Three subtypes of adenosine receptors (A(1), A(2A) and A(3) ARs) are functionally expressed in cardiomyocytes. Adenosine released during ischemia and ischemia/reperfusion plays a major role in cardioprotection. Phosphatidylinositol 3-kinase (PI-3K)/protein kinase B (PKB) and MEK/ERK1/2 pathways are involved in cell Survival. Since the role of these pathways in AR-mediated preconditioning is poorly understood, we have investigated whether PI-3K/PKB and/or MEK1/ERK1/2 pathways are involved in AR-induced card i oprotecti on in neonatal rat cardiomyocytes. Cells were pre-treated (15 min) with adenosine (non-selective), CPA (A(1)), CGS 21680 (A(2A)) or Cl-IB-MECA (A(3)) before 4 h hypoxia (0.5% 02) and 18 h reoxygenation (HX4/R). HX4/R-induced increase in LDH release was significantly reduced by adenosine (70%), CPA (59%) and CI-IB-MECA (46%). The MEK 1 inhibitor PD 98059 suppressed the effects of adenosine, CPA, and Cl-IB-MECA on LDH release, whereas the PI-3K inhibitor wortmannin did not reverse this card i oprotection. Western blotting of phosphorylated ERK1/2 and PKB during HX4/R supported the involvement of ERK1/2 and not PKB in A(1), and A(3), agonist-mediated cardioprotection. In addition, adenosine, CPA and Cl-IB-MECA inhibited HX4/R-induced caspase 3 activity by 75%,70% and 59%, respectively, and this inhibition was abolished by PD 98059. Interestingly, wortmannin inhibited by 66% the anti-apoptotic response triggered by Cl-IB-MECA but had no effect on adenosine or CPA-induced inhibition of caspase 3. CGS 21680 did not modify cell survival or caspase 3 activity. In conclusion, these data show that the preconditioning effect of adenosine requires A(1), and A(3) but not A(2A) ARs and involves an anti-apoptotic effect via MEK 1/ERK1/2 pathway in neonatal rat cardiomyocytes. In addition, A(3)AR-induced preconditioning also involves a PI-3K dependent pathway. (c) 2005 Elsevier Ltd. All rights reserved.
