Effect of adenosine receptor blockade: preventing protective preconditioning depends on time of initiation

Sites of action of adenosine in interorgan preconditioning of the heart

The mechanism underlying interorgan preconditioning of the heart remains elusive, although a role for adenosine and activation of a neurogenic pathway has been postulated. We tested in rats the hypothesis that adenosine released by the remote ischemic organ stimulates local afferent nerves, which leads to activation of myocardial adenosine receptors. Preconditioning with a 15-min mesenteric artery occlusion (MAO15) reduced infarct size produced by a 60-min coronary artery occlusion (60-min CAO) from 68 +/- 2% to 48 +/- 4% (P < 0.05). Pretreatment with the ganglion blocker hexamethonium or 8-(p-sulfophenyl)theophylline (8-SPT) abolished the protection by MAO15. Intramesenteric artery (but not intraportal vein) infusion of adenosine (10 microg/min) was as cardioprotective as MAO15, which was also abolished by hexamethonium. Whereas administration of hexamethonium at 5 min of reperfusion following MAO15 had no effect, 8-SPT at 5 min of reperfusion abolished the protection. Permanent reocclusion of the mesenteric artery before the 60-min CAO enhanced the cardioprotection by MAO15 (30 +/- 5%), but all protection was abolished when 8-SPT was administered after reocclusion of the mesenteric artery. Together, these findings demonstrate the involvement of myocardial adenosine receptors. We therefore conclude that locally released adenosine during small intestinal ischemia stimulates afferent nerves in the mesenteric bed during early reperfusion, initiating a neurogenic pathway that leads to activation of myocardial adenosine receptors.

Ischemic preconditioning: from adenosine receptor to KATP channel

Ischemic preconditioning is a phenomenon whereby exposure of the myocardium to a brief episode of ischemia and reperfusion markedly reduces tissue necrosis induced by a subsequent prolonged ischemia. It is hoped that elucidation of the mechanism for preconditioning will yield therapeutic strategies capable of reducing myocardial infarction. In the rabbit, the brief period of preconditioning ischemia and reperfusion releases adenosine, bradykinin, opioids, and oxygen radicals. The combined effect of the release of these substances on G proteins and the cell's phospholipases induces the translocation and activation of the epsilon isozyme of protein kinase C. Protein kinase C appears to be the first element of a complex kinase cascade that is activated during the prolonged ischemia in preconditioned hearts. Current evidence indicates that this cascade contains at least one tyrosine kinase and ultimately leads to the activation of p38 mitogen-activated protein kinase. p38 Mitogen-activated protein kinase phosphorylates mitogen-activated protein kinase-activated protein kinase 2. Mitogen-activated protein kinase-activated protein kinase 2 phosphorylates HSP27, a 27-kDa heat shock protein that controls actin filament polymerization, and, therefore, affects the integrity of the cytoskeleton. Finally, mitochondrial adenosine 5'-triphosphate-sensitive K+ channels open, and the latter may be the final mediator of protection for ischemic preconditioning. The protective pathway has many built-in redundancies, perhaps creating a safety factor. These redundancies may also explain some of the species-related differences seen in ischemic preconditioning in which one redundant pathway may predominate over another.

Transgenic overexpression of cardiac A(1) adenosine receptors mimics ischemic preconditioning

The role of A(1) adenosine receptors (A(1)AR) in ischemic preconditioning was investigated in isolated crystalloid-perfused wild-type and transgenic mouse hearts with increased A(1)AR. The effect of preconditioning on postischemic myocardial function, lactate dehydrogenase (LDH) release, and infarct size was examined. Functional recovery was greater in transgenic versus wild-type hearts (44.8 +/- 3.4% baseline vs. 25.6 +/- 1.7%). Preconditioning improved functional recovery in wild-type hearts from 25.6 +/- 1.7% to 37.4 +/- 2.2% but did not change recovery in transgenic hearts (44.8 +/- 3.4% vs. 44.5 +/- 3.9%). In isovolumically contracting hearts, pretreatment with selective A(1) receptor antagonist 1, 3-dipropyl-8-cyclopentylxanthine attenuated the improved functional recovery in both wild-type preconditioned (74.2 +/- 7.3% baseline rate of pressure development over time untreated vs. 29.7 +/- 7.3% treated) and transgenic hearts (84.1 +/- 12.8% untreated vs. 42.1 +/- 6.8% treated). Preconditioning wild-type hearts reduced LDH release (from 7,012 +/- 1,451 to 1,691 +/- 1,256 U. l(-1). g(-1). min(-1)) and infarct size (from 62.6 +/- 5.1% to 32.3 +/- 11.5%). Preconditioning did not affect LDH release or infarct size in hearts overexpressing A(1)AR. Compared with wild-type hearts, A(1)AR overexpression markedly reduced LDH release (from 7,012 +/- 1,451 to 917 +/- 1,123 U. l(-1). g(-1). min(-1)) and infarct size (from 62.6 +/- 5.1% to 6.5 +/- 2.1%). These data demonstrate that murine preconditioning involves endogenous activation of A(1)AR. The beneficial effects of preconditioning and A(1)AR overexpression are not additive. Taken with the observation that A(1)AR blockade equally eliminates the functional protection resulting from both preconditioning and transgenic A(1)AR overexpression, we conclude that the two interventions affect cardioprotection via common mechanisms or pathways.

Alpha 1-adrenoceptor activation mediates the infarct size-limiting effect of ischemic preconditioning through augmentation of 5'-nucleotidase activity

We have reported that ischemic preconditioning may limit infarct size by increasing 5'-nucleotidase activity. The present study tested whether alpha 1-adrenoceptor stimulation in ischemic preconditioning mediates the infarct size-limiting effect through augmentation of 5'-nucleotidase activity. The coronary artery was occluded four times for 5 min separated by 5 min of reperfusion (ischemic preconditioning) in 82 dogs. Then the coronary artery was occluded for 90 min followed by 6 h of reperfusion. Infarct size normalized by risk area was smaller after ischemic preconditioning than in the control group (40.6 +/- 2.3 vs 6.7 +/- 2.0%, P < 0.001), even though no difference existed in endomyocardial collateral flow during ischemia (8.7 +/- 1.0 vs 8.9 +/- 1.0 ml/100 g per min). Ectosolic and cytosolic 5'-nucleotidase activity was increased after ischemic preconditioning. However, prazosin blunted the infarct size-limiting effect of ischemic preconditioning (infarct size: 42.8 +/- 3.7%). Intermittent alpha 1-adrenoceptor stimulation by methoxamine mimicked the increase in 5'-nucleotidase activity and the infarct size-limiting effect, which were abolished by alpha, beta,-methyleneadenosine 5'-diphosphate. Identical results were obtained in the conscious model (n = 20). Therefore, we conclude that increases in ectosolic 5'-nucleotidase activity due to alpha 1-adrenoceptor activation may contribute to the infarct size-limiting effect of ischemic preconditioning.