Role of adenosine A₁ and A₃ receptors in regulation of cardiomyocyte homeostasis after mitochondrial respiratory chain injury

Activation of either the A₁ or the A₃ adenosine receptor (A₁R or A₃R, respectively) elicits delayed cardioprotection against infarction, ischemia, and hypoxia. Mitochondrial contribution to the progression of cardiomyocyte injury is well known; however, the protective effects of adenosine receptor activation in cardiac cells with a respiratory chain deficiency are poorly elucidated. The aim of our study was to further define the role of A₁R and A₃R activation on functional tolerance after inhibition of the terminal link of the mitochondrial respiratory chain with sodium azide, in a state of normoxia or hypoxia, compared with the effects of the mitochondrial ATP-sensitive K⁺ channel opener diazoxide. Treatment with 10 mM sodium azide for 2 h in normoxia caused a considerable decrease in the total ATP level; however, activation of adenosine receptors significantly attenuated this decrease. Diazoxide (100 μM) was less effective in protection. During treatment of cultured cardiomyocytes with hypoxia in the presence of 1 mM sodium azide, the A₁R agonist 2-chloro-N⁶-cyclopentyladenosine was ineffective, whereas the A ₃R agonist 2-chloro-N⁶-iodobenzyl-5′-N- methylcarboxamidoadenosine (Cl-IB-MECA) attenuated the decrease in ATP level and prevented cell injury. Cl-IB-MECA delayed the dissipation in the mitochondrial membrane potential during hypoxia in cells impaired in the mitochondrial respiratory chain. In cells with elevated intracellular Ca²⁺ concentration after hypoxia and treatment with NaN₃ or after application of high doses of NaN₃, Cl-IB-MECA immediately decreased the elevated intracellular Ca²⁺ concentration toward the diastolic control level. The A₁R agonist was ineffective. This may be especially important for the development of effective pharmacological agents, because mitochondrial dysfunction is a leading factor in the pathophysiological cascade of heart disease.