A3 adenosine receptor stimulation modulates sarcoplasmic reticulum Ca2+ release in rat heart

Objective: Stimulation of A3 adenosine receptors has been shown to protect cardiac myocytes from ischemic injury, but the mechanism of this action is unknown. We evaluated the effect of adenosine agonists and antagonists on the sarcoplasmic reticulum (SR) Ca2+ channels. Methods: Isolated rat hearts were perfused with control buffer or different adenosine agonists and antagonists. Hearts were then homogenized and used to determine SR Ca2+-induced Ca2+ release, assayed by quick filtration technique after loading with Ca-45(2+), and the binding of [H-3]ryanodine, a specific ligand of the SR Ca2+ release channel. In parallel experiments, hearts were challenged with 30 min of global ischemia and 120 min of reperfusion, and the extent of tissue necrosis was evaluated by triphenyltetrazolium chloride staining. Results: Perfusion with the A1>A3 agonist R-PIA and the A3>A1 agonist IB-MECA was associated with reduced [H-3]ryanodine binding, due to reduced B-max (by about 20%), whereas K-d and Ca2+-dependence of the binding reaction were unaffected. These actions were abolished by the A3 antagonist MRS 1191, while they were not affected by A1 and A2 antagonists. The rate constant of SR Ca2+ release decreased by 25-30% in hearts perfused with R-PIA or IB-MECA. Tissue necrosis was significantly reduced in the presence of R-PIA or IB-MECA. Protection was removed by MRS 1191, and it was not affected by A1 and A2 antagonists. Hearts were also protected by administration of dantrolene, a ryanodine receptor antagonist. In the presence of dantrolene, no further protection was provided by IB-MECA. Conclusion: A3 adenosine receptor stimulation modulates the SR Ca2+ channel. This action might account for the protective effect of adenosine. (C) 2001 Elsevier Science B.V. All rights reserved.