Dietary interventions in north Karelia, Finland and south Italy. Modification of thromboxane B2 formation in platelets of male subjects only.

The effects of dietary interventions, based on changes of total fat, saturated fatty acids and cholesterol contents and of the polyunsaturated/saturated (P/S) fatty acid ratio of the diet, were studied in normal male and female subjects, living in North Karelia, Finland, and South Italy. In North Karelia the increase of P/S ratio (from 0.15 to 1.2) of the diet for a 6-week period resulted in reduced thromboxane B2 (TxB2) production by collagen-stimulated platelets only in male subjects, whereas plasma total, LDL and HDL cholesterol were reduced in both sexes. After a 6-week return to the original diet, plasma lipid levels were restored in all subjects. In the South Italy study, changes in platelet TxB2 production were observed only after return to the original diet in male subjects. Total and LDL cholesterol were significantly increased during the dietary intervention and returned toward baseline levels after switch back to the original diet. These data indicate that the increase of the P/S ratio in the diet reduces platelet TxB2 formation only in men.

Biological actions and possible uses of vitamin E.

Symptoms of Vitamin E deficiency can be generally attributed to derangement of processes depending upon the integrity of cellular and subcellular membranes, following the formation of tissue-damaging products of lipid peroxidation. Antioxidants modulate also the formation of products derived from long-chain polyunsaturated fatty acids, such as arachidonic acid--which are structural components of biological membranes--through oxidative reactions involving the cyclooxygenase and lipoxygenase systems. Vitamin E inhibits the aggregatory responses of blood platelets to aggregating agents "in vitro", after a preincubation period required for the uptake of the compound by the cells. The antiaggregatory activity of alpha-tocopherol, however, does not appear to be strictly dependent upon inhibition of the formation of thromboxane, the proaggregatory compound derived from arachidonic acid through be cyclooxgenase system. The effects of Vitamin E on platelet function may be of relevance in the control of thromboembolic processes of clinical importance.

Fish oil administration as a supplement to a corn oil containing diet affects arterial prostacyclin production more than platelet thromboxane formation in the rat.

The administration to male rats of 5 en % fish oil (FO) as supplement to a diet containing 5 en % corn oil (CO), selectively and markedly decreased arterial parameters (6-keto-PGF1 alpha formation and platelet antiaggregatory activity) assessed in isolated aortic segments perfused with autologous platelet rich plasma (PRP). Platelet parameters (ADP-induced aggregation, TxB2 formation in thrombin-stimulated PRP and sensitivity to exogenous PGI2) were instead minimally affected. Eicosapentaenoic acid (EPA, 20:5 n-3) did not accumulate in plasma, platelet and aorta lipids and arachidonic acid (AA, 20:4 n-6) levels declined markedly only in the plasma compartment. When FO was given alone at the same 5 en % level, both arterial and platelet parameters were similarly affected. EPA accumulated in plasma cholesterol esters and was present in appreciable concentrations also in platelets and aortic walls. AA levels declined markedly in plasma lipids and appreciably also in platelet and aorta lipids. It is concluded that a) arterial and platelet parameters are differentially affected by FO administration depending upon the presence of n-6 polyunsaturated fatty acids in the diet, b) 6-keto-PGF1 alpha production by arterial tissue does not seem to be related to changes of PG precursor fatty acid levels in the phospholipid fraction.

Increased platelet aggregability is associated with increased prostacyclin production by vessel walls in hypercholesterolemic rabbits.

The influences of experimental hypercholesterolemia in the rabbit on platelet-vessel wall interactions have been studied by evaluating the aggregatory response of platelet rich plasma (PRP) to arachidonic acid (AA) stimulation and levels of 6-keto-PGF1 alpha in PRP from normal (N) and hypercholesterolemic (HC) animals prior and after perfusion through the corresponding aortas. In addition, the responses of N PRP to aggregation after perfusion through HC aortas and those of HC PRP perfused through N aortas, and the platelet response to the inhibitory effect of exogenous prostacyclin have been evaluated. The data indicate that in HC rabbits, on one side platelets are hyperreactive to AA and less sensitive to the inhibitory activity of prostacyclin and, on the other, the antiaggregatory activity and prostacyclin production of vessel walls is higher, suggesting compensatory mechanisms in the haemostatic balance.

In vitro effects of synthetic antioxidants and vitamin E on arachidonic acid metabolism and thromboxane formation in human platelets and on platelet aggregation.

The "in vitro" effects of alpha-tocopherol, butylhydroxytoluene (BHT) and butylhydroxyanisole (BHA) were studied on aggregation of human platelets induced by collagen and arachidonic acid (AA), on the metabolic conversion of 14C AA through the cyclooxygenase and lipoxygenase pathways and on the formation of thromboxane B2 (TXB2) in washed platelets after stimulation with collagen. Vitamin E completely inhibited AA induced platelet aggregation only at high concentration (mM) and after 10 minutes of preincubation, with limited effects on AA metabolism in platelets and no effect on TXB2 formation from endogenous substrate. BHA completely inhibited platelet aggregation in the 10(-6) M range, gave 50% inhibition of AA metabolism in the 10(-5) M range and almost complete inhibition of thromboxane formation in the 10(-4) M range. BHT was about 100 times less active on platelet aggregation and AA metabolism. The lipoxygenase and cyclooxygenase pathways were differentially affected at low concentrations of BHA and only at concentrations greater than 5 X 10(-5) M were both pathways depressed.

Differential effects of various vasoactive drugs on basal and stimulated levels of TXB2 and 6-keto-PGF1 alpha in rat brain.

Thromboxane B2 and 6-keto-PGF1 alpha (6KPGF1 alpha), the major stable metabolites of thromboxane and prostacyclin, are present in the CNS, where they appear to be mainly produced within and/or acting upon the vascular district. Their concentrations are of few pg/mg protein in rat brain cortex of animals sacrificed by microwave (MW) radiation, procedure which inactivates tissue enzymes and allows the determination of endogenous "basal" levels of eicosanoids. Levels of 6KPGF1 alpha and especially those of TxB2 increase several fold over the basal values in brain cortex of animals sacrificed by decapitation followed by a few minute interval before analysis (post-decapitation ischemia, PDI). Pretreatment of animals with the vasoactive drug papaverine, resulted in elevation of brain basal levels of 6KPGF1 alpha and with the carbochromene derivative AD6 in reduction of basal levels of TxB2, whereas the calcium antagonist nifedipine and dipyridamole did not modify basal levels of the two eicosanoids. Treatments with papaverine and AD6 reduced the accumulation of TxB2 and enhanced that of 6KPGF1 alpha occurring after PDI, to different extents, both resulting, however, in reduction of the TxB2/6KPGF1 alpha ratio. Nifedipine instead, decreased the release of both eicosanoids and resulted in elevation of the TxB2/6KPGF1 alpha ratio, whereas dipyridamole had no effect. In conclusion, the evaluation of the overall effects of drug treatments on the TxB2/6KPGF1 alpha ratio in cerebral tissue, provided useful informations on the pharmacological modulation of vascular eicosanoids in this district.

Platelet-vessel wall interactions: effects of platelets and plasma on the antiaggregatory activity and 6 keto-PGF1 alpha production in isolated perfused aortas.

An experimental model for the study of platelet-vessel wall interactions has been developed, based on perfusion of rat platelet rich plasma (PRP) through isolated rat aortas. In the perfused PRP, platelet aggregation was inhibited and levels of 6 Keto PGF1 alpha and cAMP were elevated over the values found in non perfused PRP. When PPP or buffer were perfused through the isolated artery, elevations of 6 Keto PGF1 alpha levels in the perfusate were smaller (in perfused PPP) or of shorter duration (in both perfused PPP and buffer). The presence of platelets in the perfusion fluid thus enhanced the formation of Prostacyclin by the arterial wall. Levels of 6 Keto PGF1 alpha in PRP obtained from aspirin-treated animals and in PRP from normal animals, both perfused through normal aortas, were the same, and also levels of the above metabolite in normal PRP perfused through aortas of aspirin-treated animals did not differ from those found in non perfused PRP. It is concluded, from these data, that PRP does not stimulate PGI2 formation in perfused aortas by providing cyclic endoperoxides. The experimental model developed allows the study of interactions between normal platelets and aortas from experimentally treated animals or viceversa.