Induction of apoptosis in cardiac myocytes by an A₃ adenosine receptor agonist

The effects of the selective adenosine (ADO) A₃ receptor agonist IB- MECA (N₃-(3-iodobenzyl)adenosine-5'-N-methylcarboxamide) on cultured newborn rat cardiomyocytes were examined in comparison with ADO, the ADO A₁ receptor-selective agonist R-PIA (N⁶R-phenylisopropyladenosine), or the ADO A₃ selective antagonist MRS 1191 (3-ethyl-5-benzyl-2-methyl-6-phenyl-4- phenylethynyl-1,4-(±)-dihydropyridine-3,5 dicarboxylate), using digital image analysis of Feulgenstained nuclei. At high concentration, IB-MECA (≥10 μM) and ADO (200 μM) induced apoptosis; however, R-PIA or MRS 1191 did not have any detectable effects on cardiac cells. In addition, DNA breaks in cardiomyocytes undergoing apoptosis following treatment by IB-MECA were identified in situ using the nick end labeling of DNA ('TUNEL'-like) assay. In the presence of ≥10 μM IB-MECA, disorder in the contraction waves appeared, and a decrease in the frequency of beats was observed. Analysis with light microscopy revealed that the number of contracting cells decreased in a concentration-dependent manner. The A₃ receptor agonist IB-MECA caused an increase in intracellular free calcium concentration ([Ca²⁺](i)). The drug produced a rapid rise followed by a sustained increase in [Ca²⁺](i), which lasted for 40-60 s. Finally, cessation of beating and Ca²⁺ transients were observed. Full recovery of contractile activity and rhythmical Ca²⁺ transients were observed 15-20 min after IB-MECA treatment. The induction of apoptosis in the cardiocytes by IBMECA led to the appearance of features of apoptotic nuclei: the onset of condensation, compacting, and margination of nuclear chromatin. These effects were accompanied by the disintegration of the structural framework of the nucleus and nuclear breakdown. The results suggest that activation of the A₃ adenosine receptor may participate in the process of apoptosis in cardiomyocytes.