The effect of myocardial infarction on the synthesis, concentration and receptor expression of endogenous melatonin.

We examined the time course of changes in the synthesis and levels of endogenous melatonin and in the expression of MT(1) and MT(2) melatonin receptors 1 day, 2 and 4 wk after myocardial infarction (MI) in rats. MI was produced by ligation of the left anterior descending coronary artery. Transthoracic echocardiography was performed to characterize structural and functional changes after MI. mRNA levels were measured by real-time quantitative reverse transcription-polymerase chain reaction and proteins by Western blotting. One day after infarction, MI rats had 4.3 times (P < 0.001) higher pineal melatonin synthesis, than sham-operated animals, which was associated with the increased concentration of melatonin in plasma (P < 0.001) and left ventricle (LV) (P = 0.01). The amount of MT(1) receptor protein decreased significantly in MI LVs compared with control LVs 1 day after infarction (P < 0.01), followed by recovery during the next 2 wk. Furthermore, the expression of MT(1) receptor mRNA of the MI LVs was elevated 2 wk after infarction (P < 0.01) compared with control LVs. The amount of MT(2) receptor proteins in MI LVs was higher than in sham-operated LVs 1 day (P < 0.05) and 4 wk (P < 0.01) after MI. In conclusion, melatonin synthesis in the pineal gland increased rapidly in response to the MI, supporting an important role for endogenous melatonin in protecting the heart after MI. The observed changes in the expression of MT(1) and MT(2) receptors suggest that melatonin receptors may be involved in mediating, at least, in part, the protective effects of melatonin in the heart after infarction.