| Background—Myocardial infarction present with an increased incidence during early morning, this phenomenon conrresponds with the highest activity of sympathetic nerve and the secretion of catecholamines during early morning. Therefore, the increased activity of sympathetiv nerve and increased levels of blood catecholamines may serve as a risk factor of myocardial infarction and other events of heart attack.β-adrenoceptors (β-ARs) mediate the effects of sympathetic nerve and catecholamines.β3-AR is a subtype ofβ-ARs and is a hot spot in the study of sympathetic nervous system. However, the rhythmicity and the controlling mechanism ofβ3-AR in the heart, as well as the change ofβ3-AR expression and function following myocardial infarction (MI) and the contribution of this change to the pathophysiology of MI is still unlnown. One of the aims of this study is to investigate the changes in the circadian rhythm ofβ3-AR expression and function after MI. GHRP-6 is a synthetic growth hormone releasing peptide, which can regulate feeding, sleeping and cardiovascular function via growth hormone secretagogue receptor (GHS-R). As GHS-R is expressed in the SCN, we postulate that GHRP-6 may regulate the circadian rhythm. The second aim of this study is to test this hypothesis.Methods and Results—Diurnal variation ofβ3-AR was examined in 40 rats with acute myocardial infarction (AMI), 40 rats with chronic myocardial infarction (CMI) and 40 normal control rats. We performed relative quantitative real-time reverse transcriptase-polymerase chain reaction (real time RT-PCR) and Western blot to quantify the mRNA and protein levels ofβ3-AR in rat hearts every 3 hours over a 24-h period. Then, we performed immunohistochemical staining to quantifyβ3-AR immunoreactivity in the myocardium at ZT6 and ZT18. The responses of heart rate and myocardial inotropy toβ3-AR agonist (BRL37344) were determined at ZT6 and ZT18 in a Langendorff perfusion set. The results showed that the expression ofβ3-AR showed a strong circadian rhthm in the normal rats, it significantly increased at subjective day and breakdown at subjective night in normal control rats. Similar with the expression ofβ3-AR, the responses of heart rate and myocardial contraction force toβ3-AR agonist were higher at the subjective day and lower at subjective night in the normal control rats.β3-AR agonist induced an increase in heart rate and a decrease in myocardial contraction force and these effects were more significant at the the subjective day and than subjective night in the normal control rats. In the AMI and CMI rats, however, the circadian rhythm ofβ3-AR expression diminished. Accordingly, the responses of heart rate and myocardial inotropy toβ3-AR agonist at ZT6 and ZT18 did not show significant difference. In the second part of this study, we examined the effects of GHRP-6 on the running wheel activity and the expression of PK2 in the SCN of C57BL/6 mice, and on the circadian oscillation of heart rate in rat. GHRP-6 (100μg/kg) treatment by subcutaneous injection at CT12 induced a 41-minutes the phase delay of the locomotor activity in C57BL/6 mouse. GHRP-6 also increased the expression of PK2 in the SCN of C57BL/6 mouse compared with saline-treated mice. GHRP administration at ZT8 (subjective day) decreased the heart rate at subjective night and therefore decreased the circadian oscilation of heart rate in rats. Conclusions—(1)β3-AR in normal rat hearts showed characteristic circadian pattern in their gene and protein expression. However, this circadian rhythm disappeared after AMI and CMI. (2) Contrast to the expression ofβ3-AR, the circadian oscillations of the mRNA transcription of clock genes Npas2 and Bmal1 significantly increased after AMI and CMI, suggesting that cardiac clock genes may have a negative feedback control on the expression ofβ3-AR. (3) GHRP-6 induced a phase delay of the running wheel activity in the C57BL/6 mice and a decrease in the locomoter activity. GHRP-6 also increased the protein expression of PK2 at subjective night (CT12) in the SCN of C57BL/6 mice, suggesting that the change in PK2 expression may underly the change in locomoter activity in C57BL/6 mice. (4) GHRP-6 decreased the circadian oscillation of the heart rate mainly by decreasing the heart rate at the subjective night in rats. This result suggest that the GHS-R signaling pathway may have a regulatory role in the circadian rhythm of cardiac activity in rat. |
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