Tocotrienols-induced inhibition of platelet thrombus formation and platelet aggregation in stenosed canine coronary arteries

Abstract

Background: Dietary supplementation with tocotrienols has been shown to decrease the risk of coronary artery disease. Tocotrienols are plant-derived forms of vitamin E, which have potent anti-inflammatory, antioxidant, anticancer, hypocholesterolemic, and neuroprotective properties. Our objective in this study was to determine the extent to which tocotrienols inhibit platelet aggregation and reduce coronary thrombosis, a major risk factor for stroke in humans. The present study was carried out to determine the comparative effects of α-tocopherol, α-tocotrienol, or tocotrienol rich fraction (TRF; a mixture of α-+γ-+δ-tocotrienols) on in vivo platelet thrombosis and ex vivo platelet aggregation (PA) after intravenous injection in anesthetized dogs, by using a mechanically stenosed circumflex coronary artery model (Folts' cyclic flow model).

Results: Collagen-induced platelet aggregation (PA) in platelet rich plasma (PRP) was decreased markedly after treatment with α-tocotrienol (59%; P<0.001) and TRF (92%; P<0.001). α-Tocopherol treatment was less effective, producing only a 22% (P<0.05) decrease in PA. Adenosine diphosphate-induced (ADP) PA was also decreased after treatment with α-tocotrienol (34%; P<0.05) and TRF (42%; P<0.025). These results also indicate that intravenously administered tocotrienols were significantly better than tocopherols in inhibiting cyclic flow reductions (CFRs), a measure of the acute platelet-mediated thrombus formation. Tocotrienols (TRF) given intravenously (10 mg/kg), abolished CFRs after a mean of 68 min (range 22 -130 min), and this abolition of CFRs was sustained throughout the monitoring period (50-160 min).Next, pharmacokinetic studies were carried out and tocol levels in canine plasma and platelets were measured. As expected, α-Tocopherol treatment increased levels of total tocopherols in post- vs pre-treatment specimens (57 vs 18 μg/mL in plasma, and 42 vs 10 μg/mL in platelets). However, treatment with α-tocopherol resulted in slightly decreased levels of tocotrienols in post- vs pre-treatment samples (1.4 vs 2.9 μg/mL in plasma and 2.3 vs 2.8 μg/mL in platelets). α-Tocotrienol treatment increased levels of both tocopherols and tocotrienols in post- vs pre-treatment samples (tocopherols, 45 vs 10 μg/mL in plasma and 28 vs 5 μg/mL in platelets; tocotrienols, 2.8 vs 0.9 μg/mL in plasma and 1.28 vs 1.02 μg/mL in platelets). Treatment with tocotrienols (TRF) also increased levels of tocopherols and tocotrienols in post- vs pre-treatment samples (tocopherols, 68 vs 20 μg/mL in plasma and 31.4 vs 7.9 μg/mL in platelets; tocotrienols, 8.6 vs 1.7 μg/mL in plasma and 3.8 vs 3.9 μg/mL in platelets).

Conclusions: The present results indicate that intravenously administered tocotrienols inhibited acute platelet-mediated thrombus formation, and collagen and ADP-induced platelet aggregation. α-Tocotrienols treatment induced increases in α-tocopherol levels of 4-fold and 6-fold in plasma and platelets, respectively. Interestingly, tocotrienols (TRF) treatment induced a less pronounced increase in the levels of tocotrienols in plasma and platelets, suggesting that intravenously administered tocotrienols may be converted to tocopherols. Tocotrienols, given intravenously, could potentially prevent pathological platelet thrombus formation and thus provide a therapeutic benefit in conditions such as stroke and myocardial infarction.

Figure 1

Chemical structures of isomers of tocopherols and tocotrienols used in this study.

Figure 2

Effects of α-tocopherol, α-tocotrienol, and tocotrienol rich fraction (TRF) on collagen-induced platelet aggregation: α-Tocopherol (250 μg), α-tocotrienol (250 μg) or TRF (250 μg) were dissolved in polyethylene glycol (1 mL) solvent and stored at -20°C. Plasma (40 mL) was warmed to 50°C (20 min) prior to injection. α-Tocopherol, α-tocotrienol or TRF solutions in 1 mL of polyethylene glycol were added to warmed plasma, vortexed vigorously, and then administered over 30 seconds through a femoral arterial line to anesthetized dogs. Collagen (12.5 μmol/mL) in a volume of 32 μL was added to 400 μL of PRP that had been incubated at 37°C for 2 min. The extent of collagen-induced platelet aggregation was quantitated by measuring the percent maximal aggregation 6 min after adding collagen. Data are expressed as means ± SD, n = 3 (α-tocopherol), 3 (α-tocotrienol), and 4 (TRF) dogs respectively, per treatment. Figure 2A is based on raw values and 2B is based on percentages compared to their respective pre-dose and post-dose control values. An asterisk indicates significant differences at P < 0.001 for each treatment compared to respective controls.

Figure 3

Effects of α-tocopherol, α-tocotrienol, and tocotrienol rich fraction (TRF) on adenosine diphosphate-induced platelet aggregation: α-Tocopherol (250 μg), α-tocotrienol (250 μg) or TRF (250 μg) were dissolved in polyethylene glycol (1 mL each) and stored at -20°C. Plasma (40 mL) was warmed to 50°C (20 min) prior to injection. The α-tocopherol, α-tocotrienol or TRF solution was added to the warmed plasma and vortexed vigorously, and then administered over 30 seconds through a femoral arterial line of the anesthetized dog. Adenosine diphosphate (ADP; 20 μmol/mL) in a volume of 32 μL was added to 400 μL of PRP that had been incubated at 37°C for 2 min. The extent of adenosine diphosphate -induced platelet aggregation was quantitated by measuring the percent maximal aggregation at 2.5 min and 6 min after adding ADP. Data are expressed as means ± SD, n = 3 (α-tocopherol), 3 (α-tocotrienol), and 4 (TRF) dogs respectively, per treatment. Figure 3A is based on raw values and 3B based on percentages compared to their respective pre-dose and post-dose control values. An asterisk indicates significant differences at P < 0.05 for each treatment compared to their respective control values.