Intravenous adenosine protects the myocardium primarily by activation of a neurogenic pathway

Abstract

Endogenous adenosine is a trigger for ischemic myocardial preconditioning (IPC). Although intravascular administration of adenosine has been used to further unravel the mechanism of protection by IPC, it is questionable whether adenosine and IPC employ the same signaling pathways to exert cardioprotection. We therefore investigated whether the active metabolic barrier of the endothelium prevents an increase in myocardial interstitial adenosine concentrations by intravenous adenosine, using microdialysis, and also the role of NO and activation of a neurogenic pathway in the cardioprotection by adenosine. In pentobarbital-anesthetized rats, area at risk and infarct size (IS) were determined 120 min after a 60-min coronary artery occlusion (CAO), using trypan blue and nitro-blue-tetrazolium staining, respectively. IPC with a single 15-min CAO and a 15-min adenosine infusion (ADO, 200 microg min(-1) i.v.) limited IS to the same extent (IS = 41 +/- 6% and IS = 40 +/- 4%, respectively) compared to control rats (IS = 63 +/- 3%, both P < 0.05). However, IPC increased myocardial interstitial adenosine levels seven-fold from 4.3 +/- 0.7 to 27.1 +/- 10.0 microM (P < 0.05), while ADO had no effect on interstitial adenosine (4.1 +/- 1.2 microM), or any of the other purines. The NO synthase inhibitor N(omega)-nitro-L-arginine (LNNA), which did not affect IS (IS = 62 +/- 3%), attenuated the protection by ADO (IS = 56 +/- 3%; P < 0.05 vs ADO, P = NS vs LNNA). The ganglion blocker hexamethonium, which had also no effect on IS (IS = 66 +/- 3%), blunted the protection by ADO (IS = 55 +/- 4%; P < 0.05 vs ADO and vs hexamethonium). These observations demonstrate that cardioprotection by ADO is dependent on NO, and is primarily mediated by activation of a neurogenic pathway.

Figure 1

Experimental protocol in which the effects of IPC by 15-min CAO and of 15-min ADO (200 μg min⁻¹ i.v.) on myocardial infarct size (panel a) and myocardial interstitial adenosine concentrations (panel b) were studied. In panel c we studied the effects of NO synthase inhibition with N^ω-nitro-L-arginine (LNNA, 25 mg kg⁻¹ i.v.) and ganglion blockade with hexamethonium (20 mg kg⁻¹ i.v.) on cardioprotection by ADO. IS=infarct size; BL=baseline; I=intervention (ADO or IPC).

Figure 2

Panel a displays the protective effect of IPC (n=7) and ADO (n=7) compared to control rats that underwent only the 60-min CAO (n=7 and n=9, respectively). Panel b displays the increase in myocardial interstitial adenosine concentrations from baseline produced by IPC (n=9) and the lack of increase by ADO (n=8). ^*P<0.05 vs corresponding Control or Baseline.

Figure 3

Infarct size in control rats and in rats receiving ADO, without (Sham) or after NO-synthase blockade (LNNA) or ganglion blockade (hexamethonium, HEX). Infarct size is expressed as percentage of the area at risk. The number of animals in each group is shown within the bars. ^*P<0.05 vs corresponding Control; ^†P<0.05 vs corresponding Sham.