Iloprost binding and inhibition of aggregation in platelet rich plasma. Differences between normal and type IIa hypercholesterolemic subjects

Internalization and down-regulation of the prostacyclin receptor in human platelets

The internalization of [3H]iloprost, a prostacyclin analogue, was studied in human platelets by binding studies. After incubation with [3H]iloprost at 37 degrees C, addition of unlabelled ligand at either 37 degrees C or 4 degrees C caused dissociation of 74% and 52% of the bound ligand respectively, suggesting that a portion had been internalized. The percentage of [3H]iloprost bound at equilibrium to the surface (evaluated by acid treatment) at either 37 degrees C or 4 degrees C was markedly different (80% versus 25%). Internalization was dependent on time and on the ligand nature and concentration. Energy-depleting agents (dinitrophenol and 2-deoxyglucose) completely inhibited internalization, whereas probenecid (inhibitor of organic anion transporters) did not affect it significantly. Subcellular fractionation indicated that, at 4 degrees C or in the absence of ligand, most of the receptor was present in membrane fractions (pellet at 27000 or 105000 g), whereas, when platelets were preincubated at 37 degrees C with iloprost, the receptor was found mainly in the cytosolic fraction. In platelets preincubated with iloprost at 4 degrees C, two classes of binding sites were present, whereas after preincubation at 37 degrees C only the lower-affinity sites were detected. After exposure to the agonist, iloprost-induced inhibition of platelet aggregation and activation of adenylate cyclase and cAMP production were significantly lower. Taken together, these data demonstrate that human platelets can internalize a high-affinity binding site for iloprost, presumably the prostacyclin receptor.

Platelet alpha 2-adrenergic receptors in hypercholesterolemia: relationship between binding studies and epinephrine-induced platelet aggregation

BACKGROUND

Platelets isolated from patients with hypercholesterolemia are more sensitive in vitro to various aggregating agents, including epinephrine, than those isolated from normocholesterolemic subjects. Increased platelet reactivity is one mechanism that may explain the enhanced risk of thromboembolism in hypercholesterolemia. This study assessed whether platelet hyperreactivity to epinephrine in hypercholesterolemia is associated with higher alpha 2-adrenergic receptor density or affinity for epinephrine.

METHODS

Platelet aggregation and binding studies, with use of [3H]yohimbine as ligand, were performed on platelets isolated from 30 patients with type IIa hypercholesterolemia and 23 control subjects.

RESULTS

Platelet aggregation in response to epinephrine was significantly higher in patients with hypercholesterolemia than in control subjects. A statistically significantly higher alpha 2-adrenergic receptor density was observed in a subgroup of 13 patients with hypercholesterolemia than in 13 sex- and age-matched control subjects (280 +/- 61 and 230 +/- 49 fmol/mg protein respectively; p < 0.03), but no difference was observed in receptor affinity for the ligand. In these subgroups plasma total and levels of low-density lipoprotein (LDL) cholesterol were inversely correlated with platelet aggregation but directly correlated with platelet receptor density.

CONCLUSION

Platelet alpha 2-adrenergic receptor density is increased in hypercholesterolemia and directly correlates with plasma total and levels of LDL cholesterol, providing at least a partial explanation for the enhanced platelet response to epinephrine that is observed in hypercholesterolemia.

The actions of lovastatin on platelet function and platelet eicosanoid receptors in type II hypercholesterolaemia. A double-blind, placebo-controlled, prospective study

We have studied the effects of 12 weeks of lovastatin (20 mg per day) on platelet function and thromboxane formation in 18 patients with type II hypercholesterolaemia in a double-blind, placebo-controlled, prospective study. Lovastatin significantly reduced total serum and LDL-cholesterol by 20% and 25% respectively. Washed platelets of lovastatin-treated patients had significantly reduced collagen-induced aggregation and thromboxane formation ex vivo. There was no change in ADP-induced platelet aggregation, but a significant increase in prostacyclin (iloprost)-stimulated platelet cyclic AMP concentrations in lovastatin-treated patients. This was associated with a significant increase in the number of prostacyclin receptors in platelet membranes prepared from lovastatin-treated patients. There was also an increase in platelet thromboxane receptors. There were no such changes in the placebo group. These data confirm our original observation of normalization of platelet function in hypercholesterolaemia by HMGCoA reductase inhibitors and suggest changes in platelet membrane composition at the megakaryocyte level as a possible site of action.

Influences of lipid-modifying agents on hemostasis

Drugs affecting lipid metabolism may influence, to a variable extent, the hemostatic system, that is, platelet activation, fibrinogen, and fibrinolysis. These effects may or may not be linked to the activity of these compounds on the lipid/lipoprotein profile. For this reason it may be important to consider the effects of hypolipidemic drugs on the different aspects of hemostasis, because this may allow a better understanding of their clinical use, as well as, eventually, a more proper selection in individual patients. Among the major lipid-lowering agents, fibric acids belong to a multifaceted series of abnormal fatty acids known to interact with a liver nuclear receptor, in turn activating fatty acid catabolism. A similar activity may be exerted by n-3 fatty acids from fish, as well as by other chemically related or unrelated compounds. Among fibric acids all but gemfibrozil can reduce fibrinogen levels; this last drug can, however, apparently activate fibrinolysis. Among the selective cholesterol-lowering medications, both resins and HMG CoA reductase inhibitors may reduce, in some patients, over prolonged periods of treatment, platelet sensitivity to major aggregants. This effect may be seen best with non-liver-selective agents (e.g., simvastatin), although recent data cast doubt on its constancy. A direct comparative evaluation of different HMG CoA reductase inhibitors on platelet aggregability has never been carried out. These last drugs may also reduce the circulating levels of the tissue factor pathway inhibitor (TFPI), transported by LDL in plasma, which is a potentially negative effect. A lipid-lowering molecule with antioxidant activity, for example, probucol, may also possibly play a role in controlling platelet activation.(ABSTRACT TRUNCATED AT 250 WORDS)

Cyclic nucleotides attenuate thrombin-evoked alterations in parameters of platelet Na/H antiport. The role of cytosolic Ca

In this work, we explored the role of cyclic nucleotides in modulating parameters of the Na/H antiport in human platelets. Sodium nitroprusside and iloprost, as well as cyclic nucleotide analogues, were used to raise cellular levels of cAMP and cGMP. Cyclic nucleotides reversed the thrombin-evoked alkaline shift in cytosolic pH set point and the activity of the Na/H antiport, concurrently with attenuation of thrombin-induced rise in cytosolic free Ca. No effect of cyclic nucleotides was observed in platelets not treated with thrombin, or platelets subjected to phorbol 12-myristate 13-acetate. cAMP did not reverse ionomycin-induced changes in the parameters of the Na/H antiport. Collectively, these observations indicate that cyclic nucleotides modulate the Na/H antiporter in human platelets through their effect on thrombin-evoked changes in cytosolic free Ca. Presumably, this effect holds for other agonists which stimulate phospholipase C, raise cytosolic-free Ca, and activate the Na/H antiport through protein kinase C dependent and protein kinase C-independent mechanisms.