Insights into the cardioprotective function of adenosine A₁ and A₃ receptors

Objectives: Cardioprotection (delaying of irreversible damage in hypoxia or prevention of doxorubicin [DOX] toxicity) is achieved by increasing the energy supply, or decreasing the energy demand in the cell and may be regulated through adenosine (ADO) receptor (AR) signalling. The aim of this study was to define of the protective role of ADO A₁R and A₃R against these two different kinds of stress conditions via direct action on isolated cardiomyocytes. Effects of A₁ and A₃ adenosine receptors were assessed by comparing morphological-functional tolerance, cellular energy state and contribution of the mitochondrial K_ATP channels during development of hypoxia and DOX cytotoxicity. Methods: The primary cardiac myocyte cultures were treated in a hypoxic chamber of N₂ (100%) in glucose-free media. A second group of cells were treated on day 7 in culture with 0.5 to 5 μM DOX for 18 h and then incubated in drug-free growth medium for an additional 24 h or 72 h. The hypoxic and cytotoxic damage was characterized by morphological and biochemical evaluations. Results: The A₁R and A₃R selective agonists (CCPA and Cl-IB-MECA, respectively) significantly decreased damage to cardiac myocytes under hypoxic conditions. Activation of both A₁R and A₃R (100 nM) was more efficient in protection against hypoxia than by each one alone. The A₃R agonist Cl-IB-MECA (100 nM) shows cardioprotective activity to the DOX-treated cells; however, the A₁R agonist CCPA (10 nM to 10 μM) was not effective in protection against DOX toxicity. Conclusion: Activation of both the ADO receptors (A₁R and A₃R) leads to positive beneficial effects in cultured cardiomyocytes in 90 min hypoxia, but only A₃R activation renders positive response against slowly developed DOX toxicity. Hence, the cascade of events involved in cardioprotection appears to be distinct for A₁ and A₃ receptor signalling.