect on systolic pressure was marginal (132 v 136 mm Hg; 4.0 (-1.4 to 8.6) mm Hg). Babies in the atenolol group had a significantly lower birth weight than those in the placebo group (2620 g v 3530 g; 910 (440 to 1380)g). CONCLUSION: Atenolol given from the end of the first trimester in patients with mild hypertension is associated with intrauterine growth retardation. When taken in conjunction with the results of a previous study in which methyldopa was given these findings indicate that benefit is unlikely to result from treating mild essential hypertension in pregnancy.

Evidence of in vivo platelet activation during heparin anticoagulation in pregnancy for prophlaxis or treatment of thromboembolism.

Conventional unfractionated heparin substantially enhances spontaneous platelet aggregation in pregnancy in vitro, and may cause platelet activation in healthy volunteers in vivo. It is unknown, however, whether therapeutically administered heparin affects platelet behavior during pregnancy. In a parallel group ex vivo study, 8 third trimester pregnant patients requiring anticoagulation with heparin exhibited a trend to a greater spontaneous platelet aggregation, in comparison to 11 age-matched healthy third trimester pregnant controls. This is consistent with heparin-induced platelet activation in vivo during therapeutic anticoagulation. Peak aggregation in the heparin-treated group was 48 +/- 4% compared to 37 +/- 5% in the healthy controls, (P = 0.086 ANOVA): and significant time treatment interaction (P = 0.03 ANOVA). There was also a weak positive correlation (r = 0.54) between the peak % spontaneous platelet aggregation and the activated partial thromboplastin time ratio during heparin administration.

Hypertension in pregnancy.

Hypertensive diseases of pregnancy are clinically important because they can adversely influence the health and life of both mother and baby. Hypertensive disease is the commonest cause of maternal mortality in England and Wales, accounting for 20.4% of maternal deaths. It is depressing to note that most, if not all, of these deaths are preventable. Three broadly different kinds of hypertension can be identified as potential complications of pregnancy: chronic hypertension, pregnancy-induced hypertension (PIH) and pre-eclampsia. Where chronic hypertension is treated with methyldopa and PIH is treated with atenolol, there is evidence that therapy is beneficial in terms of immediate pregnancy outcome and is not harmful to the child. Atenolol is currently being evaluated in combination with nifedipine to treat cases of early onset of severe pre-eclampsia, and preliminary results are encouraging. Prevention rather than cure should be the aim in managing hypertensive diseases during pregnancy. Early intervention can prevent serious problems later on.