Abstract: Objective: This study aimed to explore the regulatory effect and mechanism of intragastric administration of L-theanine on antioxidant capacity and the mRNA expression of antioxidant enzymes in the heart, liver, spleen and kidney tissues of rats. Methods: Sixty-four Sprague Dawley rats were randomly divided into control group (CON), low-dose group (LT), middle-dose group (MT) and high-dose group (HT) (with 16 animals in each group, half male and half female), which were lavaged daily with L-theanine solution at doses of 0, 50, 200 and 400 mg/kg, respectively for 14 consecutive days. After the last administration, the contents of malondialdehyde (MDA) and nitric oxide (NO), the activities of superoxide dismutase (SOD), glutathione peroxidase (GPX), catalase (CAT) and nitric oxide synthase (NOS), and the mRNA expression of SOD1, SOD2, SOD3 and GPX genes were determined in rat visceral tissues. Results: L-Theanine administration influenced the content of NO and the activities of SOD, CAT and NOS in the rat heart. The HT group presented the lowest NO content in the heart tissue among the four groups tested, and increased the activities of SOD and CAT to the highest levels. The NO content in the rat kidney was also impacted by L-theanine administration, and it was the lowest in the HT group. However, no significant effects were observed on the contents of MDA and NO, or the activities of SOD, CAT, GPX and NOS in rat liver, spleen, kidney and heart tissues except the above parameters. The mRNA expression of SOD1 in the liver, and SOD1, SOD3 and GPX1 in the spleen were significantly inhibited in the LT group, the mRNA expression of SOD2 in the spleen was up-regulated in the HT group, and the mRNA expression of SOD1, SOD2, SOD3 and GPX1 in the heart were increased significantly at all doses of L-theanine. Conclusion: L-Theanine administration could enhance antioxidant capacity in the heart, via a mechanism associated with increased SOD activity due to up-regulated mRNA expression of SOD2. At the same time, the effect of L-theanine administration on antioxidant capacity in rat visceral tissues was gender-dependent.

Key words: L-theanine, antioxidant capacity, visceral tissues, gene expression, rats