Purpose: To investigate the effects of melatonin on serum paraoxonase and lactonase activities in isoproterenol-induced myocardial infarction.

Materials and Methods: Rats were divided into four groups as control, melatonin, ISO and ISO+melatonin. Melatonin was injected intraperitoneally for 7 days to melatonin and ISO+melatonin groups. On 6th and 7th days, isoproterenol was injected intraperitoneally to ISO and ISO+melatonin groups. One rat for each group was kept alive for 15 days after the last ISO injection and their hearts were investigated histopathologically. On 8th day, all other rats were sacrificed and serum troponin I and malondialdehyde levels, total oxidant and antioxidant status, paraoxonase and lactonase activities were measured. Oxidative stress index was calculated from the formula.

Results: Experimental myocardial infarction was confirmed by serum TnI elevation and was supported with histopatologic examination which shows existence of fibrotic areas. Serum malondialdehyde, total oxidant status and oxidative stress index were significantly increased whereas total antioxidant status, paraoxonase and lactonase activities were significantly decreased in isoproterenol-induced myocardial infarction. Melatonin significantly decreased malondialdehyde, total oxidant status and oxidative stress index and increased paraoxonase and lactonase activities in isoproterenol-induced myocardial infarction. Melatonin increased serum lactonase activity in healthy rats.

Conclusion: Melatonin prevents a decrease in serum paraoxonase and lactonase activities seen in ISOinduced myocardial infarction.