Nitric oxide synthase activity in blood vessels of spontaneously hypertensive rats: antioxidant protection by gamma-tocotrienol

Involvement of free radicals and nitric oxide (NO) has long been implicated to the pathogenesis of essential hypertension. Several studies using antioxidants as the radical scavenger have shown to confer protection against free radical mediated diseases. This study is designed to investigate the role of antioxidant gamma-tocotrienol on endothelial nitric oxide synthase (NOS) activity in spontaneously hypertensive rats (SHR). SHR's were divided into four groups namely untreated SHR (HC), treatment with 15 mg gamma-tocotrienol/kg diet (gammal), 30 mg gamma-tocotrienol/kg diet (gamma2) and 150 mg gamma-tocotrienol/kg diet (gamma3) and studied for three months. Wister Kyoto (WKY) rats were used as the control (C). Blood pressure was recorded every fortnightly by tail plethysmography. Animals were sacrificed and NOS activity in blood vessels was measured by [3H]arginine radioactive assay. Nitrite concentration in plasma was determined by Greis assay and lipid peroxides in the blood vessels by spectrofluorometry. This study showed that gamma-tocotrienol significantly reduced systolic blood pressure (SBP) in SHRs with a maximum reduction in group treated with gamma-tocotrienol 15 mg/kg diet (HC: 210 +/- 9 mmHg, gammal:123 +/- 19 mmHg). Blood vessels from untreated SHR showed a reduced NOS activity compare to that of WKY rats (C: 1.54 +/- 0.26 pmol/mg protein, HC: 0.87 +/- 0.23 pmol/mg protein; p<0.001). Gamma-tocotrienol improves NOS activity in all the groups with more significance in group gamma2 (p<0.001) and gamma3 (p<0.05). Plasma level of nitrite was reduced in SHR from 55 +/- 3 microM/ml in WKY to 26+/-2 muM/ml (p<0.001). Plasma nitrite level was reversed by treatment with gamma-tocotrienol. (gammal: p<0.001, gamma2: p<0.005, gamma3: p<0.001, respectively). In all the treatment groups, NOS activity showed significant negative correlation with blood pressure (gammal: r=-0.716, p<0.05; gamma2: r=-0.709, p<0.05; gamma3: r=-0.789, p<0.05). For plasma nitrite, although it shows a negative correlation with blood pressure it was significant only in gammal (r=-0.676, p<0.05) and gamma2 (r=-0.721, p<0.05). From this study we found that compared to WKY rats, SHR has lower NOS activity in blood vessels, which upon treatment with antioxidant gamma-tocotrienol increased the NO activity and concomitantly reduced the blood pressure. These findings further strengthen the hypothesis that free radicals and NO play critical role in pathogenesis of essential hypertension.