Platelet membrane fluidity in pregnancy hypertension

The effect of prostanoid precursors and inhibitors on platelet angiotensin II binding

Pregnancy-induced hypertension is characterised by an imbalance of arachidonic acid metabolites: Prostacyclin (PGI2) is vasodilatory and a potent inhibitor of platelet reactivity. Thromboxane (TXA2) induces vasoconstriction and platelet aggregation. Previous intervention studies have been aimed at increasing vasodilatation and decreasing platelet aggregation using low dose aspirin or dietary manipulation of prostaglandins. The aim of this study was to investigate the value of combining low dose aspirin with dietary fatty acid supplementation and its effects on platelet angiotensin II binding in non-pregnant women. Sixty non-pregnant, healthy female volunteers were recruited and randomly allocated to one of six treatment regimens which included aspirin taken alone and in combination with fish oil or evening primrose oil. A control group took no treatment. Platelet AII binding was determined before and after treatment for 1 month. There was no change in platelet angiotensin II binding after 1 month in the control group or in those who received evening primrose oil or fish oil alone. A significant decrease in binding was found in those who took aspirin in combination with fish oil (P = 0.03). An increase in binding was seen in those who took aspirin only, although this was not statistically significant (P = 0.14). A decrease was found in those who took aspirin in combination with evening primrose oil but again this was not statistically significant (P = 0.07). This study found that the combined effect of low-dose aspirin and fish oil causes a significant decrease in platelet angiotensin II binding not caused by either compound taken alone. Given that angiotensin II exerts its effect in part by direct interaction with vascular AII receptors, (platelets being used as 'models' of vascular myocytes), and that pre-eclampsia is associated with major pathophysiological changes in prostanoid metabolism, these pilot data provide a basis for further investigation.

Platelet thromboxane A2 receptor number and function in normal and hypertensive pregnancy

PROBLEM

Our study evaluated the number and function of platelets for thromboxane A2/prostaglandin H2 (TxA2/PGH2) and the platelet response to TxA2 receptor agonists in normal and hypertensive pregnancy. In addition, correlations between platelet membrane lipid composition and TxA2 receptor number and function were evaluated.

METHODS

Ten normotensive healthy pregnant women (GC), 10 hypertensive pregnant women (PIH), and 10 normotensive nonpregnant healthy women (C) were examined. Radioligand binding and aggregation studies were performed. Lipid composition of platelet membrane was assessed as cholesterol to phospholipids ratio (CH/PL) and normalized to protein content.

RESULTS

TxA2/PGH2 receptor binding sites and affinity did not differ among the groups. An enhanced response to TxA2 receptor agonist U46619 in GC and in PIH compared to C was observed, while platelet response to thrombin was not different among the groups. CH/PL was altered in PIH respect to GC and C. No significant correlation was found between CH/PL and number and function of platelet TxA2 receptors in PIH.

CONCLUSIONS

In PIH there is no alteration of number or function of TxA2 receptors on platelet membrane. However, the enhanced TxA2 production associated with the increased sensitivity of platelet to TxA2 in pregnancy may contribute to the microcirculatory thrombotic changes of PIH.

Is pre-eclampsia preventable?

Pre-eclampsia is characterised by reduced utero-placental blood flow which in turn results from vascular damage occurring early in pregnancy. Endothelial damage and reduced prostacyclin and nitric oxide synthesis will initiate excessive platelet aggregation, fibrin deposition and vascular occlusion. Prevention of pre-eclampsia is dependent on affecting this process. Antiplatelet therapy is being assessed as a prophylaxis. Screening tests using increased platelet turnover as their basis are also being assessed.