INDUCTION OF APOPTOSIS IN CARDIAC MYOCYTES BY AN A(3) ADENOSINE RECEPTOR AGONIST

The effects of the selective adenosine (ADO) A(3) receptor agonist IB- MECA -6-(3-iodobenzyl)adenosine-5'-N-methylcarboxamide) on cultured ne wborn rat cardiomyocytes were examined in comparison with ADO, the ADO A, receptor-selective agonist R-PIA (N-6-R-phenylisopropyladenosine), or the ADO A(3) selective antagonist MRS 1191 phenyl-4-phenylethynyl- 1,4-(+/-)-dihydropyride-3,5 dicarboxylate), using digital image analys is of Feulgen-stained nuclei, At high concentration, IB-MECA (greater than or equal to 10 mu M) and ADO (200 mu M) induced apoptosis; howeve r, R-PIA or MRS 1191 did not have any detectable effects on cardiac ce lls. In addition, DNA breaks in cardiomyocytes undergoing apoptosis fo llowing treatment by IB-MECA were identified in situ using the nick en d labeling of DNA ('TUNEL''-like) assay. In the presence of greater th an or equal to 10 mu M IB-MECA, disorder in the contraction waves appe ared, and a decrease in the frequency of beats was observed. Analysis with light microscopy revealed that the number of contracting cells de creased in a concentration-dependent manner. The A(3) receptor agonist IB-MECA caused an increase in intracellular free calcium concentratio n ([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 contracti le activity and rhythmical Ca2+ transients were observed 15-20 min aft er IB-MECA treatment. The induction of apoptosis in the cardiocytes by IB-MECA led to the appearance of features of apoptotic nuclei: the on set of condensation, compacting, and margination of nuclear chromatin. These effects were accompanied by the disintegration of the structura l framework of the nucleus and nuclear breakdown. The results suggest that activation of the A(3) adenosine receptor may participate in the process of apoptosis in cardiomyocytes. (C) 1998 Academic Press.