To limit ischemia/reperfusion injury, the endogenous peptide apelin-13 (Ap) is effective if given at the concentration of 0.5 mM after ischemia, while uncertain results are obtained if given before. The difference has been attributed to an ischemia-induced increase in APJ receptors. Aim of the study: a) to see whether before ischemia the protection is induced by increasing the concentration of Ap; b) the relationship between the time of receptor increase and the protection.

Isolated rat hearts perfused with oxygenated buffer underwent 30 min of global ischemia followed by 120 min of reperfusion (Rep). Left ventricular pressure and infarct size were studied with Ap given either before or after ischemia at 1 and 0.5 mM concentrations respectively.

In other experiments, Rep was stopped at 15, 30 or 120 min and the APJ mRNA and protein level were measured with RT-PCR and Western blot respectively. Finally we studied whether Ap administration affects APJ mRNA and protein level.

Before ischemia, Ap did not cause any protective effect even if given at 1 mM concentration. APJ mRNA and protein level were increased at 15 min of Rep and were back to the control at 120 min. A similar increase was caused by Ap, no matter whether ischemia was performed or not. Since the increase in APJ level due to Ap is similar to that caused by ischemia, we conclude that the increase of the receptors is not sufficient to explain why the protection takes place only if the peptide is given after ischemia.