A1 adenosine receptors and muscarinic cholinoceptors in myocardial ischemia

Abstract

The regulation of cardiac A1 adenosine receptors and M2 muscarinic cholinoceptors was investigated in ischemic rat hearts. Ischemia was induced in isolated, perfused hearts either by stop (stop-flow) or by reduction (low-flow) of perfusion flow. Receptor densities and affinities were determined by radioligand binding. The mRNA concentrations of the receptors and of control messages were measured by quantitative polymerase chain reactions (PCR). Second messenger coupling of the receptors was evaluated by measuring their inhibition of adenylate cyclase activity. Up to 60 min of stop-flow ischemia and 6 h of low-flow ischemia, cardiac A1 adenosine receptor density and affinity, and adenosine receptor-mediated inhibition of adenylate cyclase, did not change significantly, compared to non-ischemic hearts. Receptor down-regulation, however, could be induced by perfusion with the A1 receptor agonist R-phenyl-isopropyl-adenosine (R-PIA) during normal flow. After 6 h of perfusion with R-PIA (0.1 mumol/l), A1 adenosine receptor density was reduced. Agonist-induced receptor down-regulation was not found after perfusion with R-PIA in low-flow ischemia. The density and the affinity of muscarinic cholinoceptors were not affected during stop-flow ischemia up to 1 h either, whereas the density was down-regulated to 75% of controls (P < 0.05) after 6 h of low-flow ischemia. This intervention also reduced inhibition of adenylate cyclase via muscarinic cholinoceptors. In non-ischemic hearts, perfusion with carbachol (10 mumol/l) suppressed receptor densities to 72% of control values. No significant changes in the concentration of A1 adenosine receptor or M2 cholinoceptor mRNAs occurred during normal flow, stop-flow and low-flow ischemia. Likewise, agonist stimulation with R-PIA or carbachol during normal flow did not change the respective receptor mRNA concentrations significantly.

Conclusion: Although a down-regulation of A1 adenosine receptor density was demonstrated after receptor agonist perfusion with normal flow, adenosine did not affect the density or functional activity of cardiac A1 adenosine receptors in the ischemic myocardium. In contrast, muscarinic cholinoceptor density and function was down-regulated after prolonged ischemia. The lack of an agonist-induced down-regulation of A1 adenosine receptors in the presence of decreasing activity of m-cholinoceptors suggests a growing importance of the adenosine system in myocardial ischemia.