Key result
Ramelteon-induced infarct size reduction involves activation of PKG and Akt, with PKG located downstream of Akt and both upstream of mKCa-channels.
Why the study?
Although ramelteon confers cardioprotection via mitochondrial calcium-sensitive potassium channel activation, its specific upstream signaling pathway remained unknown.
p-value: p=0.0012
Ramelteon-induced cardioprotection involves activation of PKG and Akt, with PKG located downstream of Akt and both upstream of mKCa-channels.
May inform MT agonist development for cardioprotection; leaves open clinical translation pending prospective trials.
Ramelteon is a Melatonin 1 (MT1)—and Melatonin 2 (MT2)—receptor agonist conferring cardioprotection by pharmacologic preconditioning. While activation of mitochondrial calcium-sensitive potassium (mKCa)-channels is involved in this protective mechanism, the specific upstream signaling pathway of Ramelteon-induced cardioprotection is unknown. In the present study, we (1) investigated whether Ramelteon-induced cardioprotection involves activation of protein kinase G (PKG) and/or protein kinase B (Akt) and (2) determined the precise sequence of PKG and Akt in the signal transduction pathway of Ramelteon-induced preconditioning. Hearts of male Wistar rats were randomized and placed on a Langendorff system, perfused with Krebs–Henseleit buffer at a constant pressure of 80 mmHg. All hearts were subjected to 33 min of global ischemia and 60 min of reperfusion. Before ischemia, hearts were perfused with Ramelteon (Ram) with or without the PKG or Akt inhibitor KT5823 and MK2206, respectively (KT5823 + Ram, KT5823, MK2206 + Ram, MK2206). To determine the precise signaling sequence, subsequent experiments were conducted with the guanylate cyclase activator BAY60-2770 and the mKCa-channel activator NS1619. Infarct size was determined by 2,3,5-triphenyltetrazolium chloride (TTC) staining. Ramelteon-induced infarct size reduction was completely blocked by KT5823 (p = 0.0012) and MK2206 (p = 0.0005). MK2206 with Ramelteon combined with BAY60-2770 reduced infarct size significantly (p = 0.0014) indicating that PKG activation takes place after Akt. Ramelteon and KT5823 (p = 0.0063) or MK2206 (p = 0.006) respectively combined with NS1619 also significantly reduced infarct size, indicating that PKG and Akt are located upstream of mKCa-channels. This study shows for the first time that Ramelteon-induced preconditioning (1) involves activation of PKG and Akt; (2) PKG is located downstream of Akt and (3) both enzymes are located upstream of mKCa-channels in the signal transduction pathway.
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Torregroza et al. (2020) studied Myocardial ischemia/reperfusion injury. Ramelteon vs. Ramelteon + inhibitors (KT5823, MK2206) or control was evaluated on Infarct size (p=0.0012). Ramelteon-induced infarct size reduction involves activation of PKG and Akt, with PKG located downstream of Akt and both upstream of mKCa-channels.
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