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: