All participant will exam brachial artery endothelium-dependent flow-mediated dilatation (FMD). After then, pneumatic cuff wiil be inflated to 200 mmHg for 15 minutes to induce brachial artery ischemia. At the end of ischemia, 15 minutes of reperfusion was performed to induce reperfusion injury. After ischemia-reperfusion (IR) injury, brachial artery FMD will be measured again. After randomization, sitagliptin group will be treated by single dose of sitagliptin (Januvia) 50mg. In 2 hours later, brachial artery FMD measurement, IR injury and brachial artery FMD measurement will be measured again.

After brachial artery FMD measurement, IR injury for each 15 minutes will be performed, and brachial artery FMD will be measured again. After randomization, placebo group will be treated by nothing. In 2 hours later, brachial artery FMD measurement, IR injury and brachial artery FMD measurement will be measured again.

If sitagliptin treatment show preventive effects of IR injury, the investigator will perform additional experiment to explore the mechanism (Protocol 2 study). Additional 15 healthy volunteers will be treated 5 mg of glibenclamide (Euglucon) 1 hour before administration of 50 m g of sitagliptin. In 2 hours after sitagliptin administration, FMD measurement before and after IR injury will be performed as described above.

The brachial FMD before and after IR injury will be assessed. After randomization, study medication will be treated. In 2 hours later, the brachial FMD before and after IR injury will be assessed again. All volunteers had a wash-out period of 7 days. Seven days later, the subjects returned to crossover study medication (ie, sitagliptin or placebo), and the protocol described above was repeated.

Adverse events such as hypoglycemia