Effects of Calcium Channel Blockers on In Vitro Platelet Function in Whole Blood Using Single Platelet Counting

Potential Protective Role of Blood Pressure-Lowering Drugs on the Balance between Hemostasis and Fibrinolysis in Hypertensive Patients at Rest and During Exercise

In patients with hypertension, the triad represented by endothelial dysfunction, platelet hyperactivity, and altered fibrinolytic function disturbs the equilibrium between hemostasis and fibrinolysis and translates into a hypercoagulable state, which underlies the risk of thrombotic complications. This article reviews the scientific evidence regarding some biological effects of antihypertensive drugs, which can protect patients from the adverse consequences of hypertensive disease, improving endothelial function, enhancing antioxidant activity, and restoring equilibrium between hemostatic and fibrinolytic factors. These protective effects appear not to be mediated through blood pressure reduction and are not shared by all molecules of the same pharmacological class.

Effects of VLDL, chylomicrons, and chylomicron remnants on platelet aggregability

Although influences of cholesterol-rich lipoproteins on platelet aggregation are well established, the knowledge of interactions between triglyceride-rich lipoproteins and platelets, in particular in the postprandial state, is limited and controversial. The in-vitro effects of these lipoproteins from hypertriglyceridemic subjects on the aggregation behavior of platelets from normolipemic donors were investigated with two whole-blood methods. VLDL and chylomicrons, but not chylomicron remnants, in concentrations comparable to concentrations occurring after a fatty meal, reduced platelet aggregation in a dose-dependent manner in particle counting. Impedance aggregometry failed to monitor these effects. We conclude, that triglyceride-rich lipoproteins inhibit platelet aggregation in vitro. They may, therefore, not be linked to the acute onset of myocardial infarction.

Effects of diltiazem on thromboxane B2 production from platelet-rich plasma and whole blood

The present study evaluated the effects of the calcium-channel blocking agent diltiazem on platelet aggregation and on synthesis of thromboxane B2 (the stable metabolite of thromboxane A2) from platelet-rich plasma (PRP) and whole blood samples. Our results showed that diltiazem inhibits collagen- and thrombin-induced platelet aggregation and TXB2 production from PRP. Since no significant interference with conversion of arachidonate to thromboxane A2 was demonstrated, inhibition of phospholipase A2 activity may be the prevailing mechanism of the diltiazem effect. The drug demonstrated a dose-related inhibitory activity on TXB2 synthesis from whole blood samples during spontaneous clotting or following stimulation with collagen or thrombin. The present results give further evidences for an antiplatelet activity of diltiazem and support the hypothesis that inhibition of platelet function contributes to the therapeutic efficacy of this drug in the treatment of cardiovascular diseases.

Effect of nifedipine monotherapy on platelet aggregation in patients with untreated essential hypertension

The effect of 6 weeks of nifedipine 30-60 mg/d on platelet aggregation and lipid parameters has been studied. A diminution in ADP-, adrenaline- and collagen-induced aggregation was observed. In the case of adrenaline- and collagen-stimulated aggregation the decrease was statistically significant. It was found that platelets which aggregated markedly during the placebo treatment were most strongly inhibited by nifedipine. The changes in lipid parameters were not significantly correlated with changes in aggregation.

The effects of intravenous and oral nifedipine on ex vivo platelet function

We have studied the possible anti-platelet effects of an intravenous formulation of nifedipine (0.75 mg as a bolus and an infusion of 1.2 mg.h-1 for 2 h), or equivalent volumes of the vehicle alone, or normal saline, in a double-blind crossover fashion in six healthy subjects. The effects of a standard oral formulation (20 mg sustained-release) compared to identical placebo were also studied in twelve other subjects. Platelet function was assessed by the addition of collagen, adenosine diphosphate, or adrenaline to whole blood followed by single platelet counting. Intravenous nifedipine had no effect on aggregation in response to any of the agonists, but oral nifedipine reduced aggregation caused by collagen by approximately 15%, despite similar plasma nifedipine concentrations after both formulations (18.5 ng.ml-1 after intravenous and 21.5 ng.ml-1 after oral administration). The lack of effect of intravenous nifedipine may be due to endothelial irritation caused by the vehicle. Intravenous nifedipine is unlikely to have a useful anti-platelet effect in patients who have acute coronary insufficiency.